Strong XUV irradiation of the Earth-sized exoplanets orbiting the ultracool dwarf TRAPPIST-1

TL;DR

This study reports that TRAPPIST-1's planets receive intense XUV radiation, which could significantly impact their atmospheres and habitability, providing crucial data for atmospheric evolution models.

Contribution

First X-ray measurement of TRAPPIST-1 revealing high XUV fluxes impacting its planets' atmospheres and habitability assessments.

Findings

TRAPPIST-1 is a strong, variable X-ray source with luminosity similar to the quiet Sun.

The planets experience XUV irradiation much stronger than Earth's current levels.

XUV fluxes are sufficient to alter planetary atmospheres significantly.

Abstract

We present an XMM-Newton X-ray observation of TRAPPIST-1, which is an ultracool dwarf star recently discovered to host three transiting and temperate Earth-sized planets. We find the star is a relatively strong and variable coronal X-ray source with an X-ray luminosity similar to that of the quiet Sun, despite its much lower bolometric luminosity. We find L_x/L_bol=2-4x10^-4, with the total XUV emission in the range L_xuv/L_bol=6-9x10^-4, and XUV irradiation of the planets that is many times stronger than experienced by the present-day Earth. Using a simple energy-limited model we show that the relatively close-in Earth-sized planets, which span the classical habitable zone of the star, are subject to sufficient X-ray and EUV irradiation to significantly alter their primary and any secondary atmospheres. Understanding whether this high-energy irradiation makes the planets more or less habitable is a complex question, but our measured fluxes will be an important input to the necessary models of atmospheric evolution.