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The length of the year in inhabited worlds should not vary
much, because if the planets are like ours, the stars are like the Sun, then
the radius of the orbit will approximately correspond to the radius of the Earth’s
orbit. Even if we take a star with a different mass and luminosity, the situation
will not change significantly. The “wet” planet should be much closer to the
dim red dwarf, but its orbital velocity will be less, since the gravity of the
star is weaker.
But all this is subject to planets moving in almost circular orbits. If the orbit is an elongated ellipse, the duration of the year may be longer. Of course, it makes no sense to consider worlds that whirl away from the sun so far that even atmospheric gases will turn into ice. Such planets can be inhabited only by the protists. But, on the other hand, even a visit to the orbit of Mercury will not be disastrous for higher life forms, for the stay in hell will be short. After all, only the velocity of a body in a circular orbit is constant. Moving along the ellipse, the planet rapidly sweeps through the lowest point of the orbit (perihelion) and almost freezes in aphelion. Approaching the luminary, it is accelerated by gravity, and moving away, on the contrary, it slows down by it.
...And immediately the question appears, that should always be first and foremost: is the existence of life-suitable worlds with an elongated orbit possible? And if possible, where can they come from?
We should start from the fact that the protoplanetary disk surrounding the forming star is exactly a disk. It is something round. And it breaks up into no less round protoplanetary rings lying in the same plane – in the plane of the disk. The birth of a planet is possible only in a circular orbit. And there are no options here. Turning the ring into an ellipse will reverse the accretion. The collision rate of planetesimals in such conditions will increase, the energy of the fragments will become higher, and they will begin to leave the common gravitational pit of merging bodies and even fly outside the ring. Instead of merging, the impacts will lead to fragmentation, and even the ring itself will collapse quickly.
But a ready-made planet is able to move into an elongated orbit. An example is Mercury, beating between 0.3 and 0.47 astronomical units from the Sun. All other planets have some eccentricity of the orbit. The transition of planets from circular to elliptical orbits is inevitable immediately after the completion of their formation process. For if the gas-dust rings do not interact with each other – the center of gravity of each of them coincides with the center of gravity of the entire system – then each of the planets already has its own center of gravity. And then the battle royal begins – a battle royal, which results cannot really be predicted, since the planets, before they find stable positions, go through an incalculable hell of gravitational resonances and momentum exchange. Mercury is a small fry. At this stage, a whole Neptune nearly flew out of the Solar System…
...Thus, the answer will be positive: the transition of a large, rocky inner planet to an elongated orbit is quite possible (if there are gas giants in the system). That is, this will not happen in a red dwarf system, very close by default. The giant in it will simply exclude the appearance of other planets. But in any other system – go ahead.
...If we take the most realistic option, it will be an Earth-like planet “wandering” between the orbits of Mars and Venus. Both in aphelion and perihelion, the conditions on it will not be suitable for preserving water in a liquid physical form, but the oceans will not have time to evaporate completely or freeze to a great depth.
The period of rotation in such an orbit will be about two years. Due to the difference in speeds at the upper and lower points of the orbit, three quarters of the planetary year “winter” will take. But the “summer”, which lasts no longer than on the Earth, will be remembered for extreme heat. In this case, the enclosing of the words “winter” and “summer” in quotation marks means that the nature of these phenomena will not be associated with a change in the angle of incidence of sunlight. But if the axis of rotation of the planet is tilted, the change of seasons in the traditional sense can also be observed. In this case, one of the hemispheres, the summer in which falls at the time of the perihelion passage, will be warm. The opposite hemisphere, almost certainly, will be completely covered by a glacier. In the orbit of Venus, it will also be truly hot there. Nevertheless, the ice will not have time to melt. In addition, the snow cap will reflect most of the sun’s rays.
The climate on such “vagrant planet” will be cooler than on the Earth. After all, it spends more time away from the sun, rather than near it. There will be no “comfortable” zones at all. At perihelion, the heat at the equator will be fierce. But in winter, when in high latitudes the temperature can drop to -100 Celsius and below, non-frozen areas of the sea can be preserved here.
Animals on such a planet will have to migrate annually, moving south during the period of distance from the Sun, and retreating to glaciers when the water in the rivers turns from warm to hot. Sedentary creatures and plants will need to develop other mechanisms of adaptation. Forests are likely to be rare. The hardiest trees will survive only in the subtropics. Most of the land on the planet will be covered by steppes, turning green in spring and autumn, burning out in summer and hiding under snow in winter.
Translated by Pavel Volkov, 2021
The original Russian article is here