A golden age of exoplanet scienceby Jeff Foust
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| “Kepler has transformed the field of exoplanet science,” Batalha said. |
It’s difficult to know when you’re in the midst of a golden age, and perhaps a bit hubristic to declare one exists while it’s still ongoing. Nonetheless, one can make a strong case that we’re in some kind of golden age today for the search for and study of extrasolar planets, or exoplanets. Two decades ago the only confirmed exoplanets orbited a pulsar, and just how common planets were around stars more like our Sun was very much an open question. Today, confirmed exoplanets number in the hundreds, with thousands more waiting to be confirmed.
The vibrancy of this field of astronomy was evident at the 221st Meeting of the American Astronomical Society (AAS), held in Long Beach, California, last week. The meeting was full of sessions, plenary talks, and press conferences associated with exoplanet studies, from the latest discoveries of new planets to efforts to try and characterize the ones already found and also better understand how frequent such planets—particularly those like the Earth—are around other stars.
The wealth of findings at the conference caused even veteran exoplanet researchers to gush. “It’s been an extraordinary AAS meeting already for those of you interested in exoplanets,” said Geoff Marcy at beginning of a talk a day and a half into the four-day meeting. “The results that have been described by so many speakers already I think have been stunning, actually historic.”
Kepler’s bounty
A major reason for that “stunning” perception has been NASA’s Kepler mission. The spacecraft, launched nearly four years ago, has become an exoplanet discovery machine. Staring at the same spot in the sky, simultaneously observing more than 100,000 stars, Kepler is designed to detect minute, periodic decreases in brightness in these stars as exoplanets pass, or transit, across the disc of the star.
At last week’s AAS meeting, scientists announced the discovery of 461 new “planet candidates,” bringing the total of such discoveries by the spacecraft to 2,740. The number of these potential candidates that have been confirmed as exoplanets by other observations is much smaller: only 105 as of the AAS meeting. However, project scientists believe that most of these candidates are actual planets, since the false positive rate has typically been only 10%.
“We’re learning as the years have gone by that the reliability of this catalog is quite high,” said Natalie Batalha, Kepler mission scientist at NASA Ames Research Center, during a plenary talk at the conference. “It’s likely that 90% or more of these are going to be bona fide planets.” She then showed an updated chart of exoplanet discoveries, with the Kepler planet candidates dominating the display, triggering an impromptu round of applause from the audience.
“Kepler has transformed the field of exoplanet science,” she continued. “There’s probably not a single exoplanet scientist in the country who is not working right now on Kepler data.”
| Based on an extrapolation of Kepler data so far, said Fressin, “Almost all Sun-like stars have a planetary system.” |
Much of the media attention devoted to Kepler has been on individual exoplanet discoveries, including those that, at least in size and orbit, are increasingly like the Earth. One such discovery touted at the conference was a planet candidate designated KOI-172.02, a planet located in the habitable zone of a Sun-like star. That planet has an estimated radius 1.5 times of the Earth, so it is not exactly an Earth twin. “This is certainly more in the regime of the ‘super-Earth’ category,” said Christopher Burke of the SETI Institute at a press conference about the Kepler discoveries.
Kepler’s ultimate mission, though, is not one of individual exoplanet discoveries but of statistics. By looking at one small, representative sample of the sky, project scientists hope to gather information on the relative occurrence of exoplanet systems in general. And initial data confirms the expectations of many scientists: other solar systems are commonplace. One study used Kepler data to estimate that one sixth of Sun-like stars have an Earth-sized planet close in, with a period of 85 days or less.
Francois Fressin of the Harvard-Smithsonian Center for Astrophysics (CfA) noted that there are even slightly higher occurrences of super-Earths and Neptune-sized planets in those close orbits. Taking those planets into account, and extrapolating for longer orbital periods, he estimated than 70 to 90 percent of stars have planetary systems. “Almost all Sun-like stars have a planetary system,” he said.
What remains to be determined, though, is what scientists call “eta-Earth”: the frequency of stars with planets like the Earth in both size and orbit. Determining eta-Earth remains a primary goal of the Kepler mission, but project scientists said they need more data to make that determination. “It behooves us to continue observing for another four years to get a robust statistic on the frequency of Earth-sized planets,” Batalha said.
Kepler, having completed its prime 3.5-year mission, is now in an extended mission that scientists hope will go long enough to collect enough data to make that eta-Earth calculation possible. “The Kepler spacecraft is operating very well,” Kepler project scientist Steve Howell told a town hall meeting about the mission during the AAS conference. The main concern about the spacecraft’s long-term health is regarding its reaction wheels, after one failed last year. “We are now operating on three reaction wheels. The Kepler spacecraft is doing fine on three wheels,” he said.
M-class planets around M-class stars
Although the focus of exoplanet searches has primarily been with Sun-like stars, a new theme emerged from the AAS conference: a new emphasis on smaller M-class red dwarf stars. These stars, it turns out, might hold the best chance for finding an Earth-like world—which Star Trek fans will recall, coincidentally, were designated M-class worlds in the original series—in the near future.
Studying M-class stars offer several advantages for exoplanet researchers. One is the sheer number of such stars: about 75 percent of the stars in the galaxy are M dwarfs. “You can think of them as the galaxy’s silent majority,” said John Johnson, a Caltech professor, in a plenary talk. “We’re rare,” he said of our own solar system, “simply because we don’t orbit an M dwarf. And I think that’s really profound.”
