Exoplanet May Have Formed Through Core Accretion
March 15, 2013

Distant Gas Giant Exoplanet Has Water And Carbon Monoxide In Its Atmosphere

April Flowers for redOrbit.com - Your Universe Online

An international team of astronomers has made the most detailed examination to date of the atmosphere around a Jupiter-like exoplanet using the Keck Observatory, one of the two largest optical telescopes in the world. The young exoplanet, orbiting the star HR 8799, has water and carbon monoxide in its atmosphere. It does not, however, have methane, suggesting that a particular planet-forming mechanism, known as core accretion, brought the planet HR 8799c into existence.

According to Quinn Konopacky of the Dunlap Institute for Astronomy & Astrophysics, University of Toronto, "We have been able to observe this planet in unprecedented detail because of the advanced instrumentation we are using on the Keck II telescope, our ground-breaking observing and data-processing techniques, and because of the nature of the planetary system."

The study has been published in a recent issue of Science Express, and will also be printed in an upcoming issue of Science.

Konopacky and her colleagues used data from the Keck II10-meter telescope, which has adaptive optics that corrects for distortion caused by Earth's atmosphere, to analyze the spectral features of HR 8799c, which is a gas giant with a mass about seven times that of Jupiter. Astronomers have been debating whether similar planets form via core accretion processes or by gravitational instability.

Core accretion is a multi-step process by which gas slowly accumulates onto a planetary core, while gravitational instability involves the simultaneous creation of the planet's interior and atmosphere. The composition of a planet's atmosphere, along with other planetary properties, is a clue as to whether a system formed according to one model or the other. The results of this study shed light on the formation of HR 8799c and provide clues about the formation of our own Solar System as well.

HR 8799c is one of four gas giants orbiting the host star HR 8799, and unlike most other exoplanets, these four were detected directly. This means their light was discernible from that of their host star. HR 8799c orbits its star at a distance comparable to Pluto's distance from Sol. The birth of such a massive planet so far away from its parent star conflicts with popular models of planetary formation.

"This is the sharpest spectrum ever obtained of an extrasolar planet," according to co-author Bruce Macintosh of the Lawrence Livermore National Laboratory. "This shows the power of directly imaging a planetary system. It is the exquisite resolution afforded by these new observations that has allowed us to really begin to probe planet formation."

Using a high-resolution imaging spectrograph called OSIRIS, which employs a new diffraction grating - the key component of the spectrograph that separates light according to wavelength, just like a prism, the team uncovered the chemical fingerprints of specific molecules, revealing a cloudy atmosphere containing carbon monoxide and water vapor. "With this level of detail," says Travis Barman, a Lowell Observatory astronomer, "we can compare the amount of carbon to the amount of oxygen present in the planet's atmosphere, and this chemical mix provides clues as to how the entire planetary system formed."

"We can directly image the planets around HR 8799 because they are all large, young, and very far from their parent star. This makes the system an excellent laboratory for studying exoplanet atmospheres," says Christian Marois of the National Research Council of Canada. "Since its discovery, this system just keeps surprising us."

"The exoplanet has an ideal set of properties, being both fairly bright and located far enough away from the star to allow us to acquire this amazing spectral data," explained Konopacky. "The fact that we don't see methane tells us a lot about the chemical processes at work in the atmosphere of this young gas giant."

The carbon to oxygen ratio revealed by the spectrum is consistent with the core accretion scenario. "Although we see a lot of water vapor in the atmosphere of HR 8799c, we actually detect slightly less than we would have expected if the planet had the same composition as its host star," said Konopacky. "This tells us that the planet has a slightly elevated amount of carbon compared to oxygen."

The researchers suggest that grains of water ice must have condensed in the planetary disk surrounding HR 8799 and depleted the oxygen. "These ice grains stuck together to make bigger ice chunks, a few kilometers across, that kept colliding and building up the planet's solid core," Konopacky suggested. "The atmosphere came later–from gas that the planet attracted after it got big enough. By the time that happened, some of the ice grains were gone and the gas didn't have as much water in it."

"Our results are most consistent with the planets around HR8799 forming via core accretion, much in the same way we think the planets in our own Solar System formed," explained Konopacky. "By studying the HR8799 system, we can get a peek at how Jupiter-like planets look very shortly after they form."

The team intends to continue to study the super-sized planets to learn more details about their nature and their atmospheres.

"These future observations will tell us much more about the planets in this system," says Dunlap Fellow Konopacky. "And the more we learn about this distant planetary system, the more we learn about our own."