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Home , Clyde Tombaugh, Dwarf planet, Haumea, Makemake, Observation arc, Palomar Observatory

INTERNATIONAL SPACE SCIENCE INSTITUTE SPATIUM Published by the Association Pro ISSI No. 33, March 2014

141348_Spatium_33_(001_016).indd 1 19.03.14 13:47 Editorial

A sunny spring . A green On 20 March 2013, Dr. Hermann meadow on the gentle slopes of Boehnhardt reported on the pre- Impressum Mount Etna and a handsome sent state of our knowledge of woman gathering flowers. A stran- and its cohorts, which is not ger passing by and falling in love so much when compared to the with her. in our cosmic neighbour- hood, yet impressively much in SPATIUM Next time, when she is picking view of their modest size and their Published by the flowers again, the foreigner returns gargantuan distance. In fact, ob- Association Pro ISSI on four black horses. Now, he, serving dwarf Pluto poses Pluto, the Roman god of the un- similar challenges to watching an derworld, carries off Proserpina to astronaut’s face on the . marry her and live together in the shadowland. The heartbroken We thank Dr. Boehnhardt for his Association Pro ISSI mother insists on her return; kind permission to publishing Hallerstrasse 6, CH-3012 Bern she compromises with Pluto allow- herewith a summary of his fasci- Phone +41 (0)31 631 48 96 ing Proserpina to living under the nating talk for our Pro ISSI see light of the during six months association. www.issibern.ch/pro-issi.html of a year, called summer from now for the whole Spatium series on, when the flowers bloom on the Hansjörg Schlaepfer slopes of Mount Etna, while hav- Brissago, March 2014 President ing to stay in the twilight of the Prof. Nicolas Thomas, underworld during winter. University of Bern

Pluto had become the ruler of the Layout and Publisher underworld and the gov- Dr. Hansjörg Schlaepfer ernor of the , when CH-6614 Brissago had wrested the lordship over the world from their common father Printing . These Roman gods are not Stämpfli Publikationen AG the only divinities making a fan- CH-3001 Bern tastic appearance on the following pages, rather, they team up with many more, such as Sedna, the venerated goddess of the deep of the Inuit or , the great creator god ruling the Easter Islands.

Most interestingly, a modern as- tronomer’s view of Pluto and its companions is in no way less fantastic: Having all their own peculiarities they circle the Sun at the rim of the in eternal twilight just like the souls in the underworld.

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141348_Spatium_33_(001_016).indd 2 19.03.14 13:47 Pluto and its Cohorts1 by Dr. Hermann Boehnhardt, Max-Planck Institute for Solar System Research in Göttingen/Germany

Introduction ments and suggestions. Yet, Clyde astronomer who could operate the does not get the expected com- photographic recently ac- ments, but rather a job there: his quired. That was in 1929 (Fig. 1). drawings are judged so excellent Clyde William Tombaugh2 dreams that the ’s director em- Unsurprisingly, Clyde accepts the to become a great astronomer. ploys him instantly as an amateur offer and starts working at the ob- -gazing is his passion, like his father’s and his uncle’s, who occa- sionally places his telescope at the youngster’s disposal. At the age of 17, Clyde buys a commercial low- cost 2¼-inch instrument from Sears, which has the advantage of being his own, yet the disadvantage of not fulfilling his expectations. So, he decides to build one himself: he starts grinding mirrors and as- sembling parts of discarded farm machinery and a shaft from his ­father’s 1910 Buick. To help pay for the material required, father Tom- baugh, a modest farmer in the ru- ral heart of ­Illinois, takes a job. The instrument gets finished in 1925, and Clyde doesn’t know that this instrument is to become the first of over thirty he will build over his lifetime. When time comes to decide on his professional life, Clyde intends to enter the local college to get pre- pared for the studies at the Univer- sity; yet a hailstorm destroys his family’s crops putting a sudden end to his dreams.

Not quite, though: he begins us- ing his telescope extensively, makes a lot of meticulous sketches of ­Jupiter and , and sends them to the astronomers at Lowell Ob- Fig. 1: Clyde William Tombaugh, the discoverer of Pluto in a photo of around servatory humbly asking for com- 1929. (Anonymous author)

1 The present issue of Spatium reports on the lecture given by Dr. Boehnhardt for the Pro ISSI Association on 20 March 2013. The notes were taken by Dr. Hansjörg Schlaepfer. 2 Clyde William Tombaugh, 1906, Streator, Illinois – 1997, Las Cruces, New Mexico, US American astronomer.