At a separate meeting of NASA’s Exoplanet Exploration Program Analysis Group, or ExoPAG, held in Long Beach immediately before the AAS meeting, David Charbonneau of the CfA noted that looking for terrestrial planets in habitable zones around M-class stars is considerably easier that for larger stars. Such planets offer deeper and more frequent transits because of their larger size relative to the star and closer orbits.
| “We’re rare,” Johnson said of our own solar system, “simply because we don’t orbit an M dwarf. And I think that’s really profound.” |
Charbonneau’s group is seeking to look for such planets with a project called MEarth, using small robotic telescopes to look for transits by Earth-sized planets around such stars. That project has discovered only one such planet over its first four years, designated GJ 1214b. “It’s kind of a run of the mill planet” other than that it orbits a red dwarf, said Zachory K. Berta of Harvard in an AAS conference presentation. That discovery rate is in line with the frequency of such planets MEarth could detect, based on Kepler data, he added; changes in observing strategy should increase the discovery rate by a factor of two to four.
Another Harvard student, Courtney Dressing, stated at the meeting that, based on existing data, there should be 0.06 planets in the habitable zone per small star. That means there is, with 95% confidence, an Earth-like planet within 31 parsecs of our Sun.
“If it so happens that M dwarfs make habitable zone planets,” Charbonneau said at the ExoPAG meeting, “then it’s a lock that the closest habitable zone planet to us, the closest habitable planet to us, orbits an M dwarf.”
Exocomets, exomoons, and other exotica
Our solar system, though, is more than just planets: there are dozens of moons and many more asteroids and comets. These would seem to be too small to be seen around other stars from the Earth, but astronomers at the meeting reported evidence for at least some of these smaller bodies.
Barry Welch of the University of California Berkeley announced the discovery of several “exocomet” systems, based on distinctive spectral lines seen around young stars. This is, he described at a press conference, further evidence of the formation of solar systems. “We’ve found essentially the leftover building blocks of planetary systems,” he said.
Others reported evidence of “exoasteroids” by detecting so-called “polluted” white dwarfs, whose spectra contain the signatures of asteroidal material that have accreted onto the stars. “I think the conclusion you can draw,” said John Debes of the Space Telescope Science Institute about his and others’ research, “is that there are a lot of asteroids everywhere.”
Debes and others ruled out alternative sources for the white dwarf pollution, such as the interstellar medium. Ben Zuckerman of UCLA, who reported on the discovery of a planetary system in the Hyades star cluster by detecting calcium lines in the star’s spectra, argued that discovery likely means planets are also orbiting it. “You wouldn’t expect asteroids orbiting around a white dwarf to just be changing their orbits and falling onto their star unless something was perturbing their orbits significantly,” he said. “You really need those planets to change the orbits.”
Other studies of these planetesimals suggest they’re like the material that made the Earth. “To zero order, extrasolar minor planets resemble Earth” in terms of their composition, said Michael Jura of UCLA in a plenary talk. “We have yet to discover anything truly exotic or completely unfamiliar.”
Exomoons, though, still remain beyond the grasp of scientists, at least for now. David Kipping of Harvard reported on the first results of the Hunt for Exomoons with Kepler effort, looking at a very tiny sample—seven—of planet candidates from the Kepler catalog. None of those seven showed any signs of moons, although he noted in four cases they were able to set an upper limit on the size of any moon of 4% of the mass of the planet.
Future plans
As the numbers of exoplanets grow, scientists are increasingly interested to try and learn more about them than just basic characteristics like mass, radius, and orbit. That pushes the limits of what’s possible today with ground- and space-based telescopes.
“The key diagnostic technique [in astronomy] is spectroscopy,” said Marcy at the ExoPAG meeting. That, however, is difficult for most exoplanets. “To allow the exoplanet field to mature into a domain similar to the rest of astrophysics, we have to be able to take spectra.”
There has been some spectroscopy done of exoplanets, particularly of “hot Jupiters,” gas giants closely orbiting their parent stars. That has enabled some knowledge of their atmosphere, including composition, temperature profiles, and winds, Heather Knutson of Caltech explained in a plenary talk at the meeting. “We have a lot of theories, we’ve got some interesting patterns, but we’re still trying to figure out how to fit the two together,” she said.
Studying more exoplanets, particularly smaller ones, may require new telescopes. At the ExoPAG meeting, Marcy argued for an exoplanet mission using one of the two 2.4-meter telescopes “donated” to NASA last year by the National Reconnaissance Office. “I think there’s a great opportunity to do spectroscopy of Jupiters and Neptunes around the nearest stars” with such an observatory, he said.
| “To allow the exoplanet field to mature into a domain similar to the rest of astrophysics, we have to be able to take spectra,” said Marcy. |
Immediately before the ExoPAG meeting, NASA announced it was chartering two science and technology definition teams to study concepts for exoplanet direct imaging missions. One will examine the use of an internal coronagraph to block light from the star to enable direct detection, while the other will study use of external starshades to accomplish the same thing. The goal will be to develop “probe-class” mission concepts with a total cost of $1 billion; those studies will be completed by early 2015 to support NASA planning for future missions later in the decade, after the completion of the James Webb Space Telescope (JWST).
Without a firm commitment to a dedicated exoplanet mission—and concerns about the funding for astrophysics missions in general later in the decade at NASA, given uncertainty about the agency’s long-term budget—some astronomers worry that the pace of exoplanet research could stumble, particularly when Kepler’s mission ends in several years. Another spacecraft that detected exoplanets by means of observing transits, France’s CoRoT spacecraft, suffered a malfunction in orbit last year and many fear the spacecraft is lost.
Others, though, are more optimistic about the future, seeing opportunities to continue exoplanet studies with ground-based telescopes and JWST and, later, the Wide-Field Infrared Survey Telescope (WFIRST) space telescope. It’s possible, then, that despite all the excitement about exoplanet research present at the AAS meeting, the real golden age of exoplanet studies, including finding true Earth-like worlds, may still be yet to come.