Discovering Other Earths, Far Away

JPL astronomer Dr. Mark Swain has announced a breakthrough observational technique which analyzes the atmospheres of distant planets and speeds up the search for other “Earths.”

Surprisingly, the technique uses small ground-based telescopes and does not rely upon the latest technology or space telescopes like Hubble. Dr. Swain’s team used a relatively small Earth-based telescope to identify an organic molecule in the atmosphere of a Jupiter-size planet nearly 63 light-years away. The measurement revealed details of the exoplanet’s atmospheric composition and conditions, an unprecedented achievement from an Earth-based observatory. The breakthrough was made using a relatively small 30 year-old telescope in Hawaii.

“This work suggests we may be able to detect organic molecules in the atmospheres of terrestrial planets with existing instruments very soon,” Dr. Swain wrote in a paper published yesterday in Nature magazine.

The new technique promises to further speed the work of studying planet atmospheres by enabling studies from the ground that were previously possible only through a few very high-performance space telescopes.

“The fact that we have used a relatively small, ground-based telescope is exciting because it implies that the largest telescopes on the ground, using this technique, may be able to characterize terrestrial exoplanet targets,” Swain said.

Currently, more than 400 exoplanets are known. Most are gaseous like Jupiter, but some “super-Earths” are thought to be large terrestrial, or rocky, worlds. A true Earth-like planet, with the same size as our planet and distance from its star, has yet to be discovered. NASA’s Kepler mission is searching from space now, and is expected to find several of these earthly worlds by the end of its three-and-a-half-year prime mission.

On Aug. 11, 2007, Swain and his team turned the infrared telescope to the hot, Jupiter-size planet HD 189733b in the constellation Vulpecula. Every 2.2 days, the planet orbits a K-type main sequence star slightly cooler and smaller than our sun. HD189733b had already yielded breakthrough advances in exoplanet science, including detections of water vapor, methane and carbon dioxide, using space telescopes. Using the new technique, the astronomers successfully detected carbon dioxide and methane in the atmosphere of HD 189733b with a spectrograph, which splits light into its components to reveal the distinctive spectral signatures of different chemicals. Their key work was development of a novel calibration method to remove systematic observation errors caused by the variability of Earth’s atmosphere and instability due to the movement of the telescope system as it tracks its target.

“As a consequence of this work, we now have the exciting prospect that other suitably equipped yet relatively small ground-based telescopes should be capable of characterizing exoplanets,” said John Rayner, the NASA Infrared Telescope Facility support scientist who built the SpeX spectrograph used for these measurements. “On some days we can’t even see the sun with the telescope, and the fact that on other days we can now obtain a spectrum of an exoplanet 63 light-years away is astonishing.”

In the course of their observations, the team found unexpected bright infrared emission from methane that stands out on the day side of HD189733b, indicating some kind of activity in the planet’s atmosphere. Swain said this puzzling feature could be related to the effect of ultraviolet radiation from the planet’s parent star hitting the planet’s upper atmosphere, but more detailed study is needed. “This feature indicates the surprises that await us as we study exoplanet atmospheres,” he added.

“An immediate goal for using this technique is to more fully characterize the atmosphere of this and other exoplanets, including detection of organic and possibly prebiotic molecules” like those that preceded the evolution of life on Earth, said Swain. “We’re ready to undertake that task.” Some early targets will be the super-Earths. Used in synergy with observations from NASA’s Hubble, Spitzer and the future James Webb Space Telescope, the new technique “will give us an absolutely brilliant way to characterize super-Earths,” Swain said.

Other authors are Pieter Deroo, Gautam Vasisht and Pin Chen of JPL; Caitlin A. Griffith of the University of Arizona, Tucson; Giovanna Tinetti of University College London; Ian J. Crossfield of UCLA; Azam Thatte of the Georgia Institute of Technology, Atlanta; Jeroen Bouwman, Cristina Afonso and Thomas Henning of Max-Planck Institute for Astronomy, Heidelberg, Germany; and Daniel Angerhausen of the German SOFIA Institute, Stuttgart, Germany.

The work was carried out with funding from NASA’s Office of Space Science in Washington, D.C. The NASA Infrared Telescope Facility is managed by the University of Hawaii’s Institute for Astronomy. JPL is managed by the California Institute of Technology for NASA.