NASA's Hubble Space Telescope measured the actual visible-light color of a deep blue planet, illustrated here orbiting a yellow-orange star.Illustration courtesy G. Bacon, STScI/ESA/NASA
Ker Than for National Geographic
A GIANT GASEOUS PLANET THAT'S ORBITING A STAR 63 LIGHT-YEARS AWAY IS A DEEP COBALT BLUE THAT'S REMINISCENT OF EARTH'S COLOR AS SEEN FROM SPACE, SCIENTISTS SAY, MARKING THE FIRST TIME AN EXOPLANET'S TRUE COLOR HAS BEEN DETERMINED.Astronomers deduced the color of the Jupiter-like world, HD 189733b, with the help of NASA's Hubble Space Telescope, which measured light from the planet before, during, and after it passed behind its parent star.Hubble measured a small dip in brightness-about one part in 10,000-and a slight color change in the combined light from the system when HD 189733b slid out of view behind the star. (Related: "An Explosion in the Number of Potentially Habitable Worlds?")"We saw the light becoming less bright in the blue, but not in the green or the red," study co-author Frederic Pont, an astronomer at the University of Exeter in the U.K., said in a statement."This means that the object that disappeared is blue because light was missing in the blue, but not in the red when it was hidden."SO WHAT'S NEW?The finding, to be detailed in an upcoming issue of the journal "Astrophysical Journal Letters", is the best evidence yet that the atmosphere of HD 189733b contains "clouds"-although ones very different from those found on Earth."We suspected that [this planet] had clouds of silicate particles, and the easiest way to detect clouds is through scattered light," Pont said in an interview Thursday. (Related: "Life on Exoplanets? Think About Habitat Variety.")The blue color of HD 189733b is consistent with the hypothesis that very small drops of silicates in the planet's atmosphere are scattering more blue light than red light.WHY IS IT IMPORTANT?The observations yield new insights into the chemical composition and cloud structure of HD 189733b and perhaps other so-called "hot Jupiter" exoplanets.Hot Jupiters are giant gas planets that orbit precariously close-closer even than Mercury is to our sun-to their parent stars and, as a result, get very hot.Their close proximity to their stars also means they are gravitationally "tidally locked" so that one side is always illuminated by the star and the other side is always dark.The confirmation of clouds on HD 189733b could provide clues about the planetary atmospheres of hot Jupiters, scientists say."We obviously don't know much on the physics and climatology of silicate clouds, so we are exploring a new domain of atmospheric physics," Pont said in a statement.WHAT DOES THIS MEAN?HD 189733b might have a color close to that of Earth, but that's where the similarities end.On this turbulent alien world, the daytime temperature is nearly 2,000^0F (1,093^0C), and it possibly rains glass - sideways - in howling, 4,500-mph (7,242-kph) winds.WHAT'S NEXT?Pont and his team are already investigating the colors of other exoplanets to determine whether cobalt blue might be a common color among hot Jupiter planets."We have nine other hot Jupiters that we've been observing with Hubble to answer this question," Pont said.The team's preliminary data suggest that the colors of hot Jupiters are actually quite diverse.That's not surprising, if one considers the planets in our own solar system, Pont said."If you look at the solar system's planets, it's really striking how different they are from each other... Planets tend to be very individualized."
