Mega-planet's ingredients point to solar system like ours
Rocky planets like Earth may lie undiscovered around HR 8799
A planet several times more massive than Jupiter, orbiting a star 130 light years away, is part of a solar system that is likely very similar to our own, a new study suggests.
Using ground-based telescopes, the study by Canadian and U.S. researchers has managed to reveal in detail what the giant planet known as HR 8799c is made of.
Based on the ratio of carbon to oxygen in the planet's atmosphere, the researchers think it formed the way our solar system was thought to have formed — and that may indicate there are interesting undiscovered planets in its solar system.
"Not only are we really excited that we have this tantalizing evidence of the way another planetary system may have formed, but we can even speculate that maybe there could be planets more like the Earth — terrestrial planets closer in to the star," said University of Toronto astrophysicist Quinn Konopacky, lead author of the study, at a news conference organized this week by the journal Science.
The journal published the results of the study online on Thursday.
From powerful telescopes on Earth, such as the Keck II telescope in Hawaii that was used in this study, astronomers can see four giant planets, each three to seven times more massive than Jupiter, orbiting the star HR 8799. Jupiter is the largest planet in our own solar system.
HR 8799c is the second-furthest out of the four planets in its system, located about far away from its star as Pluto is from the sun. But because it is a very young planet, it is very hot — roughly 600 C to 1000 C on its surface. Object that are very hot appear bright when seen with a telescope that measures infrared light.
Konopacky, a postdoctoral fellow at U of T's Dunlap Institute for Astronomy and Astrophysics, and her collaborators used a device called the OSIRIS high-resolution imaging spectrograph to finely divide the light from HR 8799c into many different colours.
Different chemical compounds in the planet's atmosphere absorb very specific colours of light, dimming those colours slightly relative to other colours and creating a distinctive chemical fingerprint.
By measuring the amount of light of specific colours associated with certain chemical compounds, the researchers were able to see that HR 8799's atmosphere contained large amounts of carbon monoxide and water and were even able to calculate the ratio of carbon to oxygen.
They also discovered, to their surprise, that it does not contain any methane. Travis Barman, a researcher at the Lowell Observatory in Flagstaff, Ariz., who co-authored the paper, said that suggests that there is a lot of mixing of gases in the planet's atmosphere — an indication that it is windy and turbulent.
The fact that the planet's atmosphere appears to contain a lower proportion of water vapour than the star it orbits suggests that the planet formed the way planets in our solar system are thought to have formed, Konopacky said.
The prevailing theory is that a disk of dust, gas and ice particles formed around the sun, and the ice particles in the cooler parts of the disk gradually clumped together into planetary cores. Once those cores got massive enough, their gravity would have pulled nearby gases around them, forming planetary atmospheres.
Konopacky said there had already been some evidence of an asteroid belt between HR 8799 and its inner-most giant planet, as well as an Oort cloud beyond the most distant giant planet — just as there is in our own solar system.
The researchers hope to do similar studies on other planets in the same system to see if they help bolster the evidence of how the system formed.
"The more we learn about this amazing distant planetary system, the more we might be able to say about our own solar system," Konopacky added.
The HR 8799 system, discovered in 2008, consists of some of the first planets that have ever been directly seen — not just detected indirectly — outside our solar system.
Some previous studies have obtained some information about the atmospheres of the planets, but not in very much detail.
The other co-authors of Konopacky's study were Macintosh from the Lawrence Livermore National Laboratory in Livermore, Calif., and Christian Marois of the National Research Council's Dominion Astrophysical Observatory in Victoria.