The colour of an exoplanet measured for the first time

11 July 2013

Astronomers using the NASA/ESA Hubble Space Telescope (HST) have, for the first time, determined the true colour of a planet orbiting another star.

If seen up close this planet, known as HD 189733b, would be a deep azure blue, reminiscent of Earth’s colour as seen from space. But that's where the similarities end. This "deep blue dot" is a huge gas giant orbiting very close to its host star. The planet's atmosphere is scorching with a temperature of over 1000 degrees Celsius, and it rains glass, sideways, in howling 7000 kilometre-per-hour winds.

The study is based on an HST observation program led by Prof Frederic Pont at University of Exeter. Tom Evans, a graduate student working with Dr Suzanne Aigrain at the Department of Physics of the University of Oxford, led the analysis and the resulting paper, which will appear in the August 1 issue of the Astrophysical Journal Letters.

"This planet has been studied well in the past, both by ourselves and other teams," says Frédéric Pont. "But measuring its colour is a real first - we can actually imagine what this planet would look like if we were able to look at it directly."

In order to measure what this planet would look like to our eyes, the astronomers measured how much light was reflected off the surface of HD 189733b — a property known as albedo. HD 189733b is faint and close to its star. To isolate the light reflected by the planet's from that emitted by the star, the team used Hubble's Space Telescope Imaging Spectrograph (STIS) 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.

By separating the light into bluer and redder constituents, the team were able to measure the planet's colour. "We saw the brightness of the whole system drop in the blue part of the spectrum when the planet passed behind its star," explains Tom Evans. "From this, we can gather that the planet is blue, because the signal remained constant at the other colours we measured."

The planet's azure blue colour does not come from the reflection of a tropical ocean, but is due to a hazy, turbulent atmosphere thought to be laced with silicate particles, which scatter blue light. "Earlier observations using different methods already suggested that such particles were present in the planet's atmosphere", says Oxford-based astronomer Dr Suzanne Aigrain (Evans's advisor and a co-author of the study), "but these most recent Hubble observations confirm it unambiguously."

For more information, see the HST press release ( and the scientific paper (, the Oxford Science Blog posted by Pete Wilton When other planets get the blues.