How new faculty member Emily Rauscher built an astrophysics career in the place she least expected
This fall, Michigan Astronomy welcomed exoplanet specialist Emily Rauscher, PhD, to its faculty. Rauscher brings valuable dimension to the department with a background rich in astrophysics and planetary science, as well as experience building 3D atmospheric circulation models that help bring these exotic planets to life.
In her various projects, Rauscher’s helped reveal the impact of magnetic fields on these alien worlds, plumbed their deep atmospheres, and suggested new observational techniques that could chip away at their mysteries. She even cut her teeth doing research under one of the country’s most prolific exoplanet hunters.
So it was surprising to learn that she began in the field certain she could embrace almost any aspect of it – except exoplanets.
“I’d known since high school that I had an interest in astronomy,” says Rauscher. “One summer, my parents told me I could take a class or get a job, and the choice was easy. I took an introductory astronomy class at UC/Berkeley. I think I highlighted my entire textbook.”
So when she started her undergraduate work, also at Berkeley, she was interested in using physics to understand why various astronomical objects behaved as they did – as long as they weren’t exoplanets. “At that point, I thought of them as these featureless things we knew existed around various stars” says Rauscher. “So even when I got the chance to do my senior thesis with a well-known exoplanet hunter, I didn’t even work on them. What finally drew me into the field was realizing we could measure properties of these planets – how hot they are or what their atmospheres are like. That ability to make an exoplanet a real place is what really excited me.”
Rauscher studies hot Jupiters, gas giants that sit close to their parent star. They are currently our best targets for atmospheric characterization.
Image credit: NASA, ESA, and G. Bacon (STScI).
This change of heart came upon Rauscher during graduate school. Like Michigan, Columbia University’s graduate program encourages students to sample different one-year projects before choosing a thesis topic. Assuming she’d become an observer, Rauscher thought she might have some fun with a theoretical project before settling on her “real” direction. So she chose a project trying to use an astronomical technique called “eclipse mapping” to create a simulated image of the day side of an exoplanet.
The technique had been used in various areas of astronomy, but not yet applied to exoplanets. It works by measuring the change in light we see as a transiting planet moves behind its star. “If you watch the light coming from the system as a function of time,” says Rauscher, “you see this little dip in light, which was the light from the planet. Then, if you zoom in on that part of the measurement, the exact shape of how the light decreases tells you how much light had been coming from those different regions of the planet that are slowly being blocked off as the planet moves behind the star.”
Her group applied the technique to hot Jupiters, gas giants that sit closer to their host star than Mercury does to our sun. As the most easily observable exoplanets, hot Jupiters are the best targets for atmospheric characterization. The group ran several models, creating images that showed which parts of a planet would be hotter and brighter than others.
A key question was whether current instruments were good enough to measure the detailed shape of the light curve required to constrain the models. What they learned was that the field was right on the cusp of such precision. While it might be possible with Spitzer, they said, the forthcoming James Webb Space Telescope would easily be able to gather these measurements for the closest, hottest exoplanets.
Their assessment was confirmed a few years later, when a group used eclipse mapping to observe HD 189733b, Earth’s nearest hot Jupiter.
After this project, Rauscher began exploring whether astronomers could get information that would help them not just create a static temperature map of an exoplanet, but determine how variable its temperature patterns were. “One set of models we were looking at suggested some planets might have a lot of variability, with big, cold vortices roaming around their poles,” she says. To constrain these models, her group considered the value of observing a planet not just as it passed behind its star but across several orbits to see how much its light levels changed over time. Unusual shapes in these so-called “orbital phase curves,” they concluded, could indicate dramatic weather patterns sweeping cold spots into and out of view. As before, other groups were later able to use Spitzer to make these observations for a handful of planets.
By the time Rauscher was ready to embark on her thesis, she was hooked on the subject. So she decided that instead of working with others’ models, she’d set out to develop her own 3D computer simulation of hot Jupiters’ atmospheres. Starting with a code developed for Earth, she began adapting it and by the end of her graduate program had a working model.
Rauscher's 3D computer simulations of hot Jupiters’ atmospheres allow her to address questions such as how a planet's atmosphere may respond to a magnetic field.
