The number of planets known to orbit stars beyond the Sun is approaching 1,000, but astronomers have seen only a tiny handful of those alien worlds directly. It’s not that telescopes aren’t powerful enough; it’s that stars are too bright. Trying to pick out a planet against the glare of a star is like trying to discern a candle sitting next to a 10,000-watt light bulb.
Yet even without seeing a so-called exoplanet, you can learn all sorts of things about it—whether it’s made largely of rock, water or even diamond, for example, which scientists can infer partly by how much of a gravitational tug the planet exerts on its parent star and partly by the way light passes through its atmosphere. At the right moment in an expolanet’s orbit, astronomers can even determine something about an otherwise invisible world’s color, as a 2011 study proved. And now comes one of the most impressive discoveries yet, according to a still unpublished paper: using observations from both the Kepler and Spitzer space telescopes, astronomers have detected what they believe to be a permanent shroud of cloud cover one half of a planet known as Kepler 7b.
That’s right: atmospheric haze covering one hemisphere of a planet you can’t even see—an observation that ought to be impossible. The first clue came from the fact that Kepler 7b is an unusually bright world—even brighter than you’d expect for a planet that’s one and a half times the size of Jupiter and 16 times closer to its star than the Earth is to the Sun.
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It’s still not bright enough to observe directly, but astronomers have been able to see the combination of starlight and planet light wax and wane as Kepler 7b passes in front of its parent sun, Kepler 7, then moves off to the side, then passes behind it. This is the same method used to detect the color of the exoplanet observed in the 2011 study. In both cases, by comparing the total amount of light just before the planet disappears to what’s left just after, astronomers could figure out the amount the planet alone contributes to the whole.
All of these observations about Keplet 7b were made with the Kepler telescope before it broke down last spring, but Kepler alone couldn’t determine whether the planet was so bright because it’s hot or because it reflects an unusual amount of light from the nearby star. For that, they turned to Spitzer, which sees not in visible light but in infrared. If the planet shone brightly in infrared light as well as visible, it would be evidence of searing temperatures.
But it didn’t. “It’s colder than we expected,” says co-author Julien de Wit, of MIT. The conclusion: Kepler 7b is covered with clouds, possibly made of sand-like particles, which bounce much of the star’s light back into space.
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The cloud coverage isn’t uniform though, and the way de Wit and his colleagues figured that out is the truly impressive part of their work. As exoplanets move around their stars, they go through phases, just like the Moon: they appear entirely dark to us when they’re between Earth and the star; half-illuminated when they’re off to one side or the other; and show up as “full planet” when they’re just about to go behind the star. And thanks to Kepler’s extraordinary sensitivity, astronomers managed to measure those subtle changes in the planet’s brightness as well. “It’s the first time we’ve reached such high precision,” says de Wit.
With that precision, says lead author Brice-Olivier Demory, also of MIT, “we could see that something strange was going on.” When Kepler 7b was next to the star on one side, the researchers observed, it was significantly brighter than when was off to the other side. The only plausible explanation: one hemisphere is much cloudier and more reflective than the other.
The scientists aren’t entirely sure why this should be, but they offer one possibility. Just as the Moon is tidally locked to the Earth and always presents the same side to our view, Kepler 7b always shows the same hemisphere to its star. It would be as if our sun hung permanently over the Atlantic Ocean: the eastern half of the Western Hemisphere would always be brightly lit, as would the western half of the Eastern Hemisphere. The other half of the planet would be permanently dark. Earth’s sun-facing side would, of course, always be much hotter than the one that was plunged in permanent darkness. That’s the case on Kepler 7b too, and there, the temperature difference drives strong, never-ending winds around the planet. When the upper atmosphere is on the cooler side, silicate particles condense to form clouds, which persist as they come around to the day side. Then when the clouds reach the hottest part of the day side (which in the Earth example would be right over the Atlantic), they evaporate, only to re-form once again when they get to the night side again.
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“This would go on constantly,” says de Wit, “for thousands of years.” The result: the western half of the day side of Kepler 7b would always be cloudy, while the eastern half would always be clear.
Astronomers can’t be sure if that division exists throughout the entire depth of Kepler 7b’s atmosphere, or if it’s only in the upper layers. It’s quite possible that deeper layers hold clouds too, but that these don’t reflect light very strongly in wavelengths Kepler can see. The researchers are hoping that someday they’ll be able to put together a full 3-D map of the planet’s atmosphere, though considering how difficult such observations are, that day is probably a long way off. Or not. Detecting any clouds at all on an exoplanet once seemed impossible—until all at once, it wasn’t.