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141348_Spatium_33_(001_016).indd 3 19.03.14 13:47 servatory that was founded and ground. Hence, in August 2006, full-time. During his years at Low- sponsored by Percival Lowell3, a Pluto is re-classified as a dwarf ell, the former farm boy Clyde member of a wealthy Boston planet by the IAU4. One of the writes astronomical history discov- ­family, in 1894. Lowell, based on ­criteria set up to this end is that a ering hundreds of new variable careful analyses of the irregular or- planet must be able to clean its stars, hundreds of new bits of and Neptune, had ­orbital environment from smaller and two . He finds new star predicted a so far unknown “Planet bodies, which is the case with all clusters, clusters of galaxies includ- X”, which could explain the odd eight planets but not for Pluto and ing one super cluster of galaxies. behaviour of the planets. Clyde’s its uncountable consorts. In all, he counts over 29,000 job is now to photograph one small galaxies. piece of the night sky at a time and enters the Uni- to carefully examine and compare versity of Kansas in 1932 where he On 17 January 1997, Clyde Wil- the photos in an effort to search for earns his Bachelor of Science de- liam Tombaugh dies at the age of a tiny moving point of light that gree in 1936. He signs up for a ­basic 91 while Pluto quietly continues might be the mysterious “Planet course, but is rejected orbiting the Sun. Since then, the X”. He takes pictures of most of on the grounds that his discovery dwarf planet has revealed many as- the sky, and spends thousands of of Pluto has already made him one tonishing facts, and with improv- hours examining­ the images. Af- of the world’s most famous astro­ ing observational means further ter ten months often working nomers, and it would be absurd to surprises will certainly come up. through the whole night in the un- accept such a personality for an in- heated dome, on 18 February 1930 troductory class. He continues In contrast to the planets, Pluto cir- he discovers an enigmatic object working at cles the Sun on an eccentric he names Pluto. That lonely night during the summers and after grad- that brings it at its perihelion nearer makes Clyde a great astronomer. uation, he starts working there to the Sun than neighbouring

Pluto the Sun in a mean dis- tance of some 5.9 billion km which is equivalent to 39.2 Astronomical Units. It takes it two and a half earthly centuries to complete one single orbit around the Sun. For many years, Pluto is considered the ninth planet in our solar system. However, as astronomers learn more about the planets and also about a new group of objects, dis- covered as of 1992 and known to- day as the objects, it Fig. 2: The orbit of Pluto as compared to the planetary orbits in the solar system. becomes clear that Pluto is more The orbits of all planets are more or less confined to the plane defined by the ’s orbit around the Sun. In contrast, Pluto orbits the central star in a plane like the larger objects in that belt tilted by 17°. Again in contrast to nearly all planets, Pluto’s axis of lies prac- than the eight planets in the fore- tically within its orbital plane, similar to Uranus. (Credit: NASA)

3 , 1855, Boston – 1916, Flagstaff, Arizona, USA, US-American astronomer, founder of the Lowell Observatory. 4 The International Astronomical Union (IAU) was founded in 1919 with its headquarters in Paris. Its mission is to ­promote and safeguard the science of astronomy in all its aspects through international cooperation.

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141348_Spatium_33_(001_016).indd 4 19.03.14 13:47 planet Neptune. That was the case – accompanied by They concentrate in a zone from the last time in February 1979. A some minor . some 30 to 50 Astronomical Units. further peculiarity of Pluto is its 2:3 This is the region at the edge of the orbital with Neptune: Pluto is the brightest, but not the planetary system, called the Kuiper this means that while Neptune or- largest trans-Neptunian object Belt honouring Gerard Peter Kui- bits the Sun three times, Pluto com- (TNO), see Fig. 3. The term TNO per5 who in the forties de­veloped pletes exactly two orbits. The rep- (or equivalently Kuiper Belt ob- theoretical foundations all­ owing etition of a similar orbital geometry ject) designates a body whose mean him to speculate on the existence between Neptune and Pluto helps orbital radius is larger than that of of a “ belt” where most of stabilize the orbit of the smaller Neptune. Today, more than 1,500 the comets come from. This is the partner over billions of years. TNOs are known, yet it is thought mysterious realm the present issue that many more exist out there. of Spatium is devoted to. Yet, the list of Pluto’s odd charac- teristics is much longer than this: its orbit is outstandingly inclined as compared to the ecliptic plane (Fig.) 2 , defined by the Earth’s or- bit around the Sun. Further, while most planets have poles that point roughly up and out of their orbit planes, Pluto (together with Uranus) are notable exceptions: they effec- tively rotate on their sides (see Fig. 2). Further and according to the present state of knowledge, Pluto holds five of which is by far the largest. Due to tidal forces the orbital and the rotational motion of the two big partners got ‘locked’ over time, i. e. the and the speed of rotation have been adjusted in such a way that during their orbital rev- olution Pluto and Charon always keep the same face towards each other. In addition, due to the ­relatively large size of Charon, the barycentre of their orbits lies not within Pluto but somewhere be- Fig. 3: The most detailed portrait of Pluto ever made. Its vantage point com- bined with an outstanding optical performance enabled the Hubble Space Tele- tween Pluto and Charon. The pair scope to gather the excellent pictures from which this image has been reconstructed. is therefore a true double dwarf (Credit: NASA/ESA)

