WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:270000 00:00:00.000 --> 00:00:05.940 Taking advantage of the total lunar eclipse of January 2019, astronomers, using NASA’s 00:00:05.940 --> 00:00:11.150 Hubble Space Telescope, have detected ozone in Earth’s atmosphere. 00:00:11.150 --> 00:00:16.500 In this observation, Hubble did not look at Earth directly. Instead, astronomers used 00:00:16.500 --> 00:00:21.670 the Moon as a mirror to reflect sunlight that passed through Earth’s atmosphere. 00:00:21.670 --> 00:00:26.320 This method serves as a proxy for how they will observe planets around other stars in 00:00:26.320 --> 00:00:29.410 search for worlds similar to our own. 00:00:29.410 --> 00:00:34.060 Though numerous ground-based observations of this kind have been done previously, this 00:00:34.060 --> 00:00:38.290 is the first time ultraviolet light passing through Earth’s atmosphere was observed 00:00:38.290 --> 00:00:41.660 from space. The measurements from this experiment detected the 00:00:41.660 --> 00:00:45.020 strong spectral fingerprint of ozone. 00:00:45.020 --> 00:00:50.100 On Earth, photosynthesis over billions of years is responsible for our planet’s high 00:00:50.100 --> 00:00:55.830 oxygen levels and thick ozone layer. That’s one reason why scientists think ozone or oxygen 00:00:55.830 --> 00:00:58.820 could be a sign of life on another planet. 00:00:58.820 --> 00:01:04.500 But finding ozone on distant worlds isn’t an easy task. Ultraviolet observations like 00:01:04.500 --> 00:01:09.240 this can best be conducted from space telescopes, above the limiting effects of looking through 00:01:09.240 --> 00:01:15.030 Earth’s skies, because ozone blocks most ultraviolet light from beyond the atmosphere. 00:01:15.030 --> 00:01:20.740 One of NASA’s major goals is to identify habitable and inhabited planets. But how would 00:01:20.740 --> 00:01:24.350 we know whether a distant planet has ozone or not? 00:01:24.350 --> 00:01:28.780 The atmospheres of some extrasolar planets can be probed if that distant world passes 00:01:28.780 --> 00:01:34.440 across the face of its parent star, called a transit. During a transit, starlight filters 00:01:34.440 --> 00:01:37.070 through the backlit exoplanet’s atmosphere. 00:01:37.070 --> 00:01:40.340 Chemicals in the atmosphere leave their telltale signature 00:01:40.340 --> 00:01:43.060 by filtering out certain colors of starlight. 00:01:43.060 --> 00:01:47.280 Astronomers have used Hubble to observe the atmospheres of several gas giant planets that 00:01:47.280 --> 00:01:52.390 transit their stars. But terrestrial planets are much smaller objects and their atmosphere 00:01:52.390 --> 00:01:54.680 thinner, like the skin on an apple. 00:01:54.680 --> 00:01:59.080 Therefore, teasing out these signatures is much more difficult. 00:01:59.080 --> 00:02:03.660 To prepare for future studies with larger telescopes, astronomers used Hubble to conduct 00:02:03.660 --> 00:02:10.120 experiments on a much closer and only known inhabited terrestrial planet: Earth! Our planet’s 00:02:10.120 --> 00:02:14.740 perfect alignment with the Sun and Moon during a total lunar eclipse mimics the geometry 00:02:14.740 --> 00:02:18.040 of a transiting terrestrial planet with its star. 00:02:18.040 --> 00:02:22.560 But the observations were also challenging because the Moon is very bright, and its surface 00:02:22.560 --> 00:02:27.420 is not a perfect reflector because it is mottled with bright and dark areas. 00:02:27.420 --> 00:02:32.260 The Moon is also so close to Earth that it appears to move very quickly in the sky, making 00:02:32.260 --> 00:02:35.660 it harder for Hubble to stay pointed at the same location. 00:02:35.660 --> 00:02:41.219 However, in spite of the challenges, the experiment was an incredible success, and Hubble is supporting 00:02:41.219 --> 00:02:46.620 the ongoing quest to find planets that are similar to our own, and perhaps, one day, 00:02:46.620 --> 00:02:49.280 find signs of life on other worlds. 00:02:49.280 --> 00:02:59.280 [ BEAUTIFUL MUSIC ]