A super-Earth is a planet that has more mass than Earth and is the right distance from its star to have liquid water on it. Super-Earths are thought to be the norm instead of being an extraordinary occurrence, but we do not know what types of conditions would make them so. New studies suggest that some planets might be in our habitable zone, but not exactly habitable for complex life like us.
On January 23rd, 2017, a team of researchers in France led by Jean-Luc Margot from the Laboratoire Lagrange at the University of Nice Sophia Antipolis published a paper, “A super-Earth skimming habitable zone of red dwarf,” in The Astrophysical Journal Letters. They explain that when scientists talk about where a planet is in the habitable zone of its star it is useful to think more broadly.
The habitable zone of a star is the region where it would be warm enough for water to vaporize, its radius divided by 2. This is called the inner edge of the habitable zone, but even at this distance from their star planets are affected by stellar winds. This causes them to lose their atmosphere over time, and it is likely that they would become Mars-like, with atmospheres tenuous enough to allow liquid water but no longer habitable.
Skimming the habitable zone is a new concept, proposed to help assess the habitability of a planet that has been affected by its star’s winds. It refers to a planet in the habitable zone of its star, but one that is close enough to be affected by the stellar wind.
If it is too close to its star, it will not be able to hold onto an atmosphere and would lose it over time. This concept is also related to the idea of a “super-Venus,” which is a planet without an atmosphere but with enough heat to keep water in a liquid state. If the planet is moving through the habitable zone, it will be close enough to its star to be affected by its stellar wind, and it will offset the warming due to being close enough. As it moves away from its star again, it will be less affected by stellar wind and warm up again. This can lead to variations in the temperature of these planets over time.
The planet the researchers studied is Gliese 667 Cc, a red dwarf star located about 20 light years away. Because its orbital period is longer than the planet’s year, it has never been in the habitable zone but will be in our future. If the planet is close enough to its star because of the stellar wind, it would be too hot to have liquid water on it and not habitable. If it is far enough, it will no longer be affected by the stellar wind and could potentially have liquid water and maybe even life.
The researchers studied the temperature variations of Gliese 667 Cc over time by using the Doppler shift of its star’s spectra. They compared the temperature of a star and its habitable zone to the temperature of a planet that is close enough to its star’s habitable zone and skimming it. The researchers found that without the stellar wind, Gliese 667 Cc would be able to have liquid water on it and be habitable. If it were closer than 15 AUs, though, the water would boil away into steam before there was enough atmosphere to protect it. The planet would then be too hot for liquid water again.
The researchers found that after 1 billion years, Gliese 667 Cc would be too hot for liquid water to exist, but it could conceivably have it in a very thin atmosphere, at least over its poles. Since the planet is in a binary system with a red dwarf star, the stellar wind causes the planet to move closer and farther from the star. This results in variations in temperature that are small but significant over time.
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