A giant impact as the likely origin of different twins in the Kepler-107 exoplanet system (1902.01316v1)

Abstract: Measures of exoplanet bulk densities indicate that small exoplanets with radius less than 3 Earth radii ($R_\oplus$) range from low-density sub-Neptunes containing volatile elements to higher density rocky planets with Earth-like or iron-rich (Mercury-like) compositions. Such astonishing diversity in observed small exoplanet compositions may be the product of different initial conditions of the planet-formation process and/or different evolutionary paths that altered the planetary properties after formation. Planet evolution may be especially affected by either photoevaporative mass loss induced by high stellar X-ray and extreme ultraviolet (XUV) flux or giant impacts. Although there is some evidence for the former, there are no unambiguous findings so far about the occurrence of giant impacts in an exoplanet system. Here, we characterize the two innermost planets of the compact and near-resonant system Kepler-107. We show that they have nearly identical radii (about $1.5-1.6~R_\oplus$), but the outer planet Kepler-107c is more than twice as dense (about $12.6~\rm g\,cm^{-3}$) as the innermost Kepler-107b (about $5.3~\rm g\,cm^{-3}$). In consequence, Kepler-107c must have a larger iron core fraction than Kepler-107b. This imbalance cannot be explained by the stellar XUV irradiation, which would conversely make the more-irradiated and less-massive planet Kepler-107b denser than Kepler-107c. Instead, the dissimilar densities are consistent with a giant impact event on Kepler-107c that would have stripped off part of its silicate mantle. This hypothesis is supported by theoretical predictions from collisional mantle stripping, which match the mass and radius of Kepler-107c.

The Likely Origin of Variation in Density of Kepler-107 Exoplanet Twins Due to a Giant Impact

The research article titled "A giant impact as the likely origin of different twins in the Kepler-107 exoplanet system" examines the intriguing disparity in densities between two similarly-sized planets in the Kepler-107 exoplanetary system. The paper provides compelling evidence that this discrepancy can be attributed to a past giant impact affecting Kepler-107c. This inversion of expected density—where Kepler-107c is denser despite greater exposure to XUV radiation from the host star—challenges typical explanations of photoevaporation and suggests significant insights about planet formation dynamics in exoplanetary systems.

Characterization and Analysis

Kepler-107 consists of four planets, but the focus here is on Kepler-107b and Kepler-107c, both with radii approximately 1.5-1.6 Earth radii ($R_\oplus$) yet notably divergent in density. Kepler-107c is more than twice as dense as Kepler-107b, with density measurements of approximately 12.65 g/cm$^3$ for Kepler-107c compared to 5.3 g/cm$^3$ for Kepler-107b. This substantial density difference suggests a significantly higher iron fraction in Kepler-107c, which cannot be explained merely by stellar XUV irradiation patterns.

Through observations with the HARPS-N spectrograph and asteroseismic analysis using Kepler data, detailed atmospheric parameters and mass estimates of these exoplanets have been acquired. For instance, Kepler-107c’s mass approximates 9.39 $\pm$ 1.77 Earth masses ($M_\oplus$), while Kepler-107b is estimated at 3.51 $\pm$ 1.52 $M_\oplus$, evidencing the dramatic density contrast.

Hypothesis and Implications

The paper supports the hypothesis that a giant impact event on Kepler-107c stripped a significant portion of its silicate mantle, accounting for the elevated iron content and density. This assertion is substantiated by smoothed particle hydrodynamics simulations that illustrate the potential effects of high-energy impacts resulting in the shedding of a planetary mantle.

Moreover, contrasting known mechanisms—such as atmospheric escape and photophoresis—underscores that they are inadequate explanations for the observed densities. Kepler-107c's iron-rich composition is better aligned with the collisional mantle stripping paradigm.

The implications of these findings extend beyond Kepler-107. Giant impacts have shaped objects within our Solar System, notably influencing Mercury’s composition and the Earth-Moon system formation. This paper adds a significant perspective suggesting that giant impacts may frequently influence exoplanetary bodies’ compositional characteristics, possibly leading to a clustering of exoplanets around collisional stripping curves in future mass-radius studies.

Future Directions

The findings suggest that, with more precise radius and mass determinations of exoplanets, a pattern might emerge in mass-radius diagrams, linking high-density exoplanets with prior large collision events. Continued exploration of such events could deepen understanding of planet formation and evolution. Sophisticated modeling and simulations of planet composition and orbital dynamics related to impacts will enhance interpretative frameworks in planetary science. Gathering more statistical evidence of exoplanets exhibiting similar density discrepancies will validate and potentially generalize the proposed collision hypothesis.

In conclusion, this research introduces intriguing potential dynamics in exoplanetary system evolution, encouraging further examination of how past interactions via collisions influence the current state of observed planets’ compositions, especially in resonant systems like Kepler-107.