The talks are over and the posters packed up—Day 1 of the summer meeting of the American Astronomical Society is officially over (minus the drinking and late-night carousing)—and from one perspective, NASA’s Kepler Mission, its supremely successful planet-hunting satellite, was the star of the show. But advances were everywhere you looked, from Noreen Grice’s talk on making astronomy accessible to the blind by emphasizing a tactile approach, to a presentation on the upcoming Pan-STARRS wide field imaging survey, to Alan Marscher’s vocal and guitar performance of his astronomy-themed songs.
Nevertheless, if AAS Seattle last January was Kepler’s coming out party, where it floored the astronomical community by announcing an abundance of earth-sized, potentially habitable planets in distant star systems, then today was a performance review bordering on the ecstatic, emphasizing its emerging roles in a diversity of fields, and the progress its scientists have made in analyzing its data.
“We are moving from observations to statistics,” said William Borucki, one of the lead scientists on the Kepler team, at a press conference this morning (which I did not attend—but did monitor via the #aas218 hashtag on Twitter!). The statistics are yielding a treasure trove of information that are redefining astronomers’ notions of what planetary systems look like. A veritable laundry list of findings was announced today, among them:
- Multiplanet systems are more common than previously thought, and given that Kepler searches for planets with the transit method—watching for a star’s dip in brightness as a planet crosses in front of it—multiple planets crossing the tiny disc of their star means that the planets’ orbits in these systems are aligned flat like a pancake.
- Further confirmation that Neptune-sized planets are much more common than Jupiter-sized planets—and earth-sized planets are much more common than Neptune-sized planets.
- The previously announced Kepler-10 planetary system is host to another previously undetected rocky planet, now designated Kepler 10-c. Unlike the previously announced planets which were classified as “candidates” and require follow-up observations, Kepler 10-c can be chalked up as a “confirmed”.
- The case of Kepler 10-c is particularly important as it served as a demonstration of a powerful new technique for confirming planet discoveries that the Kepler team calls “validation”. In it, transit candidates are observed simultaneously by two telescopes operating at different wavelengths—in this case, both Kepler (operating in the optical) and the Spitzer Space Telescope (which sees in the infrared). If there is really a transit, the resulting dip in brightness will look much the same at both wavelengths. If, however, the dip is really just the natural variation of the star, it will look much different to Spitzer than it will to Kepler. This technique allows for confirmation with only one set of observations of the same transit, instead of having to wait for the planet to circle around its star again for another pass.
And that was just the morning presser.
In the afternoon, Ron Gilliland gave an invited lecture in the cavernous main hall in which he spoke to what many scientists had suspected and hoped for: that Kepler, billed as a planet-hunter, has also emerged as an excellent weapon in the battle to understand stars.
The technique is known as asteroseismology (yes, there’s an e in “aster”—something I didn’t catch at first), and as the name suggests the study of earthquakes on earth, asteroseismology is the field of studying “starquakes”.
The waves of an earthquake that travel through the earth’s interior are known as acoustic waves—that is, the waves are made up of material moving forwards and backwards, compressing and decompressing, just like sound waves in air. (These are known as p-waves, for they’re propagated by the force of pressure.) The cool outer layers of a star act as a hollow sphere that rings with these waves like a bell.
Stars that are denser add to these p-waves another type of wave, the familiar buoyant, bobbing up and down waves that you see on the surface of the ocean—with the twist that not only can they appear on the surface, but throughout the interior of the star as well. (These are known as g-waves, or gravity waves, because they are formed by the force of gravity competing against buoyancy.)
Altogether, these bell-like and oceanic-like waves can give us a wealth of information about stars. For one, they can yield very accurate measurements (to within about 1%) of the sizes of stars, an impressive achievement in itself. But they can also tell us much about their interiors—how dense they are, what they’re made up of, and, in the bigger picture, how a star evolves from a contracting cloud of gas to a fiery beacon in the void of space, suspended in the night sky. On a more human level, this is how babies are made, and all other things—since we are all star stuff.
Here’s the thing: these waves, as one might expect, are impossible to observe directly across the expanse of space. But as they ripple across a star, they do cause small but periodic changes in their star’s brightness. Sound like a job for any satellite we know?
Ding ding ding!
Kepler, by design, stares constantly at 150,000 stars looking for changes in their brightness. So even when it’s not finding planets, it’s able to provide scientists with an haul of asteroseismic data, unprecedented both in quantity and in quality. (Ironically, the first thing any asteroseismologist has to do with Kepler data is subtract out any transits in the data—one man’s riches is another man’s garbage, and all that.) Impressively, Kepler has been able to easily detect these waves, including unprecedented observations of widespread g-waves in red giants.
In fact, Kepler found that these stars are acoustically “noisier” than scientists’ theories had even predicted, and twice as noisy as the sun, a fascinating finding in itself. Gilliland noted that there is probably no such thing as a star whose brightness does not change from hour to hour.
This is a brave new world for stellar astrophysics, opening up new opportunities to test our theories of stellar evolution against real world observations. The scientists who were asked to anonymously referee these papers—a part of the scientific process that is intended to invite incisive criticism in the name of upholding truth and integrity—were nothing short of effusive with praise for the Kepler work. Gilliland read from excerpts of the reviews (which are usually kept confidential) and words that were used included “landmark”, “revolutionary,” “extraordinary,” and “brilliant”.
In short, “Kepler really is living up to the hype,” said Gilliland. All told, in barely two years of operation, Kepler has already yielded an impressive track record of accomplishments. Surveying my friends and colleagues who attended the morning press conference, some noted that much of the overall scientific picture of multiplanet systems that the science team painted had already been published in the months since AAS Seattle; although the numbers were refined, to an extent, it was an repackaging of old news. But we were all quite impressed with the utility that was explained in the afternoon asteroseismology talk. The picture that’s emerging of Kepler is that, far from a one-trick pony looking for once-in-a-blue-moon transit events, it is a true scientific workhorse that is leading to breakthroughs in multiple areas of research.
Follow me on Twitter @alshain. I’ll be tweeting with the #aas218 hashtag.