Far-flung planet has cloudy skies

Artist's impression of an exoplanet

A planet orbiting a nearby star is suspected to have dusty, cloudy skies, making it about 400 degrees Celsius hotter than computer models predicted. (Artist's impression)

Astronomers have tested the atmosphere of a young gas-giant planet orbiting another star, and have found that it is unlike that of any previously studied “extrasolar” planet (one that belongs to a star other than our Sun).

By obtaining a spectrum of its light, they determined the planet’s temperature. And it’s about 400 degrees Celsius higher than expected.

Extrasolar planet HR 8799 b

The young extrasolar planet HR 8799 b (green blob in the middle), isolated from the glare of its parent star.

The reason could be dust in the planet’s atmosphere.

The planet, known as HR 8799 b, is one of three gas-giant planets orbiting the star HR 8799, 130 light-years from Earth. HR 8799 b is the lightest of the three, about 7 times the mass of Jupiter.

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 team took the planet’s temperature by using the presence or absence of gaseous methane as a “thermometer”. They found that HR 8799 b has little or no methane.

Based on the spectrum and previously obtained images of the planet, and by comparing the observations to theoretical models of low-temperature atmospheres, they estimate the coolest temperature for the planet should be about 930 degrees Celsius.

The models, however, did a poor job of fitting all the data, predicting that HR 8799 b should be about 500 degrees, based on the age of the planet and the amount of energy it is currently emitting.

W. M. Keck Observatory

The W. M. Keck Observatory operates two 10-metre optical/infrared telescopes on the summit of Mauna Kea on the island of Hawai’i.

The scientists suspect the discrepancy arises because the planet is more dusty and cloudy than the models predict. Dust would hold in the extra heat.

The HR 8799 planets 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 giant Keck II Telescope to make an ultra-sharp image of the star for many hours. Then they used a special kind of spectrograph to precisely separate the spectrum of the planet from the light of its parent star.

“Adaptive optics systems on Keck and other large ground-based telescopes make sharper images than even the Hubble Space Telescope,” said Trent Dupuy, a University of Hawaii graduate student and co-author of the study.

Adapted from information issued by W. M. Keck Observatory / Brendan Bowler & Michael Liu (IfA, Hawaii) / Pablo McLoud / NASA / ESA / G. Bacon (STScI).

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