An international team of scientists using the Hubble Space Telescope’s Imaging Spectrograph has for the first time determined the true color of an extrasolar planet.
An artist’s impression of HD 189733b, a huge gas giant that orbits very close to its host star HD 189733 (NASA / ESA / M. Kornmesser)
The planet, named HD 189733b, would be a deep azure blue, reminiscent of our planet’s color as seen from space.
Discovered in 2005, HD 189733b is a huge gas giant orbiting very close to its host star. It is located about 63 light-years away in the constellation of Vulpecula (the Fox). This world is one of the nearest exoplanets to Earth that can be seen crossing the face of its star.
This image shows the planet HD 189733b with the Sun, Sirius and Alpha Centauri in the background (NASA / ESA / G. Bacon / AURA / STScI)
“This planet has been studied well in the past, both by ourselves and other teams. But measuring its color is a real first – we can actually imagine what this planet would look like if we were able to look at it directly,” said Dr Frédéric Pont of the University of Exeter, UK, second author of a paper reporting the results in the Astrophysical Journal Letters (full paper in .pdf).
To determine the true color of HD 189733b, Dr Pont and his colleagues measured how much light was reflected off the planet’s surface, a property known as albedo. To isolate HD 189733b’s light from this starlight, they used Hubble’s Space Telescope Imaging Spectrograph to peer at the system before, during, and after the planet passed behind its host star as it orbited. As it slipped behind its star, the light reflected from the planet was temporarily blocked from view, and the amount of light observed from the system dropped. But this technique also shows how the light changes in other ways – for example, its color.
This plot compares the colors of Solar System planets to the color of the hot Jupiter HD 189733b (NASA / ESA / A. Feild / STScI / AURA)
“We saw the brightness of the whole system drop in the blue part of the spectrum when the planet passed behind its star. From this, we can gather that the planet is blue, because the signal remained constant at the other colors we measured,” said lead author Dr Tom Evans from the University of Oxford, UK.
This star field image shows the star HD 189733, center. To the right of the star is the notable planetary nebula Messier 27 (NASA / ESA / Digitized Sky Survey 2 / Davide De Martin / Hubble)
“It’s difficult to know exactly what causes the color of a planet’s atmosphere, even for planets in the Solar System. But these new observations add another piece to the puzzle over the nature and atmosphere of HD 189733b. We are slowly painting a more complete picture of this exotic planet,” Dr Pont concluded.
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Bibliographic information: Thomas M. Evans et al. 2013. The Deep Blue Color of HD 189733b: Albedo Measurements with Hubble Space Telescope/Space Telescope Imaging Spectrograph at Visible Wavelengths. ApJ 772, L16; doi: 10.1088/2041-8205/772/2/L16