5 Gerard Peter Kuiper, 1905, Harenkarspel, the Netherlands – 1973, Mexico City, US-American astronomer with Dutch roots.

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141348_Spatium_33_(001_016).indd 5 19.03.14 13:47 Trans-Neptunian ther out we find the Twotinos, Its surface is extremely characterized by a 2:1 resonance low, about −243 °C; it is covered Objects (TNOs) with Neptune. Further, there ex- with of , , and ists a variety of other , possibly . such as 5:2 and 3:1. Such resonance orbits have a stable character over Objects (SDOs): The or- While the inner boundary of the the lifetime of the solar system, i. e. bits of this category are character- TNO region is marked by Nep- most of the resonance members ized by very high eccentricity val- tune, its outer edge can loosely be may have been orbiting the Sun for ues with perihelia around 35 AU defined at a few thousand AU, a very long period of time. and aphelia reaching as much as where the inner Oort Cloud6 of 1,000 AU. While their orbits are comets starts. TNOs are consid- Classical TNOs: The members of this thought to have been more or less ered scientifically interesting since group, also called Cubewanos7 are circular initially, gravitational in- they are believed to contain widely marked with red dots in Fig. 4. The teractions with Neptune must have unaltered and pristine material Classicals are characterized by or- caused the high eccentricities we left over from the formation era of bits with low eccentricity values observe today. This scattering pro- the solar system. Yet, as a conse- and mean semi-major axes of be- cess gave the group its name. Since quence of their enormous dis- tween 42 and 50 AU. Since they SDOs get close to Neptune once tance, observations require the do not come close to Neptune their in a while, their orbits are chang- most powerful astronomical tele­ orbits are safe against gravitational ing; on the average the mean life- scope and instrument equipment. interaction and scattering by the time of their orbits is estimated to It is, therefore, approximately dur- outer . Their inclination be of the order of 10 million years. ing the last fifteen years that as- with respect to the ecliptic plane The largest representative of this tronomy has gained a clearer pic- can reach high values (up to 30 °). group is dwarf planet (136199) ture of this ­fascinating world. Let One representative of this group is Eris9 with a size similar to Pluto. us start now with a survey of the (136472) Makemake8. Discovered Its orbit is eccentric with a perihe- different groups of TNOs. in 2005 by American astronomers lion of 38 AU, an aphelion of some at the Mount Palomar Observa- 98 AU, and an inclination of 44 °. Resonant TNOs: Of all TNOs, about tory, Makemake circles the Sun on It requires 560 years to orbit the one third are in resonant orbits with an orbit with a 38.5 AU perihelion Sun once. SDOs are indicated with Neptune. Most of them exhibit a and a 52.8 aphelion with an incli- a pink hue in Fig. 4. 3:2 resonance on highly eccentric nation of 29 ° with respect to the orbits. The brightest and hence best ecliptic. It requires 309 Earth years Detached Disk Objects (DDOs) are – ac- known member of this group is to orbit the Sun once. Due to its cording to the present state of Pluto, which gave the group its relatively large size of some knowledge – not related to neither name: the , marked with 1,500 km diameter Makemake is Neptune nor to the Kuiper Belt white dots in Fig. 4. On orbits fur- also recognized as a dwarf planet. proper. These objects have orbits

6 The , named after the Dutch astronomer , is a region starting at a few thousand AU from the Sun and reaching out to the edge of the Sun’s gravitational dominance. It is thought to be the source of the long-period comets, such as Halley’s comet. 7 Cubewanos got their name after the first TNO – apart from Pluto – discovered in 1992, preliminarily designated as 1992 QB1. 8 Makemake is named after the creator god in the Easter Islands aborigines’ tradition. 9 received its name from the Greek goddess of conflict making good reference to the clashes in the astronomical community which finally led to the new definition of dwarf planets by the IAU and the de-classification of Pluto as a planet in 2006.

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141348_Spatium_33_(001_016).indd 6 19.03.14 13:47 whose points of closest approach to Sedna. Featuring a diameter of has put DDOs in their current or- the Sun (perihelion) are suffi- about 1,000 km, its mean distance bits: scenarios range from forma- ciently distant from Neptune such to the Sun is about 18 times that tion, over scattering by planetary that they are essentially unaffected of ­Neptune, its orbit is extremely embryos to extraction from the by Neptune’s and those of ­eccentric with a perihelion of 76 planetary disk by the passage of a the other giant gas planets: this AU and an aphelion of 1,012 AU. stellar object through the outer so- makes them appear “detached” When Sedna is at its aphelion the lar system. Because of the remote from the planetary disk in the so- light of the Sun requires more than distance to the Sun, DDOs are out- lar system. In 2003, astronomers at 5 days to reach it. The dwarf planet side of the plot shown in Fig. 4. the Mount takes 12,700 years to orbit the Sun Centaurs: Indicated by orange dots discovered the first DDO, (90377) once. Up to now it is unclear what in Fig. 4, the Centaurs are subject

