Determining habitability: which exoEarths should we search for life?

Abstract

Within the next few years, the first Earth-mass planets will be discovered around other stars. Some of those worlds will certainly lie within the classical ' habitable zone ' of their parent stars, and we will quickly move from knowing of no exoEarths to knowing many. For the first time, we will be in a position to carry out a detailed search for the first evidence of life beyond our Solar System. However, such observations will be hugely taxing and time consuming to perform, and it is almost certain that far more potentially habitable worlds will be known than it is possible to study. It is therefore important to catalogue and consider the various effects that make a promising planet more or less suitable for the development of life. In this work, we review the various planetary, dynamical and stellar influences that could influence the habitability of exoEarths. The various influences must be taken in concert when we attempt to decide where to focus our first detailed search for life. While there is no guarantee that any given planet will be inhabited, it is vitally important to ensure that we focus our time and effort on those planets most likely to yield a positive result.

THE McDONALD OBSERVATORY PLANET SEARCH: NEW LONG-PERIOD GIANT PLANETS AND TWO INTERACTING JUPITERS IN THE HD 155358 SYSTEM

“…Furthermore, any satellites sufficiently large to be considered habitable would also be subject to significant tidal heating from their host planet, which would likely act to increase their habitability when they lie toward the outer edge of Kasting's habitable zone (as they would have earlier in the life of the system). The induced tectonics would also potentially improve the habitability of any such moons (e.g., Horner & Jones 2010). Our dynamical analysis suggests that the two planets in the HD 155358 system are most likely trapped in mutual resonance.…”

“…In fact, the presence of two giant planets on relatively close-in orbits may indirectly increase the water content on terrestrial satellites through radial mixing of planetesimals rich in ices (Mandell et al 2007), a key requirement for such objects to be considered habitable (e.g., Horner & Jones 2010). Furthermore, any satellites sufficiently large to be considered habitable would also be subject to significant tidal heating from their host planet, which would likely act to increase their habitability when they lie toward the outer edge of Kasting's habitable zone (as they would have earlier in the life of the system).…”

The Anglo-Australian Planet Search Xxiv: The Frequency of Jupiter Analogs

“…Jupiter also hosts a large population of Trojan asteroids (Fornasier et al 2007;Vinogradova & Chernetenko 2015) and irregular satellites, both of which are thought to have been captured during the giant planet's migration (e.g., Sheppard & Jewitt 2003;Morbidelli et al 2005;Jewitt & Haghighipour 2007;Lykawka & Horner 2010). The planet has even been put forward as having played a key role in the volatilization of the terrestrial planets, driving the injection of a late veneer of volatile material to the inner solar system (Owen & Bar-Nun 1995;Horner & Jones 2010), and helps to drive periodic climate change on the Earth, in the form of the Milankovitch cycles (Hays et al 1976;Horner et al 2015).…”

Herschel imaging of 61 Vir: implications for the prevalence of debris in low-mass planetary systems

“…The delivery of debris to the inner planets may also be an important source of volatiles to the planets (e.g. Horner & Jones 2010). Furthermore, it is reasonable to expect that the initial conditions in protoplanetary discs that favour the formation of certain types of inner planetary system might also result in (or exclude) specific types of debris disc (e.g.…”