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 ]