Fig. 4: A survey of the outer plane- of dwarf planet Pluto and a great num- and in blue the short and long-period tary system and the Kuiper Belt with ber of further TNOs. The dots indicate comets. (Adapted from all the presently known objects: The or- in red the Classical Disc Objects, in Center, 28 February 2013) bits of Jupiter, Saturn, Uranus and Nep- white the Plutinos, in pink the Scattered tune are shown here together with that Disc Objects, in orange the Centaurs

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141348_Spatium_33_(001_016).indd 7 19.03.14 13:47 Fig. 5: An excerpt of the ESO12 TNO The red arrows indicate a trans-Neptu- 20 arcsec wide. (Credit: ESO and Survey 1999 showing a sequence of 5 nian object rushing through the scene. O. Hainaut) images taken over a period of 10 hours. Each image covers an area of the sky

to the Neptunian gravity field, of the mission, i. e. 9P/ From the ground TNOs can best which scatters them towards the , 19P/Borrelly, 26P/ be observed with powerful tele- other gas . Jupiter finally de- Grigg-Skjellerup, 67P/Churyu- scopes operating in the visual, near cides on their future fate: Either mov-Gerasimenko, 81P/Wild 2 or microwave wavelength they become short-period comets and 103P/Hartley 2. ranges. For a detailed analysis in the when accelerated towards the in- visual and infrared, facilities of the ner solar system or they are jetti- 8–10 m class are required, in the mi- soned back towards the outer solar Observing TNOs crowave region the Alma array10 is system. The known Centaurs are best suited. From space, the Hubble relatively small bodies with semi- The observation of trans-Neptu- Space Telescope is first choice in the major axes between those of the nian objects is an outright challenge visual and the Herschel observa- outer planets. This causes their or- due to their small size (about 2,000 tory11 in the thermal infrared range. bits to be unstable over time spans km diameter at the most) combined of a few million years as they cross with their enormous distance of At the time, when the Kuiper Belt the orbits of one or more of the some 5 billion km, see Fig. 6. In the was discovered in 1992, the search outer gas planets. It has been esti- visible and near-infrared wave- for TNOs was an extremely cum- mated that there are around 45,000 length region they reflect the sun- bersome job, as it was in Clyde Centaurs in the solar system with light at the surface. Due to their Tombaugh’s days: it consisted of diameters larger than 1 km. ­distance from the Sun, surface tem- comparing a large number of small peratures are in the order of 50 to field (order 10 arcmin) images to Short-period comets orbit the Sun 70 K (about –200 °C) or below. This detect an object that moved across close to the ecliptic plane in the places the maximum of their ther- the sky. Today, this process has same direction as the planets with mal emission between 20 to 100 µm been automatized using large field orbital periods of generally less than in the far infrared. In this region, detector arrays and powerful 200 years. They are represented the Earth’s is opaque; computers. among the blue dots in Fig. 4. observations in the thermal infra- ­Members of this group are the main red, therefore, can be performed Fig. 5 shows a sequence of CCD targets (except comet 1P/Halley) from space borne platforms only. images of one specific area of the

10 Operated by a consortium including the European Southern Observatory organization, the Atacama Large Millimeter/ (ALMA) is a group of 66 high-precision antennas, spread over distances of up to 16 kilometres for studying electro­magnetic radiation in the region between infrared and radio waves. 11 The Herschel spacecraft stopped operating in March 2013. 12 European Southern Observatory: The European Southern Observatory ESO is a European intergovernmental ­astronomy organization operating three sites in Chile (La Silla, Paranal and Chajnantor) on behalf of its fifteen member states amongst which is Switzerland.

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141348_Spatium_33_(001_016).indd 8 19.03.14 13:47 night sky. A TNO highlighted Binary and Multiple TNOs weak gravity field. It might be that with red arrows moves through the those multiple systems reflect later image which remains referenced to TNOs show an intriguing variety products of the evolution of TNOs the background stars. Yet, the pride of types: there are single TNOs as when the accompanying bodies of having found a new TNO may well as double and multiple TNOs. have been captivated by the larger last for only a short time: about half The latter are systems containing body and forced into an orbit of the TNOs detected are lost two or more objects circling one around it. It is not probable, how- again as the measured orbit arcs are another. Such multiple systems are ever, that they are the product of short resulting in unsecure deter- of special interest as they allow – impacts where the ejected material mination of their orbital parame- thanks to Kepler’s third law – to might have created the compan- ters such that they may not be determine the of the sys- ion, as is the case with Earth’s found back in the same orbit again tem and sometimes even of the Moon. Mostly, the size of the in the following year. This will partners in the system. TNO companions is similar to that change in 2019, when the US of the central body, which could Large Synoptic Survey Telescope Today some 50 multiple systems not be the case if an impact would will become operational; the tele- are known. Pluto is a good exam- have created the companion. Mul- scope will image the whole acces- ple: it consists of a major body tiple systems are somewhat more sible sky within a mere three days. ­surrounded by five satellites. This common in the class of low incli- comes as a surprise in view of its nation CDOs than with the hot

