Transit observations of exoplanets around bright host stars, together with advanced numerical modelling techniques and known astrophysical parameters (such as the host-star age and radiation environment), offer a unique tool for understanding the interaction of exoplanet upper atmosphere with the magnetospheres of their host stars. Hubble Space Telescope (HST) UV transmission spectroscopy and Spitzer secondary eclipse measurements of known, bright exoplanetary systems have been used to study a number of issues related to the upper atmospheres of exoplanets, including: space weather events (e.g. Lammer et al. 2011a; Lecavelier des Etangs et al. 2012); thermospheric structure (e.g. Koskinen et al. 2012); the exosphere-magnetosphere-stellar plasma environment (Holmström et al. 2008; Ekenbäck et al. 2010; Llama et al. 2011); outflow of planetary gas including atomic hydrogen (Vidal-Madjar et al. 2003; Ben-Jaffel 2007; Ben-Jaffel & Hosseini 2010; Ehrenreich et al. 2012), and heavy species such as carbon, oxygen and metals (Vidal-Madjar et al. 2004; Linsky et al. 2010; Fossati et al. 2010).
Moreover, transiting Earth-like or super-Earth exoplanets orbiting bright M-stars detected by PLATO can be used as a proxy for the early history of solar system planets such as Venus, Earth, and Mars. When young, these planets faced a much harsher UV radiation environment than today, closer to that produced by active M-dwarf stars (Lammer et al. 2011b; Lammer et al. 2012). PLATO detections of such terrestrial planets will allow for the study of EUV heated and extended upper atmospheres around Earth-type exoplanets follow up observations, as has already been demonstrated for other classes of exoplanets (e.g. Ehrenreich et al. 2015). The expected results of such observations are essential for testing early terrestrial atmosphere evolution hypotheses (Erkaev et al. 2013; Kislyakova et al. 2013).
To really examine the complex physics involved in the interaction of close-in exoplanets with their host stars, PLATO’s contribution of bright nearby systems will be essential. UV transmission spectroscopy, particularly examining Lyman alpha, is key for studying mass loss from hot Jupiters. Owing to the present lack of bright target stars, only four exoplanets (HD 209458b, WASP-12b, HD 189733b, and 55 Cancri b) are able to be studied in detail. WASP-12b is a particularly interesting and extreme hot Jupiter, but is too distant to be studied at Lyman alpha with the HST. By using the near-UV where the host star is much brighter, mass loss from WASP-12b was able to be detected (Fossati et al. 2010; Haswell et al. 2012), but without Lyman alpha data the quantitative comparison of the mass loss rate with models is uncertain. PLATO is expected to find hundreds of exoplanets orbiting nearby, bright stars, and follow-up of these discoveries will produce a step-change in our ability to probe the processes governing the catastrophic end-point of hot Jupiter evolution. Such observations will fundamentally improve our knowledge of the exoplanet-upper atmosphere-magnetosphere environments, and future space observatories such as the World Space Observatory-UV (WSO-UV) (Shustov et al. 2009, 2011; Gómez de Castro et al. 2011) will be able to take advantage of this legacy.
PLATO will also detect small planets (mini-Neptunes, Rplanet~2‒4 RE) around A-stars. However, it might be that mini Neptunes do not exist at small distances for these stars, if the XUV-radiation is strong enough to erode the gaseous envelope. The minimum distance at which mini Neptunes are detected around A-stars therefore constrains the erosion of planetary atmospheres for stars with extreme environments.
Among the surprising findings from Kepleris the existence of a number of extremely close-in rocky bodies orbiting their host stars at periods of less than a day. Kepler-78b is an Earth-sized planet in an 8.5h orbit (Sanchís-Ojeda et al. 2013), Kepler-42c is a sub-Earth sized planet in an 11h orbit (Muirhead et al. 2012), and KIC 12557548b appears to be a disintegrating, Mercury-like object in a 16h orbit (Rappaport et al. 2012). These objects are fascinating from an evolutionary point of view, and may be remnant cores of hot Jupiter analogues to WASP-12b that are losing mass. Alternatively, they may have been rocky bodies throughout their evolution. In either case these objects, in particular KIC 12557548b with its prodigious mass loss, give an unprecedented opportunity to study the composition of exo-rocks through transmission spectroscopy. Unfortunately, the Keplerdiscoveries are distant, and hence the signal-to-noise of any follow-up observations will limit the scope of the inferences we can draw from them. PLATO will find many similar systems around nearby, bright targets.