Secular Behavior of Exoplanets: Self-Consistency and Comparisons with the Planet-Planet Scattering Hypothesis

If mutual gravitational scattering among exoplanets occurs, then it may produce unique orbital properties. For example, two-planet systems that lie near the boundary between circulation and libration of their periapses could result if planet-planet scattering ejected a former third planet quickly, leaving one planet on an eccentric orbit and the other on a circular orbit. We first improve upon previous work that examined the apsidal behavior of known multiplanet systems by doubling the sample size and including observational uncertainties. This analysis recovers previous results that demonstrated that many systems lay on the apsidal boundary between libration and circulation. We then performed over 12,000 three-dimensional N-body simulations of hypothetical three-body systems that are unstable, but stabilize to two-body systems after an ejection. Using these synthetic two-planet systems, we test the planet-planet scattering hypothesis by comparing their apsidal behavior, over a range of viewing angles, to that of the observed systems and find that they are statistically consistent regardless of the multiplicity of the observed systems. Finally, we combine our results with previous studies to show that, from the sampled cases, the most likely planetary mass function prior to planet-planet scattering follows a power law with index -1.1. We find that this pre-scattering mass function predicts a mutual inclination frequency distribution that follows an exponential function with an index between -0.06 and -0.1.Comment: 29 pages, 3 figures, accepted for publication in A

Retention of Habitable Atmospheres in Planetary Systems

International audienceThe ability of a planet to retain an atmosphere influences whether water can be stable as a liquid at the planet's surface. A planet's atmospheric state is the result of source, loss, and modification processes that have acted on the atmosphere over time. The loss of atmosphere to space is therefore an important component in assessing planetary surface habitability. 'Atmospheric escape' is a catch-all term that refers to distinct processes that provide sufficient energy to particles for escape to space. Escape processes include thermal escape, hydrodynamic escape, ion loss, photochemical escape, and sputtering. At present, scientists who study atmospheric escape processes at Earth, solar system planets, and exoplanets each employ different and often siloed strategies to estimate escape rates. This fractured approach has hindered development of a comprehensive understanding of how atmospheric escape works at any planet. Here we present an overview of a team science effort to estimate atmospheric escape rates for a wide variety of star-planet combinations. Our goal is to determine which regions within the parameter space of stellar and planetary properties relevant for atmospheric escape are most likely to result in planets that retain habitable atmospheres. Our effort consists of four objectives: (1) We will compute stellar EUV and wind inputs for atmospheric escape for an ensemble of star-planet scenarios; (2) We will improve and link models for atmospheric escape from any planet via each major escape process, validating them against observations; (3) We will construct a multi-dimensional end-to-end model library for atmospheric escape based on more than 200 star-planet combinations, making it publicly accessible via a web portal; and (4) We will apply the model library to understand the connection between atmospheric escape, habitability, and observations