Task C: The Habitable Planet

This task addresses the nature and distribution of habitable worlds in the universe. Using observational data and models, the VPL team explores the interdependent effects of galactic, stellar, planetary processes 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.

Theoretical models enable the exploration of diverse, but plausible, exoplanetary environments in advance of more challenging observations. These models can address fundamental questions: How do habitable planets form and evolve? What is the diversity of terrestrial planetary environments? What are the characteristics of planetary systems that support habitable planets? Understanding which planets are more likely to be habitable informs the search for life, develops testable hypotheses, improves interpretation of observations, and informs future instrument development.

Factors Affecting Habitability

Planetary habitability is the outcome of the interplay of processes between the planet, planetary system, and host star, which we have studied extensively. These include gravitational interactions, which can affect volatile delivery during formation (e.g. Raymond et al., 2007), migration (Gillon et al., 2017; Luger et al., 2017b), tidal heating (e.g. Barnes et al., 2013; Barnes et al., 2018), orbital evolution (Deitrick et al., 2015; Barnes et al., 2015), spin state and obliquity (Deitrick et al.,2018a,b), and planet-planet interactions (Armstrong et al., 2014; Barnes et al., 2015); interaction between the planet and the host star’s spectrum and activity levels, which affect atmospheric and ocean loss (Raymond et al., 2006; Luger and Barnes, 2015; Luger et al., 2015, Lincowski et al., 2018), climate (Meadows et al., 2018, Lincowski et al., 2018), photochemistry and surface habitability (Segura et al., 2003, 2005, 2010; Arney et al., 2017); and interactions between aspects of the planet’s own environment, including its interior, atmosphere, and biosphere (Driscoll and Barnes, 2015). These interactions govern the planet’s evolutionary trajectory, and have a larger and more diverse impact on a planet’s habitability than its position in the habitable zone. For M dwarf habitable zone planets, models suggest that evolutionary processes may desiccate them early on (Luger and Barnes, 2015; Barnes et al., 2018), leaving behind O2-, N2- or CO2 -rich atmospheres, which can strongly affect climate. These evolutionary processes can produce uninhabitable planets in the habitable zone and leave tell-tale signs of evolutionary impact in their planetary spectra (Meadows et al., 2018, Lincowski et al., 2018). In this Task, we develop the coupled models required to simulate the formation, migration, evolution, and current atmospheric states of habitable zone terrestrial and volatile-rich planets with secondary outgassed atmospheres, providing the first multi-parameter, end-to-end picture of terrestrial planet habitability.

VPL research has made significant advances in understanding the factors and evolutionary processes affecting terrestrial planet habitability. In addition to the work described above, we have set the community’s habitable zone (HZ) standard (Kopparapu et al., 2013), and used 3D models to define M dwarf HZ limits (Kopparapu et al., 2016). We showed that M dwarf planets are less susceptible to glaciation, and may recover more easily (Shields et al., 2013, 2014), that high surface temperature CO2/H2O-rich atmospheres could exist within the HZ (Wordsworth and Pierrehumbert, 2013), and that multiple stable habitable or uninhabitable states could occur for a HZ water world (Goldblatt, 2015). We described how obliquity evolution can extend the range of habitable orbits (Armstrong et al., 2014) or eliminate habitability throughout the HZ (Barnes et al., 2015), depending on planetary architecture. We discovered that tidal heating could drive a runaway greenhouse on M dwarf HZ planets (Barnes and Heller, 2013) and may shut down the generation of a magnetic field (Driscoll and Barnes, 2015). We showed that stellar evolution might remove an M dwarf planet’s ocean early on (Luger and Barnes, 2015), or enable habitability for a migrated planet by removing its hydrogen shroud (Luger et al., 2015). We used our evolution modeling suite to assess the impact on Proxima b’s habitability due to stellar evolution, planet-planet perturbation, tidal heating, and interior and atmospheric evolution (Barnes et al., 2018). Meadows et al. (2018) used these evolutionary scenarios to generate self-consistent climate-photochemical states for a diversity of atmospheres, to assess potential habitability, generate thermal phase curves and spectra, and identify observational discriminants – such as collisionally induced absorption from O2 and strong CO2 bands – that might indicate the evolutionary trajectory and current state of the planet.

