Astronomers discover an unusually low-density super-Earth

Researchers from the Center for Astrobiology and other international centers have discovered a super-Earth with unusually low density. The explanation seems to lie in its thick and enormous atmosphere of water vapor, some 50 times greater than that of our planet.

Planets of a similar size to Earth, also called rocky, are composed of an iron core and a rock mantle in a similar proportion to what we find on our planet. Yet the universe never ceases to amaze, and recent discoveries are beginning to suggest that the compositional diversity of these worlds may be greater than previously thought. Now, an international study, led by researchers from the Center for Astrobiology (CAB, CSIC-INTA) and published in the journal ‘Astronomy & Astrophysics’, have analyzed in detail the super-Earth TOI-244 b, which orbits around a red dwarf star located 72 light years away. The results reveal that this exoplanet may be shrouded in a thick steamy atmosphere.

The TOI-244 b signal was detected in 2018 by NASA’s TESS space mission, and the authors have used its data to carry out the study, together with those collected by the ESPRESSO instrument of the Paranal Observatory, belonging to the European Southern Observatory ( ESO) and located in the Atacama desert (Chile). Thus they have been able to confirm and characterize this new exoplanet in depth. The data shows that this exoplanet is 1.5 times larger than Earth and 2.7 times more massive. These measurements correspond to a density almost two times lower than expected, placing TOI-244 b very far from the typical characteristics of other lands.

“It was a truly surprising result, and one that none of us expected before undertaking the research,” says Amadeo Castro-González, CAB predoctoral researcher and lead author of the study.

Why is it so sparse? The researchers looked at several possibilities to explain the low density of this rocky planet. In the first place, they considered that its core had little iron and, therefore, that it harbored an extensive layer of rock. However, they ruled out this possibility, since calculations indicated that even a planet made entirely of rock would not have a density as low as that observed in TOI-244 b.

Then they thought of another alternative: that the low density of the planet was due to the existence of an extensive atmosphere. Although this atmosphere might initially have been composed of light chemical elements abundant in planet-forming regions, such as hydrogen and helium, the star’s ultraviolet and X-ray radiation would have evaporated this primordial atmosphere, leaving a secondary one made mostly of water.

Based on this possibility, the researchers carried out simulations that suggest that TOI-244 b has a solid structure similar to that of Earth that would be surrounded by a hydrosphere composed of gaseous and supercritical water with a thickness of between 400 and 600 kilometers. that is, about 50 times more extensive than the Earth’s atmosphere. “Given the temperature and humidity conditions on the planet’s surface, the climate of TOI-244 b would be similar to that inside a humid sauna of planetary dimensions, which, of course, given the extremely high temperatures and pressures, would be very hostile to the planet.” most of the life forms we know of,” explains Amadeo.

Population of planets with inflated atmospheres Only a handful of planets out of the more than 5,300 found to date have properties similar to TOI-244 b. Thanks to this discovery, the authors propose the existence of a possible new population of rocky planets with inflated atmospheres. These planets tend to orbit stars that are low in metals, and which also tend to receive lower stellar radiation than denser super-Earths.

“These trends can give us very valuable information about the nature of these objects, but more well-characterized planetary systems are still needed to confirm them for sure. If confirmed, it would suggest that the presence of an extensive hydrosphere might not only be the most likely explanation for the structure of TOI-244 b, but also for an emerging new population of inflated super-Earths,” concludes Amadeo. “These results deepen the need to understand planetary genetics, the formation and evolution conditions that make us find such a rich diversity of exoplanetary systems,” says author Jorge Lillo Box, also a CAB researcher and Amadeo’s thesis supervisor. .