Stellar wind effects on the atmospheres of close-in giants: a possible reduction in escape instead of increased erosion

Abstract

The atmospheres of highly irradiated exoplanets are observed to undergo hydrodynamic escape. However, due to strong pressures, stellar winds can confine planetary atmospheres, reducing their escape. Here, we investigate under which conditions atmospheric escape of close-in giants could be confined by the large pressure of their host star's winds. For that, we simulate escape in planets at a range of orbital distances ([0.04, 0.14] au), planetary gravities ([36%, 87%] of Jupiter's gravity), and ages ([1, 6.9] Gyr). For each of these simulations, we calculate the ram pressure of these escaping atmospheres and compare them to the expected stellar wind external pressure to determine whether a given atmosphere is confined or not. We show that, although younger close-in giants should experience higher levels of atmospheric escape, due to higher stellar irradiation, stellar winds are also stronger at young ages, potentially reducing escape of young exoplanets. Regardless of the age, we also find that there is always a region in our parameter space where atmospheric escape is confined, preferably occurring at higher planetary gravities and orbital distances. We investigate confinement of some known exoplanets and find that the atmosphere of several of them, including π Men c, should be confined by the winds of their host stars, thus potentially preventing escape in highly irradiated planets. Thus, the lack of hydrogen escape recently reported for π Men c could be caused by the stellar wind.

The evolution of the solar wind

“…The second important condition to consider is the relative location between a prominence (co-rotation point) and the sonic point. Information about what is happening in the wind cannot be passed back to the star for any event that takes place above the sonic point (Del Zanna et al 1998;Vidotto and Cleary 2020). Thus, if the prominence, formed at the co-rotation radius, is formed above the sonic point, the star keeps loading the prominence with stellar wind material and the loop top becomes denser and denser, until it eventually erupts, and the cycle starts again.…”

“…This is the Chapman-Ferraro equation firstly derived for Earth's magnetosphere. Note that this equation needs to be modified for exoplanets in closer orbits, as in those cases, the magnetic pressure and thermal pressure of the stellar wind, as well as ram pressure of the planetary atmosphere, might have to be incorporated in Equations ( 72) and ( 73) (Vidotto et al 2013;Vidotto and Cleary 2020). Recently, Carolan et al (2019) studied the evolution of Earth's magnetosphere during the solar main sequence.…”

Irradiation-driven escape of primordial planetary atmospheres

“…A full understanding of photoevaporative loss is thus warranted for deciphering the full picture of planet formation and evolution. As a result, in the last two decades much effort has gone into the development of more realistic, numerical models of atmospheric escape (Lammer et al 2003;Yelle 2004;Tian et al 2005;García Muñoz 2007;Murray-Clay et al 2009;Owen & Jackson 2012;Erkaev et al 2013;Erkaev et al 2015Erkaev et al , 2016Salz et al 2015;Debrecht et al 2019;McCann et al 2019;Esquivel et al 2019;Vidotto & Cleary 2020). Specific exoplanet targets have been modeled with a great deal of sophistication, also accounting for 2D and/or 3D effects and complex chemistry (see, e.g, Ehrenreich et al 2015b andKhodachenko et al 2019 for GJ 436 b, Koskinen et al 2013, Khodachenko et al 2017, Bisikalo et al 2018and Debrecht et al 2020 for HD 209458 b, Odert et al 2020 for HD 189733 b).…”

The Impact of Eruptions from Young Stars on Environments of Rocky Exoplanets

“…While these processes can be important for evolution of stellar angular momentum and their contribution to magnetic breaking and activity, explosive events can impact environments of rocky exoplanets in the habitable zones around these stars and present challenges for their habitability (see Dong et al 2018;Yamashiki et al 2019;Airapetian et al 2017;2020 in references herein). Specifically, ionizing radiation fluxes in the X-ray [0.1-10 nm] and Extreme UV (EUV, 10-91.2 nm) bands (referred to as the XUV band) can deposit heating via photoinization in the exospheres of exoplanets, and thus cause atmospheric escape (Cohen et al 2014;Johnstone et al 2019;Airapetian et al 2020;Vidotto & Cleary 2020). The moderate ionizing fluxes (about 10 times of the current Sun's EUV flux) can also erode planetary atmospheres via production of photoelectrons that drive polarization electric field, the source of ion outflows from exoplanets (Airapetian et al2017;Garcia-Sage et al 2017).…”