If Kepler-186F

colophon EN:l'astrofilo 30/04/14 07:54 Page 2 A new astronomy magazine for you Dear Readers, Free Astronomy Magazine is pleased to welcome you. After more than 5 years of publications in Italian, the free multimedia magazine “l’Astrofilo” is now broadening its reach by publishing online the English version of the same. Our main objective is to deal in the most simple and detailed way with the latest discoveries made by professional astronomers. The choice of the topics covered takes into account the need to reach a wide as possible audience and avoid, therefore, subject matters or con- tents that are too technical. Any amateur astronomer with minimal scientific knowledge will be able to fully appreciate the articles published by Free Astronomy Magazine, but also those less-experienced beginners can surely find some interesting material to read. Accuracy of the scientific content, utmost graphic quality, ease of commu- nication and Readers’ satisfaction are the key concepts of our philosophy and publication activity. We would welcome your opinion about Free Astronomy Magazine. Have a nice read! Michele Ferrara Editor in chief editorial editorial 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. MAY-JUNE 2014 kepler186 EN:l'Astrofilo 30/04/14 07:42 Page 5 EXOPLANETS 5 stronomers have achieved another historic result in the long journey A taking them toward finding an ex- trasolar planet with characteristics very similar to those of Earth. That breakthrough step consists in the confirmation of the ex- awayaway 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 discovery 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 system 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, astronomers have identified over 3,800 candidate 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] MAY-JUNE 2014 kepler186 EN:l'Astrofilo 30/04/14 07:42 Page 6 EXOPLANETS 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 transit. [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 account 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 EXOPLANETS 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, astronomers made official the existence 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.

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