NASA's Kepler mission, designed to find extrasolar planets transiting across the faces of their stars, is off to a great start, project scientists announced Monday morning (Jan. 4, 2010) at the convention of the American Astronomical Society (AAS) in Washington, DC. Attending their presentation was S&T editor in chief Robert Naeye. He rushed me back his voluminous notes.
Kepler science team leader William Borucki announced that Kepler has already logged more than 100 transiting-planet candidates, but said most of these will probably prove to be false alarms once followups are done by other methods. The team did release detailed information on five fully confirmed Kepler exoplanets that were discovered in just the first 43 days of data-taking last May and June, following the mission's launch last March. In these cases, radial-velocity measurements made by ground-based observatories showed the transiting objects to have planet-like masses, based on their stars' slight wobbles due to the planets' gravitational influence.
Four of the five new worlds are puffed-up hot Jupiters (with 1.3 to 1.5 times Jupiter's diameter) orbiting very close to their stars. One of them, named Kepler 7b, is among the lowest-density planets yet discovered, with a measured diameter and mass that yield an average density of just 0.17 grams per cubic centimeter (compared to Saturn’s 0.69 and Earth’s 5.52). That's the density of styrofoam. The result is “something theoreticians will be delighted to look at in terms of the structure of this planet,” Borucki said.
The other planet is a hot Neptune with about our own Neptune’s density — even though it orbits so close to its star that its surface layer should be roasted to 1,900°C. All five, in fact, should have outer-layer temperatures hotter than lava, and two should be hotter than molten iron.
Such hot worlds in fast, close orbits are the likeliest to transit their stars from our viewpoint, and are the most readily detectable if they do, and are the most quickly confirmable as well. Longer-period planets orbiting farther out, in a star's habitable zone for life where climates are more reasonable, with require a few years to find and confirm.
Lots more planets are coming — probably hundreds by the time the mission is scheduled to end three years from now. The host stars in all five cases announced today are somewhat larger and brighter than our Sun, with 1.4 to 2.0 times the Sun’s diameter. This is because these stars were chosen for early followups based on showing many narrow spectral lines, good for radial-velocity measurements. Such large stars are not very common; this bodes well for greater numbers of planets to be found around smaller dwarf stars, which are much more abundant.
In the first 43 days of data-taking, Kepler found about 175 transit candidates. Fifty of these were scrutinized to find the five confirmed planets that were announced. Some 125 candidates remain, and that's just from the first six weeks of data. Tidal waves of subsequent data are already in hand.
Kepler is watching, nearly continuously, a selection of about 156,000 stars from 9th to 15th magnitude (out of 4.6 million in its field of view) in a patch of sky covering 100 square degrees a little north of the plane of the Milky Way between Vega and Deneb, as shown at right. The satellite will keep watching these stars nearly 24/7 for at least 3½ years, in order to catch at least three transits of any luckily aligned planets that are in wide, Earth-like, 1-year orbits.
Any such planet has only a 1-in-200 chance of being in an orbit that's oriented just right to cross a Sun-sized host star as seen from our viewpoint. That's one reason why Kepler is watching so many stars.
Another is statistics. Kepler is intended not just to identify a few individual exo-Earths. It was designed, Borucki stressed, to watch enough stars to give a firm statistical reading on the abundance — or rarity — of terrestrial-size planets generally, throughout the galaxy and the universe.
Kepler is routinely achieving 1-part-in-40,000 brightness precision (0.000025 magnitude) for measurements of 12th-magnitude stars. That is good enough to find transits of worlds as small as Earth, as planned.
Variable Stars, Oscillating Stars, Rotating Stars. . .
Some other news from the team's press conference and just-released papers:
• Kepler’s measurements are so precise that most “false positives,” such as an eclipsing binary star blended with the image of another star, can probably be weeded out upfront, without tedious and expensive radial-velocity measurements from the ground. This is very good news. Eclipsing binaries are the main source of false “transits.”
