New planetary candidates range from Earth-size to Neptune-size to Jupiter-size. (Image: NASA/Wendy Stenzel)

When a veteran planet hunter like Debra Fischer calls it the most momentous discovery since 51 Peg, you know it must be big.

In 1995, scientists found the first planet circling a normal star outside our solar system—an unassuming yellow dwarf called 51 Pegasi. In the 16 years since, they’ve identified more than 500 such exoplanets.

Today they tripled the total in one announcement.

Actually, the Kepler spacecraft science team is only claiming 1,253 new candidate planets, based on four months of staring at 155,000 stars in the constellation Cygnus. Most still need verification with ground-based telescopes, and perhaps 20 percent of the claims will wash out, according to Fischer, who was on the panel presenting the Kepler findings to the press today.

Still, the number is impressive. According to Kepler project scientist William Borucki, his telescope is only surveying 1/400th of the sky. So by extrapolation, half a million new planets might be out there, easily detectable with a Kepler-style telescope, which watches for dips in light as a dark planet crosses in front of a bright star.

Kepler’s new batch of 1,253 candidates includes 68 Earth-size- planets,  288 “super-Earths” (twice as big as our own world), 662 the size of Neptune, and 165 the size of Jupiter. The telescope even found one sun-like star called Kepler-11 with six planets, a record.

A whopping 54 of the new candidates orbit in the so-called habitable zone of their star, where temperatures would theoretically be moderate enough for life to exist. And five of those 54 are Earth-size.

Finding a true Earth analog—a planet the size of our own, circling a star similar to the sun at roughly the same distance—requires three years of data. That’s how long it will take to get repeated crossings of the sun’s disk for orbits the size of Earth’s.

So, says Borucki, we’ll need patience to find a close match of our own planet around another star. But that doesn’t mean some of the 54 planets announced today, or even their moons, couldn’t harbor life of some kind.

Meanwhile, you can join in analyzing the Kepler data at the “Planet Hunters” site. So far, says Fischer, some 16,000 people around the world have participated in this nifty example of “citizen science.” They’ve already turned up hundreds of candidates, most of which are likely to overlap the list released today by the professionals.

Artist's conception by L. Calçada/ESO

Having already found more than 500 planets circling distant stars, scientists are getting better at understanding what they’re made of. A group led by Jacob Bean at the Harvard-Smithsonian Center for Astrophysics reports in this week’s Nature that they’ve analyzed the atmosphere of a planet only slightly larger than our own for the first time. And they may have found water —or rather, steam.

The planet, called GJ 1214b, has a radius about 2.6 times larger than Earth’s, and orbits a star located 40 light-years away in the direction of the constellation Ophiuchus. Scientists knew from previous observations that the planet must have an atmosphere, because its density is too low for an all-rocky planet. Theoretical models suggest three possibilities: A) a cloud-free hydrogen atmosphere, B) high clouds or haze obscuring a deeper hydrogen atmosphere, and C) an atmosphere made mostly of water vapor.

Bean and his colleagues used the 3.6-meter Very Large Telescope in Chile to analyze the spectrum of starlight filtering through the planet’s atmosphere. The data led them to rule out option A and favor option C, the steam world (although B is still a possibility). And future infrared observations should be able to distinguish between B and C.

Sadly, though, “the planet would not harbor any liquid water due to the high temperatures present throughout its atmosphere,” say the authors.

Photo: JAXA

We had hoped that Japan’s IKAROS solar sail would work as advertised, and it did. Here’s an animated image of the fully deployed sail, taken by a “separation camera” from a short distance away.

In other happenings:

• The Hayabusa asteroid sample return capsule came home in spectacular style last week. Video here.
• Scientists on NASA’s Kepler planet-hunting mission have released a new batch of data, and now have 400 “objects of interest” that could turn out to be new planets. There’s some disagreement over whether the team should be able to hold on to the data until they’re sure, though.

It’s nice when an expensive new machine works as advertised—nicer still when that machine has the ability to revolutionize a whole field of science.

At this week’s meeting of the American Astronomical Society in Washington, scientists couldn’t stop gushing about the exquisite performance of NASA’s Kepler telescope, which was launched last March and is now staring at a field of 150,000 stars in the constellation Cygnus, looking for tiny (less than a hundredth of a percent) dips in brightness that signal the presence of an eclipsing planet.

The First Five

“We are doing photometry at a level never seen before,” said David Latham of the Harvard-Smithsonian Center for Astrophysics. And as the high quality of the data becomes apparent, “We’re all burning the midnight oil seven days a week,” to keep up, said planet hunter and Kepler team member Geoff Marcy of the University of California at Berkeley.

Kepler scientists announced the discovery of five new planets at the meeting, including one oddball with the density of Styrofoam. But perhaps more interesting were some of the objects that show unusual light curves but have yet to be identified. Until they’re confirmed as planets or something else, the project simply calls these “Kepler Objects of Interest,” or KOIs. KOI #74 and #81 are especially strange—only four times the size of Earth and the size of Jupiter respectively, they’re both hotter than the stars they orbit. “Does anybody know what they are?” Latham asked an auditorium full of astronomers. Nobody ventured a confident answer.

It will take three years for Kepler scientists to confirm Earth-size planets circling stars at Earth-like distances (such orbits take a year, and astronomers need a couple of orbits before making a positive ID). But it’s possible that small planets orbiting closer to their stars will turn up sooner in the data.

The real bottleneck will be follow-up observations from the ground. Just because a star’s light dims periodically, it doesn’t mean there’s a planet. It might be another star in a binary system regularly crossing in front of its companion. The best way to tell the difference is with ground-based spectroscopy.

In the first 43 days of observing, Kepler turned up 175 KOIs—light curves that could indicate the presence of planets. Examination of the first 50 yielded 5 confirmed planets, but the other 125 in the queue are waiting to be followed up, and Kepler has been looking for several months since then.

We’re going to have to be patient.