Planet Hunters are Losing Count

Anyone entering the first-level lounge of the Jackson Lake Lodge in Grand Teton National Park, Wyoming, is struck by the awesome view of the spectacular mountain range across the lake. But if you entered the lounge between September 12th and 16th, you also couldn’t have missed the dozens of astronomers flocking together here for the Extreme Solar Systems II conference, recognizable by their meeting badges, if not by their tendency to focus on laptop screens bearing news of alien worlds rather than the scenery of this world outside.

A Neptune and a Jupiter transit a Sun-like star in this artist's concept. Earth is only a quarter the diameter of Neptune; star-dimmings caused by planets that small are hard for even Kepler to detect reliably.
S&T: Steven Simpson
“It’s just amazing,” observes Michel Mayor of the Geneva Observatory in Switzerland, who, back in 1995, discovered the first-ever exoplanet orbiting a normal star. “Fifteen years ago, there were just a handful of people working in this field; now there are well over a thousand.”

As one of the very few science writers attending this exciting conference, I couldn’t help wondering whether these researchers – observers, theoretical astrophysicists, planetary scientists, and dynamicists from all corners of the world – had the same thoughts I had: somewhere in their growing databases, might there be a world with landscapes not only as impressive as the one outside the huge windows, but as friendly? With verdure-covered mountains, blue lakes, glaciers, tornadoes, waterfalls and rainbows? And if so, what creatures might move among them?

Even as of last February, NASA's Kepler spacecraft had identified 1,235 exoplanet candidates (yellow and dark-blue dots), far more than the count of transiting exoplanets known prior to the mission (lighter violet dots). Click on the image for a larger version.

Planet diameters are plotted vertically, orbital periods are plotted horizontally; these early discoveries orbit closer to their stars than Earth orbits the Sun. (The Jupiter, Neptune, and Earth illustrations at right are not to scale with each other.)

NASA / Wendy Stenzel
The main message I took away from the Jackson Lake conference is that everybody is losing count as far as extrasolar planets are concerned. Yes, the canonical Extrasolar Planets Encyclopedia lists an exact number: 687 confirmed as of September 23rd. But it changes often now, and in jumps. At a virtual press conference organized by the European Southern Observatory, Swiss astronomer Francesco Pepe announced the discovery of 16 new super-Earths in a batch of 50 new planet discoveries by the super-sensitive European HARPS radial-velocity spectrograph at the La Silla Observatory in Chile. One of these worlds, said Pepe, orbits its orange-dwarf star in the habitable zone, more or less, so liquid water might possibly exist on its surface. But at the same time, HARPS team leader Mayor, during his presentation, mentioned not 16 but 19 new super-Earths, and later that day he revised that number upward to 20.

That same day, Coel Hellier of Keele University, U.K., rushed his audience through the discovery Powerpoint slides of more than 23 new exoplanets found with the WASP-South transit survey (Wide-Angle Search for Planets), which monitors vast numbers of stars across the sky using small telescopes.

One persistent mystery that continues to perplex theorists is why so many giant planets are "puffed up" to much larger diameters and lower densities than heating by a close star can account for. This graph plots the masses and radii of a sample of transiting giants, along with Jupiter and Saturn. Most of them are much larger than theory predicts. The dotted and solid lines represent theoretical models for hot, close-in gas giants with a 20-Earth-mass rock-and-iron core and with no core.
David Charbonneau
More dramatically, Natalie Batalha of NASA’s Ames Research Center, a co-investigator on NASA’s Kepler transit-hunting space telescope, announced well over 500 new exoplanet candidates from Kepler as part of its latest data release. “I think I mentioned a total of 1,781 candidates” from Kepler, she told me later that week, “but there are many more in the pipeline. At the end of the mission, we may easily have found over 3,000 candidates.”

A Kepler "candidate" planet consists of signs that something more or less planet-size is crossing a star's face with clockwork regularity, but whose mass has not yet been confirmed as being within the planetary ballpark. This usually requires a long series of radial-velocity observations from the ground. However, the Kepler team has gotten so good at recognizing false alarms just by teasing apart its own data that its members confidently expect 90% to 95% of the candidates to prove real.

Most exciting among the Kepler finds are the so-called "multis": stars with two, three, or more candidate planets transiting their faces. “I just love these systems,” said Darin Ragozzine (Harvard University). “They’re just awesome. We’ve hit at an amazing treasure trove here.” Kepler already announced a huge number of multis last February, including the extremely compact (and extremely co-planar) system Kepler-11. At the conference Jason Rowe of NASA/Ames presented no less than 158 new multi-planet systems. “And there are many more on the horizon,” he added.