• For hot Jupiters — large bodies dazzlingly lit by their stars — Kepler can even see the "secondary eclipse": the planet's light being blocked by the star when the planet passes behind it, as shown at right. The depth of the secondary eclipse, combined with the planet's known diameter from the primary transit, can help tell the planet's albedo (reflectivity; how light or dark it is). Details of the phase effect, or how the reflected light changes as the planet waxes and wanes from crescent to full and back again, should tell more about its atmosphere. Moreover, the light curve also indicates how the shape of the star itself is slightly distorted by a close giant planet's tidal effect. (More on the secondary eclipse above.)
• For a few stars, Kepler has measured slight surface oscillations like those on the Sun. These arise from low-frequency sound (pressure) waves resonating through a star’s body. These resonant oscillations reveal a star’s diameter, mass, and state of evolution with very high precision — refining, in turn, the diameter, mass, orbit, and age of any planet it may have.
For instance, a Kepler oscillation study has refined the diameter of the star HAT-P-7 from an uncertainty of 10% to 1%, which in turn has refined the density of its previously known planet from 50% to 5% uncertainty. Ronald Gilliland, a stellar-oscillations specialist at the Space Telescope Science Institute, said at a press conference later in the day, “The data coming back from Kepler for stellar seismology is just spectacular. We expect it to reinvigorate and revolutionize our field.”
• Thousands of new variable stars are turning up, and Kepler’s extremely high-precision, near-continuous light curves offer rich material for new study. For instance, a third of the stars most similar to our Sun turn out to have tiny, short-term variabilities greater than the Sun’s. But not by much. And in time, we may be able to see the equivalent of our own star's sunspot cycle.
• A star’s placement on the detector’s array of pixels yields its position to better than a thousandth of a pixel-width, or 4 milliarcseconds. This is that way that many eclipsing binary stars are being weeded out, by their proper-motion wobbles. Such precision will also provide the best-yet parallaxes (distances) for most of the faint stars in Kepler’s catalog. Good distances are needed to help to characterize any planets the stars are seen to have.
• Slight, periodic variations can also reveal a star’s rotation rate, due to temporary starspots rotating in and out of view. In this way Kepler scientists expect to better calibrate the relation between a star’s rotation rate and its age and mass, the most widely useful way to assign ages to individual stars everywhere.
• Two strange, very hot objects that Kepler found orbiting white, type-A stars appear to be too hot to be planets but too low-mass to be stars. These may be an odd class of "not-really-white-dwarfs" — in which a normal white dwarf has shed all but about 0.15 solar mass to its companion star, and has thus enlarged beyond normal white-dwarf size due to its lower mass, gravity, and degree of compression.
Only the Beginning
So far, the Kepler team members have been holding most of the satellite's data close to the vest as they work on planet confirmations themselves. This morning Borucki said that Kepler data will be released publicly on a regular basis starting in June 2010, around the one-year anniversary of the start of its regular science observations.
At the afternoon press conference, independent commentator Caty Pilachowski (Indiana University) summed up astronomers' excitement. The new finds, she said, "represent the first of a new breed"; Kepler can, for the first time, compile an unbiased sample of transiting exoplanets free of many selection effects, to "get a true picture of what planets inhabit close-in zones and eventually farther out. What we’ll learn from Kepler five years from now will be astounding — what percentage are rocky planets, steam planets, or Jupiter-like." So, she urged the reporters present, please be patient.
One important piece of Kepler news, she said, is the finding that Sun-like stars are generally quiescent, showing only very small brightness variations. That's good news for astrobiology, and it's also good news for us. It tells us that solar-type stars spend most of their time in quiescent states, so we ourselves are in a good, safe place, not just enjoying a lucky quiet time next to a star that may act up.
"The precision is so exquisite, it makes future of stellar astronomy very exciting," she said. "What we can learn from slow rumbles of red giants is very exciting; we will understand insides of these stars, it will tell us about detailed evolution of stars going from tip of the main sequence to red giants. The statistics on the fraction of binary stars will lead to new science results . . . Kepler will give us all of that data. It will change everything about how we do astronomy."
See NASA's press release on today's announcements.
View the slides from the press conference.
Here are the past year's research papers from and about Kepler on the astronomy preprint archive, arxiv.org/astro.ph.
The five new worlds will bring the total in the Extrasolar Planets Encyclopaedia up to 420, including 69 that have been seen to transit.