Move over, Icarus. Six newly discovered exoplanets have been discovered flying so close to their host stars that they are literally evaporating—creating a ring of debris. The discovery of the planets, published today in three separate papers in Nature Astronomy, were identified using a new technique that first looked for that ring of debris. It is thus an efficient method to find small planets orbiting extremely close to their star, which have long eluded detection. In addition, follow-up studies should allow astronomers to probe the geology of these "ablating" worlds and better understand how such planets form and evolve—perhaps even shedding light on the oddities within our own solar system.

In 2009 Carole Haswell, an astronomer at the Open University in England, observed the exoplanet Wasp-12b—a Jupiter-like world that orbits its host star so tightly a year there lasts just 26 hours—when she noticed something odd about its parent star. The hot, outer layers of its atmosphere known as the chromosphere appeared to be missing. And she had an inkling that the star's close-in planet just might be the culprit. At the time, astronomers knew that this world was so hot that the outer reaches of its atmosphere were effectively boiling off into space. "They're just too close to the fire," says David Grinspoon, a scientist at the Planetary Science Institute who was not involved in the study. "It's like you're roasting your marshmallow and you put it too close to the fire—and poof!" Haswell hypothesized that the resulting trail of gas from the planet absorbed the same wavelengths of light that the star's chromosphere emits, making it appear dark.

The idea was tantalizing. It suggested that astronomers could search for stars with the same signature—a "missing" chromosphere—to target close-in exoplanets. Moreover, if astronomers used this new technique to scrutinize the already well-surveyed nearby stars, they would likely find only small worlds given that large ones had already been discovered through other methods. That would be particularly valuable because, to date, small exoplanets have proved notoriously difficult to find. So Haswell set out on a mission. She and her colleagues scoured data from 2,700 nearby sunlike stars and found that 39 appeared to be missing their chromospheres. Then, the team used a planet-finding instrument on the European Southern Observatory's 3.6-meter telescope at the La Silla Observatory in Chile to take a closer look.

"What we found was a success beyond my wildest dreams," Haswell says. Her team discovered planets around the first three stars that they were able to observe in detail. And these systems are pretty wild. The star DMPP-1 hosts multiple planets with three inner planets—3.5 to 10 times the mass of the Earth—and one outer planet heavier than Neptune. The star DMPP-2 hosts a planet with a mass roughly half that of Jupiter in a five-day orbit; the world had been overlooked because of DMPP-2's stellar pulsations. And the star DMPP-3 hosts a small planet roughly twice the mass of Earth and also a second star that orbits at a greater distance. All the newfound planets orbit their stars substantially closer than Mercury does the sun, and many of them are quite small—on par with rocky worlds like the Earth. "We think we're identifying a hidden population of planets," says co-author John Barnes, also from the Open University.

Grinspoon called the study "ingenious." He has long thought that we would know little about exoplanets. "But then you have these incredibly clever techniques that people keep devising," he says. "I read about them and I think 'I'll be damned, they figured out a way to do this.' And to me, this is another step in that progression."

Not only do the results show a new technique that will allow astronomers to uncover these planets efficiently, they also point toward a number of follow-up studies that could allow astronomers to understand these worlds in incredible detail. To confirm the planets' existence, the team used the "radial velocity" method, which looks for the wobbles in a star's movement induced by the gravitational tugs of accompanying worlds. The team suspects, however, that many of these planets will also be detectable via the "transit" technique, which spots tiny dips in starlight caused when a planet crosses in front of its host star as seen from Earth. Radial velocity measurements allow astronomers to estimate planets' masses, whereas transits allow them to measure the sizes of worlds. When combined, the two techniques can reveal a density for each planet—a crucial step in better understanding a world's composition. Moreover, astronomers can gain an even better handle on the geology of ablating planets by studying the disks of cast-off debris that encircles their host stars, looking for the presence of various chemical elements by their absorption of specific wavelengths of starlight.

Grinspoon is excited to use this technique to better understand how planets evolve—particularly in their early stages, when their young host stars may pelt them with violent outbursts of intense radiation. "This may be a window into that particular phase," he says. Take Venus as an example. Some models suggest that the planet might have held oceans for billions of years—meaning that the now-torrid and toxic world was once eminently Earth-like and habitable. But the veracity of such ideas hinges on the activity of our young sun. A newborn Venus is thought to have been a "magma world," which would have become a "steam world" as it cooled, rapidly venting its water—as steam—off into space, much like the small planets that Haswell's team has uncovered. Alternatively, Venus could have experienced an intermediate phase, in which its steam condensed and rained down on the surface, creating an ocean. When and how the sun bombarded young Venus is the most likely arbiter between these two vastly different planetary fates. And so, by better understanding this process in other systems, we might further understand what occurred early in our own solar system.

But before Haswell and her colleagues plan to conduct follow-up studies, they will keep poring over the other systems that likely host close-in planets. With only three fully observed, they have 36 left on their to-do list. Luckily, they just received telescope time for 10 nights early next year. As Barnes says, "It's a good Christmas present."

Quelle: SCIENTIFIC AMERICAN