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Aliens can exist on ‘one-sided’ planets: what’s special about these worlds

The most common planets in the Universe are those that always face their star with only one side. On one side of such a world there is an eternal hot day, and on the other there is an eternal night, where it is much colder.

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Two completely different sides of such worlds are separated by the terminator line, that is, the border between light and darkness, heat and cold.

This is a peculiar region in the form of a line of eternal twilight that encircles the entire planet. Given that it is not too hot and not too cold, extraterrestrial life could exist in this twilight zone.

“These planets have a permanent day side and a permanent night side,” Ana Lobo, a postdoctoral researcher in the UCI Department of Physics & Astronomy, who led the new work, explained in a press statement.

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“You want a planet that’s in the sweet spot of just the right temperature for having liquid water,” Lobo explained. “This is a planet where the dayside can be scorching hot, well beyond habitability, and the night side is going to be freezing, potentially covered in ice. You could have large glaciers on the night side.”

Although such worlds face many challenges in terms of habitability, this does not mean that life cannot exist on them. Such planets receive an endless flow of light and heat on one side, without any sources of external heat on the other side.

Astronomers are particularly interested in the habitability of such planets because they are incredibly common throughout the universe. The physics behind them is called tidal locking. Thanks to this, the Moon always faces the Earth with only one side.

But stars can do the same to their planets. For example, Mercury is almost tidally locked to the Sun, but Jupiter’s gravity causes the small planet to rotate, albeit very slowly.

Although astronomers suspect there are a trillion or more exoplanets in the Milky Way, only a few thousand worlds outside the solar system have been discovered so far. Among them there are rocky planets similar to Earth and most likely they always face one side to their star. Therefore, the question of their habitability occupies the first place among astronomers.

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Although such worlds face many problems in terms of habitability, this does not mean that life cannot exist on them. Such planets receive an endless flow of light and heat on one side, without any sources of external heat on the other side.

If the planet receives too much radiation on the dayside, the atmosphere could enter a catastrophic greenhouse cycle, which would likely mean the end of any life that has managed to evolve there. On the other hand, if it is too cold on the night side, the atmosphere simply collapses, turning into ice, which settles on the surface, and this is also not very good for the existence of life.

To be habitable planets, such one-sided worlds must transfer heat efficiently from the day side to the night side for equilibrium to exist. This can depend on a variety of factors, such as the composition of the planet’s atmosphere, its distance from its host star, and the abundance of water on the surface.

Recent research suggests that water-rich worlds have a wide range of habitability, both in terms of the conditions in which life can thrive and in terms of what percentage of the planet is favorable to life.

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Given enough water, powerful ocean currents, combined with evaporation and freezing of water, can effectively transfer heat from the day side to the night side. In some cases, the planet may even have a relatively uniform temperature, making it completely habitable.

But even if the planets don’t have a lot of water, they have a chance to become habitable. In this case, life can concentrate on the terminator line. The day side will be too hot to support any life, but the night side will be too cold. But between these parts of the planet a temperate climate may prevail.

But astronomers still don’t know the exact properties of tidally locked planets, making them promising targets for further study with the Webb Space Telescope. Perhaps it is on such planets that extraterrestrial life will be discovered for the first time.

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Jake Carter

Jake Carter is a researcher and a prolific writer who has been fascinated by science and the unexplained since childhood. He is always eager to share his findings and insights with the readers of, a website he created in 2013.