TIPO SUBJETIVO

While liquid water is undoubtedly an important factor when determining the habitability of a planet, there are other factors that must also be considered (e.g. Lammer et al., 2009; Arm- strong et al., 2016). For instance, the planetary mass (Kopparapu et al., 2014), the presence of tectonic activity (Jellinek & Jackson, 2015) and the presence of a gas giant within the plan- etary system (Laakso et al., 2006) may all affect the development of life on any given planet. Of course, this is by no means an exhaustive list of factors affecting planetary habitability.

In the previous section, I discussed the role that the planetary atmosphere plays in regu- lating surface temperatures. It is clear that the ability for a planet to retain an atmosphere is key to habitability. One method by which atmospheric loss can occur is hydrodynamic escape (e.g. Murray-Clay et al., 2009). Another method is erosion of the atmosphere by sufficiently strong stellar winds, flares and coronal mass ejections (Khodachenko et al., 2007; Lammer et al., 2007; Zendejas et al., 2010; Vidotto et al., 2011; Lammer et al., 2012). If enough of the planetary atmosphere is eroded away, the planet would be rendered uninhabitable. Earth has retained its atmosphere thanks to the shielding provided by its magnetosphere. In contrast,

exoplanets

θoval rms θoval rms

Figure 5.2: An illustration of how the auroral oval angle of a planet (the angle between the pole and the last open field line),_θoval, is affected by magnetosphere size,r_ms. As a crude first order estimate,

θovalcan be approximated by truncating a dipole to the size of the magnetosphere. In the figure, closed

field lines are shown with solid lines while open field lines are shown with dotted lines. The figure clearly shows smaller values ofθ_ovalfor larger magnetospheres.

Mars and Venus both currently lack a substantial intrinsic magnetic field. As a result, both suf- fer significant atmospheric losses with Mars having a much thinner atmosphere (Wood, 2006; Edberg et al., 2010, 2011). As a general rule, larger magnetospheres offer better atmospheric protection for two reasons. Firstly, it is more difficult for stellar winds to penetrate into the atmosphere at the substellar point, or “nose", of the magnetosphere. Secondly, the auroral oval, through which mass-loss can always occur is minimised for larger magnetosphere sizes (see Fig. 5.2).

In this chapter, I assess the ability of exoplanets, around solar-type stars, to maintain magnetospheres similar in size to both the present day and early Earth’s magnetospheres. The thermal plasma pressure, wind ram pressure, and stellar magnetic pressure all act to compress the exoplanetary magnetosphere. Vidotto et al. (2013) have studied how the stellar magnetic pressure affects hypothetical Earth analogues around M dwarfs. Compared to solar- type stars, M dwarfs have close-in HZs and can possess much stronger magnetic field strengths (Donati et al., 2008a; Morin et al., 2008b, 2010). The stellar magnetic pressure is therefore the dominant pressure term of the wind in the HZ. In contrast, the ram pressure dominates in

5.1. Introduction

the HZ of solar-type stars (Zarka et al., 2001; Zarka, 2007; Jardine & Collier Cameron, 2008) and so I only study the stellar wind ram pressure term in this chapter.

In order to study the interaction between stellar winds and exoplanets, I use a survey of 167 stars observed by the Bcool collaboration (Marsden et al., 2014). After excluding the subgiants and any stars that did not have all the data required for the wind models I employ, 124 solar-type stars remained, mostly with masses between 0.8M and 1.4M. I refer the reader to Marsden et al. (2014) for full details of the sample. I will assume the existence of a fictitious exoplanet orbiting in the HZ of each star in this sample. Unfortunately, typical exoplanetary magnetic field strengths are not known since there have been no direct observations of exoplanetary magnetic fields to date, although Vidotto et al. (2010) and Llama et al. (2011) hint at a possible indirect detection. In light of this, I assume that the fictitious planets have the same properties as Earth, i.e. same mass, radius, and magnetic field strength. For each Earth analogue, I calculate the ram pressure exerted on it and determine if it can maintain a present day Earth-sized magnetosphere.

Lammer et al. (2007) suggest that smaller magnetospheres may still offer adequate pro- tection and it is thought the Earth had a smaller magnetosphere in its past as a result of higher solar activity (Sterenborg et al., 2011). Tarduno et al. (2010) report that the Earth had a geo- dynamo around 3.4 Gyr ago, during the Paleoarchean, which generated a magnetic field that was roughly 50% weaker than the present day’s. Using this field strength and the empirical wind model of Wood (2006), Tarduno et al. (2010) estimate a magnetosphere size of around 5 RE. Since the Earth was able to retain its atmosphere, it is reasonable to assume that a

Paleoarchean sized magnetosphere would sufficiently protect an Earth analogue. However, the magnetospheric size estimate of Tarduno et al. (2010) is dependent on the wind model adopted. I discuss the range of possible Paleoarchean magnetosphere sizes using different models in section 5.3.

The rest of this chapter is structured as follows. Section 5.2 covers the details of the wind models used. Section 5.3 covers the results obtained using the models outlined in the previous section and their broader implications within the context of other works. A discussion and concluding remarks follow in Sect. 5.4.

exoplanets