Testing exoplanet evaporation with multitransiting systems

Abstract

The photoevaporation model is one of the leading explanations for the evolution of small, close-in planets and the origin of the radius-valley. However, without planet mass measurements, it is challenging to test the photoevaporation scenario. Even if masses are available for individual planets, the host star's unknown EUV/X-ray history makes it difficult to assess the role of photoevaporation. We show that systems with multiple transiting planets are the best in which to rigorously test the photoevaporation model. By scaling one planet to another in a multi-transiting system, the host star's uncertain EUV/X-ray history can be negated. By focusing on systems that contain planets that straddle the radius-valley, one can estimate the minimum-masses of planets above the radius-valley (and thus are assumed to have retained a voluminous hydrogen/helium envelope). This minimum-mass is estimated by assuming that the planet below the radius-valley entirely lost its initial hydrogen/helium envelope, then calculating how massive any planet above the valley needs to be to retain its envelope. We apply this method to 104 planets above the radius gap in 73 systems for which precise enough radii measurements are available. We find excellent agreement with the photoevaporation model. Only two planets (Kepler -100c & 142c) appear to be inconsistent, suggesting they had a different formation history or followed a different evolutionary pathway to the bulk of the population. Our method can be used to identify TESS systems that warrant radial-velocity follow-up to further test the photoevaporation model.The software to estimate minimum planet masses is publicly available at: https://github.com/jo276/EvapMass

Current Population Statistics Do Not Favor Photoevaporation over Core-powered Mass Loss as the Dominant Cause of the Exoplanet Radius Gap

“…gratefully acknowledge support from NASA HST Grant HST-GO-14784.001-A for this work. We sincerely thank James Owen for providing us with the code from Owen & Campos Estrada (2020). We wish to express deep gratitude for the monumental effort of the teams carrying out the Gaia, Kepler, GALEX, CKS, asteroseismology, and smaller planetary and stellar surveys that make population studies of this kind possible.…”

A Pair of TESS Planets Spanning the Radius Valley around the Nearby Mid-M Dwarf LTT 3780

“…The power of this comparative scaling of planets within the same planetary system is that certain unobservable quantities that directly affect final planet masses are scaled out. An example of this is the host star's XUV luminosity history in the photoevaporation scenario (Owen & Campos Estrada 2020). A full derivation is presented in Appendix A but here we simply state the condition for the consistency of the gaseous (i.e., nonrocky) and rocky planet parameters with the photoevaporation model.…”

“…A full derivation is presented in Appendix A but here we simply state the condition for the consistency of the gaseous (i.e., nonrocky) and rocky planet parameters with the photoevaporation model. This requires the gaseous planet's mass-loss timescale to exceed the maximum mass-loss timescale of the rocky planet (Owen & Campos Estrada 2020…”

“…A few notable caveats exist with the planetary mass limits imposed by the photoevaporation model in Equation 3 (Owen & Campos Estrada 2020). These are discussed in Appendix A.…”

“…Each of the aforementioned mechanisms makes explicit predictions for the location of the rocky/nonrocky transition in the orbital period-radius space. Measurements of planetary bulk compositions in systems of multiple planets that span the radius valley therefore offer an opportunity to resolve the precise location of the rocky/nonrocky transition (Owen & Campos Estrada 2020) and distinguish between the model predictions. Precise planetary bulk composition measurements for systems around a range of host stellar masses will enable the dependence of the radius valley on stellar mass to be resolved and, consequently, used to test competing models of the emergence of the radius valley (CM20).…”