July 11, 2013 | 2
Using data from the Hubble Space Telescope, astronomers have confirmed that a planet called HD189733b, which orbits a star 63 light years from here, is a deep blue colour.
Earth also looks blue from space. But that’s just about the only thing our planet has in common with this one.
HD 189773b is a hot Jupiter — a gas giant that orbits its star at just one thirtieth of the distance we orbit the sun. Its atmosphere has an average temperature of 1000C. And on HD 189773b, it rains glass. Sideways.
This is not the pale blue dot you’re looking for.
Astronomers think this planet looks deep blue because of the silicate particles in its hazy, turbulent atmosphere. In 2007, Nasa’s Spitzer Space Telescope mapped the infrared light from the planet, showing the temperatures on the day and night sides of the planet differ by 260C. This temperature difference causes howling winds of up to 7000kph. Silicates start to condense at temperatures above 1300C, so it is thought they could form small grains of glass in the atmosphere.
To work out the colour of this planet, astronomers used something called a secondary eclipse to work out the planet’s albedo, a measure of how bright it is. Just before a planet dips behind a star, we can see the light of the star combined with starlight that is reflected off the planet. Then, when the planet is fully eclipsed by the star, we only see the light from the star itself.
By subtracting the starlight from the combined starlight plus planet light, astronomers can get a signature of the light reflected from the planet alone.
Using data from four Hubble orbits in December last year, a team of astronomers, led by Tom Evans of the University of Oxford, did just this for HD 189773b. They saw brightness in the blue part of the visible light spectrum drop as the planet disappeared behind its star. In other words, the planet would look blue to our eyes.
“Our best-ﬁt albedos suggest that it is a deep dark blue, quite distinct from the atmosphere colors seen in our solar system,” say the researchers, in their paper to be published in the August 1 issue of Astrophysical Journal Letters.
Working out exoplanet colours can tell us an awful lot about those planets. In a post last year I explored what different features of light reflected from exoplanets could tell us about what’s going on at the planet’s surface. We’re a long way off being able to find a ‘Blue Marble‘ outside of our own solar system, but finding a deep blue dot is a good start.