By Ron Cowen
Surveying thousands of stars for telltale twinkles that signal the passage of an orbiting planet, NASA’s Kepler spacecraft has discovered a whopping 706 candidate planets beyond the solar system. If confirmed, that motherlode would boost the number of known extrasolar planets, now estimated at 460, to well over a thousand.
The trove, announced June 15, includes evidence of five stars that have full-fledged planetary systems. These exoplanet systems, if verified, would be the first known in which each planet creates a minieclipse as it transits, or passes in front, of its parent star. The amount of dimming and the duration of a transit offer information about planets, including their size, that cannot be gleaned by less direct methods of detection.
A team including Kepler lead scientist William Borucki of NASA’s Ames Research Center in Moffett Field, Calif., has posted the findings online (at lanl.arxiv.org/abs/1006.2799 and at lanl.arxiv.org/abs/1006.2763) at arXiv.org. The discoveries were made by analyzing Kepler’s first few months of data, recorded in the spring of 2009 when the telescope examined 156,000 stars.
“This is a massively historic discovery,” says study coauthor Sara Seager, a theorist at MIT. “This is showing how the Kepler mission will revolutionize exoplanets and change the way we do exoplanet science.”
The newly reported findings don’t include details about the most interesting 400 of the 706 candidate planets, which orbit the brightest stars Kepler has surveyed. These cases may offer the most promise for finding planets with masses close to Earth‘s own. Information on these 400 planets won’t be made public until next February.
Although the five planetary systems still have to be verified, “they show that Kepler will find dozens — and likely over a hundred — stars having multiple planets that all transit in front of their host star,” says veteran planet hunter and study coauthor Geoffrey Marcy of the University of California, Berkeley. “Apparently, stars commonly house multiple planets.”
One candidate system consists of three orbs, while the other four contain two. The orbiting objects range in size from twice Earth’s diameter to slightly larger than that of Jupiter. They reside relatively close to their stars, at distances ranging from roughly one-quarter to one-half Mercury’s average separation from the sun. They are not yet confirmed planets, however, because their masses have yet to be determined.
Astronomers are already attempting to measure those masses, using ground-based telescopes to discern the tiny wobble induced in the motion of a parent star due to the tug of orbiting bodies.
“We’re using the Keck telescope [atop Hawaii’s Mauna Kea] 20 nights per year just to follow up the Kepler planets,” Marcy says.
Because these stars are dim and the expected wobble signal weak, researchers may have to weigh the bodies using another method — variations in the timing and duration of transits due to the gravitational interplay among the planets in each system, says study coauthor Jason Steffen of the Fermi National Accelerator Laboratory in Batavia, Ill.
Even though the masses of the candidate planets remain unknown, the team says it has already ruled out that the bodies are companion stars that might be mimicking the minieclipses generated by transiting planets. It would be particularly unlikely that a companion star could create the pattern of eclipses seen in the candidate multiple planet systems, notes Steffen. The main confounding source that the team hasn’t entirely excluded would be a planet orbiting a neighboring star.
Once the masses of the candidates are measured and combined with their diameters from the transit observations, researchers can determine the average densities of the bodies.
“From those densities, we can distinguish rocky planets from gas giants and water worlds,” says Marcy. “Kepler is opening a future for planet hunting in which the orbits, masses, densities and architectures of full planetary systems will be captured as a quantitative family portrait.”
The findings may also bode well for finding systems similar to the Earth’s solar system and for hunting habitable planets, comments Alan Boss of the Carnegie Institution for Science in Washington, D.C., who is not a coauthor of the article analyzing the candidate planetary systems.
“The fact that multiple transiting planets are seen means that they must all orbit more or less in the same plane, like our solar system,” he says. In contrast, some recently identified planets don’t all lie in the same plane (SN Online: 5/14/10) and have probably been thrown around by gravitational interactions with other planets in the system. More sedate systems might be more likely to support life.
