Atmosphere of Far Worlds Probed with New Technique and Large Telescope

Credit: Gran Telescopio CANARIAS

Astronomers on two research teams, including an astronomer at Penn State University, have demonstrated the power of a new technique to determine the chemical composition of the atmospheres of planets far outside our solar system. Using the technique — called narrow-band transit spectrophotometry — the teams discovered the element potassium in the atmospheres of giant planets similar in size to Jupiter. "This discovery illustrates the power of new instrumentation on the worlds largest optical telescopes, to not just discover planets, but also to study exoplanetary atmospheres in detail. Such techniques could one day aid in the confirmation of the atmospheric constituents of planets more like our own," said Suvrath Mahadevan, assistant professor of astronomy and astrophysics at Penn State and a researcher in the Penn State Center for Exoplanets and Habitable Worlds.

The planets are much hotter than Jupiter, up to 1,000 degrees Centigrade (1,832 degrees Fahrenheit) or more, and they are very far away from Earth — one named HD 80606 b located at about 190 light years and the other named XO-2b located almost 500 light years away. The findings of both research groups, one based in the United States and the other in the United Kingdom, are online at http://arxiv.org, the arXiv preprint server, and have been submitted for publication to the journals Astronomy & Astrophysics and the Monthly Notices of the Royal Astronomical Society. Scientists from the U.K. group will present their findings at the ExoClimes 2010 conference to be held at the University of Exeter from 7 to 10 September 2010.

"This new technique, which measures the light absorbed by the atoms and molecules in a planet's atmosphere, works only for planets that pass in front of their parent stars as viewed from Earth. Most of the nearly 500 known planets do not, and even fewer orbit stars that are bright enough for such precise observations," said Eric Ford, associate professor of astronomy at the University of Florida and the leader of the U.S. research team. "Another challenge is that observations must be carefully timed, in order to see the planets in silhouette against the backlighting of their parent star."

When an exoplanet transits across the face of its host star atoms and gases in its atmosphere absorb some of the starlight at specific wavelengths. These absorption wavelengths form a unique fingerprint, allowing researchers to identify the presence of Potassium. Credit: European Space Agency with adaptations by David Sing

Transit spectrophotometry works like this: While the planet is backlit, astronomers measure the light that passed through its atmosphere. Atoms and molecules absorb specific wavelengths (colors) of light, providing a chemical signature that scientists can recognize. By analyzing the amount of absorption by the planet's atmosphere at specific wavelengths, astronomers can detect the presence of a particular atom or molecule; in this case, potassium.

The U.S. team, which includes astronomers at the University of Florida, the University of California at Santa Cruz, Penn State, Wesleyan University, and the Universidad de La Laguna in Tenerife in Spain, had help from another technological breakthrough. These researchers, as well as the U.K. team, led by David Sing at the University of Exeter, used one of the world's most powerful telescopes, the Gran Telescopio Canarias in the Canary Islands off the northwest coast of Africa. This observatory includes a mirror almost 35 feet wide and is situated at one of the world's best locations for star-gazing.

Sing says he's excited about future prospects for transit spectrophotometry. "The initial results from both teams have been very encouraging," he said. "We still haven't explored the full capabilities or ultimate limitations of the instrument yet."

"The new technique opens the door to comparing the abundances of multiple atoms and molecules in several more planets," Ford said. "While both planets in the U.S. and U.K. studies have potassium, there are interesting differences in the details that provide information about the structure of the plants' atmospheres.

Ultimately, astronomers want to examine smaller, Earth-like planets for molecules such as methane gas and water vapor, which are linked intimately to life on Earth.

CONTACTS

Suvrath Mahadevan: suvrath@astro.psu.edu, 814-865-0261

Barbara Kennedy (PIO): 814-863-4682, science@psu.edu