Nearly half of the known exoplanets orbit within 0.1 au from their star. At such close distances, the nature and evolution of these planets is shaped by interactions with their host star (irradiation, tidal effects, and magnetic fields). In particular, the deposition of stellar X-ray and extreme ultraviolet radiation (XUV) into an exoplanet upper atmosphere can lead to its hydrodynamic expansion and substantial escape (e.g. Lammer et al. 2003; Vidal-Madjar et al. 2003; Lecavelier des Etangs et al. 2004; Yelle 2004; García Muñoz 2007; Koskinen et al. 2010; Johnstone et al. 2015). Atmospheric loss is considered as one of the main processes behind the deficit of Neptune-mass planets at close orbital distances (the so-called hot Neptune desert, e.g. Lecavelier des Etangs 2007; Davis & Wheatley 2009; Szabó & Kiss 2011; Lopez, Fortney & Miller 2012; Beaugé & Nesvorný 2013; Lopez & Fortney 2013; Owen & Wu 2013; Jin et al. 2014; Kurokawa & Nakamoto 2014; Lundkvist et al. 2016). These planets are large enough to capture much of the stellar energy, but in contrast to hot Jupiters are not massive enough to retain their escaping atmospheres (e.g. Hubbard et al. 2007; Lecavelier des Etangs 2007; Ehrenreich, Lecavelier des Etangs & Delfosse 2011). The missing hot Neptunes could have lost their entire atmosphere via evaporation, evolving into bare rocky cores at the lower radius side of the desert (e.g. Lecavelier des Etangs et al. 2004; Owen & Jackson 2012). This scenario is strengthened by the recent observations of warm Neptunes at the border of the desert, on the verge of (Kulow et al. 2014; Bourrier, Ehrenreich & Lecavelier des Etangs 2015; Ehrenreich et al. 2015; Bourrier et al. 2016; Lavie et al. 2017) or undergoing (Bourrier et al. 2018b) considerable mass-loss. Because they survive more extreme conditions than lower mass gaseous exoplanets, hot Jupiters are particularly interesting targets to study star–planet interactions. Their upper atmosphere can be substantially ionized because of stellar photoionization (e.g. Schneiter et al. 2016), which could help the formation of reconnections between the stellar and planetary magnetospheres that would enhance stellar activity (e.g. Cuntz, Saar & Musielak 2000; Shkolnik, Walker & Bohlender 2003; Ip, Kopp & Hu 2004; Shkolnik et al. 2008; although see Poppenhaeger & Schmitt 2011; Scandariato et al. 2013; Llama & Shkolnik 2015 for the difficulties to detect such signatures). Atmospheric escape of neutral hydrogen and metal species has been detected via transmission spectroscopy for several Jupiter-mass planets, bringing information about their upper atmosphere and the stellar environment (HD 209458b, Vidal-Madjar et al. 2003, 2004, 2008; Ehrenreich et al. 2008; Ben-Jaffel & Sona Hosseini 2010; Linsky et al. 2010; Schlawin et al. 2010; Vidal-Madjar et al. 2013; Ballester & Ben-Jaffel 2015; HD 189733b, Lecavelier des Etangs et al. 2010, 2012; Bourrier et al. 2013; 55 Cnc b, Ehrenreich et al. 2012; WASP-12b, Fossati et al. 2010; Haswell et al. 2012). Shocks could for example form ahead of hot Jupiters because of the interaction between the stellar wind and the planetary outflow or magnetosphere (Vidotto, Jardine & Helling 2010; Cohen et al. 2011; Llama et al. 2013; Tremblin & Chiang 2013; Matsakos, Uribe & Königl 2015)
Citation: Kulow et al. 2014
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