Fig. 6: The eight largest known which probably stands out than four hours. This in turn gave it a trans-Neptunian objects together most: a collision with an object of sim- highly ellipsoidal shape. (Credit: Wiki- with their moons in an artist’s ilar size may have prompted it in the dis- media Commons) sketch. It is a zoo full of exotics, of tant past to swirl with a period of less

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141348_Spatium_33_(001_016).indd 9 19.03.14 13:47 CDOs in more excited orbits sug- rather because their orbit must nearer than say some 100 AU. Oth- gesting that the aggregation of the have been excited in the past by erwise, this picture would look bodies took place before the hot processes that are not yet well un- much more fuzzy and not so well CDO population was put in place derstood. As a curiosity, there are sorted and restricted to 48 AU. Fi- where it is found today. even retrograde CDOs with a in- nally, we have the Scattered Disk clination of more than 90 ° (not Objects as well as the Detached Disk shown in the plot) which means Objects which show – as a still small Orbital Dynamics that they orbit the Sun in the op- sample – a more or less random dis- posite direction as compared to the tribution of their orbital data. While the planets in the solar sys- great majority of the bodies in the tem behave more or less gently solar system. The dynamically cold with regard to their orbital eccen- CDOs, on the other hand, appear Numbers and tricity and inclination values, the to be a population that has not ex- TNOs hold many surprises in this perienced a lot of orbit excitation How many Kuiper Belt Objects respect (and not only in this re- since their formation. Mostly - exist currently and what is their spect) as we are going to outline tual gravity scattering and impacts ­total mass? Estimations from a sur- now: can explain their low inclinations vey identifying objects with a dia­ and eccentricities. They are be- meter larger than 50 km leads to Fig. 7 shows the inclination of lieved to represent the original an impressive population of some TNOs over their semi-major axis. TNOs formed quasi in-situ bil- 300,000. If the threshold is low- It starts with the Centaurs that lions of years ago. The existence of ered to 20 km (which is compara- cross the orbits of one or more of a dynamically hot and cold popu- ble to a cometary nucleus), the the outer planets. Their inclina- lation in the same distance range is number rises sharply to about tion, as a consequence of past en- a mystery that still requires a plau- 3,000,000. This means that there counters, can reach high values up sible explanation. is a nearly inexhaustible reservoir to 50 °. At an orbital radius of about for future comets. On the other 39.2 AU follow the Plutinos in At 48 AU, there seems to exist an hand, the total mass of all objects their 3:2 resonance with Neptune. effective ‘edge’, beyond which only in the Kuiper belt is strikingly low: Further out there are the Classical much fewer objects are found: this it may not exceed some 20% of the Disk Objects CDOs with semi- indicates that after the formation of Earth’s mass despite the immense major axes between 40 and 48 AU, the Kuiper Belt no star passed-by number of objects. which exhibit two subpopulations, one with inclinations below 5 ° and eccentricities below 0.1 – also called the dynamically cold CDOs – and one with much higher incli- nations and eccentricities – called the dynamically hot CDOs. The latter don’t bear their name because of their surface temperature – which is very low anyhow –, but

Fig. 7: The orbital parameter incli- nation and semi-major axis of Kui- per Belt objects. (Adapted from Glad- man et al., in ‘The Solar System beyond Neptune’, Univ. Arizona Press, 2008 )

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141348_Spatium_33_(001_016).indd 10 19.03.14 13:47 Fig. 8: An estimation of the mean surface over the distance from the Sun. The drop in the surface den- sity in the Kuiper Belt is several orders of compared to the region of the giant planets. It is thought to repre- sent an important hint towards under- standing the evolutionary history of the Kuiper Belt. (Credit: Stern et al., 1996, AJ 112; Duncan et al., 1995, AJ 110)

The multiplicity of very small ob- jects in the Kuiper Belt needs some further elaboration: Fig. 8 shows the surface mass density on hypothet- ical spheres centred at the Sun for increasing distance (in AU). The mass of the outer planets spread over their gravitational field of ­influence leads to a 1/r2 law. The same procedure made for the Kui- per Belt objects shows a much lower mass density in that zone. Obviously, there is a discontinuity original ones from the early days of as their orbits interfere with the between the planets and the Kui- the planetary system. Fig. 9 provides great outer planets. In contrast, res- per Belt which from the point of an overview to that end: we note onant objects and Classical Disk view of solar system evolution is an that the orbits of Centaurs and Scat- Objects tend to possess very stable important fact suggesting that in- tered Disk Objects are unstable over orbits over billions of years. Orbits itially there must have been much time periods of say 10 million years beyond the 48 AU barrier are sta- more mass there than today.

