colophon EN:l'astrofilo30/04/1407:54Page2 A new astronomy magazine for you editorialchief in Editor Ferrara Michele read! nice a Have Magazine. Astronomy Free about opinion your welcome Wewould activity. publication and philosophy our of concepts key the are satisfaction Readers’ and nication commu- of quality, ease graphic utmost content, scientific the of Accuracy read. to material interesting some find surely can beginners less-experienced those also but Magazine, Astronomy Free by published articles the appreciate fully to able be will knowledge tific scien- minimal with astronomer amateur Any technical. too are that tents con- or matters subject reach therefore, to avoid, and need audience possible as the wide a account into takes covered topics the of choice The astronomers. professional by made discoveries latest the with way detailed and simple most the in deal to is objective main Our same. the of version English the online publishing by reach its broadening now is multimedia “l’Astrofilo” free magazine the Italian, in publications of years 5 than more After you. welcome to pleased is Magazine Astronomy Free Readers, Dear kepler186 EN:l'Astrofilo 30/04/14 07:42 Page 4

4 EXOPLANETS JustJust aa stepstep away away frofromamanewnew EaEarthrth

For the first time, the existence of a large planet like the Earth, with an orbit entirely inside the habitable zone of its star, has been confirmed. We do not know if it has an atmosphere and if its surface is favourable to life's flourishing, but its discovery is nonetheless an important landmark in the search for other Earths.

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stronomers have achieved another historic result in the long journey A taking them toward finding an ex- trasolar planet with characteristics very sim- ilar to those of Earth. That breakthrough step consists in the confirmation of the ex- away away istence of an Earth-sized planet located in the habitable zone of its star. The main stages leading up to this extra- ordinary milestone had been, since the ‘90s: the discovery of the first planets around stars other than the Sun; the dis- covery of giant planets in the habitable zones of these stars; the discovery of super-Earths both inside and outside the w habitable zones; the discovery of Earth- sized planets and even smaller outside the habitable zones; and now the discovery of a potential new Earth. It took more than 20 years to complete the journey that led us from believing that our planetary sys- tem was perhaps unique, to the awareness that in our galaxy alone there may be mil- lions of Earth-like planets. To date, thanks mainly to the Kepler space telescope, astro- nomers have identified over 3,800 candi- date planets, of which around 1,800 have already been confirmed. Among these there is Kepler-186f, the planet that repre- sents the last step before the discovery of an ever increasingly possible “Earth-2”.

n the background, a fantasy view of Kepler- I186 system, with, in the foreground, the Earth-sized planet Kepler-186f. In the video above, an animated reconstruction of that distant planetary system, with the orbits of the four inner planets and an artist’s impres- sion of what Kepler-186f may look like. [NASA Ames/SETI Institute/JPL-Caltech]

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That planet orbits around Kepler-186 (one planets similar to our own. This is due to comparison of more than 156,000 stars which have several reasons: their habitable zones are A between the been monitored for years by Kepler) also more internal and smaller than those of size of Earth and known as KIC 8120608, KOI 571 and – after stars like the Sun and more massive, and Kepler-186f, and its sky coordinates – 2MASS J19543665 hence the most interesting planets transit between the size of the Kepler-186 +4357180. It is a red dwarf with spectral more often on the stellar disk, shortening planetary system type M1, a diameter of about 660,000 kilo- the time for their discovery; in addition, the with that of our metres (47% of that of the Sun), a surface ratio between the star’s diameter and the inner solar sys- temperature of nearly 3800 kelvin and an planets’ diameter is more favourable than tem. All elements abundance of iron which is half that of the in other cases, and consequently the drop are drawn to Sun. Kepler-186 is located scale. Kepler- 492 light-years from Earth, 186f’s orbit is as in the constellation Cyg- large as that of nus, and has an apparent Mercury. Left: an visual magnitude of 14.9 animation show- (in the extreme red end of ing the effects of the visible spectrum, while a planetary tran- sit. [NASA in the infrared it reaches a Ames/SETI Insti- magnitude of 11-12). Red tute/JPL-Caltech] dwarf stars are very com- mon as, in fact, they ac- count for 3/4 of the stellar population of our Galaxy (and perhaps of the entire universe) and are an ideal target in the search for kepler186 EN:l'Astrofilo 30/04/14 07:42 Page 7

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he graph in flux during transits is more evi- T shows the dent; finally, the reduced stellar signals of the 5 mass can facilitate the detection planets discov- of possible changes in the radial ered around Ke- velocity, allowing to trace back pler-186. The the planet's mass. For how differ- gray dots are the individual photo- ent from the Sun, it is not thus a metric measure- coincidence that a large sample ments, the blue of red dwarfs has been included dots the medians among the stars to be monitored and the solid red with Kepler. One of those red lines represent dwarfs was precisely Kepler-186, the more proba- which in the first two years of ble theoretical constant photometric monitoring light curves. The (one measurement every 29.4 star’s dimming is minutes) showed no less than 4 given in parts per separate series of transits, which million (ppm). [E. astronomers attributed to as Quintana et al.] Below: a compari- many planets, named Kepler- son between the 186b, Kepler-186c, Kepler-186d, transit of the Kepler-186e – the letter “a” is not Earth on the Sun used for the planets in that it and of Kepler- identifies the star – and with or- 186f on its star: bital periods ranging from 3.9 to the second is 22.4 days. about 4 times Such rapid revolution periods are deeper and there- indicative of short distances from fore easier to de- the red dwarf – in this case be- tect. [Wendy tween 5 and 19 million kilome- Stenzel] tres – and, therefore, of much higher surface temperatures than those on Earth. None of those four planets can consequently be considered an analogue to Earth, even though their di- mensions are very interesting in all having a diameter less than 1.5 Earth diameters, with one, the “186b”, which diameter is only 8% larger than that of our planet. But then, at the end of the third year of monitoring that distant planetary system, as- tronomers made official the ex- istence of a fifth planet, named Kepler-186f, which was observed transiting over the disk of the little star every 129.9 days at an average distance from the same of 58.7 million kilometres. Such distance places it within the out- kepler186 EN:l'Astrofilo 30/04/14 07:42 Page 8

8 EXOPLANETS

er boundary of the habitable zone, which served light curve. Those tests were perform- f Kepler-186f in the specific case of that star extends from ed by a team of about twenty researchers I had an atmo- 32.9 to 59.8 million kilometres. From the at- led by Elisa Quintana (SETI Institute and sphere compara- tenuation of the starlight during the transit NASA Ames Research Center), who made ble to that of the of Kepler-186f, it has been calculated that use of two cutting-edge astronomical re- Earth, it would generally appear the planet has a diameter 1.11 times great- search instruments, the Gemini North tele- very similar to er than the terrestrial one – hence 14,160 scope (8 meters diameter) and the Keck II the latter. In the km – with a possible error margin of ± 1,800 (10 meters diameter), of the neighbouring comparison (it could therefore be exactly as large as the observatories on Mauna Kea, Hawaii. above, the new Earth). As with all candidate planets discov- Since Kepler-186f is not sufficiently massive planet is imag- ered by Kepler, also for the fifth Kepler-186 to produce detectable effects on the radial ined with wide planet had been necessary to carry out tests velocity of its star, and neither capable to and deep oceans. from the ground with telescopes capable of dynamically perturb the four inner planets, [PHL/UPR Arecibo, excluding any alternative interpretation of the Quintana’s team could only prove the NASA] From the the planetary transit, relatively to the ob- actual existence of that planet and exclude animation on the the existence of other side it can be objects capable of pro- seen how, viewed from the Earth, ducing the same light the transits of the curve. Regrettably, no 5 planets of Ke- telescope is today pow- pler-186 alter the erful enough to directly light curve of the show an Earth-size plan- star. [Gemini Obs. et at such a short dis- E. Quintana et al.] tance from a red dwarf, but it is though possible to verify if there are any other small and even fainter stars, prospective- ly close to Kepler-186, that can “mimic” the ef- fects of a planetary tran- kepler186 EN:l'Astrofilo 30/04/14 07:42 Page 9

