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00:00 Planet Earth has been home to life for an estimated 3.7 billion years, and now hosts
00:05 8.7 million species of animals and plants.
00:09 Naturally, we have long wondered if other planets in the universe could similarly harbour
00:13 life.
00:14 In the search for such planets, researchers strive to find the ones most similar to Earth…
00:19 but some wonder if our little blue dot isn't the best there is, and theorise that we could
00:24 soon find something even better.
00:27 This is Unveiled, and today we're answering the extraordinary question; did scientists
00:31 just find planets that are better than Earth for life?
00:35 Do you need the big questions answered?
00:37 Are you constantly curious?
00:38 Then why not subscribe to Unveiled for more clips like this one?
00:41 And ring the bell for more thought-provoking content!
00:44 There's only one planet that we're certain has life, and that's our home, Earth.
00:49 Since at least the era of classical antiquity over two millennia ago, humans have pondered
00:54 the possibility of life on other worlds.
00:57 The famed Greek philosophers Democritus and Epicurus, who held that everything was composed
01:02 of infinite atoms in an infinite void, believe there must be other, similar worlds out there.
01:08 According to Epicurus' disciple, Metrodorus of Chios, if our world was the only one, it
01:14 would be like a single ear of wheat growing on a vast plain.
01:19 Of course, this was long before the invention of the telescope, making the idea quite abstract
01:24 for the time.
01:25 It came at a time when the dominant view was still geocentric, putting the Earth at the
01:29 centre of the universe.
01:30 Now we're closer to finding extraterrestrial life than ever before.
01:34 The international search effort has been well underway since the 1980s, utilising a plethora
01:40 of advanced tools, from radio telescopes to infrared instruments on satellites.
01:45 We have currently over 5,500 confirmed discoveries of exoplanets.
01:51 Most of these were discovered by the Kepler Space Telescope, and are all compiled in NASA's
01:56 Exoplanet Archive.
01:57 Once catalogued, they are assessed for habitability, and if there's potential, they're added
02:02 to the Habitable Exoplanets catalogue.
02:04 This work is carried out primarily by the Planetary Habitability Laboratory at the University
02:09 of Puerto Rico in Arecibo.
02:12 What they look for first is if the exoplanet has the conditions needed for liquid water
02:16 on the surface.
02:18 The range around a star where planetary orbits are ideal for liquid water is called the "habitable
02:23 zone", or the "Goldilocks zone".
02:26 This is thought of as the most important requirement, since on Earth, where there is water, there
02:31 is life.
02:32 It's assumed that if a world can support liquid water, life won't be too far away.
02:37 There are numerous other factors that need accounting for, however, such as radiation
02:41 from the host star, and atmospheric composition, alongside various geophysical factors.
02:48 All exoplanets on the catalogue are assigned an Earth Similarity Index, or ESI.
02:53 This number ranges from zero to one, with zero meaning no similarities and one being
02:59 identical.
03:00 The rationale for this is that Earth is the only place we know life exists.
03:04 Thus, researchers think that the closer an exoplanet is to our home, the stronger the
03:08 chance of life.
03:10 Not all astronomers are in agreement on this, however, and in the past decade some have
03:14 argued that the assumption is misguided.
03:17 While Earth is indeed a hotspot for life, we can't be certain that it is the most ideal
03:21 home for life.
03:22 It's often said by astronomers that the more you learn about Earth, the more you realise
03:26 just how lucky we are to be here.
03:29 It's not just liquid water that makes our planet a paradise; there are also an abundance
03:33 of other factors.
03:35 For example, the magnetic field that absorbs most of the sun's harmful ultraviolet radiation,
03:40 or the tectonic plates that form continents and bring heat to the surface.
03:44 We also can't forget Jupiter, whose gravity may capture asteroids and comets hazardous
03:50 to life.
03:51 So, there are good reasons to describe our home as perfect for life.
03:55 However, in 2014, physics professor John Armstrong and astrophysicist Renee Heller challenged
04:01 this claim.
04:02 They proposed that certain kinds of planets with characteristics very different to Earth
04:07 could be even more habitable.
04:09 In fact, could be what they call "superhabitable".
04:12 They define "superhabitable" as a planet that can sustain a more diverse selection
04:17 of plants and animals.