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. 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:

The Evolution of Terrestrial Planets

VPL has produced VPLanet, a software suite that simulates planetary system evolution, with a focus on habitability. Parameterized physical models, typically consisting of ordinary differential equations, are coupled together to simulate evolution over the entire age of a planetary system. These models currently include galactic tidal perturbations, stellar evolution and a planet's thermal atmospheric escape, the orbital evolution of the planetary system, including circumbinary planets (i.e. a planet orbiting around binary stars), planetary rotational axis evolution, tidal heating, interior radiogenic heating, and an energy balance climate model.

VPLanet has been used to comprehensively assess the potential for habitability of Proxima Centauri b, particularly its potential galactic evolution considering nearby stars Alpha Centauri A and B, tidal locking of b, and considering the atmospheric escape of any primordial atmosphere and the planet's potential internal thermal history (Barnes et al., 2018). This model has also been used in studies targeting the possibility of planets in binary star systems, demonstrating possible mechanisms for the lack of observed circumbinary planets (Fleming et al., 2018). Other orbital factors can affect habitability, such as the oscillation of a planet's obliquity (its axial tilt) and orbital eccentricity (Milankovitch cycles) and can induce a variety of climate states, which could curtial a planet's habitability in the habitable zone (Deitrick et al.,2018a,b).

Tidal effects on planetary habitability have been studied extensively by VPL. 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 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., 2013). 

Planetary Environmental States and Observational Discriminants

VPL leads the field in forward modeling terrestrial planet environments, their atmospheres, factors affecting habitability, biosignatures, and observational discriminants (spectral signatures). We include factors such as geological outgassing, biological surface fluxes, the wavelength-dependent reflectance of different surfaces and the associated effects on habitability and detection of biosignatures in reflectance spectra, photochemistry-induced changes to planetary atmospheres as a result of different host stars, including flares, and the affects of aerosols, such as condensed clouds and photochemical hazes, on both climate and detectability.

TRAPPIST-1 e from Lincowski et al., 2018

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.

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.

Repeated flares over time can completely depress the ozone abundance, particularly if the UV radiation of the flares is accompanied by the generation of protons, which can destroy ozone through NOx catalytic cycles (Tilley et al., 2019).

A Diversity of Planetary Atmospheres

Planet discoveries by Kepler and other campaigns have shown that planets very unlike those in our Solar System exist in the Galaxy. The large number of M dwarf stars and likely associated planets indicates that small planets unlike the bodies in our Solar System may be very abundant. VPL has considered different types of planetary environments, particularly those unlike Earth that could harbor life, and those that may not be habitable, but exist in the habitable zone.

In comprehensive studies of Proxima Centauri b (Meadows et al., 2018) and the seven-planet TRAPPIST-1 system (Lincowski et al., 2018), we have considered both O2- and CO2-dominated atmospheres that may form and be stable for these planets, even in the habitable zone. The dense O2 atmospheres could be false positives for life, but we have demonstrated observational discriminants, particularly O2-O2 collision-induced absorption, that may serve to identify these atmospheres (Schwieterman et al 2016). Venus-like planets may be a common evolutionary outcome (e.g. Kane et al., 2014, Meadows et al., 2018, Lincowski et al., 2018), and in these climate and spectral studies, we have shown how difficult it may be to identify these planets with JWST. Venus-like photochemical processes may be robust around M dwarf stars, but a planet like TRAPPIST-1 b – with only twice the insolation of Venus – may be too hot to condense Venus-like sulfuric acid aerosols, leaving open the possibility of a clear-sky Venus-like planet, which could be easily characterized by JWST (Lincowski et al., 2018).