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he diagram sit. In particular, we T on the left, must be able to ex- besides providing clude the possibility a scale of the or- that a companion bits of Kepler-186 star partially eclipses planets, compares the primary, or that the habitable zones (in gray) of a background vari- four planetary able star contami- systems, showing nates the light curve the amount of of Kepler-186. energy received Just to dispel any from the various doubt, the team led planets (Earth = 1). by Quintana has car- [S. Raymond/E. ried out observa- BOLMONT et al.] tions at high spatial Below: Elisa resolution in the im- Quintana, the re- mediate area sur- search scientist rounding the red who, with about twenty colleagues, dwarf. Using the confirmed the speckle imaging existence of Ke- technique with the pler-186f. The re- Gemini telescope, sults of the work they were able to were published in move – in the visible the April journal range – as far as 4.2 Science. astronomical units (AU) toward the star without finding any

suspicious object; a ally exists. The remaining 0.02% is the prob- result achieved also ability that a very small companion star in the infrared with exists at between 1.4 and 4.2 AU from Ke- the Keck II (fitted pler-186. At distances below 1.4 AU there with the Natural cannot be anything significant as the plan- Guide Star Adaptive etary system would not otherwise be as Optics system and the stable as it appears – a stability which has Near-Infrared Cam- lasted several billion years (8-10 and maybe era 2). The images more). obtained with the Having ascertained that Kepler-186f exists, two telescopes have the researchers then attempted to under- allowed to conclude stand the extent to which it is similar to that there are 99.98 Earth. Its location towards the outer edge chances out of 100 of the habitable zone guarantees to it that Kepler-186f re- about 1/3 of the energy that our planet kepler186 EN:l'Astrofilo 30/04/14 07:42 Page 10

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receives from the Sun; not much, but enough to ensure that any wa- ter on its surface stays liquid, at least in warmer regions and during periods of max- imum insolation. (It must be considered that around noon Ke- pler-186 is as bright in the sky of Kepler- 186f as our Sun appear to us around sunset.) A favourable ground temperature and the resultant presence of liquid water are, how- ever, strongly depen- dent on the existence, composition and thick- ness of the atmo- sphere, of whose pos- sible existence we know nothing. The models of the thermal evolution of planets predict that an object mass should not diverge too much from f the nearly with a diameter less than 1.5 Earth diame- 1.44 Earth masses. O 1,800 exo- ters has little chances to preserve for long From all this and hypothesize that Kepler- planets known, an atmosphere dominated by hydrogen 186f may be a place suitable to host life only a handful and helium. At Kepler-186f’s distance, these there is a lot in between, since many other orbit in the habit- able zone of their elements would evaporate during the ear- unknown-to-us factors might intervene to stars and have di- ly stages of a red dwarf‘s existence, since radically worsen the scenario. We do not mensions not too those stars are subject to a very lively sur- know, for example, if the planet rotates on dissimilar from face activity and emit intense ultraviolet ra- its axis, or if it always shows the same hemi- those of Earth. diations. It is therefore likely that Kepler- sphere to its star, as the other Kepler-186 This diagram 186f retained an atmosphere not too dis- planets most probably do. Neither we do shows them ac- similar to that of the Earth. know if it has a magnetic field capable of cording to size, More difficult, instead, is to make any as- protecting a possible biosphere from cos- distance from the sessment on the mass of that planet, which mic radiations. In fact, we only know that stars and energy can only be estimated within a wide values Kepler-186f exists, that it orbits in a poten- received from range that has at its extremes a composi- tially favourable region and that its size is them (Earth = 1). tion exclusively based on water and a com- comparable to that of the Earth. Despite [Chester Harman, PHL/UPR Arecibo, position of pure iron. In the first case Ke- knowing only these few things, its discov- NASA/JPL] pler-186f would have a mass equal to 1/3 ery is the most important step that could that of Earth, while in the second case it still be taken towards the discovery of a would weigh almost as much as 4 Earths. second Earth, and in the coming years, If, as it seems more plausible, the planet when the instruments available to astron- were to have a composition closer to that omers will allow to directly investigate its of the rocky planets of our solar system atmosphere and that of similar planets, we (about 2/3 of silicates, 1/3 of iron-nickel, will know if and to what extent those envi- traces of water and other elements), its ronments can host life as we know it. n shergottiti EN:l'Astrofilo 30/04/14 07:30 Page 12

12 SMALL BODIES TheThe MojaveMojave CraterCrater andand the the origin origin ofof shergottitesshergottites

According to some researchers, about ust north-east of Meridian Planum, where a decade ago the rover Opportu- 3 million years ago a small asteroid Jnity landed, there is a large impact crater called Mojave. It has a slightly oval shape crashed into Mars’ surface, creating a and its significant size, roughly between 55 large crater and sending rocks of var- and 60 km across and about 2600 meters deep, means that it was created by the high- ious sizes hurtling into space. Part of speed collision of an asteroid a few kilome- them ended up falling on Earth and tres in diameter. Located at 7.5° above the Martian equator, the Mojave Crater is part once recovered and analyzed were of the region called Xanthe Terra, marked by classified as shergottites. To what ex- the confluence of two huge outflow chan- nels, Simud Vallis and Tiu Vallis. The crater tent is this scenario realistic? lies therefore within a geological context,

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SMALL BODIES 13 CraterCrater gingin ofof ss

n the background already in itself interesting, and I and in the video the clearly noticeable and well- on the side, a sim- preserved rays of ejecta surround- ulation of the Mo- ing it suggest that it is a relatively jave Crater based recent formation. The whole sce- on images taken nario has made the Mojave Crater by the MRO, pro- an ideal candidate for a study con- cessed in 3D ducted by Stephanie C. Werner thanks to the ste- reoscopic effect (University of Oslo), Anouck Ody provided by diffe- (Université de Lyon) and François rent camera an- Poulet (Université Paris Sud). The gles. [NASA/JPL objective of the three researchers Caltech/Univer- was to discover the area of origin sity of Arizona] of a well-known group of Martian