04:18 The researchers still assume that life needs water, but believe that there could be planets
04:23 better optimised than Earth for biodiversity.
04:26 They reject the idea that the Goldilocks Zone is a good indicator of habitability.
04:31 A lot of Rocky World's inhabitable zones aren't habitable.
04:35 And geothermal processes can make planets outside of the zone habitable.
04:40 One such example is Jupiter's icy moon, Europa.
04:43 While its surface is covered in ice, scientists believe that a subsurface ocean is hiding
04:48 below, kept warm by tidal heating.
04:51 According to Heller and Armstrong, the search for extraterrestrial life should be less human-centric
04:57 and more biocentric.
04:59 Biodiversity criteria should include a planet's age, mass, location in its system, host star's
05:05 spectral type, and a few other features.
05:08 The concept isn't limited to just exoplanets, but also includes exomoons.
05:13 Superhabitable planets, the researchers suggest, are likely slightly larger, more massive,
05:18 and older than Earth.
05:20 A larger surface area would allow for more shallow seas, which warm more easily than
05:25 deep oceans, making them comfortable habitats.
05:28 In terms of mass, the ideal would be roughly twice Earth's - optimal for plate tectonics,
05:33 a strong magnetic field, and a thick atmosphere.
05:37 Created by the rotation of a liquid outer core, Earth's magnetic field shields us
05:41 from cosmic radiation.
05:43 Theoretically, a more massive world could have a stronger magnetosphere, a thicker atmosphere
05:48 would make the surface warmer, and historically, warmer epochs on Earth have encouraged biological
05:54 diversity.
05:55 Ideally, a superhabitable planet should orbit a different kind of star to the sun.
06:00 The sun is a yellow dwarf, but orange dwarfs, which have lower luminosities and are less
06:05 massive, have significantly longer lifespans.
06:09 Called K-type main-sequence stars, orange dwarfs remain stable for 17 to 70 billion
06:15 years - far longer than our sun's 10 billion year shelf life.
06:19 This would allow more time for life to originate and evolve.
06:23 As a bonus, they emit much less UV radiation, too.
06:26 Currently, there are 24 exoplanets deemed potentially superhabitable, although only
06:31 two of these are validated planets.
06:34 However, it's thought that there might also be other contenders already catalogued and
06:39 overlooked.
06:40 This leads us to ask the exciting question, what would life be like on such a planet?
06:45 Of course, we can only speculate about the specifics, but rainfall would likely be more
06:49 common… and native flora very different.
06:52 With a denser atmosphere and higher mass, life might be larger and more common.
06:57 Plants might follow different processes of photosynthesis, due to the different spectral
07:02 output of other host stars.
07:04 Orange dwarfs are cooler and redder than the sun, and plants might evolve pigments optimized
07:10 to absorb those wavelengths.
07:12 Their leaves might be blue, or some other color, rather than green.
07:16 If we managed to discover a superhabitable planet, it would have major implications.
07:21 After one thing, it could provide invaluable insight into the hypothetical "Great Filter".
07:26 This is the theory that life is rare in the universe, because there's some unknown barrier
07:30 between the earliest stages and the highest levels of development.
07:34 Perhaps, for example, life tends to destroy itself.
07:37 Or, the universe, with its asteroids and rogue black holes, is far more dangerous than we
07:42 think.
07:43 We're currently unsure whether such a Great Filter exists at all, and whether it's behind
07:47 or ahead of us.
07:49 Life on a superhabitable world could shed some light on this topic.
07:53 If we find that life tends to get stuck in its simplest form, that barrier might be behind
07:58 us.
07:59 If, on the other hand, we find a planet teeming with complex life, having comfortably survived
08:04 for billions of years, well, maybe we should figure out how they did it.
08:08 Perhaps, they had no desire to leave their homeworld, and avoided some risk of collapse
08:12 inherent in interplanetary expansion.
08:15 Alternatively, we might find the ruins of a once-great extraterrestrial civilization,
08:20 which could also suggest some answers.
08:23 Both scenarios would put the barrier ahead of us, and give us some idea of what to watch
08:27 out for.
08:28 What do you think?
08:29 Is there anything we missed?
08:30 Let us know in the comments, check out these other clips from Unveiled, and make sure you
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