Perfect world: why Earth is not the best place to live
Ask anyone: which planet is the coolest in the Universe? Most, of course, will name the Earth. In fact, of all the worlds we know, this is the only one that is comfortable for life and for people. The sun is located at an acceptable distance, providing a constant moderate flow of energy. The dense atmosphere retains heat and moisture, and the magnetosphere protects the surface from radiation.
Plate tectonics slowly mixes and updates the lithosphere with new minerals. Vast oceans calm the climate, the tilt of the planet's rotation axis creates seasons, and the massive Moon stabilizes this movement. You can list for a long time, reaching up to Jupiter, which is supposed to protect the Earth from frequent meteor strikes, "catching" them in a huge number of its powerful gravity. It would seem that what could be better than such an ideal set?
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But it all depends on what is considered the best. If we consider the habitability of the planet from the point of view of humanity adapted to its native land, then there may not be another candidate. But let's look at things more broadly and think, is it so perfect? After all, from the point of view of habitability, as such, our planet is in many ways a borderline, extreme case, and suitable planets near distant stars can support a much richer and more diverse biosphere than ours.
Life on the border
The key conditions for life (at least for the life forms we understand) are heat and moisture. Therefore, the area around the star where the temperature is quite moderate and allows you to keep on the surface of the planet melted liquid water is called the habitable zone. Its boundaries depend on the size and brightness of the star, and calculations carried out in 2013 showed that for the Sun this region is between 0.99 and 1.7 astronomical units. Recall that 1 a. e. it corresponds to the average radius of the earth's orbit, and it turns out that our planet is located at the innermost edge of the habitable zone, far from its optimal center.
Moreover, the brightness of the Sun gradually increases. Four billion years ago, when life began on Earth, it shone almost a third fainter. The planet was outside the habitable zone: there wasn't enough radiation to melt its oceans. It is assumed that the earth then received additional heat from volcanic and greenhouse processes. Only over time, the star "accelerated" and "flared up", making the planet really comfortable. It is possible that this is why life lingered so long on the simplest forms, and the first multicellular organisms appeared only about a billion years ago.
Complex animals appeared "recently", during the Cambrian explosion. Over the past 540 million years, the biosphere has become extremely diverse, has mastered the land, and has gone from primitive shellfish to smart parrots and bureaucracy. It is possible that a better location within the habitable zone would give earth life a few additional billions of years. Or even a few dozen.
Bigger Yes better
Or take the dimensions. The earth is the largest of the stony planets in the Solar system, and its size allowed it to retain internal heat for a long time so that over time the lithosphere moved, and plate tectonics started. There is no such thing on Venus, Mars, or mercury. Meanwhile, even here, the Earth has barely "slipped" into the appropriate boundaries: models predict that plate tectonics should occur more easily on larger stony planets with a mass of about two Earth masses.
On our planet, tectonics was facilitated by an excess of water that mixed with silicate minerals of the lithosphere, changing their melting temperature. If the Earth were more massive, this would not be necessary, and life would have much more space and various conditions for development. Perhaps the same can be said about climate: paleontological records show that the biosphere became especially diverse during periods when the planet became warmer than usual. It is possible that the optimal temperature for the biosphere should be slightly higher than here. Similarly, the ideal oxygen content should be 30-35% instead of the current 21%.
This is how astrophysicists Rene Heller and John Armstrong reasoned, who is 2014, in an article published in the journal Astrobiology, subjected our planet to comprehensive criticism and put forward the concept of "super habitable" worlds. However, they started with the Sun itself, assuming that the optimal conditions for life can be found in a dimmer and quieter star.
Shallow water stars
Such a star should belong to the spectral class K-orange, and not a yellow dwarf like our Sun (it belongs to the class G, but the neighboring Alpha Centauri B - just an orange dwarf). K-stars have been around for several times longer than G — almost like even dimmer red dwarfs, but unlike them, they don't show as frequent, unpredictable, and powerful flares as they do. All this creates the basis for exceptionally long and stable conditions in the orbit of such a star.
Of course, class K is not so large and bright: these stars have a mass of 0.5 to 0.8 solar masses and luminosity of no more than 0.6 solars. Therefore, the habitable zone is much closer to them, and for super habitability, the planet must move in a shorter orbit, closer to the center of this area. It is desirable that there are several such worlds in the system, which will ensure that they constantly exchange the "germs of life" — as may have already happened between Earth and Mars, while (and if) it was inhabited.
So, it is better to take a larger body, optimally-two earth masses, and 1.2-1.3 of its radius. The increased size will not only provide plate tectonics and more living space. Powerful gravity will hold more water and a denser atmosphere, making it easier to maintain a consistently high temperature (preferably around 25 °C, about 10 degrees above our own). The terrain of a massive planet will also become smoother and smoother. It will have fewer depths poor in light and food, but more warm and lively shallow waters. According to one observer, the ideal world will present fewer diverse ecosystems, but each of them will reveal its full potential for biodiversity.
The pursuit of an ideal
Heller and Armstrong note that almost all the characteristics necessary for "super habitability" follow from one main thing — the increased size in comparison with the Earth. Theoretically, such measures should be even more numerous in The galactic spaces than our own. And there are more orange dwarfs than yellow solar-type stars-according to some estimates, they account for up to 9% of the entire stellar population. Especially since at least one suitable world is already known.
The exoplanet Kepler-442b, located in the constellation Lyra 1200 light-years from the Sun, meets the conditions of "super habitability" almost perfectly. It orbits a K-class star with a radius of 1.3 earth and a mass of 2.3 earth. Unfortunately, the average temperature on Kepler-442b is far from optimal: about -2.5 °C. However, this seems to be just the first example, and we will find many truly perfect planets in the future.
It is possible that vegetation will help to identify such a world — of course, lush and not green at all. The spectrum of K-stars differs from that of the sun, and the sky of a super-inhabited planet will not be as blue as our own. The areas where earth plant pigments absorb light are in the red, but most of all in the blue-purple area (green light is reflected). However, class K emits more actively in the red and infrared bands, but in blue and purple — weaker.
Therefore, it is assumed that under this light, plants will reflect more blue, so that their leaves will be darker than earth's, closer to the purple color. There are even hypotheses that over a long time of evolution in an ideal world, plants should learn to effectively absorb all the light falling on them and become completely black. One day people will be able to walk through such a dark and dense Gothic forest. As far as we know, with proper training, our body is quite capable of moving at a gravity of up to 4 earth. The ideal world is almost perfect here.