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meteorites, classified as shergot- tites. Together with nakhlites and chassignites, shergottites make up the total of more than 140 samples of Martian rocks known to date, ascribed to 69 different episodes of impacts against the Earth's surface. Even though they were in each case only small objects (just 3 above 10 kg), they are nonethe- less extremely important since they represent different mo- ments of the geological evolu- tion of Mars. In order to make the most of this evidence, it is however necessary to know the exact place of origin of the pre- cious rocky material, consisting basically in identifying an im- pact crater with features compa- tible with the properties of cer- tain Martian meteorites. The start- ing point is to obviously analyze the mineralogical properties of these latter ones, which is in it- self relatively simple since they are physically available and be- cause they can be subjected to various laboratory tests. From the countless analyses of recent decades has emerged that near-

ly all those meteorites are mineralogically re- n these images it lated to the three groups mentioned above, I is possible to ob- that three-quarters of them belong to the serve the morphol- shergottites group and that the time spent ogical characteris- tics of the rays of in space by the original rock material before ejecta surrounding falling to Earth is about 11 million years for the Mojave Crater the nakhlites and chassignites, and 1 to 5 and the manner in million years for the shergottites. Since the which they extend. time elapsed from reaching Earth and their [Science/AAAS] shergottiti EN:l'Astrofilo 30/04/14 07:30 Page 15

mpressive 3D discovery can be measured in thousands of the cratering rate of the material scattered I renderings of years at most (all previous falls are irretriev- around (ejecta) and of the internal floor. the perimeter bor- ably lost), it is clear that the time spent in By applying a model developed within the der of the Mojave space, estimated by the action of cosmic rays Apollo program and adapted to rocky bod- Crater highlighted on exposed surfaces, is almost equivalent to ies other than the Moon, the researchers in the image the time elapsed from the impact on Mars have concluded that the crater must have above. In order to emphasize the that sent those rocks into space. formed about 3 million years ago and it is elevations, the Werner and her colleagues have identified therefore compatible with the time of origin vertical dimen- as the only source of shergottites the Mo- of the shergottites. By considering, more- sions have been jave Crater, following a careful analysis of over, that an impact capable of creating a exaggerated by a factor of 3 com- pared to the hori- zontal ones. These terrains could be closely related to the shergottites. [NASA/JPL-Caltech Univ. of Arizona] shergottiti EN:l'Astrofilo 30/04/14 07:30 Page 16

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crater of this size occurs on Mars only about connaissance Orbiter. As a result of the ob- he coloured every 35 to 50 million years, it becomes sta- servations by the two spectrometers, both T pixels over- tistically unlikely the existence of other via- inside and around the wide perimeter bor- laid on this image of the Mojave ble shergottites progenitors, unless proving der of the Mojave Crater, it was possible to Crater show the that also far more modest impacts are capa- detect the presence of chemical compounds regions where ble of sending rocks flying into space. that are typical of shergottites, such as py- the OMEGA and A mere time coincidence is not however roxene and, more in particular, olivine. The CRISM instru- enough, as it is also necessary to show a min- correspondence between the proportion ments detected eralogical correspondence. To pursue this and abundance of individual compounds the highest con- goal, the Werner’s team analyzed data would seem to leave little doubt about the centrations of pyroxenes and collected by the OMEGA instrument (also correlation between the Mojave Crater and olivine, whose known as the Visible and Infrared Mineralo- shergottites. In fact, that is the only structure abundances are gical Mapping Spectrometer) aboard the in terms of size, age and mineralogy that can consistent with Mars Express spacecraft, and by the CRISM be a candidate site as the place of origin of those found in instrument (Compact Reconnaissance Ima- shergottites. But there is a shergottites. [C. ging Spectrometer for Mars) on the Mars Re- problem, by no Werner et al.] On the side: a section of the Dar al Gani 476 shergottite. [Steve Jurvetson]

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hergottites means minor, as regards the Sfound in the shergottites’ age which Dar al Gani re- turns out to be much youn- gion, Libya. The ger than that of the terrains typical reddish- from which they broke brown colour of away. The Xanthe Terra re- the surface is due to the oxidized gion is in fact 4.3 billion iron contained in years old, while the radio- the combination carbon dating performed of minerals that on the available shergottites characterises samples (all volcanic rocks) them. The several sets their age at between dark looking in- 150 and 600 million years clusions scattered ago. How can this huge age on the surface difference be explained? are chondrules. One of the main opponents [Utas collection] to the hypothesis put for- ward by Werner and her col- heat generated by previous small asteroids im- pacts, or as a result of the action of flowing water. Susceptible to this type of alterations would be for example an iron sulfide mineral called pyrrhotite, which was one of the most used for dating the shergottites. The isotopic clock could therefore be turned back, but the doubts remain. According to Mc- Ewen there are other craters which are more appropriate to be linked to shergottites, but it remains to evaluate whether the impacts that created them were able to transfer to the sur- face rocks enough energy to accelerate them to the escape velocity from Mars, i.e. more than 5 km/s. Some age-dating doubts have also emerged as regards the Mojave Crater, which leagues is Carl Agee (Institute of Meteoritics, University of New Mexico), who argues that such discrepancy is sufficient evidence to invalidate the hy- pothesis itself. Of the same opinion is also Alfred McEwen (University of Arizona), Princi- pal Investigator for the HiRISE instrument, the leading edge camera aboard the Mars Re- connaissance Orbiter. But Werner’s team contends that some of the elements con- stituting the shergottites may have undergone some alter- ations caused by the strong shergottiti EN:l'Astrofilo 30/04/14 09:11 Page 18

according to many geologists would be much older than the age (from a minimum of 1 to a maximum of 5 million years) suggested by the cra- tering model adopted by Werner. In fact, on the perimeter border and in- side the crater are recognizable some hy- drogeological formations that should date back to the time when the water flowed abundantly and regularly on the Martian surface; that is to say at least hundreds of millions of years ago. According to Werner are significantly inconsistent though, the impact may have led to the with the age of the crater, which, creation of a local hydrothermal system, as we recall, is about 3 million years which would have remained ac- old. The meteorites that were exposed to cos- little quiz for tive for decades or mic rays for a longer period of time may have A our readers even accumulated some radiation when they experts in meteor- were still Martian rocks ites. The sample (or more likely that you see pho- tographed front the crater is and back here, somewhat can it be a sher- older), gottite? It mea- w h i l e sures 5.5 cm on t h o s e the longest side that are and weighs 85 younger grams. Anyone may be who would like fragments to answer can t h a t write to while in info@astropub- s p a c e lishing.com broke away from the main body about 1 or more million years after leaving Mars. Such fragmenta- tion could be the result of pre-existing fractures, upon which then acted the planets’ grav- centuries; a itational forces and/or other factors. In con- period of time clusion, the connection between the Mojave considered sufficient Crater and shergottites is perhaps not as to produce the formations still visible today. clear-cut as the recent work by the Werner’s Another issue to be clarified is the amount team, published in the journal Science, would of time that the shergottites spent in inter- like to show. At present there are, however, planetary space, which, as previously said, no other more convincing alternatives and varies between 1 and 5 million years. If we neither there are at this stage any better sug- consider that in order to be recovered and gestions as regards nakhlites and chassigni- recognized, all the known samples must tes, whose age is about 1.3 billion years: their have fallen to Earth no more than a few connection with such an old crater could well thousand years ago, the two extreme values remain an impossible undertaking. n shergottiti EN:l'Astrofilo 30/04/14 09:11 Page 19 chariklo EN:l'Astrofilo 30/04/14 07:36 Page 20

20 SMALL BODIES TwoTwo ringringsfsforor CharikloChariklo

Ring systems are no longer the prerogative of the giant plan- ets of our solar system; we now also know of the existence of an asteroid sur- rounded by that kind of structures. This discov- ery opens up new horizons on the evolution of mi- nor bodies and poses some rather interesting questions.

