A mysterious exoplanet located just 138 light-years from Earth is perhaps in a strange phase of transformation

 An analysis of an exoplanet, called HD-207496b, revealed that the celestial body, with a mass and radius of 6.1 and 2.25 times the mass and radius of the Earth, has a gaseous atmosphere, a global ocean or a combination of both - and it could soon shrink to become a super-Earth. This could help astronomers solve a mystery in the measurements of exoplanets, a gap between the masses of the largest rocky planets on Earth and the smallest gaseous planets on Neptune. But it will take a closer look at the enigmatic exoplanet to characterize its atmosphere. Astronomers have discovered and confirmed about 5,300 worlds outside the Solar System, with almost twice as many unsubstantiated candidates. 

With this information, scientists can conduct statistical analysis to understand trends in planetary systems. And an interesting thing that we’ve learned is that there’s an obvious scarcity of exoplanets between 1.5 and 2 times the mass of Earth with orbits shorter than about 100 days. It is the "small valley of the radius of the planet". Below it, rocky worlds such as Earth, Venus and Mars are generally discovered; called super-Earths. Above are worlds with dense atmospheres, such as miniature Neptune, referred to as mini-Nettunos. The reasons for the "valley" are not entirely clear, but increasing evidence is beginning to suggest that proximity to the host star has something to do with this phenomenon. It is possible that below a certain critical threshold, an exoplanet does not have enough mass to maintain a gravitational grip on its atmosphere (the gas is evaporated by the star’s radiation).

When TESS detected an exoplanet near the ray valley, with a radius 2.25 times that of the Earth and an orbit of 6.44 days with an orange dwarf star, called HD-207496, scientists used HARPS, a spectrograph for radial velocities, for a closer look. HARPS data revealed that HD-207496b has a mass about 6.1 times that of Earth. This means that the density of the exoplanet is about 3.27 grams per cubic centimeter. It is considerably less dense than the 5.51 grams per cubic centimetre of the Earth and this implies that its composition is not entirely rocky. So the researchers made the modeling to see what the object could be made of. "We found that HD-207496b has a lower density than Earth, and so we now expect it to have a significant amount of water and/or gas in its composition," the researchers write in their article. "From the modelling of the internal structure of the planet, we conclude that the planet has a water-rich envelope, a gas-rich envelope, or a mixture of both". Evaporation modeling reveals that if the exoplanet has an atmosphere rich in hydrogen gas and helium, it is a temporary state: the star will completely depopulate the exoplanet within 520 million years. It is also possible that the atmosphere has already disappeared and HD-207496b is already a bare ocean world. "In general", the researchers write, "we expect the planet to have both water and an H/He envelope and be between these two models". The star, HD-207496, is relatively young, about 520 million years old. This means that it represents a rare opportunity to study the youth of one of these exoplanets before the transformation into a naked super-Earth, if this is really in store for HD-207496b. Subsequent studies to characterize the atmosphere are expected to reveal the true nature - and ultimate destiny - of this mysterious world.