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

• Published in: Monthly notices of the Royal Astronomical Society. Letters Vol. 465; no. 1; pp. L74 - L78
• Main Authors: Wheatley, Peter J., Louden, Tom, Bourrier, Vincent, Ehrenreich, David, Gillon, Michaël
• Format: Journal Article Web Resource
• Language: English
• Published: Blackwell Publishing 01.02.2017
• Subjects: Aérospatiale, astronomie & astrophysique; Physical, chemical, mathematical & earth Sciences; Physique, chimie, mathématiques & sciences de la terre; Space science, astronomy & astrophysics
• ISSN: 1745-3925; 0035-8711; 1365-2966; 1745-3933
• DOI: 10.1093/mnrasl/slw192

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 LX/Lbol = 2–4 × 10−4, with the total XUV emission in the range LXUV/Lbol = 6–9 × 10−4, 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 subjected 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.