A Dusty, Cloudy ExoPlanet
Spectrum of Young Extrasolar Planet Yields Surprising Results
By obtaining a spectrum of its emitted light, the astronomers determined the temperature of the planet. As a result, they found that current theoretical models of gas-giant planets did a poor job of explaining all the data. The team suspects that the reason is dust in the planet’s atmosphere. Models with normal amounts of dust do not resemble this planet, but models with exceptionally thick dust clouds do a much better job. It therefore appears that young gas-giant planets are extremely cloudy.
“We are at a point where not only can we directly image planets around other stars, but we can begin to study the properties of their atmospheres in detail. Direct spectroscopy of exoplanets is the future of this field,” said Mr. Brendan Bowler, a graduate student at the University of Hawaii and the lead author of the study.
The planet, known as HR 8799 b, is one of three gas-giant planets orbiting the star HR 8799, located 130 light-years away from Earth in the constellation Pegasus. (For reference, the distance to the nearest nighttime star from Earth is about 4 light-years.) HR 8799 b is the lowest-mass planet around the star, about 7 times the mass of Jupiter. This multiplanet system was discovered by direct imaging in 2008, and now, only a year and a half later, astronomers have obtained a spectrum of one of its planets. The spectrum of a planet contains much more information than a single image: it can reveal the temperature, chemical composition, and cloud properties of the planet.
The models, however, did a poor job of reproducing all the data. Current theoretical models predict HR 8799 b should be about 400 Kelvin cooler than they measured, based on the age of the planet and the amount of energy it is currently emitting. The team suspects the discrepancy arises because the planet is much more dusty and cloudy than expected by current models.
“Direct studies of extrasolar planets are just in their infancy. But even at this early stage, we are learning they are a different beast than objects we have known about previously,” said University of Hawaii astronomy professor Michael Liu, coauthor of the study.
The planets around HR 8799 are incredibly faint, about 100,000 times dimmer than their parent star. To obtain the spectrum of HR 8799 b, the team relied on the adaptive optics system of the Keck II Telescope to make an ultra-sharp image of the star for many hours. Then they used the facility instrument called OSIRIS, a special kind of spectrograph, to precisely separate the spectrum of the planet from the light of its parent star.
Although over 500 planets have been discovered around other stars, only six planets have been directly imaged. Three of these are around HR 8799 and were discovered in 2008 by Christian Marois of Canada’s National Research Council and collaborators. When it was announced, the discovery represented one of the first direct images of light emitted from extrasolar planets.
A paper describing the study will be published in the Astrophysical Journal later this year.