Orbital Stability

The stability over time of the TNO’s orbital parameters has al- ready been addressed several times. The question is whether the cur- rently observable orbits reflect the

Fig. 9: Orbital stability of Kuiper Belt objects. Colours indicate the dynamic lifetime according to the scale on the right part of the plot. Orbital stability tends to increase either with increasing distance from the gravity fields of the outer giant planets or on resonant orbits with Neptune. (Credit: Duncan et al., AJ 110, 1995)

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141348_Spatium_33_(001_016).indd 11 19.03.14 13:48 ble as well as there is no major ob- Fig. 10: over size of Kuiper ject known out there whose grav- Belt objects. While the smaller objects tend to exhibit low albedo values caused ity field could influence their by snow/rock surfaces, the larger TNOs orbits. reach albedo values indicating a snow cover. (Credit: Stansberry et al, in ‘The Solar System beyond Neptune’, Univ. Arizona Press, 2008 ) Size and Albedo

While the diameters of Pluto, as well as some other objects, such as which serves as a proxy for the size Charon and Sedna, can be mea­ of the objects. The large number sured directly with the Hubble of small bodies suggests that the Space Telescope, the two para­ size of the smaller TNOs (some meters albedo and size of all other 100 km radius) is due to collisions. objects have to be inferred from In a phase spanning billions of photometric observations in the years they suffer a continuous mass made. The large TNOs show ab- regimes of the reflected and ther- loss as a consequence of collisions sorption bands by (methane, mal emitted light which in many with other TNOs of similar or even some nitrogen, cases also allows us to estimate the smaller size. The large objects un- and ) which is in line with surface of the objects. dergo collisions with smaller ones their high albedo. Fig. 12 shows the Sizes between 20 and 2,500 km as well, but do not loose (signifi- spectra for a selection of TNOs. dia­meter are found for the objects cant) mass thereby. We conclude in the TNO populations. The lat- that the original distribution of While the spectra of the medium ter value is already larger than sizes is probably represented in the size objects in Fig. 12 (left panel) do Pluto, which is at best only second large objects, such as Pluto, while not show major differences in the in size of all Kuiper Belt objects. the smaller objects are a secondary continuum slope of the near in- The albedo turns out to be quite population caused by collisions that frared region beyond 1 µm, the vis- diverse: while most objects have occurred in the mean time. ual range reveals a larger individ- values between 3% and 25%, uality in the spectral gradients. The which is fairly dark, a few have an steeper this slope, the more the ob- albedo of 60 to 90%. As shown in Spectral Analysis ject appears reddish as compared to Fig. 10, those are the large TNOs, the Sun. Statistical analysis of the such as Pluto, Charon and Eris. Spectroscopic analyses provide various populations among the Their reflectivity is similar to that some information on the surface TNOs reveals that Plutinos, Scat- of snow. This leads to the conclu- material of which the TNOs are tered Disk Objects, Detached Disk sion that their surface must be cov- ered by snow or ice while the smaller bodies are thought to have Fig. 11: Cumulative number of TNOs a mixture of icy, rocky and carbon over the magnitude scale. Magni- rich materials at the surface. Addi- tude 14 denotes a larger/brighter object tional darkening of their surfaces than mag. 22. This plot shows, that the vast majority of TNOs are quite small can be the result of the high energy as compared to Pluto. They most prob- radiation impinging on the bodies ably constitute the results of collisions over millions of years. between larger bodies of which only a few have survived up to now. (Credit: M.E. Brown, in ‘The Solar System be- Fig. 11 shows the cumulative num- yond Neptune’, Univ. Arizona Press, ber of objects over the magnitude 2008 )

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141348_Spatium_33_(001_016).indd 12 19.03.14 13:48 Fig. 12: Reflectivity spectra of TNOs. observing time at one of the largest tel- strong absorptions of methane ice on the Left panel: medium sized objects. escopes on Earth. (Credit: M. A. Ba- surfaces at 1.2, 1.4, 1.7 and 2.2 µm. The weak absorption dips around wave- rucci et al., in ‘The Solar System beyond (Credit: M.E. Brown, in ‘The Solar Sys- length 1.6 and 2.1 microns are due to Neptune’, Univ. Arizona Press, 2008). tem beyond Neptune’, Univ. Arizona water ice on the surface. Every spectrum Right panel: large sized objects. The re- Press, 2008) represents a few hours to a full night of flectivity spectra are dominated by

