WEBVTT 00:00.000 --> 00:14.240 The scientists are looking for exoplanets, but despite all their efforts, they couldn't 00:14.240 --> 00:15.360 find any so far. 00:15.360 --> 00:19.920 Isn't it very extraordinary that the universe is so big, but our planet seems to be the 00:19.920 --> 00:25.640 only one with conditions right for life and that it looks very specially created and very 00:25.640 --> 00:28.080 well protected? 00:28.080 --> 00:36.000 Prior to 1995, there had been many attempts for detection of planets around other stars, 00:36.000 --> 00:40.040 and these would be extrasolar planets or exoplanets. 00:40.040 --> 00:47.100 And every attempt or every discovery was proven to be either subject to doubt or just incorrect. 00:47.100 --> 00:55.860 But in 1995, two teams discovered planets, a planet around 51 Pegasi. 00:55.860 --> 00:59.720 This was a real observation of a planet around another star. 00:59.720 --> 01:04.780 At that point, there was a very great sense of euphoria in the scientific community because 01:04.780 --> 01:09.660 for the first time, we had begun to have techniques that are sensitive to seeing planets around 01:09.660 --> 01:10.780 other stars. 01:10.780 --> 01:16.860 So in 1995, the expectation was that there are planets abundantly around other stars and 01:16.860 --> 01:20.700 many planets like the Earth will be very easily observable. 01:20.700 --> 01:28.620 If you look at statements made, for example, to national academies or to the Congress of the 01:28.620 --> 01:34.620 United States by American scientists, they talk about billions of Earths just within our galaxy. 01:34.620 --> 01:41.540 But in fact, since 1995, as our techniques have even more improved, yes, what we have seen is thousands 01:41.540 --> 01:42.540 of planets. 01:42.540 --> 01:44.540 We have seen thousands of planets around other stars. 01:44.540 --> 01:50.540 But then when you ask to compare those planets with the Earth, what you find is that planets, 01:50.540 --> 01:56.540 yes, there are, but Earth-like planets, there essentially are not because the requirements for 01:56.540 --> 02:00.740 an Earth-like planet are many, many requirements. 02:00.740 --> 02:05.860 The planet itself has to be situated in what is called the habitable zone. 02:05.860 --> 02:12.740 A habitable zone is essentially a distance from the sun where the temperature of the, the 02:12.740 --> 02:17.860 mean temperature of the planet is one in which liquid water can exist at least somewhere on 02:17.860 --> 02:19.180 the planet. 02:19.180 --> 02:25.780 So if you're too close, what you end up having is that there is excess heat and the heat evaporates 02:25.780 --> 02:28.820 the water which causes a greenhouse effect. 02:28.820 --> 02:35.500 This causes an effect that planetary geologists call the moist greenhouse that then leads to 02:35.500 --> 02:40.460 a runaway greenhouse and so that all the water essentially evaporates. 02:40.460 --> 02:45.820 When it evaporates, it gets to the upper reaches of the atmosphere where a particle process called 02:45.820 --> 02:51.560 sputtering essentially sends particles from the sun and knock off the hydrogen off the upper 02:51.560 --> 02:55.040 atmosphere of the planet and the planet becomes desiccated. 02:55.040 --> 02:58.980 So it doesn't just stay moist, it just actually gets desiccated. 02:58.980 --> 03:05.960 On the other hand, if the planet is too far from the sun, the star, then the moisture will 03:05.960 --> 03:12.260 snow down on the, onto the surface of the planet where the albedo, which is the reflectivity 03:12.260 --> 03:14.080 of the planet, goes up. 03:14.080 --> 03:19.160 As it does that, the planet temperature drops which causes more snowing and you end up with 03:19.160 --> 03:21.640 a situation called the snowball Earth. 03:21.640 --> 03:26.160 So you have a runaway greenhouse on one end and a snowball on the other end and only in 03:26.160 --> 03:28.240 the middle is a perfect state. 03:28.240 --> 03:33.900 Now calculations have been done to say from the Earth, how far closer can I go before I 03:33.900 --> 03:35.660 get into a runaway greenhouse? 03:35.660 --> 03:38.580 Some of these calculations put it at 1%. 03:38.580 --> 03:42.180 We're 1% closer and we get into a runaway greenhouse. 03:42.180 --> 03:44.800 So we are very, very perfectly situated. 03:44.800 --> 03:47.880 And that's just from the standpoint of distance. 03:47.880 --> 03:50.740 Then the point is that not all stars are the same. 03:50.740 --> 03:57.380 We have a very stable, energetic star, a massive, energetic, stable star. 03:57.380 --> 04:02.520 If our star was one of the more common stars, like an M star, which would be a cooler star, 04:02.520 --> 04:04.820 the habitable zone is closer. 04:04.820 --> 04:08.440 I have to be closer to be habitable, to have the moisture. 04:08.440 --> 04:12.480 But when I'm that close, it turns out I'm very close to a star which is very unstable. 04:12.480 --> 04:19.240 M stars have a lot of strong energy output changes that also damage the planet. 04:19.240 --> 04:25.160 The other thing that happens is that the planet's day and night cycle, which is governed by its 04:25.160 --> 04:31.300 spin, as you get closer there's a gravitational effect called tidal friction and tidal locking, 04:31.300 --> 04:36.380 which essentially locks the planet so that one side of the planet is the only site that's 04:36.380 --> 04:37.380 seen. 04:37.380 --> 04:42.160 So one side of the planet totally burns up, the other side just, you know, gets just super 04:42.160 --> 04:43.160 chilled. 04:43.160 --> 04:48.160 These are just two of a great number of conditions that have come together for the Earth.