Heather Knutson, assistant professor of planetary science at Caltech, has been awarded the Newton Lacy Pierce Prize of the American Astronomical Society (AAS). The prize, given annually to young astronomers for outstanding achievement in observational astronomical research, was awarded to Knutson for her "transformational work in the characterization of exoplanet atmospheres," according to the award citation.
"I was delighted to hear that I had won the award," she says. "It was a great way to start off the new year."
Knutson studies the structure, chemistry, and atmospheric dynamics of extrasolar planets—those outside our own solar system. In the last two decades, astronomers have identified more than 1,800 exoplanets, which come in a variety of shapes and sizes, and orbit their host stars at different distances. Learning about the atmospheres of these planets is important for determining how the worlds evolved, what kind of weather they experience, the chemistry that is taking place at their surface, and whether any of these planets should be considered potentially habitable.
"It's an exciting time for exoplanets," Knutson says. "We now know that the majority of planetary systems look quite different than our own familiar solar system."
When a planet is discovered, measurements of its mass and radius allow it to be classified into a broad category, such as "hot Jupiters," gas giant planets closely orbiting their stars, or "super-Earths," planets larger than Earth but smaller than Neptune. Knutson has added detail to these general classifications through her measurements of exoplanet temperatures and characterizations of atmospheric compositions.
Knutson has helped develop many of the techniques that are now used to study atmospheric dynamics on these exoplanets. As visiting an exoplanet is out of the question, she draws conclusions by observing the eclipse created when the planet passes in front of its host star, called a transit. By measuring the depth of this eclipse at different wavelengths, astronomers can determine the composition of the planet's atmosphere.
Knutson found that information about a planet and its atmosphere could also be revealed by taking spectra at a seemingly odd point in an exoplanet's orbit: when it is not in front of but behind—and eclipsed by—its star, an event known as a secondary eclipse.
"Secondary eclipses tell us about the atmospheric composition and how the planet's temperature changes with height in the atmosphere," Knutson says. "In theory, we can detect clouds via reflected light during secondary eclipse, but usually we see them first when they scatter or block the starlight passing through the planet's atmosphere during the transit."
Using this technique, Knutson has sampled and analyzed a diverse range of exoplanets including the Neptune-sized GJ 436b. Spectra from the planet were featureless, indicating either a high cloud layer or a dense hydrogen-poor atmosphere of mostly heavy molecules like water vapor, carbon monoxide, and carbon dioxide. Knutson and her team have also constructed the first "map" of temperature distributions across an exoplanet.
Before joining the Caltech faculty in 2011, Knutson earned her PhD from Harvard and was a Miller Fellow at UC Berkeley. In 2012, she was awarded the Annie J. Cannon Award in Astronomy by the AAS. She is one of the founding members of Caltech's Center for Planetary Astronomy.
Knutson will accept the Pierce Prize at the 227th annual meeting of the AAS, which will be held in Kissimmee, Florida, in January 2016.