According to lead author Quinn Konopacky, an astronomer with the Dunlap Institute for Astronomy & Astrophysics, University of Toronto, “We have been able to observe this planet in unprecedented detail because of Keck Observatory’s advanced instrumentation, our ground-breaking observing and data-processing techniques, and because of the nature of the planetary system.” The paper is being published in the March 22 issue of the journal Science.
“This is the sharpest spectrum ever obtained of an extrasolar planet,” said co-author Bruce Macintosh, an astronomer at Lawrence Livermore National Laboratory. “This shows the power of directly imaging a planetary system—the exquisite resolution afforded by these new observations has allowed us to really begin to probe planet formation.”
The team used a high-resolution imaging spectrograph called OSIRIS, fitted on the Keck II telescope on the summit of Mauna Kea, Hawaii, to uncover the chemical fingerprints of specific molecules, revealing a cloudy atmosphere containing water vapor and carbon monoxide. “With this level of detail,” says coauthor Travis Barman, an astronomer at the Lowell Observatory, “we can compare the amount of carbon to the amount of oxygen present in the atmosphere, and this chemical mix provides clues as to how the planetary system formed.”
There has been uncertainty about how planets in other solar systems formed, with two leading models, called core accretion and gravitational instability. When stars form, they are surrounded by a planet-forming disk. In the first scenario, planets form gradually as solid cores slowly grow big enough to start absorbing gas from the disk. In the latter, planets form almost instantly as parts of the disk collapse on themselves. Planetary properties, like the composition of a planet’s atmosphere, are clues as to whether a system formed according to one model or the other.
Although the planet’s atmosphere shows clear evidence of water vapor, that signature is weaker than would be expected if the planet shared the composition of its parent star. Instead, the planet has a high ratio of carbon to oxygen—a fingerprint of its gradual formation in the gaseous disk tens of millions of years ago. As the gas cooled with time, grains of water ice formed, depleting the remaining oxygen gas. Planetary formation started when ice and solids collected into planetary cores—very similar to the way our solar system began.
“Once the solid cores grew large enough, their gravity quickly attracted surrounding gas to become the massive planets we see today,” said Konopacky. “Since that gas had lost some of its oxygen, the planet ends up with less oxygen and less water than if it had formed through a gravitational instability.”
The planet is one of four gas giants known to orbit a star called HR 8799, 130 light-years from Earth. The authors and their collaborators previously discovered this planet, designated HR 8799c, and its three companions back in 2008 and 2010. Unlike most other planetary systems, whose presence must be inferred by their effects on their parent star, the HR 8799 planets can be individually seen.
“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,” said coauthor Christian Marois, an astronomer at the National Research Council of Canada. “Since its discovery, this system just keeps on surprising us.”