Young planets under extreme UV irradiation

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 MJ . 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⊕.

Grid of upper atmosphere models for 1–40 M⊕ planets: application to CoRoT-7 b and HD 219134 b,c

“…The influence of the equilibrium temperature on the velocity at the Roche radius is however unclear, possibly because variations in the equilibrium temperature imply simultaneous changes in the atmospheric structure and in the orbital separation, thus in the Roche radius. In Kubyshkina et al (2018), we showed that, keeping planetary mass and radius fixed, the effects of T eq variations on the velocity are negligible for planets with Λ greater than 20, while for planets with lower Λ the velocity may significantly decrease with increasing temperature.…”

“…Our hydrodynamic code includes X-ray heating and H + 3 cooling, which are relevant for some of the planets close to our grid boundaries. The addition of X-ray heating provides us also with a further important degree of freedom relevant for young, close-in planets, which are subject to strong blow-off (e.g., Kubyshkina et al 2018). We provide below a detailed description of the modelling scheme.…”

“…In contrast, for the less massive planet, the XUV stellar flux does not penetrate deep enough into the planetary atmosphere to cause thermospheric heating, thus the atmosphere expands adiabatically, driven by its internal heat and by the low planetary gravity. In general, the profiles of planets with low Λ values do not develop steep gradients (see e.g., Kubyshkina et al 2018), making the definition of R eff and of the position of the maximum dissociation and ionisation ambiguous. We will come back to this point in Section 5.1.2.…”

“…The same equilibrium temperature for different stellar masses occurs at rather different orbital separations (e.g., within the grid, a T eq of 300 K corresponds to distances ranging from 5×10 −3 to 1.52 AU). Our analysis (see Section 5 and also Kubyshkina et al 2018) indicates that between T eq and d 0 , the latter has the larger influence on the results, thus for the interpolation we substitute T eq with d 0 . Therefore, the routine interpolates the output parameters over d 0 j for the pair of M * i .…”

Radial velocity confirmation of K2-100b: a young, highly irradiated, and low-density transiting hot Neptune

“…Theoretical evolution models predict that the most significant changes in the bulk and orbital parameters of exoplanets occur in the first few hundred Myr of their evolution (e.g., Adams & Laughlin 2006;Kubyshkina et al 2018a;Raymond et al 2009). Planets orbiting stars in young open clusters are thus particularly valuable tests of these models.…”