WASHINGTON, D.C.--In a major step toward reading the signature of life on planets circling other stars, researchers today announced the first chemical analysis of a planet's atmosphere outside our solar system. So far, only the element sodium has been detected, and the planet--a "hot Jupiter" circling searingly close to its star--is no place to find life. But the technical achievement of measuring part-per-million concentrations on an invisible body at light-year distances bodes well for the search for life beyond the solar system.
Scientists pulled off this first in long-distance chemical analysis by running an interstellar experiment much like one done in the laboratory. There, scientists can analyze gas by passing light through it and separating the light into a spectrum. Taking the experiment into space, astronomers David Charbonneau of the California Institute of Technology in Pasadena, Timothy Brown of the National Center for Atmospheric Research in Boulder, Colorado, and their colleagues used the sunlike star HD 209458, 150 light-years away, as their light source. Every 3.5 days, the star's lone planetary companion orbits across the face of the star for about 3 hours, letting some of the starlight pass through its atmosphere and on to Earth.
Back in Earth orbit, the Hubble Space Telescope picked up the light from the star and spread it into a spectrum, just as in a lab. In the yellow-green part of the spectrum, the researchers detected a one-part-in-10,000 increase in light absorption by the element sodium as the planet crossed the star, showing that the planet's atmosphere contained sodium. The results were announced today at a NASA press conference.
"It's a nice milestone that promises much more in the future," says astronomer Adam Burrows of the University of Arizona in Tucson. Astronomers are working toward measuring more biologically relevant compounds in the atmospheres of extrasolar planets, such as water, oxygen, and methane. Earth's oxygen is entirely the product of life, and methane may have been abundant in Earth's atmosphere before oxygen-producing organisms gained the upper hand. Expanding the list of compounds analyzed shouldn't be too difficult in the case of hot Jupiters, as many as a dozen of which are being discovered each year. Analyzing the atmospheric chemistry of Earth-like planets--none of which have yet been found--will take some more doing.