2023 Author: Bryan Walter | [email protected]. Last modified: 2023-05-21 22:24
Low-mass exoplanets (less than Mars) completely covered with water, contrary to expectations, do not experience an uncontrolled greenhouse effect and can remain in the habitable zone for billions of years. The simulation results are sent for publication in The Astrophysical Journal, the preprint can be read at arXiv.org.
Astronomers believe that a large number of low-mass exoplanets can exist, which, despite their low gravity, have large reserves of water, and at the same time are in the habitable zone of their star. One of the sources of bodies of this kind can be the ex-moons of giant planets in other systems, outside the solar system. It is known from observations that giant planets often migrate closer to their stars. In the solar system, large planets have many satellites, many of which are largely composed of water ice.
It is assumed that as a result of the migration of the host exoplanet, some of these satellites will inevitably be lost and turn into low-mass independent planets (some researchers suggest calling them plunets). Moreover, due to the large amount of water in their composition, they will be covered by deep oceans. Today, the main criterion for the potential habitability of celestial bodies is the existence of liquid water on their surface.
It remains unclear whether low-mass bodies close to their star can hold their atmosphere and oceans for a long time, despite weak gravity. For low-mass bodies, this can be difficult due to the so-called uncontrolled greenhouse effect. Its difference from the usual greenhouse effect is that the heating of the atmosphere is so strong that it leads to the ingress of water vapor into the stratosphere (on Earth, the stratosphere is essentially waterless, water vapor cannot reach it due to low temperatures in the upper troposphere). In the stratosphere, water vapor under the influence of ultraviolet radiation decays into lighter hydrogen and oxygen, and the former quickly leaves bodies with moderate gravity. The planets close to the stars may heat up to a temperature when all the oceans boil away, making the celestial body unsuitable for life of the terrestrial type.
Researchers from Harvard University and the Massachusetts Institute of Technology, led by Constantin Arnscheidt, tried to establish whether the long-term existence of the atmosphere and, accordingly, liquid water on the surface of such bodies is possible. To do this, they took the scenario that a low-mass body has a large amount of water (40 percent by mass) and an atmosphere entirely consisting of water vapor (the option most easily amenable to the uncontrolled greenhouse effect). Then scientists calculated how the area of that part of the atmosphere that absorbs the radiation of the luminary, and the area of that part of the gas envelope that re-radiates into space in the infrared range, will change.
For low-mass and water-rich bodies, due to their low gravity, the atmosphere in the habitable zone will extend significantly higher than for bodies with high gravity. In this case, already the uppermost layers of the atmosphere will re-emit infrared radiation into space, cooling the planet. But the effective absorption of short-wave radiation from the star will take place in the lower atmosphere. In fact, a low-mass body will be covered with a large gas bubble (that is, "swollen" upper layers of the atmosphere) cooling it (due to re-radiation into space). According to the authors' calculations, for temperatures of the planet's surface between 273 and 440 Kelvin (approximately from zero to +167 Celsius), an uncontrolled greenhouse effect will not be observed at all. That is, planet-oceans with a mass of 0.1 terrestrial to at least 0.03 terrestrial may well remain inhabited for a period of time from a billion to more than one hundred billion years.
The conservative width of the habitable zone for them, according to the calculations of the researchers, will be 0.08 AU for planetary systems in yellow dwarfs (for example, the solar system), and 0.08 AU for planetary systems in red dwarfs. The researchers note that this is precisely a conservative estimate - if there are geochemical cycles (for example, carbon) on such bodies that stabilize the climate, the width of the habitable zone for them will be much larger. Conversely, a significant amount of carbon dioxide in the atmosphere without an effective carbon cycle (as on Venus today) can lead to a narrowing of such a habitable zone. The authors note that the worlds described by them, due to the presence of an extended atmosphere, can have a radius of more than 0.3-0.4 Earth. This lies within the observational capabilities of space telescopes, such as the recently out-of-service Kepler or future space telescopes. Thus, their conclusions about the possibility of long-term existence of liquid water and the atmosphere in low-mass bodies in the habitable zone can be verified by observations.
Astronomers have repeatedly adjusted their ideas about the size of the habitable zone for certain unusual conditions. We recently wrote about how they extended it to planets around closely spaced binaries.