Young planets under extreme UV irradiation

I. Upper atmosphere modelling of the young exoplanet K2-33b

¹ Space Research Institute, Austrian Academy of Sciences, Schmiedlstr. 6, 8042 Graz, Austria
e-mail: daria.kubyshkina@oeaw.ac.at
² Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
³ Institute of Computational Modelling, FRC “Krasnoyarsk Science Center” SB RAS, 660036 Krasnoyarsk, Russian Federation
⁴ Siberian Federal University, 660041 Krasnoyarsk, Russian Federation
⁵ University of Vienna, Department of Astrophysics, Türkenschanzstrasse 17, 1180 Vienna, Austria

Received: 22 August 2017
Accepted: 5 December 2017

Abstract

The K2-33 planetary system hosts one transiting ~5 R_⊕ planet orbiting the young M-type host star. The planet’s mass is still unknown, with an estimated upper limit of 5.4 M_J. The extreme youth of the system (<20 Myr) gives the unprecedented opportunity to study the earliest phases of planetary evolution, at a stage when the planet is exposed to an extremely high level of high-energy radiation emitted by the host star. We perform a series of 1D hydrodynamic simulations of the planet’s upper atmosphere considering a range of possible planetary masses, from 2 to 40 M_⊕, and equilibrium temperatures, from 850 to 1300 K, to account for internal heating as a result of contraction. We obtain temperature profiles mostly controlled by the planet’s mass, while the equilibrium temperature has a secondary effect. For planetary masses below 7–10 M_⊕, the atmosphere is subject to extremely high escape rates, driven by the planet’s weak gravity and high thermal energy, which increase with decreasing mass and/or increasing temperature. For higher masses, the escape is instead driven by the absorption of the high-energy stellar radiation. A rough comparison of the timescales for complete atmospheric escape and age of the system indicates that the planet is more massive than 10 M_⊕.