Kepler-10c: INAF discovers a possible "Water World"

An international research team led by the National Institute for Astrophysics (INAF) has made a highly precise measurement of the mass of the exoplanet Kepler-10c, suggesting that it may be largely composed of water ice . The study, published in the journal Astronomy & Astrophysics , used data collected by the High Accuracy Radial velocity Planet Searcher for the Northern hemisphere (HARPS-N) spectrograph, installed at the Telescopio Nazionale Galileo (TNG) located in the Canary Islands . The analysis also confirmed the presence of a third planet within the Kepler-10 system , providing valuable new information for understanding the processes of planetary formation and the origins of our Solar System.

Using nearly 300 radial velocity measurements from HARPS-N, researchers were able to estimate with unprecedented accuracy the mass and density of Kepler-10c, an exoplanet orbiting the star Kepler-10. This planetary system is known to host Kepler-10b, the first rocky super-Earth discovered by NASA's Kepler mission with an orbital period less than one Earth day , and Kepler-10c, a planet classified as sub-Neptune with an orbital period of 45 days. The mass of Kepler-10c has long been a matter of debate, with conflicting estimates making it difficult to determine its composition.

The HARPS-N data were processed through a new method that corrects for instrumental effects and variations in the magnetic activity of the parent star, even when low intensity. The analyses, conducted independently by three groups within the team, produced consistent results . This work led to the conclusion that Kepler-10c is highly likely a "water world", a planet whose mass is largely made up of solid water (ice) and perhaps, in a small percentage, also liquid . The researchers hypothesize that the planet formed beyond the water condensation line, about two to three astronomical units from its star, and then progressively migrated towards its current innermost orbit.

A significant result of the study is also the confirmation of the existence of a third planet in the Kepler-10 system . This celestial body, not detectable through transits, was identified thanks to the small anomalies it induces in the orbit of Kepler-10c, observable in the variations of the transit times of the latter. This method is analogous to the one that led to the discovery of Neptune, based on the perturbations observed in the orbit of Uranus. The "ghost" planet, whose existence had been hypothesized previously, now has an accurately determined orbital period of 151 days and an estimated minimum mass, thanks to the precision of the radial velocity measurements of HARPS-N. Luca Borsato of INAF in Padua, second author of the paper, comments : " The analysis of the radial velocities and the variations of the transit times, both individually and in combination, provided results in excellent agreement on the parameters of the third planet, correcting previous imprecise estimates of its properties ."

Aldo Bonomo of INAF in Turin, first author of the article, underlines the importance of the discovery in the theoretical context : " The existence of 'water worlds' has been predicted by models of planetary formation and migration, but we do not yet have definitive confirmation. However, about fifteen planets around stars similar to the Sun, such as Kepler-10c, seem to have exactly the composition predicted by these models. The final verification of the existence of 'water worlds' could come from the study of their atmospheres with the James Webb space telescope, since we expect them to be particularly rich in water vapor ."

The study of the Kepler-10 system offers important insights into the formation of planets around their stars. Super-Earths like Kepler-10b and sub-Neptunes like Kepler-10c, which are common in the Galaxy but absent from our Solar System, are a key piece of our understanding of the diversity of worlds that orbit other stars . In particular, analyzing the composition of sub-Neptunian planets to determine whether they are ice-rich or ice-poor can provide crucial insights into their origins and the early formation of planetary systems, including our own. Understanding how and where these planets form and their migration toward their parent stars means looking back in time to uncover more details about the origins of Earth and, perhaps, of life itself.