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SMALL BODIES 21 forfor CharikloChariklo

hiron’s wife (and daughter of Apollo), Chariklo, has two rings. In the Greek C mythological context this would not be unusual, but if with Chariklo we mean the asteroid catalogued with the number 10199, then the fact that it has two rings turns into something amazing. And that’s exactly the case. The news of the unexpect- ed discovery of a ring system orbiting the distant asteroid (2.4 billion kilometres from the Sun) was made official in late March by Felipe Braga-Ribas (Observatório Nacional/MCTI, Rio de Janeiro), with a press conference and an article published in the weekly journal Nature. It all started over a year ago, by drawing up a list of possible stellar occultations by the Centaurs group of asteroids and trans- Neptunian objects. The calculations had indicated that the largest known centaur, Chariklo, on 3 June 2013 would have passed in front of a star of magnitude 12.4 (UCAC4 248-108672). The centre of the hariklo’s rings as they would be seen v i s i b i l i t y Cby an astronaut from the surface of b a n d the asteroid.[ESO/L.Calçada/NickRisinger]

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would have been in South America and, consequently, Braga-Ribas along with several of his colleagues decided to plan an observing campaign to follow with various instru- ments the photometric trend of the event. The project in- volved the telescopes of 8 different observatories in Brazil, Argentina, Uruguay and Chile, in- cluding the TRAPPIST, a Danish robotic tele- scope (1.5 metre diameter), located at La Silla Observatory (ESO) in Chile, specialized in planetary transits and thus suitable for stellar occultations. The need for more ob- servational points along the visibility band of the phenomenon derives from the fact that in spite of how well the position of the occulted star and the orbit of the asteroid is known, the diameter and shape of the latter are, in most cases, not known with the same degree of precision and it is there- fore not possible to predict with absolute certainty which areas will be covered by the shadow. Since this principle works both ways, if we are able to detect the bounda- ries of that shadow, on the basis of the oc-

he Transiting Planets and Planetesimals Small T Telescope (TRAPPIST) – the instrument that al- lowed us to discover Chariklo's rings. [E. Jehin/ESO]

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particular interest are rare, and one of these was precisely that where Chariklo play- ed the leading part. Like all Centaurs, also Chariklo has an unsta- ble long-period orbit, shaped in such a way as to make it move between the giant planets of the solar system, between Sat- urn and Uranus in this case, with a “gravity predilection” for the latter to which is al- most bound by a 4:3 orbital resonance (a Chariklo year lasts 63.17 terrestrial years while that of Uranus 84.32 years, hence 4 years of the first last nearly 3 years of the second). When discovered in 1997 by James Scotti he Chariklo cultation timings it is possible to determine (Spacewatch program), Chariklo demoted T system “seen” the diameter and approximate shape of the from its status of largest centaur the already from a few thou- asteroid (i.e., that projected during the few mentioned and better-known Chiron: 250 sand kilometres minutes duration of the event). It is conse- kilometres across the first and 230 kilome- away. The aster- quently important to arrange observing tres the second. Besides the need to verify oid has a very points also along the borders of the theo- the dimensions of the asteroid, initially esti- low albedo, about half that of the retical outer boundary of the cast shadow, mated from the thermal properties of al- rings. In the since the missed oc- video to the right cultation on the part a simulation of of some observers is the asteroid’s equally important as approach (depic- its recording by ted in its most others. Observations likely shape). of this type of phe- [ESO/L. Calçada nomena are com- M. Korn-messer monplace activities N. Risinger] for astronomers as, in fact, they are quite recurrent during the year, with some of them also well within the reach of small telescopes. But those involving objects of chariklo EN:l'Astrofilo 30/04/14 07:37 Page 24

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n the illustration to the right and in the video Ibelow, a spectacular reconstruction of the Chari- klo system, with a transit between the two rings discovered by the Braga-Ribas’ team. We also note the presence of a small satellite that with its action provides the gravitational confinement that gives to the rings their well-defined shape. [ESO/L. Calçada/M. Kornmesser/N. Risinger]

bedo (and then actually confirmed by the occultation observation), astronomers also wanted to use the event to understand the causes of the unusual spectrophotometric behaviour to which Chariklo had been sub- jected between 1997 and 2008, when its brightness had been gradually dimming by nearly a factor of 2 and from its spectrum had disappeared the absorption band at 2 μm of the water ice. Then, from 2008 to 2013 its brightness started to pick up again to almost the level of '97 and the presence of water ice became detectable again. Mys- teries that the predicted occultation would have probably allowed to unravel. Of all the telescopes dedicated to the obser- vation of the phenomenon, 3 sited in Chile recorded the event in line with predictions and 7 in total recorded a dozen unexpected events in the form of secondary interrup- biting Chariklo. This is not the first time that tions of the luminous flux of the star. Due ring systems around solar system bodies are to the high acquisition rate of images by the discovered through stellar occultations, as it Lucky Imager camera of the Danish tele- had already happened in 1977 with Uranus scope at ESO, two secondary events (one and in 1984 with Neptune, but it is certainly preceding and the other following the main the first time that rings are discovered one) were each resolved into two sub- around an asteroid. Chariklo – for however events, which the team of researchers led by small – becomes therefore the fifth plane- Braga-Ribas interpreted as a pair of rings or- tary body to have a system of rings after the chariklo EN:l'Astrofilo 30/04/14 07:37 Page 25

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four giant planets. From the timings and respectively of 6.6 km and 3.4 km, with a di- depth of the secondary occultations attribu- vision wide almost 9 km, inside which no table to the rings (symmetric with respect to material was detected. The mass of the first the central event), the researchers were ring is comparable to that of a frozen object able to mark out with high accuracy that of about 1 kilometre in diameter, while the system. In summary, both rings are com- mass of the second is significantly smaller. plete, rather dense and have well-defined The fact that all the secondary drops in flux edges. The inner ring has a radius of 391 km are consistent with a system of homoge- and the outer one of 405 km; the width is neous rings throughout their length, makes chariklo EN:l'Astrofilo 30/04/14 07:37 Page 26

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it highly improbable any alternative interpretation for those structures. They cannot, for example, re- present traces of cometary activity such as that observ- ed on Chiron. On the contrary, those sec- ondary events perfectly suit the interpretation that saw the two rings reach the more favourable inclina- tion for their visibility (60°) in 1996-97, and presented, instead, edgewise to us in 2008 (in keeping with the relative orbital motion of Chariklo and Earth). This scenario provides the sim- plest possible explanation for the aforementioned photometric variability of the asteroid and for the dis- appearance-reappearance of the absorption band of water ice, element that evidently dominates urn’s A-ring (0.3). Depending on the thick- n the diagram the rings and falls below the threshold of ness that’s attributed to Chariklo’s rings and Iabove we can visibility when the same are not visible on the models adopted to interpret its evo- see how the var- (since edgewise). lution, we find out that their existence ious observers If we assume that the photometric variabil- should last anywhere from a few thousand recorded the stel- lar occultation in ity of the Chariklo system is entirely attribu- to a few million years. Consequently, either which Chariklo table to the geometric variations of the they are very young structures or acting had the leading rings, it follows that their albedo (surface upon them there is a confinement mecha- role. The green reflectivity) is about 0.09, a value higher nism similar to that of Saturn’s F-ring, but segments repre- than that of the asteroid and of the rings of in this case provided by a “shepherd” satel- sent the most re- Uranus (0.05), but lower than that of Sat- lite with a mass comparable to that of the liable recordings same rings. (their length is So far no satellite has proportional to been discovered in or- the margin of bit around Chariklo, error) [F. Braga- but the assumption is Ribas et al.] plausible, given that In the video: re- presentation of at least 5% of Cen- the occultation taurs host satellites. seen from a par- In addition to the is- ticularly favour- sue of the long per- able position. sistence of the rings, [ESO/L. Calçada] there is also another, no less important, mat- ter to solve. How did they form? Braga-Ribas and col- chariklo EN:l'Astrofilo 30/04/14 07:37 Page 27

