1 00:00:03,000 --> 00:00:09,000 Astronomers know that planets around other stars beyond the Solar System are common. 2 00:00:10,000 --> 00:00:15,000 But these planets are very hard to see and even harder to study. 3 00:00:16,000 --> 00:00:19,000 Fortunately, there is a clever trick 4 00:00:19,000 --> 00:00:22,000 that helps to separate the feeble glow of a planet 5 00:00:22,000 --> 00:00:25,000 from the dazzling glare of its parent star: 6 00:00:26,000 --> 00:00:31,000 exploiting the polarisation of the light reflected from the planet. 7 00:00:33,000 --> 00:00:39,000 This method will allow future instruments on ESO’s Very Large Telescope in Chile, 8 00:00:39,000 --> 00:00:43,000 and the European Extremely Large Telescope, 9 00:00:43,000 --> 00:00:46,000 to see otherwise invisible planets 10 00:00:46,000 --> 00:00:51,000 and even to search for signs of life beyond the Solar System. 11 00:00:56,000 --> 00:00:58,000 This is the ESOcast! 12 00:00:59,000 --> 00:01:02,000 Cutting-edge science and life behind the scenes of ESO, 13 00:01:02,000 --> 00:01:05,000 the European Southern Observatory. 14 00:01:05,000 --> 00:01:08,000 Exploring the ultimate frontier with our host Dr J, 15 00:01:08,000 --> 00:01:12,000 a.k.a. Dr Joe Liske. 16 00:01:13,000 --> 00:01:18,000 In this episode of the ESOcast we’ll talk about a very special feature of light 17 00:01:18,000 --> 00:01:22,000 and how we can use it to detect planets around other stars. 18 00:01:23,000 --> 00:01:28,000 And, we’ll talk about about a powerful new instrument that will exploit this feature: 19 00:01:28,000 --> 00:01:30,000 the planet-finder SPHERE 20 00:01:30,000 --> 00:01:35,000 which will be installed at ESO’s Very Large Telescope in early 2014. 21 00:01:39,000 --> 00:01:42,000 Light is an electromagnetic wave. 22 00:01:44,000 --> 00:01:49,000 Usually the plane containing a light wave can be in any direction, 23 00:01:49,000 --> 00:01:53,000 but sometimes one direction is more likely than others, 24 00:01:53,000 --> 00:01:56,000 and the light is said to be polarised. 25 00:01:56,000 --> 00:02:01,000 Several of ESO’s telescopes can measure this polarisation, 26 00:02:01,000 --> 00:02:06,000 offering exciting opportunities to find and study distant objects, 27 00:02:06,000 --> 00:02:10,000 including planets around their host stars. 28 00:02:14,000 --> 00:02:16,000 Take any star in the sky. 29 00:02:16,000 --> 00:02:20,000 Chances are that this star hosts several planets. 30 00:02:21,000 --> 00:02:25,000 One of these planets may even be similar to the Earth. 31 00:02:26,000 --> 00:02:31,000 But these planets are very hard to see in the glare from the bright star, 32 00:02:31,000 --> 00:02:34,000 as they are more than a billion times fainter. 33 00:02:37,000 --> 00:02:42,000 Fortunately, we can use polarisation to help us tease out the very weak light 34 00:02:42,000 --> 00:02:46,000 of the planet from the dazzling light of its parent star. 35 00:02:46,000 --> 00:02:49,000 So how does this work? 36 00:02:49,000 --> 00:02:54,000 In many cases, the light we receive from the planet is actually reflected starlight 37 00:02:54,000 --> 00:02:57,000 that is scattered in the planet’s atmosphere. 38 00:02:57,000 --> 00:03:02,000 The scattering process produces polarised light just like the light 39 00:03:02,000 --> 00:03:05,000 we receive from the blue sky here on Earth. 40 00:03:05,000 --> 00:03:08,000 The point is that we can detect this polarisation, 41 00:03:08,000 --> 00:03:11,000 that is, the preferential alignment of the light 42 00:03:11,000 --> 00:03:14,000 caused by the scattering in the planetary atmosphere, 43 00:03:14,000 --> 00:03:18,000 using state-of-the-art instrumentation on big telescopes. 44 00:03:21,000 --> 00:03:23,000 Such an instrument 45 00:03:23,000 --> 00:03:25,000 — called SPHERE — 46 00:03:25,000 --> 00:03:31,000 has been built and will be installed on ESO’s Very Large Telescope in 2014. 47 00:03:33,000 --> 00:03:36,000 SPHERE will take images of exoplanets. 