Sen— Earth is unique in the Solar System as a beautiful, water-covered world. Seen from the distant space probe Voyager 1 in 1990, it resembled a “pale blue dot”, the term given to it by legendary populariser of science Carl Sagan.
Now another “blue dot” has been observed in orbit around a nearby star. Astronomers say it is the first time they have been able to discern the true colour of an exoplanet and they did it using the Hubble Space Telescope operated by NASA and ESA.
Superficially it resembles Earth, but it could hardly be more different from our own life-friendly home world. This cobalt-coloured planet is a “hot Jupiter” gas giant with 1,000 C temperatures and where it rains glass - sideways - in howling 7,000 km-per-hour winds.
Dubbed HD 189733b, the planet lies right on our cosmic doorstep, only 63 light-years away from Earth, and orbits an orange dwarf star with a red dwarf companion in the constellation of Vulpecula, the Fox.
HD 189733b lies only 4.7 million km (2.6 million miles) from its own sun, a distance so close that it zips around it in only 2.2 days. It must be tidally locked, meaning it keeps the same face directed towards the star throughout its orbits, just as the Moon does to the Earth.
The planet was discovered by French astronomers in 2005 but it has taken years to build up a picture of its properties. To separate the light it reflects from the brilliant glow of its host star, astronomers had to use an instrument called an imaging spectrograph attached to Hubble.
They used it to monitor the planet at various stages of its orbit including when it passed in front of and behind the star. The difference in the appearance of the light when the planet was hidden behind the star allowed them to tell its colour.
Tom Evans of the UK’s University of Oxford, is lead author of a paper on the findings to be published in Astrophysical Journal Letters. He said: “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 colours we measured.”
The planet’s colour is thought to be due to a hazy, turbulent atmosphere laced with silicate particles - glass in other words - which scatter blue light. The Hubble observations are conclusive evidence that earlier suspicions about the blue nature of the planet were correct.
In 2007, NASA’s Spitzer Space Telescope made a temperature map of HD 189733b and showed that they vary between the two sides by hundreds of degrees, producing ferocious winds.
Frédéric Pont of the University of Exeter, UK, who led the Hubble observing programme, said: “This planet has been studied well in the past, both by ourselves and other teams. 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.
He added: “It’s difficult to know exactly what causes the colour 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.”
A video zooms in on the region of sky where the blue planet lies. Credit: Hubble/ESA