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ight curve of Lthe occultation, recorded in the red band with the TRAPPIST. Outside the occultation, the sum of the flux composed of the contribution by the star and the asteroid is consid- ered = 1. Besides the very sharp drop in flux caused by the passage of Chariklo over the star, we can clearly see the two sec- ondary drops in flux attributable to the rings. The small asymmetries observable in the curve are within the margins of error of the re- cording. [F. Braga- leagues propose several possible scenarios, disk may have derived from a past cometary Ribas et al.] all relying on the preliminary existence of a activity of Chariklo. Below: video of disk of debris, including one sufficiently In the light of current knowledge, the most the occultation massive to condition with its presence the likely hypothesis seems to be that the ma- with the related evolution of the entire structure. That disk terial was ejected from the asteroid as a re- light curve. The drop in flux due could have been originated from material sult of an impact – and then subsequently to the rings sys- coming from the surface of the asteroid, confined by a relatively massive shepherd- tem is easily rec- which entered into orbit following a meteor ing body not yet discovered. ognizable. [ESO/ impact, or from the destruction (for inscru- The small Chariklo’s mass and resultant low F. Braga-Ribas/ table reasons) of one or more satellites. A escape velocity (only 360 km/h) though re- M. Kornmesser] (not very convincing) alternative is that the quire that the impact occurred at a rela- tively low speed, much lower than those typical of today’s interplanetary space occu- pied by Centaurs and by Kuiper belt objects (KBOs) – probable place of origin of the asteroid in question. Prior to that system being perturbed by the initial arrangement of the planetary orbits, the relative veloci- ties among KBOs were lower and the im- pact that favoured the formation of the disk first, and the rings later, could have occurred in that ancient time and in those remote regions of the solar system. This scenario does however raise new questions about the persistence of the rings and about their apparent uniqueness among asteroids. n planetX EN:l'Astrofilo 30/04/14 07:32 Page 28

28 PLANETOLOGY

Planet X and its variants; a shattered myth

The latest infrared sky mapping has confirmed that there is no planet of significant mass with an orbit between that of Neptune and the Oort Cloud. Among the thousands of new objects discovered around the Sun, no one seems to correspond to Planet X or a variant thereof.

lanet X is without a doubt one of the that has the potential to enchant and in- ew and more greatest myths of astronomy of re- spire, also that of Planet X has over time Nrestrictive lim- P cent centuries. Its as much mysterious been fuelled by the most diverse arguments its were placed as hypothetical existence has always at- and interpretations, so as to be identified on the presence beyond Neptune’s tracted the general public’s attention, becom- at times as a mere planetary body, some- orbit of giant plan- ing in time the connecting link between times as a small star, or as something half- ets like the one science fiction and astronomical disciplines, way between the two. Some more daring shown here. If and favouring at times an increased inter- theories have ended up distorting in many any planets exist, est towards these latter ones. Like any myth ways a basic reality, which roots lie in pure they are dwarfs.

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its variants; myth

astronomy. The Planet X hypothesis did in fact originate as a scientific need following the biggest success of celestial mechanics, namely the discovery of Neptune in 1846, based on the observed gravitational pertur- bations in Uranus' motion. The presence of Neptune did not seem to completely jus- tify the behaviour of Uranus, and the mo- tion of Neptune itself did not appear to

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30 PLANETOLOGY

match that predicted or non-existence is not provable. (In a high- by the astronomers’ ly developed civilization it should not be calculations. The dis- necessary to prove the non-existence of crepancy between the something whose existence has not been theoretical and the ob- demonstrated, but evidently our civiliza- served positions, led to tion is not sufficiently advanced.) The open assume that there might discussions about Planet X ended (in the be an additional outer plan- scientific world) when, from the data col- et which, due to its mass and lected by Voyager 2, astronomers realized thus its gravitational force, was that Neptune’s mass was different responsible for the inconsistencies found. from that previously estimated and The extensive searches for that object re- that the new value measured by sulted in the almost accidental discovery in the probe no longer required 1930 of Pluto, a planet whose mass soon the existence of a large proved to be largely insufficient to justify planet beyond Pluto to such discrepancy. Planet X started from account for the sup- that point on to become a real challenge, posed residual in- but the instruments, methods and amount consistencies of time available to astronomers in the on the mo- half century following Pluto’s discovery tions of were insufficient to verify the existence of Ura- orbiting objects at such great distances nus from the Sun, and this helped to transform and that hypothetical presence in a topic on Nep- which to speculate freely, just like with tune. As everything else whose often hap- existence pens when the truth is disappointing, many devotees of mat- ters not strictly scien- tific have however continued to be- lieve in the existence of Planet X,

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cheering each time that, starting from the substellar, object was considered respon- ‘90s, new trans-Neptunian objects such as sible for supposed cyclical mass extinctions Quaoar, Sedna, Eris and many others were on Earth, occurring approximately every discovered; all invariably found to be of 26 million years and triggered by its pas- insignificant size, comparable at most sage in the Oort Cloud, which would have to that of Pluto and thus included sent a large number of comets into the together with this in the dwarf inner solar system and some toward our planets’ category. Since the planet. After more recent and accurate pa- possible nature and location leontological analyses have shown that of Planet X have adapted there is no evidence of periodicity in mass over time to our astronom- extinctions, also the Nemesis variant lost ical knowledge and to the credibility, not without, however, some oc- need to explain phenom- casional rekindled interest for this theory. ena (also not strictly as- The last one is of 2010 and doubles the fre- tronomical), from the quency of the extinctions, without though ‘80s onward also oth- finding any confirmation in the sky where er alternative hy- an object such as Nemesis would have not potheses to the escaped the many orbiting infrared tele- classic planetary body scopes that in recent decades have scoured were put forward. One the entire sky. Another Planet X variant of the better known is that was suggested in the late ‘90s to interpret admitting the existence of a an anomaly in the trajectories of the dark companion of the Sun, per- long-period comets coming from the Oort haps a very faint red dwarf, per- Cloud. It was predicted that those comets haps a brown dwarf, which was called were randomly coming from any direction, Nemesis. The choice of the name of a ven- but after verifying that it was not so, some geance goddess of Greek mythology lies in astronomers suggested that a perturbing the fact that the hypothetical stellar, or object several times more massive than

n the back- I ground, a ren- dering of the WISE satellite or- biting around Earth. Left: this graph shows the observational limits of WISE, based on the mass (Jupiter=1) and distance from the Sun of plane- tary bodies. [Penn State University]

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Jupiter could be re- sponsible for the lack of homoge- neity within the cloud and, conse- quently, of the pref- erential paths fol- lowed by the com- ets. Also in this case no direct evidence was found, but it was nonetheless de- cided to give a use- less name to the hypothetical plan- et, Tyche, the Greek goddess of fortune. In short, on several occasions and for various reasons it had been necessary to ever possible to consider the Planet X issue ummary table hypothesize the presence of a giant planet conclusively settled without at the same Sof the outer so- or a stellar body, positioned somewhere time also remove any slightest doubt, lar system show- between Pluto’s orbit and the Oort Cloud, astronomers have been waiting for indis- ing groups of with a sufficiently eccentric orbit to bring putable proof before putting an end to this Centaurs, Pluti- nos and objects it to directly or indirectly interact with centuries-old myth. of the Kuiper other bodies of our solar system. In some These proofs have emerged from the data- belt. Much more cases it has perhaps been more an attempt base of the second all-sky survey perform- further away to try to lend substance to a myth than to ed by the WISE satellite (Wide-field Infra- known objects propose a viable solution to unsolved prob- red Survey Explorer), released by NASA last are at present a lems, given that for these same problems November and immediately examined by small number. were later found much more simple and researchers who were eagerly waiting to Below: the orbit convincing explanations. In not being how- compare its astrometric data with that of of Tyche, the hy- pothetical Planet X whose exis- tence was predict- ed in the late ‘90s. planetX EN:l'Astrofilo 30/04/14 07:32 Page 33

