Colour Clues
Apart from their beauty, the colours of a planet can actually tell us something about the conditions there. Mercury and Mars have very little atmosphere, so the colours that we see are that of their surface. Mars' reddish colour is due to iron oxide (rust!), but there are differing views about how the iron on its surface came to go rusty.
Cloudy skies
The other planets in our solar system have substantial atmospheres, so although we cannot see their surfaces (with the exception of Earth), the colour of these planets tells us about the gasses that surround them. On Jupiter, the white clouds signify ammonia, whereas the orange-brown bands are due to ammonium hydrosulphide. Saturn is also surrounded by white ammonia clouds, with traces of other compounds including hydrocarbons. Venus is cloaked by extremely dense clouds of sulphuric acid, which appear pale yellow.
The ice giants Neptune and Uranus are predominantly blue, due to methane in their atmospheres. In contrast, Earth's overall blue colour is due to the oceans, and to scattering of blue sunlight by particles in the atmosphere. With both surface and atmosphere being visible, green, brown and white also make a contribution. In fact, as you can see in the graphic above, Earth is much more diverse and quite different in colouring from all the other planets.
So much for our neighbours, but how about planets around other stars? Is it possible to learn more about them from their colours? This is a trickier proposition than it sounds, because generally it's extremely difficult to image these exoplanets directly – they are drowned out by the far brighter light of their host star.
However, in 2013, scientists working with images from the Hubble Space Telescope succeeded in determining the colour of an exoplanet known as HD 189733b. As the planet passed behind the star, they noticed a that the brightness of the whole planet-star system dipped, but only in the blue part of the spectrum, meaning that the planet must be a deep cobalt-blue colour, as you can see in this illustration. The planet is far too hot to be another Earth, and it's believed that the colour could be due to scattering of light by tiny droplets of silicate in the atmosphere – in other words, by particles of glass!
In the future, observations of exoplanet colours could even be used to spot possible alien life! Researchers have recently put together a catalogue of 137 'bio-signatures' of different micro-organisms with a diverse range of pigmentation. Thr project is basd on the idea that if a planet's surface is dominated by a particular life-form, it would would refect a colour characteristic to that organisim. Thus the aim of the catalogue is to assist astronomers in recognising potential life, by enabling comparison of any colours observed with those of the organisms listed.
An intriguing approach in theory – but the fact is still a very long way off!
1. NASA/GSFC
2. NASA, ESA, M. Kornmesser
3. Hegde et al. / MPIA