“We have been able to observe this planet in unprecedented detail because of Keck Observatory’s advanced instrumentation, our groundbreaking observing, and data-processing techniques, and because of the nature of the planetary system,” said Quinn Konopacky from the University of Toronto.
“This is the sharpest spectrum ever obtained of an extrasolar planet,” said Bruce Macintosh from the Lawrence Livermore National Laboratory in California. “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, 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,” said Travis Barman from Lowell Observatory in Flagstaff, Arizona.
There has been uncertainty about how planets in other solar systems formed, with two leading models — core accretion and gravitational instability. When stars form, a planet-forming disk surrounds them. In the first scenario, planets form gradually as solid cores slowly grow big enough to start absorbing gas from the disk. In the latter, as parts of the disk collapse on themselves, planets form almost instantly. 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, grains of water ice formed, 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 that is 130 light-years from Earth. Scientists 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 seen individually.
“We can directly image the planets around HR 8799 because they are all large, young, and very far from their parent star,” said Christian Marois from the National Research Council of Canada. “This makes the system an excellent laboratory for studying exoplanet atmospheres, 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 1,000° 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 Macintosh.
