Ice-cold exoplanets like Enceladus or Europa are unlikely to support life, as the rise in their surface temperature will turn them not into an ocean world, but a giant greenhouse, according to an article published in the journal Nature Geoscience.
“Many of our colleagues believe that ice worlds can become habitable if the brightness of their luminary increases and their ice shell is melted. We have shown that this is actually impossible for exoplanets that do not have land and are similar in size to Enceladus and Europa. They immediately turn from a “ice” into a “greenhouse” bypassing the phase during which they resemble the Earth,”write Jun Yang from the University of Beijing (China) and his colleagues.
The Kepler telescope and a number of ground-based observatories have discovered several dozen planets in recent years, which are not small or large analogues of the Earth or gas giants, but a kind of “water worlds” consisting almost entirely of water. These planets can be composed of liquid water or be covered with a thick crust of ice that hides a giant subglacial ocean beneath.
Today, scientists are actively debating whether such planets can support life. Some of them believe that the oceans of such "water worlds" are no different from the seas and lakes of the Earth, while other planetary scientists assume that these planets will be dominated by a powerful greenhouse effect, which in principle does not allow life to exist in their waters.
The authors of the article drew attention to the fact that the climate of such water worlds will depend on two distinctive properties of water - the fact that it reflects heat and light well in a frozen state and retains heat in a vaporous state. The more ice or steam appears on the planet, the more it will reflect or retain heat in itself, due to which the amount of ice and steam will increase by itself at an ever faster rate.
Guided by similar considerations, Jan and his colleagues created a computer model of an analogue of the solar system, in which a planet revolving around a star, similar in size and properties to Enceladus or Europa. Scientists have gradually increased the brightness of the star, imitating what happened to the Sun in the last 3-4 billion years, during which its brightness has increased by 30%.
These calculations led to relatively unexpected results - it turned out that "water worlds" can exist only in two forms - in the form of a completely frozen ball of ice and in the form of a giant greenhouse, in which all the water has turned into steam.
Analogs of terrestrial oceans practically never appear on such worlds, since the process of transformation of the planet from a “snowball” into a “steam boiler” takes moments in geological terms. To launch this transformation, as it turned out, an unexpectedly large amount of energy is needed, about 1.7-2 times more than our planet receives from the Sun today.
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How did the Earth, given its large reserves of carbon dioxide and water, avoid such a fate? Scientists believe that the unique fate of our planet is due to the fact that its surface is covered not only with water, but also with land areas.
The interaction between molecules of CO2, water and silicate rocks, as scientists note, removed excess greenhouse gases from the atmosphere and returned them back along with volcanic eruptions and other types of geological activity, preventing the Earth from turning into a greenhouse and helping it to "unfreeze" when turning into an ice ball.
All of this, as noted by Jan and his colleagues, calls into question the habitability of many "water worlds" discovered by planetary scientists in recent years, such as Kepler-22b, Kepler-62e and Kepler-62f. These planets, which scientists considered to be ocean worlds, may in fact be either greenhouses or "ice", completely unsuitable for supporting life.
