Influence of Stellar Flares on the Chemical Composition of Exoplanets and Spectra

Abstract

More than three thousand exoplanets have been detected so far, and more and more spectroscopic observations of exoplanets are performed. Future instruments (JWST, E-ELT, PLATO, Ariel, . . . ) are eagerly awaited as they will be able to provide spectroscopic data with a greater accuracy and sensitivity than what is currently available. This will allow more accurate conclusions to be drawn on the chemistry and dynamics of the exoplanet atmospheres, on condition that the observational data are processed carefully. An important aspect to consider is temporal stellar atmospheric disturbances that can influence the planetary composition, and hence spectra, and potentially can lead to incorrect assumptions about the steady-state atmospheric composition of the planet. In this paper, we focus on perturbations that come from the host star in the form of flare events that significantly increase the photon flux impingement on the exoplanets atmosphere. In some cases, and particularly for M stars, this sudden increase may last for several hours. We aim at answering the question to what extent a stellar flare is able to modify the chemical composition of the planetary atmosphere and, therefore influence the resulting spectra. We use a one-dimensional thermo-photochemical model to study the neutral atmospheric composition of two hypothetic planets located around the star AD Leo. We place the two planets at different distances from the star, which results in effective atmospheric temperatures of 412 K and 1303 K. AD Leo is an active star that has already been observed during a flare. We therefore use the spectroscopic data from this flare event to simulate the evolution of the chemical composition of the atmospheres of the two hypothetic planets. We compute synthetic spectra to evaluate the implications for observations. The increase of the incoming photon flux affects the chemical abundances of some important species (such as H and NH 3 ) down to altitudes associated with an atmospheric pressure of 1 bar, that can lead to variations in planetary spectra (up to 150 ppm) if performed during transit. We find that each exoplanet has a post-flare steady-state composition that is significantly different from the preflare steady-state. We predict that these variations could be detectable with both current and future spectroscopic instruments if sufficiently high signal-to-noise spectra are obtained.

A VERY BRIGHT, VERY HOT, AND VERY LONG FLARING EVENT FROM THE M DWARF BINARY SYSTEM DG CVn

“…The effect of such large variations in the stellar ionizing flux may be significant, especially early in the life of the star and planetary system. The previous studies examining the impact of stellar flares on exoplanet atmospheres, like Segura et al (2010) and Venot et al (2016), used only UV observations, ignoring the potential increase in UV emission incident on the planet due to high-energy photons in the manner described in Smith et al (2004). Hence they are underestimates at best of the impact of these flares on exoplanet atmospheric chemistry.…”

“…One critical open question in this area is whether the scalings observed in solar eruptive events between photons and particles holds during stellar events. A more recent paper on the effects of stellar flares on exoplanetary atmospheres (Venot et al 2016) determined that planets around very active stars would likely never achieve a steady state due to the frequent photon bombardment of the exoplanetary atmosphere from stellar flares. Smith et al (2004) considered the transport of ionizing radiation in terrestrial exoplanet atmospheres, and found that while a thick atmosphere can protect the planetary surface from incident X-rays and γ-rays, up to 4% of the incident ionizing radiation received at the surface in the 2000-3200 Å wavelength range comes from atmospheric transmission and reprocessing of the high-energy radiation.…”

Frequent Flaring in the TRAPPIST-1 System—Unsuited for Life?

“…That complex flare event on AD Leo, which has by chance very similar light curve shape to the largest flare seen in the K2 light curve of TRAPPIST-1, has an amplitude of V ≈ 0.5 magnitude. Venot et al (2016) concluded, that atmosphere of the two super-Earth-like hypothetical planets, that would orbit AD Leo, would be altered irreversibly and significantly after such eruption. Their model suggests that the post-flare steady state would only return on the scale of ≈ 30000 years, thus, the planetary atmosphere would be constantly altered by eruptions due to the high flaring rate.…”

The MUSCLES Treasury Survey. IV. Scaling Relations for Ultraviolet, Ca ii K, and Energetic Particle Fluxes from M Dwarfs

“…Strong overlying magnetic fields likely confined the eruption (Sun et al 2015;Thalmann et al 2015). As more exoplanetary atmosphere models consider the important influence of stochastic flares (Lammer et al 2007;Segura et al 2010;Airapetian et al 2016;Venot et al 2016), improving constraints on the fluxes and energies of associated particles is necessary.…”