Effects of X-ray and extreme UV radiation on circumbinary planets

Abstract

Context. Several circumbinary planets have recently been discovered. The orbit of a planet around a binary stellar system poses several dynamic constraints. In addition to these constraints, the effects that radiation from the host stars may have on the planet atmospheres must be considered. We here evaluate these effects. Because of the configuration of a close binary system, these stars have a high rotation rate, even for old stars. The fast rotation of close, tidally locked binaries causes a permanent state of high stellar activity and copious XUV radiation. The accumulated effects are stronger than for normal exoplanets around single stars and cause a faster evaporation of their atmospheres. Aims. We evaluate the effects that stellar radiation has on the evaporation of exoplanets around binary systems and on the survival of these planets. Methods. We considered the X-ray and EUV spectral ranges (XUV, 1-912 Å) to account for the photons that are easily absorbed by a planet atmosphere that is mainly composed of hydrogen. A more complex atmospheric composition is expected to absorb this radiation more efficiently. We used direct X-ray observations to evaluate the energy in the X-rays range and coronal models to calculate the (nondetectable) EUV part of the spectrum. Results. We considered in this problem different configurations of stellar masses, and a resonance of 4:1 and 3:1. The simulations show that exoplanets orbiting close binary systems in a close orbit will suffer strong photoevaporation that may cause a total loss of atmosphere in a short time. We also applied our models to the best real example, Kepler-47 b, to estimate the current mass-loss rates in circumbinary planets and the accumulated effects over the time. Conclusions. A binary system of two solar-like stars will be highly efficient in evaporating the atmosphere of the planet (less than 6 Gyr in our case). These systems will be difficult to find, even if they are dynamically stable. Still, planets may orbit around binary systems of low mass stars for wider orbits. Currently known circumbinary planets are not substantially affected by thermal photoevaporation processes, unless Kepler-47 b has an inflated atmosphere. The distribution of the orbital periods of circumbinary planets is shifted to much longer periods than the average of Kepler planets, which supports a scenario of strong photoevaporation in close-in circumbinary planets.

The Degree of Alignment between Circumbinary Disks and Their Binary Hosts

“…Indeed the two CB disks in our sample around P < 7 day binaries, V4046 Sgr and CoRoT 2239, have small mutual inclinations and do not host tertiaries capable of driving significant Lidov-Kozai oscillations (V4046 Sgr hosts a companion orbiting ∼12,000 au outside the disk; Kastner et al 2011). Two promising alternative explanations for the shortfall of CB planets around the shortest-period binaries are 1) a primordial phase of binary orbit expansion driven by pre-main-sequence tidal evolution which destabilizes planetary orbits (Fleming et al 2018) and 2) increased X-ray and EUV flux from tight, tidally locked binaries photoevaporates circumbinary planets to smaller planet radii (Sanz-Forcada et al 2014), which would remain undetected in the Kepler light curves.…”

The BEBOP radial-velocity survey for circumbinary planets

“…This third star is suspected to either disrupt circumbinary planet formation (Muñoz & Lai 2015;Hamers et al 2016;Xu & Lai 2016) or bias the planet sample to long-period, misaligned orbits, both of which would have been missed by the Kepler transit survey. Alternate explanations for the dearth of planets around the tightest binaries, which do not invoke the presence of a third star, have also been proposed: tidal expansion of the binary orbit, causing the planet to become unstable (Fleming et al 2018) and UV evaporation of exoplanet atmospheres, shrinking them to an undetectable size (Sanz-Forcada et al 2014).…”

The Athena X-ray Integral Field Unit (X-IFU)

“…[89][90][91]. The X-IFU will allow us to investigate the initial phase of the intense flares including mass motions as well as their influence on circumstellar disks and/or early planetary evolution.…”