A mysterious planet has been found that may be turning into a water world

An analysis of the exoplanet, called HD-207496b, has shown that this world, which has a mass and radius respectively 6.1 and 2.25 times greater from the mass and radius of the Earth, has either a gaseous atmosphere, or is comprised of water, or has a mixture of both. And the most interesting thing is that, apparently, this planet is gradually decreasing in size, and over time it can turn into a super-Earth, a planet that is larger than Earth in mass, but smaller than Neptune.

The study, published in the Astronomy & Astrophysics journal and available on the arXiv website, may help astronomers solve the mystery of exoplanet discovery.

To date, astronomers have discovered and confirmed the existence of about 5,300 worlds outside the solar system, with almost twice as many unconfirmed candidates. With this information, scientists can perform statistical analysis to identify trends in planetary systems. It turns out that there is a "deficit" of exoplanets with orbits shorter than 100 days and a mass of 1.5-2 times greater than the Earth. This deficit is known as the "valley of planets with small radius".

The causes of this "valley" origin are not entirely clear, but growing evidence suggests that a nearby star has something to do with it. It is possible that with a mass below a certain critical threshold, the exoplanet simply cannot maintain its atmosphere, and the gas evaporates under the influence of the star's radiation.

When NASA's Transiting Exoplanets Survey Satellite (TESS) discovered an exoplanet with a radius 2.25 times greater than that of Earth’s radius and which orbits the orange dwarf HD-207496 star every 6.44 days, scientists enlisted the help of the HARPS telescope to study it in more detail. HARPS data showed that HD-207496b has a mass of about 6.1 times greater than that of Earth’s mass.

This means that the density of the exoplanet is about 3.27 grams per cubic centimeter. This is significantly less than Earth's density of 5.51 grams/cubic centimeter; it is likely that HD-207496b is not entirely rocky. Therefore, the researchers ran simulations to find out what this world is made of.

"We found that HD-207496b has a lower density than Earth, and therefore it is expected to have a significant amount of water and/or gas in its composition," the researchers wrote in their paper. Based on the internal structure modeling, we concluded that either the planet is rich in water, gas, or has a mixture of both."

Evaporation simulations have shown that if the exoplanet has a gas-rich atmosphere (hydrogen and helium), then this state is temporary; in 520 million years, the star will completely strip the exoplanet of its atmosphere. It is also possible that the atmosphere has already disappeared, and HD-207496b is already a purely water world.

"In general," the researchers write, "we expect the planet to consist of both water and gas, and to exist between these two models."

The star HD-207496 is relatively young, about 520 million years old. This means it offers a rare opportunity to study the early stages of the evolution of one of these exoplanets before it transforms into a super-Earth, if that happens at all with HD-207496b.