An exoplanet candidate with at least 4 times the mass of the Earthprobably rocky like ours, potentially capable of hosting liquid water on its surface and – so as not to miss anything – very close to the Solar System in astronomical terms (“just” 18 light years): this is the new discovery made by an international team of astronomers and published with a study in the journal The Astrophysical Journal. The exoplanet is called GJ 251 c and it has, at least on paper, all the elements to be able to host life. This makes it an object of great interest, although for now it is a case of keeping enthusiasm at bay. Among astronomers there is indeed a certain skepticism on its real possibility that an exoplanet like this is really habitable: we don’t know – and for now we can’t know – if it has aatmosphereand the nature of its parent star could put it in less than ideal conditions for the development of life.
What we know about GJ 251 c: the study
The planet was discovered by measuring and analyzing the star’s oscillations with cutting-edge computational models GJ 251. As planets orbit their star, they shift – albeit minimally – the center of gravity of the planetary system in which they find themselves. As a result, the star undergoes displacements – always minimal – the speed of which can be measured with very high precision here on Earth along the direction of the line of sight between us and the star. The more massive the planet and closer to the star, the more important these shifts are: this method of discovery – called radial velocity method – therefore preferably identifies large planets “in the face” of their star.
In this case, we are talking about a planet that at least has a mass 3.84 times that of Earth at a distance of approximately 29 million km from its starthat is, about 20% of the distance between the Earth and the Sun. We are therefore faced with what astronomers call “super Earth”a category of planet intermediate in size between that of Earth (the largest rocky planet orbiting the Sun) and that of Neptune (the smallest giant planet in the Solar System). Assuming that GJ 251 c is on the 4 Earth masses, it is likely that it is a rocky planet rather than gaseous, therefore compatible with the development of life. Its orbit also places it in the habitable range of its star, i.e. that region in which the temperatures on the planet are between 0 °C and 100 °C thus making it – in principle – capable of having liquid waterwhich is an indispensable ingredient for life as we know it.
What Not we know about the exoplanet: does it have an atmosphere?
At this point there is a crucial aspect about GJ 251 whether or not it has an atmosphere. The reason? Having an atmosphere is a fairly indispensable requirement for an exoplanet like the one just discovered to host life. In fact, atmospheres protect planets from cosmic rays high-energy vehicles traveling in space and from ionizing radiation (ultraviolet rays, X-rays and gamma rays) produced by their stars, in addition to the fact that they guarantee the surface pressure necessary to prevent any basins of liquid water from evaporating, leaving the planet deserted and dry.
The problem is that at the moment we are unable to determine whether or not GJ 251 c has an atmosphere. The only instrument we have available that can resolve the issue would be the space telescope James Webbbut to do so it would need the planet to be closer to its star. Webb can in fact identify chemical compounds that can indicate the presence of an atmosphere, but to do so the planet must be well hit by starlight, and unfortunately this is not the case for GJ 251 c.
Not all is lost, however, because fortunately the exoplanet is very close to us in astronomical terms, just 18.2 light years away. The authors of the study calculated that, thanks to its proximity, next-generation 30-meter-class telescopes – such as the Extremely Large Telescope under construction in the Chilean Andes, which could be operational as early as 2030 – will be able to produce direct images of GJ 251 c. This will allow us to study in unprecedented detail what in fact is currently the best candidate potentially habitable exoplanet in the northern sky so it will be possible to have direct images in the next decade.
The reasons for skepticism about the actual habitability of GJ 251 c
While on the one hand the scientific community shows a justified interest in this extrasolar planet, on the other hand it is better not to hold your breath too long waiting to find extraterrestrial life on GJ 251 c. In fact, there are still many variables at play that can exclude the habitability of the planet.
First of all, we need to ask more stringent limits for its mass. Remember that 3.84 Earth masses is the limit minimum estimated at GJ 251. If it turns out to be a beast of 8-10 Earth masses, it might not even have a rocky surface: this means practically saying goodbye to the possibility of it hosting life forms.
Then there is the question of the atmospherewhich we talked about before: if future telescopes were to detect that the planet is “naked”, the probability that it could be habitable would collapse almost instantly.
However, one weighs on this question very serious question markwhich is the nature of its parent star, that is, one red dwarf. Red dwarfs are low-mass stars (in this case we are talking about 35% of the solar mass) which, unlike the Sun which is an absolutely stable and quiet star, they are particularly “nervous” and they tend to produce flares, stellar eruptions and other forms of intense activity that are often incompatible with the tranquility necessary for a planet to be able to develop some biosphere undisturbed.
Violent flares, for example, can literally strip a sufficiently nearby planet of its atmosphere. This is something we can affirm with full knowledge of the facts, since rocky planets have already been discovered in the habitable belt of red dwarfs (for example around Proxima Centauri – the star closest to us – e TRAPPIST-1) and the data collected by the James Webb telescope have allowed us to exclude that they have an atmosphere. For this reason, astronomers have become rather skeptical about the habitability of planets orbiting red dwarfs.
We just have to wait for the first light from the next generation telescopes and analyze the data they will collect. Beyond the specific case of GJ 251 c, there is no doubt that the possibility of obtaining direct images of extrasolar planets will open up completely new avenues for astrobiology.
