– January 26, 2020 – Science

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Some of the exoplanets discovered are called super-Earths. What does that expression mean? Could these stars have something more, or even better, than our planet? It is actually a question of size.

An exoplanet has recently been discovered in the neighboring planetary system Proxima Centauri. If its existence is confirmed, it could well be a super-Earth. But what do scientists really mean by that? No, this is not a planet wearing a cape to do justice in the universe – despite the beautiful illustration in this article by our DA Claire Braikeh. Nor does the expression mean that we would be in the presence of a “better” planet than ours.

“In the expression ‘super-Earth’, the word super is not to be understood in the usual sense. It only means that we’re talking about a planet bigger than the Earth, implying also that it is rather composed of rocks and metals, as opposed to a gaseous planet like Jupiter or Neptune,” explains Hervé Cottin, a teacher at the University of Paris-Est Créteil and researcher at the Inter-University Laboratory of Atmospheric Systems (LISA), to Numerama. The expression does not indicate the possibility that life exists or could exist on such a star. “This term says nothing about his habitability. You might as well say ‘super-Venus‘,” says the scientist.

  • To read: Was Venus really the twin sister of Earth?

La taille de la Terre comparée à celle de la super-Terre 55 Cancri e. // Source : Wikimedia/CC/NASA/JPL-Caltech/R. Hurt (SSC) (photo recadrée)

The size of the Earth compared to that of the super-Earth 55 Cancri e. // Source: Wikimedia/CC/NASA/JPL-Caltech/R. Hurt (SSC) (cropped photo)

A super-Earth is bigger than the Earth: but how much bigger? Not all scientists who use the term have the same appreciation. The criteria for precisely defining a super-Earth vary according to scientific publications,” says Hervé Cottin. It says that a super-Earth would have between 5 and 10 times the mass of the Earth. As for its radius, it would be between 1.5 and 2.5 times that of the Earth. »

Super-Earth, mini-Neptune: overlapping categories

Clearly identifying that we are in the presence of a super-Earth is tricky, because there is another category of planets to consider: the mini-Neptunes. “Mini-Neptune is the term used for planets that are smaller than Neptune and are gaseous, with no solid surface. To speak of a mini-Neptune, the radius of the exoplanet must generally be between 1.7 and 4 times that of the Earth. The ‘super-Earth’ and ‘mini-Neptune’ categories overlap,” says the astrochemist.

The case of the exoplanet K2-18 b has recently been well illustrated: water vapour was detected in its atmosphere at the beginning of September by NASA. This detection, which may have been wrongly associated with a sign of life, showed that the differentiation between super-Earths and mini-Neptunes may be important.

Vue d’artiste de COROT-7c, une mini-Neptune. // Source : Wikimedia/CC/MarioProtIV (photo recadrée)

Artist’s view of COROT-7c, a mini-Neptune // Source: Wikimedia/CC/MarioProtIV (cropped photo)

The observation methods do not always allow to decide

But is it always possible to distinguish between the two categories? “For the same radius, the mass of a planet can differ depending on its composition. If we know the size of a planet, but not its mass, we can’t decide whether it’s a super-Earth or a mini-Neptune,” explains Hervé Cottin. However, two methods are used to observe exoplanets and they give different information.

  • The method of transits: “We deduce that there is a passage of an exoplanet in front of its star, thanks to the decrease of luminosity of the star. From this decrease in luminosity, we can estimate the size of the object, which very partially masks the star as it passes in front of it, but not its mass,” says the scientist.
  • The radial velocity method: “We study the gravitational effect exerted by the planet on its star. In this case, we can deduce its mass, but not its size. These two methods can sometimes be combined.

In any case, the planets are not really “seen” by researchers. “These are indirect measurements, we have never seen an exoplanet directly, except on very rare occasions. The images broadcast are artists’ visions,” the astrochemist explains.

The discovery of the TRAPPIST-1 system in February 2017 revealed the existence of a system with seven planets comparable in size to the Earth orbiting a single star. Several of them are described as super-Earths. “The discovery of the TRAPPIST system was a big news story, because we had a measure of both the size and mass of the planets. This made it possible to draw up a hypothesis on their composition, particularly the presence of water. But we don’t know if it’s present in solid or liquid form. Moreover, even if water were to be present in a liquid state, there is nothing to suggest that life would necessarily appear there,” concludes Hervé Cottin.

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