The Curious Case of the Evaporating Exoplanet

In the past quarter century, astronomers have found a smorgasbord of worlds beyond our own: icy exoplanets and fiery exoplanets, planets the size of the moon and planets bigger than Jupiter. Some have surfaces that resemble toffee; others are like cotton candy. This cosmic candy store now carries more than 4,000 worlds, and the inventory is only expanding. The pace of discovery is so fast that the numbers cited in this story will probably be out of date by next month.

That means the latest additions are not, on their own, noteworthy. The novelty lies, instead, in how they were found, and the process is anything but sweet. It’s downright gnarly: Astronomers recently detected six exoplanets by looking for signs that they’re being shredded to pieces by their stars.

It wasn’t long ago that astronomers had no proof that planets outside of our solar system even existed. Now they’re finding distant worlds in the midst of their slow demise.

The idea for this technique came to Carole Haswell, an astrophysicist at the Open University in the United Kingdom, about a decade ago. At the time, she was studying telescope data for a distant, sunlike star called WASP-12, located in the pentagon-shaped constellation of Auriga. But when Haswell analyzed the telescope data, the star seemed to be missing something pretty important: a chunk of starlight.

Haswell had expected to detect the kind of emission that stars like our sun are known to exhibit. “When we looked at WASP-12, at the wavelengths where you could expect to see that, we were seeing absolutely zero light,” she says. She suspected that the explanation lay not with the star itself, but with its companion, a planet about the size of Jupiter. The planet completes one loop around WASP-12 in a single Earth day. The telescope observations showed that the planet, scorched to temperatures hotter than some actual stars, was being torn apart.

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Haswell hypothesized that some of the eroded material, instead of plunging into WASP-12, remained in space, trailing behind the planet like the tail of a comet. Such a stream of debris could absorb some of the star’s light, making the luminous object appear partially dark to telescopes.

The astrophysicist decided to examine other sunlike stars that behaved similarly. Some of them had planets like the one around WASP-12, and they were a breeze to find: Jupiter-sized worlds with tight orbits are some of the easiest planets to detect. But other stars with missing bands of light weren’t known to have planets at all. “If they were hosting really hot, close-orbiting giant planets, then probably they would have been found already,” Haswell said. If these stars had any worlds, they must be low-mass planets, which are more difficult to discern from a star’s blinding glare.

Haswell and her team targeted this population of darkened stars with a telescope instrument that hunts exoplanets the traditional way. Planets can cause their stars to wobble ever so slightly in space, changing the color of the starlight. Astronomers study these tiny shifts for regular patterns, which indicate whether an exoplanet is lurking nearby. Haswell’s hunch was right: Her team found six exoplanets around three different stars, all many light-years from Earth.

The exoplanets, which are probably rocky worlds, range in mass from a few times that of Earth to half that of Jupiter. They’re closer to their stars than Mercury is to the sun. One planet circles its star in five days, a world that would make keeping your New Year’s resolutions much easier, if you didn’t mind the heat. After some millions of years, they’ll be whittled down to nothing—surprisingly quick in astronomy terms.

The planets are far from their stars’ habitable zones, where temperate climes could give liquid water a chance. But they make exciting research targets for scientists curious about the hidden interiors of alien worlds. “When observing a planet from light-years away, there is virtually no way to study the planet's interior composition,” says Jens Hoeijmakers, an astronomer at the Universities of Bern and Geneva in Switzerland, who studies exoplanets but was not involved in Haswell’s research. “By slowly evaporating these planets—or what's left of them—these stars are helping us study what rocky exoplanets are made of, even if they are many light-years away, beyond the reach of spacecraft equipped with scoops, drills, or seismometers.”

In the ’90s, astronomers usually unveiled exoplanet discoveries one by one, careful to temper their excitement should another team of scientists decide to take a look and end up debunking the findings. In the past decade, with the launch of space telescopes specifically designed to search for exoplanets, new finds are announced in batches. The cosmos is sprinkled with planets, some huddled around their stars in groups of eight, like our own. Most of these worlds are invisible to us; astronomers see them only in tiny dips and wobbles of starlight. But with powerful instruments and careful study, scientists can work out their sizes and compositions, and even catch whiffs of water vapor in their atmospheres.

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Haswell’s technique can save exoplanet seekers quite a bit of time, says Matthew Kenworthy, an astronomer at the Leiden Observatory who was not involved in the work. For example, Kepler, the now-defunct space telescope that uncovered the majority of the known exoplanets, soaked in the light of 100,000 stars at once, waiting to catch the tell-tale dimming that meant a passing planet. “The big disadvantage of this is that you don’t know what stars to look at, so you have to look at many tens of thousands of stars for months at a time to spot the distinctive, small signal,” Kenworthy said. Haswell, he said, can “cherry-pick the best candidates using this technique, and then spend the more expensive follow-up measurements to confirm the exoplanets’ existence.”

Haswell and her colleagues still have 30-something star systems to probe for potential exoplanets. So far, their chances of finding more look good. “We’ve got some more in the pipeline,” she said.

Last week, an astrophysicist at NASA tweeted that his New Year’s resolution was to “discover a few exoplanets.” These days, that’s a pretty realistic goal.