KEPLER-10c: A water world in a three-planet system unveiled with HARPS-N
An INAF-led team has measured the mass of the planet Kepler-10c with unprecedented accuracy, defining it as a possible world largely composed of water ice. The study, published today in the journal Astronomy & Astrophysics and made possible thanks to the data collected with the HARPS-N spectrograph at the Telescopio Nazionale Galileo, offers new information for understanding planet formation and the origins of our Solar System.
An international team led by researchers from the Italian National Institute for Astrophysics (INAF) determined the mass and density of the planet Kepler-10c with unprecedented precision and accuracy. Thanks to about 300 radial velocity (RV) measurements collected with the High Accuracy Radial velocity Planet Searcher for the Northern hemisphere (HARPS-N) spectrograph installed at the Telescopio Nazionale Galileo (TNG), it was possible to estimate its composition - mostly water in solid but possibly also liquid state - and to understand how it might have formed. The study was published today in the journal Astronomy & Astrophysics.
Kepler-10 is a historical exoplanet system: it hosts Kepler-10b, the first rocky super-Earth discovered by NASA's Kepler space mission with an orbital period of less than one day, and Kepler-10c, a planet with an orbital period of 45 days, classified as a sub-Neptune, that is, a planet with a radius and mass less than those of Neptune. For years, the mass of Kepler-10c was the subject of great uncertainty: conflicting estimates had made it difficult to figure out what it was made of.
The data acquired with HARPS-N were reduced with a new method, which corrects for instrumental effects and variations due to the magnetic activity of the parent star, even if of low intensity, and were analyzed independently by three groups inside the team, reaching the same results. This work showed that Kepler-10c is probably a water world, that is, a planet with much of its mass in water ice. The researchers believe that the planet formed beyond the so-called water condensation line at about two or three astronomical units from its star, and then migrated inwards to its current orbit.
But that's not all: the team also confirmed the existence of a third planet, not visible in transits but revealed due to a small anomaly it induces on the orbit of Kepler-10c, found in the transit time variations (TTVs) of the planet Kepler-10c, similarly to the discovery of Neptune thanks to the anomalies in the orbit of Uranus. This “ghost” planet had been hypothesized previously, but only now could its orbital period of 151 days and minimum mass be accurately determined, thanks to the exceptional quality of the HARPS-N radial velocities. “The analysis of the HARPS-N RVs and Kepler TTVs, both separately and in combination, gave results in excellent agreement on the parameters of the third planet. We also used an innovative method to show what is the most likely period of the third planet from the TTVs alone, and the same period was independently found with the RVs.” comments Luca Borsato of INAF-Padua, second author of the paper.
Aldo Bonomo, senior researcher at INAF-Turin, first author of the paper, adds “The existence of the water worlds has been predicted theoretically by models of planet formation and migration, but we do not yet have certain confirmation. However, about 15 planets around solar-type stars such as Kepler-10c seem to have precisely the composition predicted by these models. The litmus test of the existence of water worlds should come from the study of their atmospheres with the James Webb Space Telescope, because we expect them to have atmospheres that are particularly rich in water vapor.”
The study of the Kepler-10 system helps us understand how planets form around their stars. Super-Earths such as Kepler-10b and sub-Neptunes such as Kepler-10c, so common in the Galaxy but absent in our Solar System, represent a crucial building block for understanding the variety of worlds orbiting other stars. In particular, studying the composition of sub-Neptune planets and understanding whether they are ice-rich or ice-poor can provide insights into the formation of planetary systems and thus of our own Solar System. Knowing how and where these planets formed and how they migrated towards their star means looking back in time to discover more about the origins of Earth and perhaps even life.
Artistic view of how Kepler-10c, a sub-Neptune water world, might look like. Credits: INAF M. Galliani
Mass-radius diagram of small (Rp ≤ 4 R⊕ ) planets orbiting FGK dwarfs, with mass and radius determinations better than 4σ and 10σ, respectively, color-coded by planet equilibrium temperatures Teq ≤ 600 K. The different solid curves, from bottom to top, correspond to planet compositions of 100% iron, 33% iron core and 67% silicate mantle (Earth-like composition), 100% silicates, 50% rocky interior and 50% water, 100% water, rocky interiors and 1% or 2% hydrogen-dominated atmospheres. The gray dark circles indicate Venus (V), the Earth (E), Uranus (U), and Neptune (N). Kepler-10 c is indicated with square and joins the fifteen or so possible water worlds located in between the 100% silicate and 100% water isocomposition curves.
For further information:
The paper “In-depth characterization of the Kepler-10 three-planet system
with HARPS-N radial velocities and Kepler transit timing variations”, by A.
S. Bonomo, L. Borsato, V.M. Rajpaul, L. Zeng, M. Damasso, N.C. Hara, M.
Cretignier, A. Leleu, N. Unger, X. Dumusque, F. Lienhard, A. Mortier, L.
Naponiello, L. Malavolta, A. Sozzetti, D.W. Latham, K. Rice, R. Bongiolatti,
L. Buchhave, A.C. Cameron, A.F. Fiorenzano, A. Ghedina, R.D. Haywood, G.
Lacedelli, A. Massa, F. Pepe, E. Poretti e S. Udry has been published online
in the journal Astronomy & Astrophysics.