Scientists have identified, at this point, more than 900 exoplanets -- planets that exist outside our solar system, orbiting other stars. We know those planets are out there; but we know that, in large part, through indirect measurements rather than through simply peering at them through telescopes.
One of them is HD 189733b, located 63 light-years away from us, and one of the nearest exoplanets to Earth that can be seen crossing the face of its star. Because of that, HD 189733b has been studied by teams of astronomers hoping to learn more about the bodies that orbit other stars.
Recently, the Hubble Space Telescope turned its attention to the planet, and they've announced a new discovery based on that research: they've determined the color of HD 189733b. Which marks the first time that scientists have determined the true color of a planet in another solar system.
"This planet has been studied well in the past, both by ourselves and other teams," says Frédéric Pont, leader of the Hubble observing program and an author of the paper announcing the find. "But measuring its color is a real first -- we can actually imagine what this planet would look like if we were able to look at it directly."
That color? Blue. Rich blue. If seen up close, the Hubble team puts it, HD 189733b "would be a deep azure blue, reminiscent of Earth's color as seen from space."
So how did the team make that determination? They measured the light reflected off the surface of HD 189733b. Since the planet is both faint and close to its star, the team needed to isolate the planet's light from its star's light -- which they did using Hubble's Space Telescope Imaging Spectrograph (STIS). They examined HD 189733b before, during, and after the planet passed behind its host star. And "we saw the brightness of the whole system drop in the blue part of the spectrum when the planet passed behind its star," Oxford's Tom Evans, first author of the paper, explains it.
"From this, we can gather that the planet is blue, because the signal remained constant at the other colors we measured."
That blue color does not mean, the team is quick to note, that the Earth-hued planet is earthly in other ways. This out-of-our-solar-system version of the "pale blue dot" is, in fact, an enormous gas giant -- think Jupiter, Saturn, Neptune, and Uranus -- that orbits very close to its host star (think Mercury). Its atmosphere clocks in at over 1,800 degrees Fahrenheit, and features periodic hazes and violent flares. Its winds whip at more than 4,000 miles per hour. Oh! And it rains glass. Yes, glass.
So HD 189733b is not habitable. But it is classifiable -- as, in particular, a "hot Jupiter," a gas giant that orbits closely to its host star. Our solar system lacks a hot Jupiter of its own -- Jupiter being, simply, Jupiter -- so we have to rely on studies of bodies like HD 189733b to learn more about that class of planets. And color is its own kind of data. While "it's difficult to know exactly what causes the colour of a planet's atmosphere, even for planets in the Solar System," Pont puts it, Hubble's 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."