Research

 
Current Research:

My research focuses on exploring the possible climates and potential habitability of planets orbiting in multiple-planet systems around low-mass stars. Smaller, lower-mass stars are not only more likely to host multiple planets per star, but also smaller planets within their systems. This means that these types of systems may be the first that we look at in our search for another habitable planet like the Earth.

I use computer models as well as actual data from recently-discovered planetary systems to determine the atmospheric and orbital conditions necessary for surface liquid water to exist on the most promising candidates for habitable planets in these systems.


Dissertation work:

My PhD research focused on the effects on climate and habitability of interactions between a star and an orbiting planet. In graduate school I became fascinated by the Snowball Earth episodes of ~600 and ~800 million years ago, where our planet may have been covered in ice from Pole to Pole. If someone had been able to observe our planet from space during these episodes, it may have looked something like this:

snowball_earth

An artist’s impression of Snowball Earth.

 

As an astronomer, I wondered if something like Snowball Earth could happen on planets orbiting stars other than our Sun, which we call Extrasolar Planets. For my dissertation I explored the effect of the spectrum of a host star on the climate of a planet. I found that the interaction between the host star’s spectrum and an orbiting planet’s atmosphere and surface will affect the manner in which the planet achieves global energy balance through a combination of reflected, absorbed, and/or emitted shortwave (incoming from the star)) and longwave (outgoing from the planet) radiation. 

For example, here is a schematic showing the global energy balance of the Earth around the Sun:

Shields_Sun_schematic

Schematic diagram of global mean energy balance for an aqua planet orbiting the Sun (Shields 2014).

You can see that about 30% of the light the Earth receives from the Sun is reflected back to space.

Now here is a similar diagram with an F-dwarf star as the host star, instead of the Sun:

Shields_F_schematic

Schematic diagram of global mean energy balance for an aqua planet orbiting an F-dwarf star (Shields 2014).

With an F-dwarf star as the host star, much more radiation is reflected by the planet. This is largely because F-dwarf stars emit more near-UV radiation than stars like the Sun (as seen below).

Shields_seds

The spectral energy distributions for F-, G-, K-, and M-dwarf stars, normalized by their peak flux (Shields et al. Astrobiology, 2013).

And ice and snow on the surfaces of planets reflects strongly in the visible and near-UV, while they absorb strongly in the near-IR:

Shields_albedos

The spectral distribution of fine-grained snow, blue marine ice, and 25%, 50%, and 75% mixtures of the two end-members. Ocean and land spectral distributions are also plotted (Shields et al. Astrobiology, 2013).

So a schematic diagram of a planet around an M-dwarf star, which emits strongly in the near-infrared tells a different story:

Shields_M_schematic

Schematic diagram of global mean energy balance for an aqua planet orbiting an M-dwarf star (Shields 2014).

In this case, a large fraction of the incoming radiation the M-dwarf planet receives from its star is absorbed by CO2 and water vapor in the planet’s atmosphere, and ice and snow on its surface.

We used a hierarchy of climate models to show that M-dwarf planets are harder to freeze, and easier to thaw out of global ice cover (Snowball states):

Shields_paper2_plot

Mean ice line latitude as a function of stellar flux for an aqua planet orbiting an M-, G-, and F-dwarf star (Shields et al. Astrophysical Journal Letters, 2014).

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