Found: A Blue Planet That Rains Glass

One of the strangest and loveliest exoplanets yet gives the galaxy another pale blue dot

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Back in the early 90’s, the Voyager 2 space probe, already out beyond Neptune and on its way toward the edge of the Solar System, swiveled its camera around to look back at its home world. It was difficult to see, but there, nearly three billion miles (4.8 billion km) away, lay Earth. Our entire planet, with all its flora and fauna and history and civilizations was nothing more than a blip of blue against the background of stars—an object so tiny and frail that the phrase “pale blue dot” inspired the great astronomer-communicator Carl Sagan to write a book about humanity’s future in space.

Now astronomers are abuzz with news of a second pale blue dot—a planet orbiting the star HD 189733, about 60 light-years from Earth. It’s a tiny bit less hospitable than Earth: known as HD 189733b, the planet is a giant, gaseous world resembling Jupiter, but much hotter. With a surface temperature of 1800°F (980°C), rainstorms of glass (yes, glass) and winds that reach 4,000 m.p.h. (6,400 k/h), it’s not even remotely likely to be home to flora and fauna.

Measuring the color of this so-called exoplanet is a major scientific milestone nonetheless: it provides important clues to what the atmosphere is made of, which can in turn tell scientists something about the origins and composition of the planet itself. And while it’s not possible with current technology, observations like this one are a warmup for studies of true Mirror Earths—smaller, more temperate worlds like ours, where life could plausibly exist.

Unlike the 90’s-vintage pale blue dot, the 2013 version wasn’t photographed directly. It’s much too close to its star, and too comparatively faint, to isolate its light from that of the much brighter HD 189733. Instead, astronomers used the Hubble Space Telescope to watch as the planet, whose orbit is perfectly edge-on as seen from Earth, ducked behind its star and then re-emerged.

When planet and star are side by side, the Hubble picks up the light of the star itself plus the reflected starlight bouncing off the planet. When the planet moves behind the star, the reflected light disappears—and by making before-and-after comparisons, astronomers can calculate what the reflected light would look like in isolation.

In this case, it’s what scientists call an “azure blue”—a little more poetic-sounding than “pale blue,” but very much in the same ballpark. On Earth, the color is produced because the blue part of the Sun’s light is scattered in all directions by air molecules—eventually reaching our eyes from so many angles that the entire sky looks blue. That blue light is reflected back into space by our oceans. On HD 189733b, the same thing evidently happens—except that it’s glassy silicate particles that do the scattering, not air, and there’s no ocean to enhance the effect through reflection.

This observation is important not just because it keeps astronomers in practice for future studies, of course: it’s also crucial to understanding planets like HD 189733b—so-called “hot Jupiters,” whose very existence was completely unexpected when planet-hunters began finding them in droves in the mid-1990’s. “These new observations add another piece to the puzzle over the nature and atmosphere of HD 189733b,” said Frédéric Pont of the U.K.’s University of Exeter, lead author of the study, in a press release. “We are slowly painting a more complete picture of this exotic planet.”

The picture would be far more complete, of course, if, astronomers could send a Voyager-like probe to visit the HD 189733 system—and 60 light-years isn’t all that far away in a Milky Way that spans 100,000 light-years. If Voyager were headed in the right direction, we could even wait for that already-flying probe to get there. The bad news: at Voyager’s current speed, that would take about 1.2 million years.