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he red and the first survey, con- T blue circles in cluded six months this simulation before the start of show all the stars the second. The aim closest to the Sun of the comparison and all the brown was (and it is) that dwarfs discover- ed by WISE. None of spotting un- of these latter is known objects, pos- located at a dis- sibly having a de- tance so close tectable slow prop- that they can dis- er motion, or any- turb the orbits of how located within objects that pop- short distances and ulate the Oort thus affected by the Cloud. [NASA/JPL- parallax phenome- Caltech] non. Those objects had to be found among the 750 mil- lion stars, galaxies and asteroids cap- tured in the WISE images. The results of the first two stud- ies – based on that huge amount of survey dwarfs located within a 500 light-years ra- data – by Kevin Luhman (Penn State Uni- dius from the Sun. Among them, none is versity) and a team led by Davy Kirkpatrick nearer than similar previously known ob- (California Institute of Technology, Pasa- jects and, more importantly, none is locat- dena) were recently published in The As- ed in close proximity of the Oort Cloud or trophysical Journal. From the total of over more inside the solar system. Of all the 22,000 objects displaying a significant prop- brown dwarfs (overall less than expected), er motion, the researchers, based on their none can be considered in terms of size specific requirements, have extracted two more similar to a planet than to a star. different groupings of 762 objects (Luh- Taking into account the observational lim- man) and 3,525 objects (Kirkpatrick et al.). its of WISE, from the results published They are in all cases small stars or brown in the ApJ it can be confidently said that f at the bound- no planet as large as Saturn exists within I aries of the 10,000 astronomical units from the Sun, no solar system planet as big as Jupiter within 26,000 astro- there were a sub- nomical units and no proportionally larger stellar object it planet at greater distances. The existence could appear as of gas giants below the limits reachable by WISEA J204027.30 WISE is however unlikely, and if they exist +695924.1; red they cannot certainly have formed at those dot nearly in the distances from the Sun in not having ever centre of this existed out there enough material for their image. This is a accretion. The theory of the “tenth” planet, tiny star, but its dimensions are and of all its variants does therefore seem similar to those to irreversibly fall apart, and since the exis- of a brown tence of that object would not in any case dwarf. help solving any of the current issues rela- [DSS/NASA/JPL- ted to the solar system, is it worth conti- Caltech] nuing to fantasize about it? n nane bianche EN:l'Astrofilo 30/04/14 08:02 Page 34

34 STELLAR EVOLUTION Planetary debris found on white dwarfs

The presence of metals in the atmo- spheres of white dwarfs has for decades been a tricky issue for researchers, who could not find an explanation for the observed abundances. The solution stood in envisaging an external origin of these metals, which, according to a recent study, should be linked with the destruction of an untold number of planets.

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For about a quarter of nimation of them the atmospheric A the scenario composition is well depicted in the defined and includes opening illustra- several metals, among tion. [NASA, ESA, STScI, and G. which nitrogen, car- Bacon (STScI)] bon, iron, nickel, oxy- Below: a plane- gen and silicon. But tary system de- the measured abun- stroyed by the dances do not at all evolution of a correspond to those solar-type star, theoretically predic- which, through ted, and this regard- the red giant less of the tempera- phase has reached ture range used and the white dwarf excluding any prefer- stage. Only plan- in the spectra of some white dwarfs, it was ences introduced by the methods used to ets outside the Roche limit re- thought that those signals originated from study individual stars. The metals could there- main intact. circumstellar material interposed between fore largely derive from external sources. [NASA/JPL- Cal- the star and the observer. A few years later, The temperature is a key parameter in radia- tech/T. Pyle] the soundness of this explanation was chal- tive levitation, since the stronger the heat lenged as a result of more recent observa- flux rising from the centre of white dwarfs tions by another satellite, the EXOSAT (Eu- (where it can reach more than 20 million ropean X-ray Observatory Satellite) which al- kelvin), and more abundant, in terms of lowed to determine that the metals were in type and quantity, are the metals that in the dwarfs’ atmospheres and not in orbit rising to the surface leave their “fingerprint” around them. in the stars’ spectra. For “cooler” white To explain this new unexpected scenario, re- dwarfs, those with surface temperatures searchers began to consider more favourably below 20,000 kelvin, radiative levitation has the hypothesis according to which the de- little or no significant effect and any possi- finitive sinking of the metals may have been ble metal contamination of external origin hindered by the radiation pressure genera- should disappear in a few days as predicted ted inside those very hot de- generate stars – a mechanism already proposed in the late ‘70s and known as “radiative levitation”. Overall, though, the sample of white dwarfs examined at the end of the ‘80s was still too small to draw general conclusions. The broad picture began to emerge with some clarity in the ‘90s, when the satellite for high energy astronomy ROSAT (Röentgen Satellite), the afore- said IUE and the Hubble Space Telescope allowed to investi- gate with sufficient accuracy the ultraviolet and X-ray re- gions of the spectrum of near- ly a hundred white dwarfs. nane bianche EN:l'Astrofilo 30/04/14 08:02 Page 37

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he most well- by Schatzman. The more the tem- T known ex- perature rises, the more the metals ample of white persist or reappear in the atmo- dwarf is that of sphere – especially if of internal ori- Sirius B, which gin – since above a certain energy can be seen top flow threshold (with surface tem- right in a realistic visualization dom- peratures above 50,000-100,000 inated by the bril- kelvin), the foreign material in liant Sirius A. outer space is not longer able to [NASA, ESA and fall on the dwarf, in that it is evap- G. Bacon (STScI)] orated. (A brief note on the tem- On the side: a perature scale. At such high heat real image of that levels, the kelvin (K) are basically stellar system equivalent to degrees Celsius; only taken by the at low temperatures the dissimi- Hubble Space larities become significant.) Since Telescope. [NASA, white dwarfs give off to the external envi- synthesized in the core of the progenitor ESA, H. Bond ronment their stored heat, which in the ab- stars. But once again, theoretical predictions (STScI), and M. Barstow (Univer- sence of a thermonuclear process is not re- and reality differ: where radiative levitation sity of Leicester)] plenished, they are destined to cool down is more efficient it could happen to not ob- in line with their initial physical properties. serve traces of metals, while the opposite is If the origin of the metals was internal (or sometimes true where no metal should be mostly internal), we should therefore expect able to rise to the surface, and this regardless that the type and abundance of metals in of the aforementioned fundamental factors. the atmospheres of white dwarfs vary in ac- This lack of consistency has, at various times, cordance with the mass, temperature and led to suppose that the source of the metals age, and that they were consequently those was external (or mostly external), but up un- nane bianche EN:l'Astrofilo 30/04/14 08:03 Page 38

