They may be completely covered in ice and their oceans do not receive starlight.
Some of the potentially inhabited exoplanets orbiting red dwarfs may be completely covered in ice and not have open surface oceans of liquid water even on the side facing the star, which, under certain conditions, makes them unsuitable for life, scientists say in a study presented in Nature Astronomy.
“Perhaps some of the tidal traps of extrasolar worlds that were previously considered potentially inhabitable are frozen snowballs and have no open areas with liquid water. As a result, the starlight necessary for photosynthetic organisms does not reach the ocean, sealed under a global ice shell, which greatly limits the chances of the development and prosperity of life, ”says Jun Yan, lead author of the study from Peking University (China).
Potentially inhabited tidal rocky extra-solar worlds in red dwarf systems, especially the Proxima b, TRAPPIST-1e, and LHS 1140b, which are especially close to us, are the main goals for future studies of exoplanetary atmospheres, which can provide clues about their ability to support life.
Current models predict that if such a planet contains a vast surface ocean, then on its warmer day side there should be an ice-free area, locked on all sides by a global ice shell. However, as noted by Jun Yan, previous simulations did not fully take into account the important component of the climate system – ice dynamics – which did not allow revealing the issue of ocean resilience to global glaciation.
“In our work, we show that ice drifting from the night side of the exoplanet flows into an open area, gradually cooling, reducing, and ultimately completely sealing it.
It should be noted that such a scenario is applicable only to worlds living in the outer and, possibly, in the middle regions of the inhabited zone, but not to those located at its inner edge, since for them the flux of stellar radiation is high enough to melt ice and snow on the surface and maintain liquid water on the day and even night sides. In addition, the presence of greenhouse gases in the atmosphere plays a small role, since a large number of them will save the exoplanet from turning into a snowball, ”Jun Yan explained.
But, in spite of the fact that planets completely covered with ice are extremely inhospitable for life, one should not put an end to their potential for habitability, because the Earth itself in the past twice experienced periods of global glaciation: 2.2 billion years ago and 630 million years ago.
“For example, photosynthetic organisms can develop in areas with thin ice, where stellar radiation nevertheless reaches liquid water, or in local unsealed areas where active geothermal processes occur, or, finally, in some“ bays, ”where access to drifting ice is limited.” – concluded Jun Yan.