1 00:00:03,000 --> 00:00:06,000 Astronomers have accurately measured the size 2 00:00:06,000 --> 00:00:09,000 of the remote dwarf planet Eris for the first time. 3 00:00:10,000 --> 00:00:13,000 They caught it as it passed in front of a faint star, 4 00:00:13,000 --> 00:00:17,000 using the Belgian TRAPPIST telescope at ESO’s La Silla Observatory, 5 00:00:17,000 --> 00:00:20,000 along with two other telescopes in Chile. 6 00:00:20,000 --> 00:00:26,000 The new observations show that Eris is an almost perfect twin of Pluto in size. 7 00:00:26,000 --> 00:00:29,000 Eris also seems to be extremely reflective, 8 00:00:29,000 --> 00:00:33,000 probably because it is covered in a thin layer of frozen atmosphere. 9 00:00:37,500 --> 00:00:39,000 This is the ESOcast! 10 00:00:39,500 --> 00:00:44,500 Cutting-edge science and life behind the scenes of ESO, the European Southern Observatory, 11 00:00:44,500 --> 00:00:51,000 exploring the ultimate frontier with our host Dr J, a.k.a. Dr Joe Liske. 12 00:00:57,000 --> 00:01:02,000 In this episode, we are going to look at new observations of the distant dwarf planet Eris 13 00:01:02,000 --> 00:01:04,000 as it passed in front of a background star 14 00:01:04,000 --> 00:01:07,000 — an event called an occultation — 15 00:01:07,000 --> 00:01:09,000 and what these results have revealed. 16 00:01:11,000 --> 00:01:14,000 Occultations are rather like eclipses 17 00:01:14,000 --> 00:01:19,000 — the background star disappears behind the object and reappears on its other side. 18 00:01:20,000 --> 00:01:24,000 As viewed from Earth, the brightness of the background star suddenly drops 19 00:01:24,000 --> 00:01:28,000 and then returns equally suddenly to its previous level. 20 00:01:28,000 --> 00:01:30,000 By looking at these two events, 21 00:01:30,000 --> 00:01:35,000 astronomers can measure the size and shape of the occulting foreground object. 22 00:01:35,000 --> 00:01:39,000 If they also know the mass of this object they can then determine its density. 23 00:01:42,000 --> 00:01:47,000 Occultations of stars by distant objects in the Solar System are very hard to observe 24 00:01:47,000 --> 00:01:51,000 because of the small size of the objects. 25 00:01:51,000 --> 00:01:55,000 But occultations are often the only way to learn about these remote specks, 26 00:01:55,000 --> 00:01:58,000 as they are too distant and too small to be seen as anything more 27 00:01:58,000 --> 00:02:03,000 than faint points of light, even through powerful telescopes. 28 00:02:04,000 --> 00:02:08,000 The occultation technique has now enabled astronomers to learn a lot more 29 00:02:08,000 --> 00:02:11,000 about the dwarf planet Eris. 30 00:02:11,000 --> 00:02:16,000 Eris was identified as a large object in the outer Solar System in 2005. 31 00:02:16,000 --> 00:02:21,000 Its discovery was one of the factors that led to the creation of a new class of objects 32 00:02:21,000 --> 00:02:23,000 called dwarf planets 33 00:02:23,000 --> 00:02:28,000 and the reclassification of Pluto from planet to dwarf planet in 2006. 34 00:02:30,500 --> 00:02:34,000 Eris is three times farther from the Sun than Pluto at the moment, 35 00:02:34,000 --> 00:02:38,500 and until now was believed to be about 25% bigger. 36 00:02:38,500 --> 00:02:44,000 But the new observations show that Eris is in fact almost exactly the same size as Pluto, 37 00:02:44,000 --> 00:02:49,000 with a diameter of around 2330 kilometres. 38 00:02:50,500 --> 00:02:54,000 Because Eris also has a moon, called Dysnomia, 39 00:02:54,000 --> 00:02:57,500 astronomers have also been able to calculate the mass of Eris 40 00:02:57,500 --> 00:03:01,000 by a careful study of this moon’s orbit. 41 00:03:01,000 --> 00:03:07,000 Using the new diameter and known mass, they then calculated the density of Eris, 42 00:03:07,000 --> 00:03:11,000 which now appears to be greater than astronomers had previously thought. 43 00:03:11,000 --> 00:03:16,000 Eris seems to be a rocky body surrounded by a thick mantle of ice. 44 00:03:17,000 --> 00:03:21,500 The dwarf planet turns out to reflect almost all of the light that falls on it 45 00:03:21,500 --> 00:03:25,500 — its surface is even brighter than fresh snow on Earth. 46 00:03:25,500 --> 00:03:30,000 Eris is probably covered in a very thin layer of frozen atmosphere 47 00:03:30,000 --> 00:03:34,500 that is likely to consist of frozen nitrogen mixed with methane. 48 00:03:34,500 --> 00:03:38,000 It is probably the result of the freezing of Eris’s atmosphere 49 00:03:38,000 --> 00:03:43,000 as the dwarf planet’s elongated orbit takes it far away from the Sun. 50 00:03:44,000 --> 00:03:48,000 These important new observations, made with relatively small telescopes, 51 00:03:48,000 --> 00:03:53,000 have allowed astronomers to measure Eris’s properties better than ever before. 52 00:03:53,000 --> 00:03:56,500 This is another step towards understanding the mysterious objects 53 00:03:56,500 --> 00:04:00,000 that lie in the remote parts of our own Solar System. 54 00:04:00,500 --> 00:04:03,000 This is Dr. J signing off for the ESOcast. 55 00:04:03,000 --> 00:04:06,500 Join me again next time for another cosmic adventure. 56 00:04:08,000 --> 00:04:12,000 ESOcast is produced by ESO, the European Southern Observatory. 57 00:04:12,000 --> 00:04:14,000 ESO, the European Southern Observatory, 58 00:04:14,000 --> 00:04:16,000 is the pre-eminent intergovernmental science and technology organisation in astronomy, 59 00:04:16,000 --> 00:04:18,000 designing, constructing and operating the world’s most advanced ground-based telescopes. 60 00:04:21,000 --> 00:04:25,500 Transcription by ESO; translation by — 61 00:04:38,000 --> 00:04:40,500 Now that you've caught up with ESO, 62 00:04:43,000 --> 00:04:46,000 head 'out of this world' with Hubble. 63 00:04:48,500 --> 00:04:51,500 The Hubblecast highlights the latest discoveries of the world´s 64 00:04:51,500 --> 00:04:55,000 most recognized and prized space observatory, 65 00:04:57,000 --> 00:05:00,000 The NASA/ESA Hubble Space Telescope.