Illustration courtesy G. Bacon, STScI/ESA/NASA

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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

For First Time, Astronomers Read Exoplanet's Color

Astronomers have determined that an alien planet is blue like Earth.

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.

The Jupiter-like world, HD 189733b, might share Earth's complexion, but that’s where the similarities with our pale blue dot ends: Its daytime temperature is nearly 2,000°F (1,093°C), and its blue color comes is the result of nonstop raining glass on the planet.

Scientists think it's possible the alien world's atmosphere is filled with tiny silicate particles, much smaller than a grain of sand, that get blown sideways in howling, 4,500-mile (7,242-kilometer) per hour winds. The silicate particles are thought to form hazy clouds that scatter blue light and give the planet its unique color.

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."

Sara Seager, an exoplanet researcher at MIT who was not involved in the study, said she was thrilled to "to be living in a time when we can study at least one exoplanet atmosphere in this much detail."

Public reaction to the news of Earth's color doppelganger has been more mixed, if social media is any indication. Twitter user @Grant_D_Foster tweeted "Hubble spots azure planet that rains glass... But still not sure why we call planets weird names," while Facebook user Michael Buxton quipped "Raining glass! What sort of umbrella do you need for that!"

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.

MIT’s Seager noted that it's not just the scattering of blue light by the planet's silicate particles that contributes to its color. It's also "the absorption at visible wavelengths by sodium," she explained. "I would say that the strong absorption at visible wavelengths is a common feature [of hot Jupiters]."

What does this mean?

While the discovery of an alien world with an Earth-like hue is interesting, perhaps more important scientifically is what it can tell astronomers about the planet's atmosphere, said Marc Swain, an astronomer at NASA’s Jet Propulsion Laboratory in Pasadena, California.

"What excites me the most is probing what ... establishes the atmospheric radiation on these planets, which is very important for understanding them," said Swain, who was not involved in the research.

"Understanding what absorbs the heat from the parent star and heats that planet's atmosphere is very important."

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."