Objects, Centaurs and short-pe- ganic compounds. Very likely, stronger internal pressure elim­ riod comets appear relatively pale large bodies like Pluto may have inating initial pores in their to moderately red, while the hot experienced a kind of differentia- bodies. Classicals show a clearly reddish tion during their evolution, which, hue and the cold Classicals appear from a homogeneous mixture led to be by far the reddest population to a concentric density gradient of minor bodies in the solar system. bringing the lighter ice up to the Fig. 13: Analysis of the density of TNOs as a function of their size. This observation will help us when surface, while heavier material Large TNOs tend to have higher den- it comes to sketching their evolu- sank towards the core. This is com- sities than smaller objects which might tion in the solar system. parable to the processes we have in be the result of a higher compression the early evolution of planets. The and elimination of initial pores during their evolution. (R. H. Brown, in ‘The dominating ice component on the Solar System beyond Neptune’, Univ. Internal Structures surface of Pluto is nitrogen with Arizona Press, 2008) admixtures of methane and carbon Unsurprisingly, there is not much monoxide ices. known regarding the inner struc- ture of TNOs. At least for the larg- The larger objects seem to be ge- est objects, however, some conclu- ologically active leading to a very sions may be drawn. As outlined thin, likely only temporary atmos- in Fig. 13 density values span over phere, at least in the case of Pluto. the 1 to 3 g/cm3 range. This, how- It is thought that their densities ever, hints to a composition not were low initially, while the higher containing (water) ices only, but density values we observe today also significant additions of heav- were acquired during their evo­ ier materials like silicates and or- lution as a consequence of the

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141348_Spatium_33_(001_016).indd 13 19.03.14 13:48 The Trans-­ out in an apparently sparsely pop- itial formation of the solar system ulated regime of the solar system; which since then has not changed Neptunian therefore, they are exposed to the significantly. This leads us to the highest radiation dose, and expe- conclusion that various zones of the ­Region and rience practically no impacts. Yet, must have had instead of being very red, they are different reddish hues and that the the Planetary statistically indistinguishable from classes of objects with different System the pale to moderately red popula- hues stem from different regions of tions. With respect to colour they the protoplanetary disk. This could can’t even be distinguished from be interpreted in the sense that the objects that suffer collisions like large objects came from zones The research of TNOs is fuelled by the Plutinos and Scattered Disc where such large objects could their potential to provide further Objects. Furthermore, there are grow thanks to a sufficient mass information about the formation the hot and the cold Classicals density in the protoplanetary disk and evolution of the solar system which are in the same distance re- at that time. In contrast, the cold as a whole. Even though our pre- gion and hence should experience Classicals grew in a zone with sent knowledge of TNOs is still the same frequency of collisions re- lesser mass density allowing only fragmentary, some interesting con- sulting in similar colour properties smaller bodies to evolve. The mix- clusions can now be drawn. – provided that the material com- ture of objects we observe today in position is the same. Yet, hot and the same distance region must First, the different colours of the cold Classicals are statistically di- therefore not necessarily be the various categories may give us verse for their colour properties. ­initial state. To summarize, one some indication on their evolution. On the other hand, the intrinsic finds that the various populations Up to a very short time ago it was colours of hot CDOs are undistin- in the Kuiper Belt stem from dif- thought that the reddening is the guishable from Plutinos, Scattered ferent parts of the initial planetary result of high energetic radiation Disc Objects and Detached Disk system and that after formation – impinging on the objects. Labora- Objects. Moreover, the population with a few exceptions – they may tory tests showed indeed that sur- of hot CDOs contains larger ob- have migrated to where we find faces of ice tend to become more jects than those found among the them now. reddish upon irradiation. This, cold CDOs. Then, we have the however, leaves the question open short-periodic comets, most of Simulations have shown that the as to why there are also white ob- which should come from the Kui- initial population of smaller objects jects. It was thought, that those per Belt. They show no colour dif- near the outer giant planets may white objects had fresh ice on their ference as compared to the Pluti- have been shifted outward within surface as a consequence of im- nos, the Scattered Disk Objects and about 500 million years after for- pacts, that would become reddish the distant Detached Disk mation of the planets. This way again after some 10 to 100 million Objects. they may have migrated to the re- years. The large TNOs that have a gion of the Kuiper Belt joining as life of their own are an exception: Putting all these conflicting find- a dynamically hot population the their geologic activity continu- ings together one comes to a new original and dynamically cold ously brings fresh material from interpretation in the sense that the TNOs in this distance range. A the interior up to the surface thus observed colours are less, or not at large number of them were also keeping them white. all, due to the evolution of the ob- jettisoned out to the Oort Cloud jects but rather reflect a property by the gravity fields of the large This interpretation did, however, which is due to their formation. planets. There, in a distance of leave questions open: for instance According to this explanation, the some 10,000 AU, the gravity of the the Detached Disk Objects are far TNOs got their hue during the in- solar system’s interstellar neigh-

