1 00:00:00,000 --> 00:00:14,240 The scientists are looking for exoplanets, but despite all their efforts, they couldn't 2 00:00:14,240 --> 00:00:15,360 find any so far. 3 00:00:15,360 --> 00:00:19,920 Isn't it very extraordinary that the universe is so big, but our planet seems to be the 4 00:00:19,920 --> 00:00:25,640 only one with conditions right for life and that it looks very specially created and very 5 00:00:25,640 --> 00:00:28,080 well protected? 6 00:00:28,080 --> 00:00:36,000 Prior to 1995, there had been many attempts for detection of planets around other stars, 7 00:00:36,000 --> 00:00:40,040 and these would be extrasolar planets or exoplanets. 8 00:00:40,040 --> 00:00:47,100 And every attempt or every discovery was proven to be either subject to doubt or just incorrect. 9 00:00:47,100 --> 00:00:55,860 But in 1995, two teams discovered planets, a planet around 51 Pegasi. 10 00:00:55,860 --> 00:00:59,720 This was a real observation of a planet around another star. 11 00:00:59,720 --> 00:01:04,780 At that point, there was a very great sense of euphoria in the scientific community because 12 00:01:04,780 --> 00:01:09,660 for the first time, we had begun to have techniques that are sensitive to seeing planets around 13 00:01:09,660 --> 00:01:10,780 other stars. 14 00:01:10,780 --> 00:01:16,860 So in 1995, the expectation was that there are planets abundantly around other stars and 15 00:01:16,860 --> 00:01:20,700 many planets like the Earth will be very easily observable. 16 00:01:20,700 --> 00:01:28,620 If you look at statements made, for example, to national academies or to the Congress of the 17 00:01:28,620 --> 00:01:34,620 United States by American scientists, they talk about billions of Earths just within our galaxy. 18 00:01:34,620 --> 00:01:41,540 But in fact, since 1995, as our techniques have even more improved, yes, what we have seen is thousands 19 00:01:41,540 --> 00:01:42,540 of planets. 20 00:01:42,540 --> 00:01:44,540 We have seen thousands of planets around other stars. 21 00:01:44,540 --> 00:01:50,540 But then when you ask to compare those planets with the Earth, what you find is that planets, 22 00:01:50,540 --> 00:01:56,540 yes, there are, but Earth-like planets, there essentially are not because the requirements for 23 00:01:56,540 --> 00:02:00,740 an Earth-like planet are many, many requirements. 24 00:02:00,740 --> 00:02:05,860 The planet itself has to be situated in what is called the habitable zone. 25 00:02:05,860 --> 00:02:12,740 A habitable zone is essentially a distance from the sun where the temperature of the, the 26 00:02:12,740 --> 00:02:17,860 mean temperature of the planet is one in which liquid water can exist at least somewhere on 27 00:02:17,860 --> 00:02:19,180 the planet. 28 00:02:19,180 --> 00:02:25,780 So if you're too close, what you end up having is that there is excess heat and the heat evaporates 29 00:02:25,780 --> 00:02:28,820 the water which causes a greenhouse effect. 30 00:02:28,820 --> 00:02:35,500 This causes an effect that planetary geologists call the moist greenhouse that then leads to 31 00:02:35,500 --> 00:02:40,460 a runaway greenhouse and so that all the water essentially evaporates. 32 00:02:40,460 --> 00:02:45,820 When it evaporates, it gets to the upper reaches of the atmosphere where a particle process called 33 00:02:45,820 --> 00:02:51,560 sputtering essentially sends particles from the sun and knock off the hydrogen off the upper 34 00:02:51,560 --> 00:02:55,040 atmosphere of the planet and the planet becomes desiccated. 35 00:02:55,040 --> 00:02:58,980 So it doesn't just stay moist, it just actually gets desiccated. 36 00:02:58,980 --> 00:03:05,960 On the other hand, if the planet is too far from the sun, the star, then the moisture will 37 00:03:05,960 --> 00:03:12,260 snow down on the, onto the surface of the planet where the albedo, which is the reflectivity 38 00:03:12,260 --> 00:03:14,080 of the planet, goes up. 39 00:03:14,080 --> 00:03:19,160 As it does that, the planet temperature drops which causes more snowing and you end up with 40 00:03:19,160 --> 00:03:21,640 a situation called the snowball Earth. 41 00:03:21,640 --> 00:03:26,160 So you have a runaway greenhouse on one end and a snowball on the other end and only in 42 00:03:26,160 --> 00:03:28,240 the middle is a perfect state. 43 00:03:28,240 --> 00:03:33,900 Now calculations have been done to say from the Earth, how far closer can I go before I 44 00:03:33,900 --> 00:03:35,660 get into a runaway greenhouse? 45 00:03:35,660 --> 00:03:38,580 Some of these calculations put it at 1%. 46 00:03:38,580 --> 00:03:42,180 We're 1% closer and we get into a runaway greenhouse. 47 00:03:42,180 --> 00:03:44,800 So we are very, very perfectly situated. 48 00:03:44,800 --> 00:03:47,880 And that's just from the standpoint of distance. 49 00:03:47,880 --> 00:03:50,740 Then the point is that not all stars are the same. 50 00:03:50,740 --> 00:03:57,380 We have a very stable, energetic star, a massive, energetic, stable star. 51 00:03:57,380 --> 00:04:02,520 If our star was one of the more common stars, like an M star, which would be a cooler star, 52 00:04:02,520 --> 00:04:04,820 the habitable zone is closer. 53 00:04:04,820 --> 00:04:08,440 I have to be closer to be habitable, to have the moisture. 54 00:04:08,440 --> 00:04:12,480 But when I'm that close, it turns out I'm very close to a star which is very unstable. 55 00:04:12,480 --> 00:04:19,240 M stars have a lot of strong energy output changes that also damage the planet. 56 00:04:19,240 --> 00:04:25,160 The other thing that happens is that the planet's day and night cycle, which is governed by its 57 00:04:25,160 --> 00:04:31,300 spin, as you get closer there's a gravitational effect called tidal friction and tidal locking, 58 00:04:31,300 --> 00:04:36,380 which essentially locks the planet so that one side of the planet is the only site that's 59 00:04:36,380 --> 00:04:37,380 seen. 60 00:04:37,380 --> 00:04:42,160 So one side of the planet totally burns up, the other side just, you know, gets just super 61 00:04:42,160 --> 00:04:43,160 chilled. 62 00:04:43,160 --> 00:04:48,160 These are just two of a great number of conditions that have come together for the Earth.