Image credit: Emily Rauscher
She would continue to refine this model during her postdoc period, while adding new knowledge and methods to her repertoire. With the support of a NASA Carl Sagan Postdoctoral Fellowship, Rauscher decided to augment her astrophysics background with two years at the University of Arizona’s Department of Planetary Sciences in order to discover, she says, “the fundamental ways we learn about planets” – something generally not covered in astronomy curricula.
One of her projects involved working with planetary atmosphere expert Adam Showman, PhD, to study the deep atmospheres of hot Jupiters “These parts of the atmosphere are very unique,” says Rauscher. “In our solar system the atmospheres of the giant planets are quite thin. But for hot Jupiters they’re thick – and there’s this deep layer from which no light is emitted. Depending on the structure of that deep atmosphere, it might influence global properties of the planet in subtle but interesting ways. But because it doesn’t exist anywhere else we know of, we had to create a new model that could treat this strange, invisible part of the planet.”
Rauscher says she enjoyed the project because it was her first chance to delve into how all parts of the planet – the atmosphere, the deep atmosphere, and the interior – interact, forming a unique whole.
Though Sagan is a three-year fellowship, Rauscher had organized to spend two years at Arizona, followed by another two at Princeton – one of them supported through the university’s local Spitzer Fellowship. Princeton’s strength in theoretical astrophysics was an enticing complement to her prior work, and she used the opportunity to delve deeper into a topic she’d considered earlier: magnetic effects in hot Jupiters’ atmospheres.
The atmospheres of these planets can be so hot they become slightly ionized, says Rauscher. With enough charged particles, an atmosphere may begin to respond to a planet’s magnetic field. And this can have consequences for a planet’s temperature structure, its wind patterns – even its size. “A lot of hot Jupiters are larger than we would expect them to be,” says Rauscher, “implying there’s some mechanism heating the planet.”
To see if magnetism might be this mechanism, Rauscher and her collaborators set out to both model its effects and to consider what types of observations might illuminate them. Using a piece of code that could turn on or off a magnetic field, they found that the field could have a profound effect on certain planets – specifically those hot enough to be sufficiently ionized.
While Rauscher says our understanding of magnetic effects is still in its early stages, it’s become an essential modeling consideration for hot Jupiters. But to progress, modelers still needed observational data with which to test their work. To address this, Rauscher teamed up with a fellow postdoc to explore another observational method for exoplanet atmospheres – planets’ Doppler spectral shift.
Rauscher identifies observational techniques that can provide important data on exoplanet atmospheres, such eclipse mapping, orbital phase curves, and Doppler spectral shift.
Image credit: Toward Eclipse Mapping of Hot Jupiters. Emily Rauscher et al. 2007 ApJ 664 1199.
Rauscher explains the concept this way: When an exoplanet transits its star, it blocks a small portion of the star’s light and filters another small portion through the planet’s atmosphere. This filtered portion has a spectral signature, which can be separated from that of its parent star. As the planet moves variously toward and away from us in its orbit, the planet’s spectrum will be Doppler-shifted, becoming slightly bluer as it approaches and slightly redder as it moves away. And while most of this shift represents the planet’s orbit, a small amount reflects the motion of the planet’s atmosphere – a combination of how the atmosphere is rotating with the planet and how the winds are blowing.
Using Rauscher’s 3D atmospheric simulations and her colleagues’ detailed radiative transfer model, the pair calculated hot Jupiters’ expected Doppler shifts under various conditions. They were the first to come out with results on the topic, showing that with just slightly improved observations, this measurement could help distinguish which planets’ winds were being slowed by magnetic drag. They also found that it might be useful, when paired with supplemental measurements, in constraining planets’ rotation rates.
Rauscher is quick to share credit with strong advisors and collaborators for the substantial progress she’s made. She also credits some of it to her modeling approach. “I tend to work on models at intermediate levels of complexity,” she says. “The goal is to capture enough of the physics to accurately reflect reality, while leaving things sufficiently clear to get a handle on the underlying processes.”
And while she says she will always love hot Jupiters, she is eager to take advantage of better instrumentation and new exoplanet discoveries to apply her models to planets that are smaller and farther from their host stars.
She feels Michigan is an excellent place to do it. “This department is excited about exoplanets, so they’re really supportive of my work,” she says. “And there are so many people doing complementary work: John Monnier on the observational side; Fred Adams, who’s worked extensively on exoplanets; and a huge Atmospheric, Oceanic and Space Sciences Department. Plus, the students are great. This is a good place to get stuff done.”