Astrophysicist Will Use FEST Award to Model Extrasolar Planets

May 30, 2008 — Phil Arras, assistant professor in the University of Virginia's Department of Astronomy, recently received an internal Fund for Excellence in Science and Technology Distinguished Young Investigator grant from U.Va. for his pioneering research to build theoretical models of "hot Jupiters."

"Hot Jupiters" are a recently discovered class of extrasolar planets that resemble the planet Jupiter in terms of their size and physical structure. But unlike Jupiter, which is about 500 million miles from the Sun, hot Jupiters orbit much closer to their parent stars — within 5 million miles — making them extremely hot.

Hot Jupiters have surprised researchers because they are much bigger and hotter than the familiar gas giants in our solar system. Their surfaces are about 10 times hotter than Jupiter or Saturn — approaching the melting temperature of iron, said Arras. And their atmospheres are sometimes much bigger than researchers would expect — meaning that their planetary structure is likely much different.

"There are many observations of these planets which are difficult to explain based on our experience with the solar system planets," said Arras. "And so there is a need to take a new look at the physical processes going on in these planets and their atmospheres and try and understand how things are different from what we are used to in the solar system."

Arras' FEST-funded project will attempt to answer a number of fundamental questions that will help elucidate the origin and structure of hot Jupiters. His project has a number of components. First, Arras will seek to understand the energy source responsible for the anomalously large size of hot Jupiters. "From our models of the structure of these planets and how they radiate away energy and cool over time, it's very hard to understand how they could be as big as they are," he said. Arras suspects that the parent stars of the hot Jupiters may play a role in pumping energy into the planets to keep them big.

Another aspect of the project will look at the unusually large upper atmospheres of hot Jupiters. Since the amount of radiation that the planets get from their stars is hundreds to thousands times more than the amount of radiation that we receive on Earth from the sun, new models are needed to comprehend how radiation from the parent star is being absorbed. "We don't see this level of heating in the solar system, and so there are new attempts to try to understand how an atmosphere responds to this very intense heating from the star," said Arras.

Finally, Arras will explore the tidal forces at work on hot Jupiters. In contrast to the approximately 3-foot ebb and flow of ocean waves that we are accustomed to on Earth as a result of the gravitational pull of the moon, the height of tides on hot Jupiters — which are influenced by the pull of their parent star — reach over 100 miles. These massive tides create high-speed winds that Arras thinks may contribute to overall planetary heating.

Arras' research approach is to use physics to theoretically model what is occurring on the planets. He will use a combination of analytic and numerical simulations of fluid flow as well as create models of atmospheric structure to compute density, temperature and height. These models will be compared to current and future observations of hot Jupiters in an attempt to understand their atmospheric structure, but more importantly to theorize and predict other observable properties.

Arras' research is timely because of the new extrasolar planet observation capabilities that updates to the Hubble Space Telescope and the launch of the Kepler mission will enable in the very near future. His models will be significant as there are not other means to interpret the data from these new observational capabilities.

"Clearly we are never going to send a mission to a hot Jupiter," admits Arras. "And so what we would like to know is, can our theories of planetary structure and evolution help us to understand just the objects that we are able to see? And what we've found is that just based on our knowledge of the solar system, we were unprepared to explain the objects that we are seeing now. And so the goal now is to understand what's missing from these theories."

The FEST Distinguished Young Investigator Grant Program is administered by the Office of the Vice President for Research and Graduate Studies and supports junior faculty research in the sciences, engineering and medicine.