WEBVTT FILE 1 00:00:00.010 --> 00:00:06.580 [ music ] 2 00:00:06.600 --> 00:00:12.040 Since the early 1990's, astronomers have known that extrasolar planets, or "exoplanets," 3 00:00:12.060 --> 00:00:15.560 orbit stars light-years beyond our own solar system. 4 00:00:15.580 --> 00:00:21.080 Because most exoplanets are too far away to be directly imaged, characteristics such as size, 5 00:00:21.100 --> 00:00:26.880 composition, and atmospheric makeup must be determined through a variety of indirect methods. 6 00:00:26.900 --> 00:00:31.030 For instance, when an exoplanet passes in front of its star, or transits, 7 00:00:31.050 --> 00:00:35.330 it blocks a fraction of the star's light and causes a dip in brightness. 8 00:00:35.350 --> 00:00:41.380 Large planets block more light, so the size of the dip can be used to determine the size of the planet. 9 00:00:41.400 --> 00:00:47.630 By observing an exoplanet's gravitational pull on its star, astronomers can also determine the planet's mass, 10 00:00:47.650 --> 00:00:54.740 and thus calculate its density, to see if it is composed of rock like Earth, or gas like Saturn. 11 00:00:54.760 --> 00:00:58.920 But to fully understand an exoplanet, astronomers must study its atmosphere, 12 00:00:58.940 --> 00:01:02.550 and the information that they need is encoded during a transit. 13 00:01:02.570 --> 00:01:07.980 As the planet crosses its star, its atmosphere absorbs certain wavelengths of light, or colors, 14 00:01:08.000 --> 00:01:13.940 while allowing other wavelengths to pass through. Because each molecule absorbs distinct wavelengths, 15 00:01:13.960 --> 00:01:19.480 astronomers spread the star's light into its spectrum of colors to see which wavelengths have been absorbed. 16 00:01:19.500 --> 00:01:25.380 The dark absorption bands act as molecular fingerprints, revealing the atmosphere's chemical makeup. 17 00:01:25.400 --> 00:01:29.250 Knowing the depth and density of the atmosphere is also important. 18 00:01:29.270 --> 00:01:33.980 To figure this out, astronomers observe the transit at many different wavelengths. 19 00:01:34.000 --> 00:01:37.560 At wavelengths where more absorption occurs, the planet will appear larger, 20 00:01:37.580 --> 00:01:43.850 with the change in size indicating how deeply the atmosphere extends, and its density at different altitudes. 21 00:01:43.870 --> 00:01:49.370 Measuring the depth of absorption at each wavelength gives astronomers the planet's transit depth curve, 22 00:01:49.390 --> 00:01:56.530 which allows them to model the composition, height, and density of the atmosphere, providing a detailed picture of the planet. 23 00:01:56.550 --> 00:02:01.830 Recent studies suggest that exoplanets and their atmospheres come in a wide variety. 24 00:02:01.850 --> 00:02:08.580 At one extreme are "hot Jupiters" like WASP 19 b, a boiling gas giant that orbits its star 25 00:02:08.600 --> 00:02:15.300 far closer than Mercury orbits our Sun. Visitors who could survive the heat might complain about the air quality: 26 00:02:15.320 --> 00:02:21.780 planet WASP 19 b's jagged transit depth curve suggests a deep atmosphere of poisonous hydrocarbons, 27 00:02:21.800 --> 00:02:26.180 with methane and hydrogen cyanide far more abundant than water. 28 00:02:26.200 --> 00:02:32.380 By contrast, planet Gliese 1214 b is a comparatively inviting "waterworld." 29 00:02:32.400 --> 00:02:37.150 Its nearly flat transit depth curve hints at a shallow atmosphere of pure steam, 30 00:02:37.170 --> 00:02:46.050 enveloping an ocean thousands of kilometers deep, with an interior of hot ice: water solidified by extreme pressure rather than cold. 31 00:02:46.070 --> 00:02:51.280 As detection methods improve, astronomers will search the atmospheres of Earth-size planets 32 00:02:51.300 --> 00:02:55.280 for signs of life such as water vapor, oxygen, and methane, 33 00:02:55.300 --> 00:03:03.980 taking us one step closer to finding a world like our own, all thanks to some flickering starlight. 34 00:03:04.000 --> 00:03:13.250 [ music, satellite beeping ] 35 00:03:13.270 --> 00:03:21.081 [ music, low rumble, wind chimes ]