"This planet reveals the astounding diversity of worlds out there," said Nikku Madhusudhan of the Massachusetts Institute of Technology, Cambridge, lead author of a report in the Dec. 9 issue of the journal Nature. "Carbon-rich planets would be exotic in every way -- formation, interiors and atmospheres."
This artist's concept shows the searing-hot gas planet WASP-12b (orange orb) and its star. NASA's Spitzer Space Telescope discovered that the planet has more carbon than oxygen, making it the first carbon-rich planet ever observed. Our planet Earth has relatively little amounts of carbon -- it is made largely of oxygen and silicon. Other gas planets in our solar system, for example Jupiter, are expected to have less carbon than oxygen, but this is not known. Unlike WASP-12b, these planets harbor water, the main oxygen carrier, deep in their atmospheres, where it is difficult to measure.
Concentrated carbon can take the form of diamond, so astronomers say that carbon-rich gas planets could have abundant diamond in their interiors.
WASP-12b is located roughly 1,200 light-years away in the constellation Auriga. It swings around its star every 1.1 days. Because the planet is so close to its star, the star's gravity stretches it slightly into an egg shape. The star's gravity also pulls material off the planet into a disk around the star (shown here in transparent, white hues).
Image credit: NASA/JPL-Caltech
It's possible that WASP-12b might harbor graphite, diamond, or even a more exotic form of carbon in its interior, beneath its gaseous layers. Astronomers don't currently have the technology to observe the cores of exoplanets, or planets orbiting stars beyond our sun, but their theories hint at these intriguing possibilities.
The research also supports theories that carbon-rich rocky planets much less massive than WASP-12b could exist around other stars. Our Earth has rocks like quartz and feldspar, which are made of silicon and oxygen plus other elements. A carbon-rich rocky planet could be a very different place.
"A carbon-dominated terrestrial world could have lots of pure carbon rocks, like diamond or graphite, as well as carbon compounds like tar," said Joseph Harrington of the University of Central Florida, in Orlando, who is the principal investigator of the research.
Carbon is a common component of planetary systems and a key ingredient of life on Earth. Astronomers often measure carbon-to-oxygen ratios to get an idea of a star's chemistry. Our sun has a carbon-to-oxygen ratio of about one to two, which means it has about half as much carbon as oxygen. None of the planets in our solar system is known to have more carbon than oxygen, or a ratio of one or greater. However, this ratio is unknown for Jupiter, Saturn, Uranus, and Neptune. Unlike WASP-12b, these planets harbor water -- the main oxygen carrier -- deep inside their atmospheres, making it hard to detect.
WASP-12b is the first planet ever to have its carbon-to-oxygen ratio measured at greater than one (the actual ratio is most likely between one and two). This means the planet has excess carbon, some of which is in the form of atmospheric methane.
"When the relative amount of carbon gets that high, it's as though you flip a switch, and everything changes," said Marc Kuchner, an astronomer at NASA Goddard Space Flight Center, Greenbelt, Md., who helped develop the theory of carbon-rich rocky planets but is not associated with the study. "If something like this had happened on Earth, your expensive engagement ring would be made of glass, which would be rare, and the mountains would all be made of diamonds."
Madhusudhan, Harrington and colleagues used Spitzer to observe WASP-12b as it slipped behind its star, in a technique known as secondary eclipse, which was pioneered for exoplanets by Spitzer. These data were combined with previously published observations taken from the ground with the Canada-France-Hawaii Telescope at Mauna Kea, Hawaii. Madhusudhan used the data to conduct a detailed atmospheric analysis, revealing chemicals such as methane and carbon monoxide in the planet's atmosphere.
WASP-12b derives its name from the consortium that found it, the Wide Angle Search for Planets. It is 1.4 times as massive as Jupiter and located roughly 1,200 light-years away from Earth. This blistering world whips around its star in a little over a day, with one side always facing the star. It is so close to its star that the star's gravity stretches the planet into an egg-like shape. What's more, the star's gravity is siphoning mass off the planet into a thin disk that orbits around with it.
Signature of a Carbon-Rich Planet
This plot of data from NASA's Spitzer Space Telescope indicates the presence of molecules in the planet WASP-12b -- a super-hot gas giant that orbits tightly around its star. Spitzer measurements suggest this planet's atmosphere has carbon monoxide, excess methane, and not much water vapor. The results demonstrate that WASP-12b is the first known carbon-rich planet.
Spitzer made these measurements as the planet circled behind the star, in an event called the secondary eclipse. The telescope collected the infrared light from the star and the planet, then just the star as the planet disappeared behind the star. This allowed astronomers to calculate the amount of infrared light coming solely from the planet. The observations were performed at four different wavelengths of infrared light. These data were then combined with previously reported measurements taken by the Canada-France-Hawaii Telescope atop Mauna Kea, Hawaii, at shorter infrared wavelengths to create this plot.
The yellow dots show the data, along with the observational uncertainties. The blue curve is a model of the planet's light, or spectrum, showing the fingerprints of chemicals in the atmosphere. The blue dots represent the blue model curve averaged to cover the same wavelengths as the data, as shown by the gray lines at the bottom of the plot.
Image credit: NASA/JPL-Caltech/CFHT/MIT/Princeton/UCF
The Spitzer data also reveal more information about WASP-12b's temperature. The world was already known to be one of the hottest exoplanets found so far; the new observations indicate that the side that faces the star is 2,600 Kelvin, or 4,200 degrees Fahrenheit. That's more than hot enough to melt steel.
Other authors of the paper are Kevin Stevenson, Sarah Nymeyer, Christopher Campo, Jasmina Blecic, Ryan Hardy, Nate Lust, Christopher Britt and William Bowman of University of Central Florida, Orlando; Peter Wheatley of the University of Warwick, United Kingdom; Drake Deming of NASA Goddard Space Flight Center, Greenbelt, Md.; David Anderson, Coel Hellier and Pierre Maxted of Keele University, United Kingdom; Andrew Collier-Cameron of the University of St. Andrews, United Kingdom; Leslie Hebb of Vanderbilt University, Nashville, Tenn.; Don Pollacco of Queen's University, United Kingdom; and Richard West of the University of Leicester, United Kingdom.
The Spitzer observations were made before it ran out of its liquid coolant in May 2009 and began its warm mission. NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA. For more information about Spitzer, visit http://spitzer.caltech.edu/ and http://www.nasa.gov/spitzer . More information about NASA’s search for exoplanets is at: http://planetquest.jpl.nasa.gov/ .
Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.