A team of international scientists using the W. M. Keck Observatory has made the most detailed examination yet of the atmosphere of a Jupiter-size planet beyond our Solar System.
According to lead author Quinn Konopacky, an astronomer with the Dunlap Institute for Astronomy & Astrophysics, University of Toronto and a former Lawrence Livermore National Laboratory (LLNL) postdoc, “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 appears online March 14th in Science Express, and March 22nd in the journal Science.
“This is the sharpest spectrum ever obtained of an extrasolar planet,” said co-author Bruce Macintosh, an astronomer at LLNL. “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, using the OSIRIS instrument fitted on the mighty Keck II telescope on the summit of Mauna Kea, Hawaii, has uncovered 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 formation in the gaseous disk tens of millions of years ago. As the gas cooled with time, grains of water ice form, depleting the remaining gas of oxygen. Planetary formation began when ice and solids collected into planetary cores—very similar to how our solar system formed.
“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 is inferred by their effects on their parent star, the HR8799 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 and another former LLNL postdoc. “Since its discovery, this system just keeps on surprising us.”
Although the planet does have water vapor, it’s incredibly hostile to life—like Jupiter, it has no solid surface, and it has a temperature of more than a thousand degrees Fahrenheit as it glows with the energy of its original formation. Still, this discovery provides clues as to the possibility of other Earth-like planets in other solar systems. “The fact that the HR8799 giant planets may have formed the same way our own giant planets did is a good sign—that same process also made the rocky planets close to the Sun,” said Dr. Macintosh.
The HR 8799 four planet system:
All four planets are more massive than any in our Solar System, with masses three to seven times that of Jupiter. Their orbits are similarly large when compared to our system. The system is believed to be young, of the order of 30 million years. HR 8799c orbits 40 times farther from its parent star than the Earth orbits from the Sun; in our Solar System that would put it beyond the realm of Neptune.
The OSIRIS instrument:
The team analyzed the distant giant’s atmosphere using a high-resolution imaging spectrograph called OSIRIS. Just as Keck’s adaptive optics technology gives astronomers a sharp image of HR 8799c, OSIRIS enables an extremely detailed analysis of the spectrum of the light from the planet—much more detailed than ever before—and allows astronomers to separate the star’s light from the planet’s. This in turn provides a more detailed understanding of the composition of the gas giant’s atmosphere.
The telescope’s adaptive optics system corrects for distortion caused by the Earth’s atmosphere, making the infrared view through Keck II sharper than through the Hubble Space Telescope.