Task C: The Habitable Planet

This task addresses the nature and distribution of habitable worlds in the universe. Using observational data, models, and orbital dynamics, the VPL team explores the interdependent effects of galactic, stellar, and planetary environments on planetary habitability. These include gravitational interactions between the Galaxy and planetary system, gravitational and radiative interaction between the planet, planetary system and host star; and interactions within elements of the planet’s own environment, including the planetary interior, atmosphere and biosphere.

While habitability studies have widespread applicability to planetary systems orbiting stars of several spectral types, we focus significantly on terrestrial-sized planets orbiting M dwarfs, as these are the first planets to be found in the habitable zone (e.g. Proxima Centauri b, TRAPPIST-1 e, f, and g, and LHS 1140 b), and potentially amenable to telescopic follow-up observations, particularly by the James Webb Space Telescope.

The VPL has made extensive progress on understanding the diverse phenomena and interactions that affect habitability, particularly for M dwarf planets. These include the effect of incident stellar radiation on the planet (insolation or instellation) as a primary metric for habitability (Kasting et al. 1993; Segura et al. 2005; Abe et al. 2011); the role of stellar flares in atmospheric retention and chemistry (Segura et al. 2010); and the formation of planets in a mass range to support habitability (Raymond et al. 2004, 2007; Barnes et al. 2009). We have also reanalyzed the role of stellar gravitational effects on planetary habitability (Barnes et al. 2008, 2009, 2012; Bolmont et al 2011) and studied the orbital evolution and stability of multiplanet systems (Barnes and Quinn, 2001, 2004; Kopparapu et al. 2009; Raymond et al. 2009).

Topics that we are continuing to explore include planet formation around low-mass stars, the role of stellar migration through the galaxy, the orbital stability of planetary systems, and studying exoplanet habitability using a diverse set of climate and photochemistry models. These studies will help our team form an interdisciplinary framework for assessing habitability that we will use to prioritize newly-discovered planets for spectroscopic follow-up, to search for signs of habitability and life.

Recent VPL scientific highlights from this research area include: 

Stellar Radiative Effects on Planetary Habitability

We have calculated the effects of a large flare from a cool M dwarf star (AD Leonis) on an Earth-like planetary atmosphere (Segura et al. 2010). A similar approach was applied to atmospheres with 0.2 bars of CO2 for an abiotic planet, with no biogenic surface fluxes (Sánchez-Flores and Segura, 2011). The results from these “early Earth” type atmospheres suggest that planets around active M dwarfs with CO2 atmospheres may be better protected from the UV radiation of their parent stars than planets around sun-like stars even without the presence of biologically produced O2. This is because the quiescent flux from the star photolyzes CO2 to produce O2 and O3 with column depths an order of magnitude larger than for an Earth-like planet around the Sun. During a stellar flare the abundances of O2 and O3 do not change significantly, and the UV stellar radiation that reaches the planetary surface is less than that for a similar planet around the Sun.

In addition to modeling the effect of extreme flares on planetary habitability and UV surface fluxes, we are also working to understand the typical flaring rate for different types of stars.   We used the first quarter of Kepler observations to identify flare events in the cool K and M dwarfs in the Kepler sample, stars that were not preselected for their level of activity. We found many such events and, most interestingly, found that stars with long-duration flares (four hours or more) spend less time flaring overall (Walkowicz et al. 2011). This indicates that some planets may be subjected to less frequent but longer duration changes in the spectral energy distribution of the stellar flux, while others experience more frequent but short duration changes. We intend to explore the comparative effects of these different flaring behaviors on planetary habitability in future work.

Dynamical Effects on Planetary Habitability

One principal VPL scientific focus is the relationships between orbital properties of planetary systems and habitability. In addition to radiative effects on planetary habitability, including climate effects and the effects of flares on UV surface habitability, we also explore tidal effects, the gravitational interaction between a planet and its parent star.   We have pioneered the concept of “The Tidal Habitable Zone”, a region around an M dwarf in which a planet does not gain too much or too little planetary heating due to tidal effects (Barnes et al., 2008), as these planets with significant tidal heating can be forced into a runaway greenhouse within the classical habitable zone for up to a billion years (Barnes et al., 2013). We have also explored the effects of tides on planetary obliquity (Heller et al., 2011) and determined that the spin properties of habitable planets can be modified by tides if the planets have large eccentricities, even for planets orbiting in the habitable zone of solar-mass stars where tidal forces are typically weaker. For brown dwarfs we find that planets have relatively short residence times in the habitable zone and hence have a decreased likelihood for habitability (Bolmont et al., 2011). 

We have explored the role of orbital architecture on the obliquity evolution of exoplanets in systems with large mutual inclination (Barnes et al., 2011). These planets may undergo wild obliquity swings (Armstrong et al., 2010), which actually increase the width of the habitable zone by suppressing the ice-albedo feedback at the outer edge (Domagal-Goldman et al., 2011). We are also exploring the possibility that tidal heating may be strong enough on planets orbiting low-luminosity M dwarfs that a runaway greenhouse is triggered on planets in the traditional radiative habitable zone (Barnes et al., 2012). 

VPL team members also proposed that Jupiter performed an inward-then-outward migration which can explain the masses and orbits of the terrestrial planets (crucially explaining the mass of Mars) as well as the asteroid belt (Pierens and Raymond, 2011; Walsh et al., 2011). We also examined how orbital migration of our Sun through the galaxy may have impacted the structure of the Kuiper Belt. In particular we find that the orbit of Sedna has a 25% chance of resulting from this migration. (Kaib et al., 2011). 

Planetary Environmental States and Observational Discriminants

Flares aren't the only stellar barrier to habitabilility, particularly for M dwarfs. We have demonstrated that the high luminosity of young M dwarfs is likely to result in a prolonged runaway greenhouse state for up to a billion years for planets that are discovered in the habitable zone, potentially dessicating the planet and resulting in a substantial, abiotic O2-dominated atmosphere (Luger and Barnes, 2015). However, this same effect could strip the gaseous envelopes from migrating mini-Neptunes, transforming them into potentially-habitable, Earth-sized rocky bodies (Luger et al., 2015).

As likely terrestrial planets have recently been discovered in the habitable zones of nearby M dwarf stars, VPL has turned its terrestrial modeling framework to actual planetary assessments to predict observational discriminants that could be observed in the coming years by JWST. The discovery in 2016 of Proxima Centauri b led to comprehensive work considering the various environmental states that could be possible and how JWST may be able to observe the planet's atmosphere and distinguish between the plausible environments (Meadows et al., 2018).