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til 2002 none of the researchers who had been studying radiative levitation in white dwarfs attached the right importance to a possible external origin of the metals. The only cases in which this possibility was taken into consideration were related to particularly cold, and/or metal-rich white dwarfs for which the radiative levitation mechanism was not sufficient to account for what’s observed. As external source of those metals it was simply cho- sen the interstellar medium, or ra- ther, its local concentrations of gas and dust that exist in the space be- tween the stars, which would oc- casionally transfer into the atmo- spheres in question. This solution though set off at least two objec- tions: 1) the white dwarfs with ex- cesses of metals (compared to theoretical (especially in cooler white dwarfs); 2) the endering of predictions) are not located in close proxim- white dwarfs with atmospheres dominated R the Far Ultra- ity to condensations of the interstellar me- by helium show a very unbalanced ratio be- violet Spectro- dium, and if in the past they ever crossed tween hydrogen and calcium in favour of scopic Explorer, any, the metals acquired would have been the latter, while it should instead reflect the the instrument which gathered long ago dispersed in the subsurface layers proportion between those elements in the the data used by interstellar medium where hydrogen is de- the team led by cidedly the most abundant. The pieces of the Martin Barstow puzzle started to come together in recent to identify the years with the discovery of metal-rich white source of the met- dwarfs, surrounded at the same time by or- als present in the biting rings of dust and gas (visible in the in- atmospheres of frared). It is precisely this material compos- white dwarfs. ing the rings that infalling into the atmo- [JHU FUSE Project] spheres of white dwarfs brings the metals Left: Professor detected. But what is its origin? According Martin Barstow to a recent work by Martin Barstow and of the University of Leicester. others, published in the Monthly Notices of the Royal Astronomical Society, to form that material would have been the remains of rocky planets orbiting around stars before their transformation into white dwarfs. The stellar disruptions leading to that final evo- lutionary state can cause planetary orbits to decay and, as a result, bring the planets at short distances from white dwarfs. If those distances are less than the Roche limit (about 1 solar radius in this case), planets are torn apart and their debris (ranging in size be- tween those of dust particles and those of small asteroids) will eventually line up along nane bianche EN:l'Astrofilo 30/04/14 08:03 Page 39

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hite dwarfs W are stars that although rather common are very difficult to discover and study. In this example on the right, some of them present in a small portion of the globular clus- ter M4 have been highlighted with circles. [H. Bond (STScI), NASA] The video below shows a white dwarf transiting over the disk of a red dwarf. De- spite its size, the an accretion disk that supplies metals to the their abundances and the physical properties ultra-dense white atmosphere of degenerate stars; until the of white dwarfs. As expected, only in hotter dwarf can signifi- supply lasts, that is. The great variety that atmospheres (the temperature of those sur- cantly deform the the inner planets of a planetary system may veyed ranges from 16,000 to 77,000 kelvin) space around it have in terms of number, mass, distance and the accretion of elements from the outside and alter the composition, would justify the randomness appears to be seriously hindered by the image of the red of the abundances of metals in the atmo- strong stellar radiation. The strongest evi- dwarf. [NASA] spheres of white dwarfs, regardless of the dence in favour of a planetary origin of the physical properties of these latter and the metals observed in the white dwarfs’ spectra efficiency of radiative levitation. consists in the relative abundances of carbon, The team led by Barstow has come to this silicon, phosphorus and sulphur, which are conclusion by examining the spectra of 89 quite comparable to those typically found in white dwarfs (the largest sample surveyed to rocky planets of our solar system. date), taken by the Far Ultraviolet Spectro- The solution proposed by Barstow and col- scopic Explorer (FUSE), which allowed them leagues seems to finally resolve a tricky issue to establish that at least 23% of dwarfs of that has lasted for decades, as well as offer- type DA contain metals and to confirm that ing at the same time interesting points for there are no direct relationships between discussion, such as, for instance, the fact that if about a quarter of white dwarfs appear to be contaminated by planetary remains, it is likely that, on average, at least 1 out of 4 stars below 8-10 solar masses possesses pla- nets. Furthermore, future acquisitions of spectra at much higher resolution than those supplied by FUSE, will allow to precisely de- fine the chemical composition of no longer- existing planets and to understand how they have evolved within their systems. Finally, those white dwarfs are also providing us a preview of how the Sun will appear in 5-6 bil- lion years or more, when in its light a possible future alien civilization will perhaps discover what is left of our beautiful planet. n VP113 EN:l'Astrofilo 30/04/14 07:29 Page 40

40 DWARF PLANETS

20122012 VPVP113 is further away than Sedna

The discovery of a dwarf planet, smaller but more distant than Sedna, expands the boundaries of the planetary system up to 80 astronomical units from the Sun. But what’s more interesting is a parameter of that new dwarf planet’s orbit which seems to indi- cate the existence of a super-Earth at an even greater distance.

he spotlight was not yet off Luhman hoped for at least a dwarf planet larger his artistic and Kirkpatrick’s works (see article than Pluto and Eris. But in the end not even T glimpse of T page 28), that rumours of a discovery that. In fact, on 26 Mar 2014 it was disclos- the Plutonian capable of recalling into question the scena- ed the content of an article to be published landscape can be rio just outlined by these same works were the day after in Nature (the International utilized to get an already starting to circulate. The rumours weekly journal of science), in which it was idea of how the Sun (top right) proclaimed the existence of something very going to be announced the official discovery would appear, interesting beyond the orbit of Neptune. of a dwarf planet with a diameter of only seen from the The allusion to a planet was clear, but since 450 km. But this is the only disappointing more remote ob- we had just about finished commenting the aspect emerging from that article, since ev- ject of the plan- latest results from the observations of erything else is of considerable interest and etary system,

WISE, we knew very well that we could not has direct implications for understanding 2012 VP113. expect the discovery of a large planet, and better the evolution of the solar system. [ESO-L. Calçada]

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DWARF PLANETS 41 further Sedna

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between 1.5 and 2 comparison hours; a sufficiently A on different adequate strategy scales of Sedna for detecting the and 2012 VP113’s movement of di- orbits with those of the planets, stant objects up to the asteroids 300 astronomical belts and the in- units (or AU) and ner Oort Cloud. even more. Pictures 2012 VP113 is cur- taken on November rently the plane- 5 confirmed the exi- tary body with stence of an object the furthest peri- of magnitude 23, helion, but Sed- whose movement na’s aphelion is was slow enough to more than double place it well beyond that reachable by 2012 VP . any known body in 113 [NASA/JPL- Cal- orbit around the tech/R. Hurt (SSC- Sun: that is to say, Caltech)] Below: 80 AU. To that ob- an animation ject was given the showing the provisional name vastness of 2012

(still in use) of 2012 VP113‘s orbit. VP113. In order to de- [Seth Jarvis, Clark termine the orbit of Planetarium]

2012 VP113 and in- vestigate its surface Let's see why by taking a small step back in properties, Trujillo and Sheppard studied it time. It is the autumn of 2012, Chadwick for about a year with the Magellan tele- Trujillo (Gemini Observatory) and Scott scope (6.5 m diameter) of the Las Campanas Sheppard (Carnegie Institution for Science) Observatory. By combining the apparent mag- are keeping under control 52 square de- nitude with an albedo estimated between grees of sky with the telescope Blanco (4 m 0.1 and 0.4, the two researchers concluded

diameter) of the Cerro Tololo Inter-Ameri- that 2012 VP113 has a diameter of about 450 can Observatory, located about 80 km East km, while the spectral analysis of the light re- of La Serena, Chile. The purpose of the re- flected from it suggest that it may have for- search is to find objects in slow motion which could belong to the inner part of the Oort Cloud, an environment where until then only one planetary body with known orbit appeared to exist, Sedna, a dwarf plan- et approximately 1500 km in diameter, to whose discovery, occurred in 2003, contri- buted the same Trujillo. For their new observing campaign started with the Blanco telescope, Trujillo and Shep- pard used the most powerful CCD camera in the world, the DECam, which had recently come into service (see l’Astrofilo, Oct 2012 issue). Every single region of the sky framed by the powerful 570-megapixel sensor was photographed three times, at intervals of VP113 EN:l'Astrofilo 30/04/14 07:29 Page 43

