Exoplanet Hunters Almost Losing Count

         “It’s just amazing,” remarked exoplanet pioneer Michel Mayor, looking around in a free moment during the Extreme Solar Systems II conference in Wyoming last September 12–16. “Fifteen years ago, there were just a handful of people working in this field. Now there are well over a thousand.”

The number of extrasolar planets has risen even faster, if you count both the confirmed and likely ones. The canonical Extrasolar Planets Encyclopedia listed 687 confirmed as of the end of September. Then there’s NASA’s Kepler mission. Last February the Kepler team announced 1,235 planet “candidates” transiting the faces of stars in the spacecraft’s field of view.

Team members expect 90% to 95% of these to prove real. At the September conference, Natalie Batalha of the Kepler team announced more than 500 additional exoplanet candidates as part of Kepler’s latest data release, bringing the new total to 1,781. (Of these, about 200 of all types lie within their stars’ habitable zones.) And, she said, “There are many more in the pipeline. At the end of the mission, we may easily have found over 3,000 candidates.”

Kepler represents the big-science end of the world’s exoplanet searches. Elsewhere at the conference, Coel Hellier of Keele University, U.K., was presenting 23 new giant planets found by the WASP-South transit survey. This South Africa-based project watches star fields across wide areas of the sky using off -the-shelf tele-photo camera lenses.Most exciting among the Kepler finds are the so-called “multis” — stars with two, three, four, fi ve, or six candidate planets crossing their faces, each at its own clock-work pace. “I just love these systems,” said Darin Ragozzine (Harvard-Smithsonian Center for Astrophysics). “We’ve hit at an amazing treasure trove here.” Kepler announced 170 multi systems last February.

At the Wyoming conference, Jason Rowe (NASA/Ames Research Center) added 158 more. And, he said, “there are many more on the horizon.”One reason why the multis are considered so exciting is that they’re almost certainly all real planets, not just candidates, explained theorist Jack Lissauer (NASA/Ames).

It’s virtually impossible that spurious signals could team up to produce the observed patterns — especially when transit timing variations (TTVs) reveal the planets’ mutual gravitational tugs on one another, providing their masses in the pro-cess. “This is an extremely highfi delity sample of candidates,” Lissauer said.

The Quest for Terra II

A running theme throughout the confer-ence was the quest for a holy-grail number: what fraction of main-sequence stars host Earth-like worlds? Determining this number is the Kepler mission’s main goal.At a lively panel discussion, the prevailing conclusion was that we don’t know the number yet, despite the widely publicized.

A Jupiter, a Neptune, and an Earth transit a Sun-size star in this artist’s concept. Star-dimmings caused by planets as small as Earth are turning out to be harder than expected for Kepler to measure reliably. Conclusion by Wesley A. Traub (JPL/Caltech) a few days later that, extrapolating from Kepler statistics, 20% to 50% of Sun-like stars (F, G, and K dwarfs) have at least one terrestrial planet in their habitable zones. Panelists pointed out that 80% to 90% of exoplanet systems seem to have gone through a period of early orbital chaos, leaving orbits a mess.

Theorist, Alessandro Morbidelli (Observatoire de la Côte d’Azur, France) said this uncertain dynamical history suggests that we can’t be sure whether the so-called super-Earths now being confirmed — planets between 2 and 10 times Earth’s mass — are rocky terrestrials or gassy mini-giants. Explained planet hunter Geoff  Marcy (University of California, Berkeley), “We can’t just extrapolate from the Jupiters and the mini-Neptunes down to the real, rocky Earths.”

NASA will decide this spring whether it can afford $20 million per year to extend the Kepler mission for 4 years beyond its planned 3.5 years. An extended mission will be necessary if Kepler is to obtain good data on planets that are as small and far from their stars as Earth is. The problem is that most stars are turning out to show more microvariability than expected, adding noise to transit light curves .

Batalha is worried about the outcome. “We live in difficult times,” she said, “when scientific progress is not limited by technology, but by economics.”Asked whether the Kepler team has thought about private funding for a mission extension, she replied that the thought had crossed the team’s mind. “We’re talking about at most $20 million per year,” she said. “When I mentioned this while lecturing to a group of entrepreneurs, they just laughed — it’s really not that much. On the other hand, we really feel the government should do this.”     

SOURCE : SKY & TELESCOPE MAGAZINE DECEMBER 2011