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141348_Spatium_33_(001_016).indd 14 19.03.14 13:48 bourhood gets influence and that Outlook major step is expected to be taken of the galactic centre begins to play in 2015 when NASA’s New Hori- a role as well by forming the Oort zons Pluto Kuiper Belt Mission cloud. (Fig.) 14 will start to explore repre- The icy objects in the Kuiper Belt sentatives of the trans-Neptunian The process of scattering is well are planetesimals13, whose growth region. Launched on 19 January understood: the giant planets’ stopped at sizes much smaller than 2006 on a rapid Jupiter gravity as- gravity fields disperse smaller ob- those of full-grown planets. They sist trajectory, it will reach Pluto, jects leading to a thinning out of are ancient relics that evolved be- Charon and their companions af- the original small body population fore the planets developed more ter a journey of no less than nine in their local orbits. Scattering than 4 billion years ago. Because years meeting the dwarf planet sys- largely takes place towards the Sun, they literally preserve the material tem on 14 July 2015 for closest ap- which absorbs most incoming ob- out of which the larger bodies ac- proach. After that, a series of fur- jects. On the other hand, a small cumulated, the icy dwarfs have a ther encounters with one or more part is scattered toward the outer great deal to teach us about plane- other Kuiper Belt objects is antic- rim of the solar system. Now, in tary formation and the history of ipated. And here our story comes order to maintain the solar system’s the solar system as a whole. full ­circle: the NASA’s New Hori- the outer zons spacecraft carries aboard a planets had to migrate outwards. Much has been learnt already by small portion of the ashes of Plu- This is thought to be the basic studying the Kuiper Belt, yet much to’s ­discoverer, Clyde William driver of more remains to be discovered. A Tombaugh … which ended at the time when no more significant amounts of mass were around to be scattered. Ob- viously, when Neptune reached its present orbit, the Kuiper Belt was already thinned out, which in turn kept it on the orbit we see today.

Fig. 14: The NASA spacecraft in the clean room. After a formidable­ journey spanning nine years it will encounter Pluto and its moons on 15 July 2015. The spacecraft carries a suite of seven instruments to meet the scientific goals of the mission, such as for instance mapping Pluto and Charon, analyses of Pluto’s atmosphere and geomorphological structure. (Credit: NASA)

13 are buildings stones of planets.

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The Author

tist for the FOcal Reducer and low unique platform for high angular dispersion Spectrograph (FORS) resolution astronomy. In 2004 he instrument for the Very Large went to the Max-Planck Institute ­Telescope (VLT) of the European for Solar System Research in Southern Observatory (ESO) in Katlen ­burg-Lindau/Germany (mo- Chile. ved to Göttingen in February 2014), now returning to his expert field The prospect of using one of the of ­research on comets, asteroids, most sensitive instruments at a ­Kuiper Belt objects. Here, he large telescope led him to embark became leader of the small bodies on the observation and analysis of group and is currently responsible the faint population of small for the experiment ­bodies in the outer solar system, contributions and teams of the namely on studies of the physical ­institute. This includes also the ­properties of Kuiper Belt objects. ROSETTA lander for In 1997 he moved to ESO in Chile, which he acts as one of the two lead working first as staff astronomer scientists. Rosetta is currently on and team leader of the medi- its final approach to comet 67P/ um-size tele­scope team at the La Churyumov-Gerasimenko where Hermann Boehnhardt received his Silla Observatory and with the the landing of Philae is scheduled diploma in physics in 1981 and the start-up of the VLT in early 1999 for 11 November 2014. PhD in 1985 with a thesis at the Paranal Observatory in the on electrostatic charging and frag- Atacama desert. During his work Dr. Boehnhardt is the author of mentation of dust in the solar sys- for ESO he had responsibilities as ­numerous scientific papers and a tem. Then, he joined the cometary instrument scientist for the FORS member of the International As­ research group of Prof. Rahe at the instruments and OMEGACAM, a tronomical Union, the Commit- Bamberg Observatory in Ger- wide-field camera for the VLT tee of (COSPAR), many, focusing on dust fragmen- Survey Tele­scope as well as deputy and others. He was engaged as a tation phenomena and comet ob- of the VLT Science Operations member of the programme com- servations. In 1990, he went to the group and for the ESO director of mittees for the Observing Pro- European Space Operations Cen- science in Chile. gramme at ESO telescopes and ter ESOC in Darmstadt where he ESA astronomical satellites. The focused on co-location of geosta- After his return to Germany in most outstanding award is tionary satellites and the fly-by of 2002 he worked at the Max-Planck 1989GB1 which was named by the the spacecraft at comet Institute for Astronomy in Heidel- International Astronomical Union Grigg-Skjellerup in 1992. There- berg as project manager of the IAU “8010 Boehnhardt”. after, he joined the instrument LINC-NIRVANA instrument, the team at the University Observa- near-IR Fizeau interferometer of tory in Munich/Germany and the Large Binocular Telescope became project manager and scien- LBT at Mt. Graham/Arizona, a

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