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n the side: Oimages of the 2012 VP113 discov- ery. The three photographs were taken at intervals of about two hours. The dwarf planet’s movement can be clearly dis- cerned when con- sidering that the med in the innermost regions of the solar na, it would have been impossible to spot arrow is stationary. system, probably in the gas giants region. such a dwarf planet at distances greater Below: one of the What most attracted the attention of the than the current ones. possible orbits of researchers, are however the parameters of In order to have a more precise idea of the the super-Earth the orbit of 2012 VP , a very long ellipse context in which 2012 VP is located, it is that according to 113 113 with eccentricity close to 0.7 which takes it worth reminding that in the current archi- Trujillo and Shep- pard affects the or- from its current 80 AU – coinciding with the tecture of our planetary system there are: bital motions of perihelion – up to 472 AU (aphelion). the rocky planets and the main asteroid belt located between 0.39 and 4.2 AU from 2012 VP113 and This means that 2012 VP113 needs no less Sedna. [C. Trujillo than 4600 years to complete one revolution the Sun; gas giants between 5 and 30 AU; and S. Sheppard] around the Sun and, as happened with Sed- frozen objects of the Edgeworth-Kuiper belt, which includes Pluto and other dwarf planets, between 30 and 50 AU. Prior to the dis- covery of Sedna it was believed that the planetary system ended more or less there and that no- thing significant existed until the Oort Cloud, a vast reservoir of cometary nuclei, on which the gravitational influence of the Sun is decidedly weak. But then the discovery of Sedna – whose orbit takes it further than all other orbits (76 AU peri- helion, 975 AU aphelion) – and

other minor objects (2004 VN112 and 2010 GB174 in particular) led astronomers to hypothesize the existence of a new population of icy bodies orbiting in a vast re- gion, at a distance of more than 50 AU (where the gravitational interactions with Neptune are no longer significant) and about 1500 AU; limit beyond which the galactic tides start to become im- portant in the formation proces- ses of planetary bodies. Beyond 1500 AU starts the outer Oort Cloud, which could perhaps ex- tend as far as to 1-2 light years VP113 EN:l'Astrofilo 30/04/14 07:29 Page 44

44 DWARF PLANETS

population. Circular iagrams help- orbits are much more Dful for under- efficient in the accre- standing the inner tion process of a pla- Oort Cloud. In the left one it can be netary mass, starting noticed that ob- from the material jects less than 150 available in the pro- AU from the Sun toplanetary disk, and have random ar- this is much more so guments of peri- the more these orbits helion (ω), while are smaller. What me- for those at great- chanism has thus fa- er distances they voured their forma- tend to be distrib- uted around 0° tion and/or location (or 360°). The one in the regions where below represents we can observe them the libration of ω today? The simula- for 2012 VP113 pre- tions have allowed dicted over a pe- the researchers to riod of 2 billion build two main forma- years. There is co- tion models capable herence with the of describing the crea- potential presence tion of the inner Oort of a super-Earth at 210 AU. [C. Trujillo Cloud. The first is and S. Sheppard] based on the ejection from the solar system

from the Sun. Sedna and 2012 VP113 are therefore the first representatives of the newly discovered population of icy bodies, whose total number (including smaller co- metary nuclei) should exceed that of all other populations of small bodies of the solar system. Trujillo and Sheppard estimate that in the inner part of the Oort Cloud could exist from 400 to 1700 dwarf planets with a diameter greater than 1000 km, this according to the size distribution models al- ready applied to the main asteroid belt and the Edgeworth-Kuiper belt. The numerical simulations suggest that the total mass of the newly found population could be closer to 1/80 of the Earth’s mass (for comparison, all the objects in the Edgeworth-Kuiper belt amount to 1/100 of the Earth’s mass). From the highly elongated shape of Sedna’s

and 2012 VP113’s orbits, it is possible to con- strue that these two dwarf planets cannot have formed by gathering material on their trajectories, and this applies also to a doz- en of other candidate objects currently wait- ing to be classified as belonging to the new

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of a planet of consider- in the company of its “sisters” birthed from able size, which in transit- a common molecular cloud. A variant of this ing in the Edgeworth-Kuiper second model presumes that objects like

belt would have perturbed the or- Sedna and 2012 VP113 might have been ac- bits of countless icy bodies, driving creted by planetesimals ripped away by the them in the outer space. The ejected plan- Sun from another star, also in this case et might have definitively abandoned the when the two stars were still part of the sphere of influence of the Sun, or it may same cluster. have positioned itself in an extremely wide Given that at present the objects for sure orbit, thus remaining invisible even in the belonging to the new population are only a most powerful instruments. The second couple, it is too early to say which model is model, instead, predicts that the inner Oort applicable, but since the different scenarios Cloud was formed by the dis- presume different orbital configurations, it ruption caused will be sufficient to wait for the discovery by a of other same-type objects to see which prediction is more realistic. While Trujillo and Sheppard do not exclude any hypothesis, they point out that the key to accurately defining the physical char- acteristics of the inner Oort Cloud may already exist. In fact, Sed-

na and 2012 VP113 have, more than anything else, similar values of argument of perihelion (ω): 311° for the first, 293° for the second – a mini- mum difference that made re- searchers s u s p i - cious.

n the back- I ground: the ap- proximate posi- tions of 2012 VP and Sedna 113 star on their orbits. passing In the box: the three images of at a relative- the discovery ly close distance combined in the from the solar system. colour version This would have happened (RGB). [C. Trujillo within the first 10 million years of and S. Sheppard] the system’s life, when the Sun was still

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The argument of perihelion is the angular distance between the perihelion point (closest to the Sun) and the ecliptic plane (the imaginary plane contain- ing the Earth's orbit). If the or- bits of the two dwarf planets were randomly distributed, that near coincidence of values would be highly unlikely. But that’s not all. Those values are al- so shared, with good approximation, by various other trans- Neptunian objects whose orbits have semi-major axes ex- ceeding 150 AU. Whichever mecha- nism may have led to the formation of the inner Oort Cloud, its members’ orbits should have today ran- domly distributed argu- ments of perihelion. This though is not so: which means that there must be a perturbing body whose con- siderable mass affects that distri- bution, acting like a shepherd herd- ing his sheep at a certain hour of the evening. According to Trujillo and Shep- pard, a planet of about 10 Earth-masses or- biting at 200-250 AU from the Sun could account for the coincidences detected. But the same thing would also be true for more search Institute), if that planet has been urious repre- massive planets located at greater distances. ejected from our solar system (hypothesis C sentation of Very recent Luhman and Kirkpatrick’s works, more plausible than others) it would have the Oort Cloud, based mainly on the survey by the WISE had only a 2% probability of settling into highlighting the telescope, place stringent upper mass limits an orbit consistent with Trujillo and Shep- inner and outer to a possible trans-Neptunian planet, but do pard’s hypothesis. This means that either regions. The dis- not exclude the existence of a super-Earth. we are dealing with a rare event, or that tance scale gives an idea of their This would, however, be beyond the reach the planets ejected from the solar system sizes, even though of the most powerful instruments currently were numerous (but this seems highly un- for graphic rea- available to astronomers, given its temper- feasible). To understand how their orbits sons the planetary ature of only few tens of kelvin. The fact are perturbed we will just have to wait for region has been that the hypothetical perturbing planet further discoveries of dwarf planets similar greatly oversized. is not observable, does not automatically to Sedna and 2012 VP113. At that point it [Andrew Z. Colvin] mean that it really exists, in fact, the proba- should be possible to determine the mass, bilities that it is there are rather limited. As distance and – above all – the position of pointed out by Hal Levison (Southwest Re- the perturbing body. n VP113 EN:l'Astrofilo 30/04/14 07:29 Page 47