2011 Annual Science Report

VPL at University of Washington Reporting  |  SEP 2010 – AUG 2011

Stellar Radiative Effects on Planetary Habitability

Project Summary

Habitable environments are most likely to exist in close proximity to a star, and hence a detailed and comprehensive understanding of the effect of the star on planetary habitability is crucial in the pursuit of an inhabited world. We looked at how the Sun’s brightness would have changed with time. We used models to study the effect of one very big flare on a planet with a carbon dioxide dominated atmosphere, like the early Earth’s, and found that these types of planets are well protected from the UV flux from the flaring star. We have also looked at the first quarter of Kepler data to study flare activity on “ordinary” cool stars, that have not been preselected for their tendency to have large flares. We find that these cool stars fall into two categories: stars that have long duration flares of several hours, but flare less frequently overall, and stars that have short duration flares, but more of them. In future work we will explore the comparative effect on a habitable planet of these two patterns of flaring activity.

4 Institutions
3 Teams
5 Publications
0 Field Sites
Field Sites

Project Progress

Last year, our group 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). This type of atmosphere is similar to what we expect from early Earth terrestrial planets in general. The results 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. (Figure 1). The initial stellar flux, corresponding to the stellar quiescent state, produces O2 and O3 via photolysis of CO2 that results in column depths for these compounds one order of magnitude larger than what is produced on a similar planetary atmosphere around the Sun. During a 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 (Fig. 1).

Figure 1. Temporal evolution of the surface ultraviolet flux on a planet around AD Leonis.. ​Surface UV flux at different times during a big flare from the M dwarf star AD Leonis. The planet is located at the habitable zone of AD Leonis (0.16 AU from the star). The atmosphere has 1 bar surface pressure and a bulk composition of 0.2 CO2 and 0.8 N2. The dotted line shows the UV flux at the surface of a planet with the same bulk atmospheric composition around the Sun.

While these calculations were a major step towards understanding how extreme stellar flares affect habitability, we only modeled a single event. Active stars flare many times during their (and their planetary companions’) lifetimes, and the rate at which flares occur differs amongst stars. In order to compute how a planetary atmosphere may be affected by multiple flare events, we must determine realistic flare rates and energy distributions for stars. Thus far, much of the work on stellar flare rates has focused on a few very active stars, whose flares are easily observed, but these represent only extremely active, young 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 in future work.

This year we also continued our study of the evolution of the Sun’s flux. Understanding changes in the solar flux over the age of the solar system is vital for understanding the evolution of planetary atmospheres, as the solar flux drives atmospheric chemistry and escape processes on the planets. We combined published data from the Sun and solar analogs to estimate enhanced far-UV (FUV) and X ray fluxes for the young Sun, described a new parameterization for the near-UV where both the chromosphere and photosphere contribute to the flux, and used Kurucz models to estimate variable visible and infrared fluxes. Our numerical parameterization for wavelength dependent changes to the nonattenuated solar flux is appropriate for most times and places in the solar system (Claire et al. 2011). This work may also be applied to other stars to represent the degree of chromospheric activity throughout the life of stars like the sun.

Studying the effect of planetary water abundance on the radiative limits of the habitable zone, Abe et al. (2011) examined habitable zone limits for dry planets where there is insufficient water to form a global ocean. They found that the habitable zone extends further inward toward the star than for planets with oceans. In this scenario, polar regions act as cold traps and help to maintain low humidity.