A fourth world in the habitable zone of a solar system with planets of unusually low density

Since three planets with an extraordinarily low density were discovered around the star Kepler-51, the mystery about them has not ceased.

Those three planets have sizes that are broadly similar to Saturn, but their masses are only a few times that of Earth, resulting in a density like cotton candy. They are believed to have tiny cores and huge atmospheres of hydrogen and helium, but how these strange planets formed and how their atmospheres have not been destroyed by the intense radiation of their young star remains an enigma.

Planets with a density as low as these are very unusual, and when one forms, it is usually the only one of its kind in a planetary system. Trying to explain how three planets of this type formed in the same system is quite a challenge.

And now it turns out that this unusual planetary system, located about 2,600 light-years away from Earth, has at least one more planet, according to new research. The discovery of this fourth planet, and the possibility that more exist, make this solar system even more complex and strange.

The study was carried out by a team including, among others, Jessica Libby-Roberts, from Pennsylvania State University in the United States, and Kento Masuda, from Osaka University in Japan.

The team set out to study Kepler-51d, the third planet in the system, with the James Webb Space Telescope (JWST) of NASA, ESA and CSA, respectively the US, European and Canadian space agencies, but almost missed his chance when the planet unexpectedly passed in front of its star two hours earlier than models predicted. After analyzing new and archival data from various space and ground-based telescopes, researchers discovered that the best explanation is the presence of a fourth planet, whose gravitational pull influences the orbits of the other planets in the system.

Artist’s recreation of the Kepler-51 solar system. (Illustration: NASA / ESA / L. Hustak, J. Olmsted, D. Player and F. Summers (STScI). CC BY-SA)

When a planet passes in front of its star, from the visual perspective with which we see it from Earth or its surroundings, it blocks part of the star’s light, causing a slight decrease in its brightness. The duration and magnitude of that decline give clues to the planet’s size and other characteristics. These planets can pass in front of their star every time they complete an orbit around it, but sometimes they do so a few minutes earlier or later because the gravity of other planets in the system pulls on them. These small differences are known as variations in transit time and are incorporated into the models used by astronomers so that they can accurately predict when a planet will cross in front of its star.

After analyzing all the data, the team came to the conclusion that the explanation that best agrees with what was observed is that the fourth planet, which has been given the name Kepler-51e, has a mass similar to those of the three other planets and follow a fairly circular orbit that makes it make a complete revolution around its star in about 264 days. It is unknown, for now, if the density of this fourth planet is as low as that of the other three known ones. In any case, Kepler-51e orbits within the habitable orbital band around Kepler-51, that is, the area in which the heat received from that star is neither excessive nor insufficient to allow the existence of liquid water on the surface. of a planet. Since liquid water is an essential ingredient for life as we know it, it is plausible that a planet with liquid water on its surface could host life.

The study is titled “A Fourth Planet in the Kepler-51 System Revealed by Transit Timing Variations.” And it has been published in the academic journal The Astronomical Journal. (Fountain: NCYT by Amazings)