48 00:03:36,000 --> 00:03:39,000 It will combine polarimetry 49 00:03:39,000 --> 00:03:43,000 with other methods to suppress the overwhelming light from a star 50 00:03:43,000 --> 00:03:47,000 and allow the very feeble light from any planets orbiting that star 51 00:03:47,000 --> 00:03:51,000 to be picked up and studied. 52 00:03:54,000 --> 00:03:59,000 The first requirement is to have a large telescope such as the VLT, 53 00:03:59,000 --> 00:04:01,000 able — in principle — 54 00:04:01,000 --> 00:04:03,000 to take pictures that are sharp enough 55 00:04:03,000 --> 00:04:08,000 to allow us to spot any planets next to the star. 56 00:04:09,000 --> 00:04:13,000 But the Earth's atmosphere blurs the view, 57 00:04:13,000 --> 00:04:18,000 so we also need a clever optical system — adaptive optics — 58 00:04:18,000 --> 00:04:22,000 to take out this blurring effect as much as possible 59 00:04:22,000 --> 00:04:26,000 and bring most of the starlight together into one bright dot. 60 00:04:27,000 --> 00:04:32,000 The centre of this bright dot is then blocked out by introducing a mask 61 00:04:32,000 --> 00:04:37,000 into the light beam to avoid swamping the fainter nearby objects. 62 00:04:38,000 --> 00:04:43,000 But even after all these tricks a halo of starlight remains — 63 00:04:43,000 --> 00:04:46,000 much brighter than the planets that we are looking for. 64 00:04:47,000 --> 00:04:51,000 However, this halo is unpolarised, 65 00:04:51,000 --> 00:04:55,000 whereas the light from the planets is generally polarised. 66 00:04:58,000 --> 00:05:00,000 The new SPHERE instrument 67 00:05:00,000 --> 00:05:04,000 will be able to pick out a planet’s faint signal of polarised light 68 00:05:04,000 --> 00:05:06,000 from the unpolarised stellar halo. 69 00:05:06,000 --> 00:05:09,000 This trick — along with several others — 70 00:05:09,000 --> 00:05:15,000 will help SPHERE to take images of Jupiter-like planets around other stars. 71 00:05:18,000 --> 00:05:23,000 However, we don’t just want to take pictures of the large exoplanets, 72 00:05:23,000 --> 00:05:28,000 we also want to get to the smaller rocky planets close to their parent stars. 73 00:05:28,000 --> 00:05:31,000 But to do that we need a MUCH bigger telescope, 74 00:05:31,000 --> 00:05:36,000 one that collects much more light and provides even sharper images: 75 00:05:36,000 --> 00:05:42,000 the 39-metre European Extremely Large Telescope, or E-ELT. 76 00:05:42,000 --> 00:05:48,000 This giant telescope will be equipped with the next generation of exoplanet imagers. 77 00:05:50,000 --> 00:05:56,000 They will use all the same techniques as SPHERE, but take them to the next level. 78 00:05:56,000 --> 00:06:00,000 By using polarimetry, as well as other methods, 79 00:06:00,000 --> 00:06:03,000 astronomers will be able to image rocky planets 80 00:06:03,000 --> 00:06:07,000 in the habitable zones around nearby stars. 81 00:06:07,000 --> 00:06:12,000 The polarised signal can also give astronomers vital clues 82 00:06:12,000 --> 00:06:16,000 on whether or not a planet has oceans and clouds of liquid water. 83 00:06:17,000 --> 00:06:20,000 And for larger Jupiter-like planets 84 00:06:20,000 --> 00:06:24,000 it should be possible to study the light in enough detail 85 00:06:24,000 --> 00:06:27,000 that we will be able to actually see what the planet looks like. 86 00:06:28,000 --> 00:06:33,000 The ultimate goal is to one day spot the signatures of life 87 00:06:33,000 --> 00:06:36,000 on worlds beyond the Solar System 88 00:06:36,000 --> 00:06:42,000 by finding evidence of oxygen, or the typical green signature of vegetation. 89 00:06:47,000 --> 00:06:52,000 Looking at exoplanets in polarised light may well turn out to be key 90 00:06:52,000 --> 00:06:56,000 in providing us with our very first signs of extraterrestrial life. 91 00:06:57,000 --> 00:07:00,000 This is Dr J signing off for the ESOcast. 92 00:07:00,000 --> 00:07:03,000 Join me again next time for another cosmic adventure. 93 00:07:12,000 --> 00:07:15,000 Transcription by Phillip Keane; 94 00:07:15,000 --> 00:07:18,000 translation by —