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Physical Chemistry

Distant Planet Yields To Chemical Characterization

Space: Sharp spectral lines allow astronomers to determine origins of a planet outside our solar system

by Elizabeth K. Wilson
March 18, 2013 | A version of this story appeared in Volume 91, Issue 11

Credit: Courtesy of Dunlap Institute
An artist’s rendering of the gas giant planet HR 8799c orbiting its sun.
Artist’s rendering of the planetary system of HR 8799 at an early stage in its evolution, showing the planet HR 8799c, as well as a disk of gas and dust, and interior planets.
Credit: Courtesy of Dunlap Institute
An artist’s rendering of the gas giant planet HR 8799c orbiting its sun.

Of the thousands of known planets outside our solar system, most have been discovered by looking at the gravitational wobbles they generate in their parent stars or at the faint dimming of starlight as the planets pass in front of those stars. Since 2008, a few such exoplanets have been detected directly, but chemical species in their atmospheres have only been broadly characterized.

Astronomers now have obtained the sharpest, most sensitive spectrum yet of the atmosphere of a distant exoplanet and unambiguously identified water and carbon monoxide as part of its makeup (Science, DOI: 10.1126/science.1232003). The work not only breaks new ground in the study of the chemistry of distant planets, but also hints at how the solar system where this particular planet is located was formed.

Astronomy postdoc Quinn M. Konopacky at the University of Toronto’s Dunlap Institute; Christian Marois at the National Research Council of Canada in Victoria, British Columbia; and their colleagues used Hawaii’s Keck Observatory to study the planet HR 8799c. It is one of four planets orbiting a bright young star 130 light-years from Earth. They are all gas giants, like Jupiter, but several times larger.

The ratio of carbon to oxygen in the atmosphere fit with that predicted by one popular model for solar system formation—the core accretion model, where cooling ice and dust form the cores of planets.

“The astonishing result of this paper is that the authors have used the chemical signatures of CO and H2O to get a first estimate of the place and method of formation of one of these planets,” says Wesley A. Traub, chief scientist of the Jet Propulsion Laboratory’s Exoplanet Exploration Program.

Coincidentally, a group led by astronomer Ben R. Oppenheimer of the American Museum of Natural History in New York City reports obtaining simultaneous spectra from all four planets of the HR 8799 system in an upcoming issue of the Astrophysical Journal. Their spectral observations, using the Hale Telescope at Caltech’s Palomar Observatory, suggest the planets’ atmospheres may also contain compounds such as ammonia and acetylene.

The two reports are “a sure sign that we are making progress on the chemical composition of these strange new worlds,” Traub says.



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