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My 2 Suns: Bounty of New Exoplanet Discoveries Includes a World Orbiting a Binary Star

Leading planet hunters from around the world announced the discovery of some 75 extrasolar planets, and hints of many more
Artist's conception of a planet orbiting twin suns



NASA/JPL-Caltech

The hundreds of distant worlds, some large and some small, that are known to dot the galaxy provide plenty of intrigue for the scientists who hunt them. But the catalogued planetary population has just gotten a lot larger and more diverse, thanks to word this week of a newly identified planet orbiting two suns, more than a dozen newfound "super-Earths," and strong indications that the Milky Way Galaxy is home to an almost unfathomable number of planets awaiting discovery.

More than 350 researchers from around the globe gathered at the Extreme Solar Systems (ESS) II conference in Grand Teton National Park, Wyo., to share their findings on these newfound exoplanets, or planets outside our solar system, of every size and configuration.

The most exotic of the latest batch of exoplanets is the world with two suns, like Tatooine of Star Wars or Dr. Who's Gallifrey. The planet, named Kepler 16 b for the NASA Kepler spacecraft that spotted it, revolves around two stars locked in a tight binary pairing; the planet's wide, nearly circular orbit keeps it well outside the stars' orbital dance. The trio is not the only so-called circumbinary system known, but it is the first for which researchers have been able to measure the properties of both stars and the planet so precisely, and the first system where the planet has been directly detected, rather than inferred. [Read more about other planets Kepler has found.]

The researchers, led by Laurance Doyle of the SETI Institute in Mountain View, Calif., announced the finding at the ESS conference and with a September 15 press conference in California that included a visual-effects supervisor from Lucasfilm, the makers of Star Wars. Doyle, Joshua Carter of the Harvard–Smithsonian Center for Astrophysics (CfA), and their colleagues also described the Kepler 16 system in a study in the September 16 issue of Science.

Kepler 16 b is reminiscent of Saturn in its dimensions and mass, but slightly denser, implying that its makeup skews more to elements heavier than hydrogen and helium. The giant planet orbits its parent stars every 229 Earth days. As curious as Kepler 16 b is, the stellar binary that hosts it is noteworthy as well. The smaller of the two stars is just one fifth the mass of our sun, making it the smallest main-sequence star whose physical properties are well known.

"We thought we understood stellar evolution for a long time, but when you get to the end of the main sequence," the data do not fit the models, Doyle says. "Now any kind of excuse of 'you’re not measuring very well' goes out the window. This one, we nailed."

The astronomical oddball was not the only news from the Kepler mission at the ESS meeting. Sarah Ballard, a CfA graduate student and member of the Kepler team, described on September 12 a newly uncovered pair of planets orbiting the star Kepler 19.

Kepler 19 b, only twice as Earth's diameter, is among the smallest exoplanets known to date. But it is likely a miserable place: at just 13.5 million kilometers from its host star, the planet's surface is likely a toasty 480 degrees Celsius.

The Kepler spacecraft detects planets such as Kepler 19 b by watching them dim the light of their host star as the planets pass in front, or "transit." Ordinarily, those transits will occur at regular intervals, like celestial clockwork, but oddities in Kepler 19 b's transit times suggest the influence of the other half of the pair—a larger, unseen planet also orbiting Kepler 19 and perturbing the motion of its neighbor.

"This is uncharted waters," Ballard says. This is the first time that anomalies in the occurrence times of transits have been used to make a solid claim for the discovery of an alien world, she says. The unseen planet, Kepler 19 c, is still mysterious, Ballard notes, adding that it "could be a rocky planet on a five-day orbit, or it could be a gas giant on an oblong, 100-day orbit."

Kepler is hardly the only planet-finding campaign meeting with success and making news this week at the Wyoming confab. A search based at institutions in the U.K. and Spain, the Wide-Angle Search for Planets (WASP), reported the discovery of some two dozen new planets. And a European contingent from the High Accuracy Radial velocity Planet Searcher (HARPS) had an even bigger haul to unveil.

There seem to be plenty of targets to go around: most of the tens of billions of yellow-orange stars in the galaxy might harbor planets, new HARPS data suggest. Previous HARPS results had implied that up to half of these stars could have planets, but a new statistical analysis shows the percentage to be even higher, about 70 percent, Francesco Pepe of the Observatory of Geneva in Switzerland said during a press conference announcing the HARPS results on September 12.

The HARPS team announced about 50 new planets, of which 16 are super-Earths—planets larger than our own but smaller than Neptune or Uranus. Statistical models suggest that four in 10 stars harbor super-Earths. One of the newfound HARPS planets, HD 85512 b, orbits on the edge of the habitable zone, a temperate band surrounding a star where temperatures could support liquid water and just maybe extraterrestrial life.

HARPS is an instrument that measures the wobble caused by a planet's gravitational tug on its host star, so it can be used to estimate planetary mass. It is not able to measure the diameter of planets, so these "super-Earths" could be large and gaseous, like Neptune, or small and rocky, like Earth, the researchers say.

Studying super-Earths is a particularly fruitful area of planetary science, says David Latham of the CfA, who was not involved in the HARPS work. "It's a kind of planet we don't have in our own system," he says. In fact, the existence of super-Earths has come to light only in the past few years.

Between the super-Earths, the circumbinary planet, and the dozens of other new discoveries, researchers are now turning up exoplanets in unprecedented numbers. "We are really in the age of discovery of new worlds," said Lisa Kaltenegger of the Max Planck Institute for Astronomy in Heidelberg, Germany, and the CfA during the press conference.

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