[Astronomy] What colour is a planet?

Some astronomy news today that I thought was too cool not to share. Astronomers have, for the first time, determined the colour of an exoplanet! (I've linked to the free PDF preprint there, as the journal article itself is behind a paywall here)

Using the Hubble Space Telescope, a group led by astronomers from the University of Oxford observed secondary eclipses of the exoplanet HD189733. A secondary eclipse if the opposite of a transit event: the planet passes behind the star, leasing to a slight dimming of the total light from the system. In this case though, the group were observing the light that was reflected from the planet's atmosphere. Generally this is mixed in with light being given out by the planet thanks to its high temperature, but at the short wavelengths used for this work these 'thermal' effects are minimal, so the only light is the reflected starlight. 

I'm very excited by this piece of science. A blue planet, with clouds made of particles of glass. That's

some great sci-fi material right there. (Credit: artist's impression from sbs.com.au)

The measured spectrum of this light depends mostly on whether there are reflective clouds in the atmosphere of the planet. The team measured the Albedo (the reflectance) of the atmosphere, and found that it was much higher at shorter wavelengths. This suggests that shorter wavelengths are scattered more within the atmosphere of the planet (possibly by clouds made of silicate particles, such as glass), allowing astronomers to make a guess at its colour. They think that HD189733b might be "a deep, dark blue, quite distinct from the atmosphere colours seen in our Solar system".

Personally I think this is amazing. Ok, so it's not a direct measurement of the planet's colour, only an inference. But it's a well-grounded inference, and still a brilliant piece of science that required really careful, precise measurements and some very complicated data analysis. Fingers crossed that it's only the beginning.