If massive planets are at a decent distance from their star, they can retain atmosphere and water for an incredibly long time. If they were ejected from their parent system, then longer than the universe has existed.
Conditions on Earth became suitable for life a few billion years ago, and it will be uninhabited again in a few billion years. However, some exoplanets, massive super-Earths, are theoretically capable of supporting life for much longer, sometimes more than 80 billion years. This is the conclusion reached by the authors of a new paper published in the journal Nature Astronomy.
Super-Earths are large exoplanets with a solid surface mass up to 10 Earth masses – many times larger than ours, but smaller than small gas planets like Neptune. There are no super-Earths in the solar system – perhaps the appearance of such a planet was prevented by Jupiter, “swallowed” its embryo. However, telescopes find a lot of super-Earths in other stars in the Milky Way. Mol Lous (Mol Lous) and her colleagues from the University of Zurich (Switzerland) have considered a subclass of “cold super-Earths”, which rotate at a decent distance from their stars and maintain a moderate temperature.
Calculations show that such cool worlds are able to hold a primary atmosphere (consisting mainly of hydrogen and helium) for billions of years, and its density is 100-1000 times greater than the Earth’s. Under this dense envelope, liquid water can exist on the surface, and new modeling has shown that it can persist for a very long time, more than long enough for the emergence and development of life.
It is worth noting that searches for exoplanets are usually led by changes in the brightness or the exact position of the star, which are caused by the rotation of a nearby planet. Therefore, most of the super-Earths found by telescopes have rather tight orbits, and the new study was conducted theoretically – without observation, using only mathematical models. Swiss astronomers have conducted more than a thousand simulations of the evolution of super-Earths of different masses, with different atmospheres and orbits around solar-type stars.
The work has shown that too close an orbit leads to gradual erosion and loss of atmosphere under the influence of the flow of stellar wind particles. However, at a sufficient distance – more than the orbit of Mars in the solar system – the hydrogen-helium atmosphere can not only persist for a long time, but also heat the planet due to the greenhouse effect. According to scientists’ estimates, in this state, the super-Earth is able to remain for five to eight billion years, until its parent star is nearing the last stages of life and begins to turn into a red giant.
However, more exotic options are also possible. A random play of gravity can throw the super-Earth away from its parent system and send it on a free flight as an “orphan planet”, not connected with any star. Calculations suggest that with sufficient mass (10 Earths) and a sufficiently dense atmosphere, such a single super-Earth could remain potentially habitable for an incredible 84 billion years. This period is much longer than the age of our Universe and the entire period during which stars will exist in it.
Life on a free-flying super-Earth must be drastically different from ours. If it evolved on such a planet, it adapted to its conditions, which are completely different from Earth’s, with the absence of light and enormous atmospheric pressure. However, even on our planet there are known almost isolated ecosystems that exist under similar conditions – for example, “black smokers” at the bottom of the oceans. The main source of energy for local organisms is chemosynthesis, not photosynthesis, and they do not need light.