If massive planets are a decent distance from their star, they can retain an atmosphere and water for an incredibly long time. If they have been ejected from their parent system, longer than the universe has existed.
Conditions on Earth became suitable for life a few billion years ago and it will be uninhabitable again in another 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 Nature Astronomy.
Super-Earths are defined as large exoplanets with solid surfaces up to 10 Earth masses - many times larger than our own, but smaller than small gas planets like Neptune. No super-Earths exist in the solar system - perhaps Jupiter, which "swallowed" its embryo, prevented such a planet from forming. However, telescopes have detected many super-Earths in other stars in the Milky Way. Mol Lous (Mol Lous) and her colleagues at the University of Zurich (Switzerland) have examined a subclass of "cold super-Earths" that orbit at a decent distance from their stars and retain a moderate temperature.
Calculations show that such cool worlds are capable of holding a primary atmosphere (consisting mostly of hydrogen and helium) for billions of years, with a density 100-1000 times greater than Earth's. Under this dense envelope, liquid water may exist on the surface, and new simulations show that it can persist for a very long time, more than long enough for life to emerge and develop.
It's worth noting that searches for exoplanets are usually led by changes in the brightness or exact position of the star, which are caused by the rotation of a nearby planet. That's why most of the super-Earths found by telescopes have rather tight orbits, and the new study was conducted theoretically - without observations, 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 orbits that are too tight lead to gradual erosion and atmospheric loss due to stellar wind particle flux. 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 up the planet through the greenhouse effect. Scientists estimate that the super-Earth could stay that way for five to eight billion years, until its parent star reaches the final stages of life and starts turning into a red giant.
However, more exotic variations are also possible. A random gravitational play could eject a super-Earth away from its parent system and set it free to fly as an orphan planet, unbound to any star. Calculations suggest that with enough mass (10 Earths) and a dense enough atmosphere, such a lonely super-Earth could remain potentially habitable for an incredible 84 billion years. This time frame far exceeds the age of our Universe and the entire time frame in 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 quite different from those on Earth, with the absence of light and enormous atmospheric pressure. However, there are also nearly isolated ecosystems on our planet that exist under similar conditions, such as the black smokers at the bottom of the oceans. The main source of energy for these organisms is chemosynthesis, not photosynthesis, and they do not need light.