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2015 Annual Science Report

VPL at University of Washington Reporting  |  JAN 2015 – DEC 2015

Stellar Effects on Planetary Habitability and the Limits of the Habitable Zone

Project Summary

In this task, VPL team members studied the interaction between stellar radiation (including light) and planetary atmospheres to better understand the limits of planetary habitability and the effects of stellar radiation on planetary evolution. Work this year spanned climate modeling to atmospheric escape. We showed that multiple stable states of climate could exist for water-rich worlds, including both habitable and uninhabitable states, suggesting that water-rich planets in the habitable zone are not necessarily habitable. Atmospheric escape models were used to illustrate how the pre-main sequence evolution of M-dwarf stars could strip the gaseous envelopes from mini-Neptune planets, transforming them into potentially-habitable, Earth-sized rocky bodies. We also showed that pre-main sequence evolution could lead to strong atmospheric escape of water on otherwise habitable worlds, potentially rendering them uninhabitable. We defined the first metric to rank an exoplanet’s potential to support surface liquid water based on fundamental data from transit observations. Observational work was also undertaken to characterize the frequency and characteristics of stellar flares on M dwarf stars from Kepler data, as input to future work on characterizing the effect of stellar flares on habitability.

4 Institutions
3 Teams
10 Publications
0 Field Sites
Field Sites

Project Progress

In this task, team members explored interaction of the host star’s radiation and wind with the atmospheres of habitable planets. In particular, we performed climate simulations with various assumptions, atmospheric escape driven by stellar radiation, and the frequency of flaring events on stars. These phenomena can significantly affect the possibility that life can arise and survive on exoplanets.

Barnes, Meadows, and Evans (2015) proposed a novel method to compare transiting exoplanets in terms of potential habitability. Their approach is markedly different from classic habitable zone calculations in two important ways. First, it is directly tied to transit observables, like transit duration, rather than just being a function of stellar mass and orbital distance. Second, it assigns a number to a planet that represents the likelihood of potentially habitable conditions, as opposed to the binarity of the habitable zone. This “habitability index for transiting exoplanets” (HITE) can be used to rank planets for their potential to support liquid surface water. The HITE value can then be used to prioritize exoplanets for follow-up observations, including spectral characterization by JWST.

Figure 1. HITE values for known KOIs as a function of the incident stellar radiation scaled to Earth's value, and assuming a circular orbit (S_circ). +’s indicate planets that may be uninhabitable due to too much absorbed energy, X's too little, and □’s indicate both limiting cases are possible. Symbol size is proportional to planetary radius. The locations of Venus, Earth, and Mars are labeled with a V, E and M, respectively. This new metric can be used to rank an exoplanet’s potential to support surface liquid water based on fundamental data from transit observations.

To perform a comprehensive assessment of habitability and in particular to calculate constraints on planetary orbits for newly discovered exoplanets, Barnes, Dietrick, Luger and Quinn worked on the development of the VPLanet framework, which can calculate the coupled effects of orbital, rotational, stellar, geophysical, atmospheric and climate evolution of planetary systems with potentially habitable planets.

VPL researchers also simulated the role of atmospheric escape from potentially habitable worlds. Lugar, Barnes, Fortney, Meadows, and collaborators demonstrated that small gaseous worlds (mini-Neptunes) in the habitable zones of red dwarfs may have their envelopes blown away by their host star, revealing a “habitable evaporated core” (Luger et al, 2015). Luger and Barnes (2015) also showed that because of the long pre-main sequence phase of M dwarfs, planets that currently reside in the habitable zones of these stars are likely to have experienced prolonged runaway greenhouses early on. During the runaway period, these planets could have lost most or all of their surface water via hydrogen escape, while building up large amounts of oxygen in their atmospheres.

Figure 2. Maximum amount of oxygen (in bars) that can accumulate in the atmosphere of an Earth-size planet as a result of the water loss. Planets in the habitable zones of all M dwarfs may accumulate hundreds to thousands of bars of photolytically-produced oxygen.
Figure 3. Maximum water loss (in Earth oceans) during the runaway greenhouse phase of an Earth-size planet in the habitable zone as a function of semi-major axis and stellar mass. Planets in the habitable zones of low mass M dwarfs can lose upwards of ten Earth oceans.

Team member Abbot studied climate stabilizing mechanisms for distant planets with H2-greenhouse atmospheres. The goal was to show that biological feedbacks could maintain habitable conditions on a planet with an H2-dominated atmosphere and greenhouse warming provided primarily by H2-H2 collision-induced absorption. The underlying assumption was that the biological processes are temperature-dependent such that there is a maximum rate of functioning at some temperature (and consequently functioning decreases at temperatures above or below this temperature). The implication of this study is that habitable conditions can potentially be maintained much further away than the traditional outer edge of the HZ (Abbot, 2015).

Team members Kasting and Kopparapu, along with an undergraduate student (Kasting et al. 2015), studied the viability of atmospheric water loss due to moist-greenhouse mechanisms. They showed that even though stratospheric temperatures can be well below 200 K on planets near the inner edge of the habitable zone, water vapor can still penetrate into the stratosphere as the surface temperature warms. Therefore, planets like Venus may indeed lose water through the ‘moist-greenhouse’ effect, and moist-greenhouse may, for all practical purposes for habitability, be the inner edge of the HZ.

We also showed that multiple stable states of climate could exist for a water world, including both habitable and uninhabitable states, at the same level of incident radiation, suggesting that water-rich planets in the habitable zone are not necessarily habitable (Goldblatt 2015). Zahnle and colleagues explored conditions under which a wet, Earth-like habitable planet can evolve into a dry, Dune-like habitable desert planet. Desert planets have broader habitable zones than ocean planets like Earth, and hence their origins and evolutions are pertinent to characterizing habitable zones in general (Kodoma et al. 2015).

In progress work by team members Haqq-Misra and Kopparapu used a 3-D climate model to examine the role of geothermal heating on planets orbiting M-dwarfs. They found that geothermal heating can enhance asymmetric distribution of energy transport to the night side of the planet, melting the ice near the anti-stellar point and potentially create an additional habitable area on the night-side. They examined the presence of a cross-polar circulation that transports energy and mass from the sub-stellar to anti-stellar point across the northern and southern poles. This contributes toward maintaining climate stability and avoiding atmospheric freeze-out with a circulation pattern atypical of those observed on Earth.

In other work in progress, Lincowski, Meadows, Robinson and Crisp examined the radiative effect of CO2 ice clouds on the outer edge of the habitable zone, using the VPL’s new 1-D RCE general purpose climate model. They confirmed that CO2 clouds don’t necessarily warm the planetary surface, and are calculating the effect that CO2 cloud cooling will have on the outer habitable zone limits.

Team member Hawley and collaborators examined the flaring and activity of M dwarfs stars in the Kepler field, including the long-term tracking of star spots (Davenport et al. 2015), as well as quantifying the flare rates for M dwarfs as a function of rotation period (Lurie et al. 2015). Tilley, Meadows and Hawley are currently using observed flare sequences provided by Hawley and Davenport to explore the impact of multiple flares on the photochemistry and surface UV flux of an Earth-like planet orbiting an M dwarf.