a mysterious exoplanet just 138 light-years from Earth it may be in a curious phase of transformation.
An analysis of an exoplanet, called HD-207496b, revealed that the celestial body, with a mass and radius of respectively 6.1 and 2.25 times the mass and radius of Earth, has agaseous atmospherea global ocean, or a combination of both – and it may soon shrink to become one super-Terra. This could help astronomers solve a mystery in exoplanet detections, a gap between masses of rocky planets larger than Earth and gaseous planets smaller than Neptune. But it will take a closer look at the enigmatic exoplanet to characterize its atmosphere. Astronomers have discovered and confirmed approx 5,300 worlds outside the Solar System, with almost twice as many unconfirmed candidates. With this information, scientists can conduct statistical analyzes to understand trends in planetary systems. And an interesting thing we have learned is that there is a clear scarcity of exoplanets between 1.5 and 2 times the mass of the Earth with orbits shorter than about 100 days. It is about 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 we find worlds with atmosfere dense, like Neptune in miniature, referred to as mini-Neptune. The reasons for the valley aren’t entirely clear, but a growing body of evidence is starting to suggest that proximity to the host star has something to do with this phenomenon. It’s possible that below a certain critical threshold, an exoplanet doesn’t have enough mass to maintain a gravitational grip on its atmosphere (gas is evaporated by the star’s radiation).
When TESS detected an exoplanet near the valley of the beam, with a radius 2.25 times that of Earth and an orbit of 6.44 days with an orange dwarf star, called HD-207496, the scientists used HARPS, a spectrograph for radial speeds, for a close look. THE HARPS data they revealed that HD-207496b it has a mass about 6.1 times that of the Earth. This means that the exoplanet’s density is approximately 3.27 grams per cubic centimeter. It is considerably less dense than Earth’s 5.51 grams per cubic centimeter, implying that its composition is not entirely rocky. Then the researchers did the modeling to see what the object might be made of. “We have found that HD-207496b has a lower density than Earth, and therefore we now expect it to have a significant amount of water and/or gas in its composition“, the researchers write in their paper. “From modeling the planet’s internal structure, 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 aatmosphere rich in hydrogen and helium gasesthis is a temporary state: the star will completely strip the exoplanet within 520 million years. It is also possible that the atmosphere has already disappeared and HD-207496b is already a world bare ocean. “In general“, write the researchers, “We expect the planet to have both water and an H/He envelope and be in between these two models“. The star, HD-207496, is relatively young, about 520 million years old. That means it presents a rare opportunity to study the youth of one of these exoplanets before transforming into a bare super-Earth, if that’s indeed in store for HD-207496b. Further studies to characterize the atmosphere should reveal the true nature – and ultimate fate – of this mysterious world.