One reason why the multis are considered so exciting is that they’re almost certainly all real planets, not just candidates, theorist Jack Lissauer (NASA/Ames) explained. Statistically speaking, it’s all but 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 each other. “This is an extremely high-fidelity sample of candidates,” Lissauer told the audience. Added an excited Ragozzine: “There are hundreds of planets here!”

As for losing count, even the low numbers could be misleading. On the same day the widely-reported circumbinary "Tatooine" planet Kepler-16b was announced at a press conference in California, to worldwide media attention, Bill Welsh of San Diego State University was presenting convincing evidence at the conference for at least three and possibly a dozen other circumbinary planets in the Kepler field of view. These are worlds that orbit not a single star, but a close binary pair. “I would still call these candidates,” Welsh told me, “but I would be extremely surprised if they weren’t real. We will know for sure within just a few months.”

Hopes for Terra II

A running theme throughout the meeting was, of course, the quest for a holy-grail number: what fraction of main-sequence stars host an Earth-size world. Determining this number is the main goal of the Kepler mission. At a lively panel discussion, the prevailing (if somewhat disappointing) conclusion was that we don’t know the answer yet. Given the uncertainties of the dynamical history of the observed exoplanet systems (for instance, some 80% or 90% seem to have gone through a period of early orbital chaos, leaving the orbits a mess), we just can’t be sure whether or not the so-called super-Earths (planets between a few and ten times the mass of the Earth) are rocky terrestrials or gaseous mini-giants, said theorist Alessandro Morbidelli of Nice Observatory in France.

“We have to be careful,” planet hunter Geoff Marcy of the University of California at Berkeley told me at the end of the week. “We can’t just extrapolate from the Jupiters and the mini-Neptunes down to the real, rocky Earths.”

Obviously more observations are needed, and Kepler will provide these, assuming the mission is extended past its nominal 3.5-year operational lifetime; unexpected microvariability in stars has been slowing down detections of small, Earth-comparable transiters. NASA will decide on Kepler’s future next spring. Natalie Batalha is worried about the outcome. “We live in difficult times,” she said, “where 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 the thought had crossed the team’s mind. “We’re talking about at most 20 million dollars per year,” she said. “When I mentioned this while lecturing for 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.”

So it may take a while before we can be sure about the existence of true Earth-analogs – rocky, terrestrial worlds orbiting in the habitable zones of Sun-like stars. Only then can we really start to fantasize about scenic lakes and spectacular mountain ranges. Not mentioning elk, buffalo and moose. I can’t wait to early December, when I will attend the first Kepler Science Conference in California. Was astronomy ever this exciting?

Sky & Telescope contributing editor Govert Schilling wrote his first book on extrasolar planets (in Dutch) back in 1997. He is considering a vastly updated sequel, in English.

4 thoughts on “Planet Hunters are Losing Count

  1. Rod

    I found this an objective review of the current situation or a SITREP :) My own tracking using NASA KOI list and list, the trend shows explosive growth since 2003 and histograms and descriptive stat indicators are the Milky Way may teem with hostile exoplanets compared to our earth.

  2. Barry Scholles

    "Was astronomy ever this exciting?" Absolutely not. I started out with an interest in astronomy in second grade in 1951 with the receipt of the Golden Nature Guide, "Stars" for Christmas. Back then Jupiter had twelve moons and Saturn had nine. There was a two page article about whether or not we would ever get to the moon. It featured a photo of a one stage dark green rocket headed toward the moon and the phrase, "Given enough time and money a trip to the moon surely will be made."

    Now we’re discovering new planets every day. Simply amazing!

    Barry Scholles
    Strasburg, OH

  3. Anthony Barreiro

    Thanks for this informative report, Govert. But you should have gotten out of the lodge more often. Those large shaggy brown creatures are bison, not buffalo.

  4. Anton Szautner

    Re the ‘puffed-up planet mystery’, I’ve been wondering for quite some time why the possibility of planetary rings hasn’t been looked into more carefully. Saturn, for example, presents quite a profile to an extraterrestrial observer looking at its transit across the Sun’s disk that makes it seem much larger (and therefore less dense if correlated with doppler data) than it actually is.