The planet-hunting Kepler Space Telescope released its second batch of data today, revealing more possible new planets than have been spotted to date. The cornucopia of planetary systems includes a star orbited by six worlds, two of which appear to be watery "miniature Neptunes."

The data cover the telescope's first four and a half months of staring wide-eyed at 156,000 stars near the constellation Cygnus, and watching for telltale winks that signal the passing of an orbiting planet.

The first bundle of data, released last June, revealed 306 target planets, but deliberately held back the smallest and most tantalizing candidates. Now, those promising planets – 1,200 altogether, pending final data analysis – are ready to go public.

"We’re just opening up the treasure chest," said exoplanet expert Daniel Fabrycky of the University of California, Santa Cruz, a member of the Kepler team. "There's lots of nice planetary systems there for the taking."

"We think this is the biggest thing in exoplanets since the discovery of ... the first exoplanet."Kepler finds planets by watching for their host stars to dim slightly as the planet crosses, or transits, in front of the star. By measuring the amount of light the planet blocks, astronomers can figure out how big the planet is. So far, Kepler has found planets ranging from many times the radius of Jupiter to just 1.4 times the radius of Earth.

But most of those planets were solo performers. Until today, Kepler-9, which has one Earth-sized world and two Saturns, was the only known star with more than one transiting planet. Among the most exciting systems revealed today is a family of six planets circling a sun-like star named Kepler-11.

Those planets were detected because they gravitationally tug each other to and fro. If a star has just one planet, it should grow brighter or dimmer like clockwork as the planet transits. But adding more planets mean the transits are never on time.

Multi-planet systems are especially valuable because the deviations from clockwork let astronomers compute the planets' mass, a measurement that would normally require hours or days of observations with the biggest telescopes on Earth.

"We think this is the biggest thing in exoplanets since the discovery of 51-Pegasi b, the first exoplanet, back in 1995," said astronomer Jack Lissauer of the NASA Ames Research Center in a press teleconference Jan. 31. The system is described in a paper in the Feb. 3 Nature.

In order for all six planet-crossings to be visible from the vantage point of Earth, the planets must all lie in a plane flatter than a CD, Lissauer says.

The five inner planets all orbit the star once every 46 days or less, and the outer planet orbits once every 118 days. If this system were dropped into our solar system, all the planets would be closer to the sun than Venus.

Compared to Earth, the planets are remarkably light for their size. The five inner planets range from 1.97 to 4.52 times Earth's radius and 2.3 to 13.5 Earth's mass. The outer planet, whose radius is 3.66 times Earth's, is too far away for its mass to be completely determined, but astronomers know it is less massive than Jupiter.

These planets are unlike anything in our solar system. From the planets' masses and radii, astronomers can calculate their density, a clue to composition. Although the planets around Kepler-11 are fairly small, they're all much less dense than Earth, making them more like mini-Neptunes than super-Earths. The inner two could be mostly water, with a thin atmosphere of hydrogen and helium. The farther-out planets may have much thicker hydrogen and helium atmospheres that comprise up to 20 percent of the planet's mass.

Planetary scientist Jonathan Fortney, a coauthor of the paper describing Kepler-11, suggests imagining these planets as a "marshmallow with a ball bearing in the center," where the thick atmosphere makes up most of the volume, but not much of the mass."

If you could stand on the surface of these planets, "you would not be able to see the sky," Fortney told Wired.com in an email. "The hydrogen-dominated atmospheres that we are talking about are extremely thick. You literally would be 1/2 to 2/3 of the way down into the planet, and the atmosphere would be opaque."

How such strange planets formed, and how they ended up in such a tightly packed configuration, is a puzzle.

"This is sending me back to the drawing board," Lissauer said. "It’s just totally unexpected to be able to get a planetary system where planets can be this close to one another, that there can be so many of them, that they can be so flat. It’s really a sense of extremes there."

The new data dump includes an abundance of other multi-planet systems, including one with 5 planets, 8 quartets, more than 100 doubles and triples.

Of the 1,235 new planet candidates, 68 are Earth-sized, 288 are super-Earth sized, 622 are similar to Neptune, and 165 are as big as Jupiter. 54 of the candidates orbit in the habitable zone, the right distance from their stars to sustain liquid water.

One of those candidates is smaller than Earth. Four are super-Earth-sized, and most are Neptunes or Jupiters. But all the moons of those Jupiters would also be in the habitable zone, noted Kepler science principal investigator William Borucki of NASA Ames Research Center in a press conference Feb. 2.

"For your Christmas vacation you could go from one moon to another, and have a vacation on that different moon," Borucki said. "I'm not saying that happens every day, but it's possible."

"All of these new Kepler discoveries, not just the six-planet system but all of them, are showing us that there is a bounty of low-mass planets around other stars," said astronomer Debra Fischer of Yale University, who was not involved in the Kepler observations. "We can see more securely that these low-mass planets not only exist, but they’re much more numerous than the big gas giant Jupiter like planets."

The new trove of observations also brings Kepler closer to its main goal: Finding Earth-sized planets that orbit in the habitable zone, where they might host life.

"We can be pretty sure we will find rocky planets in the habitable zone, because of how the numbers are coming out for smaller and smaller planets," Fabrycky said.

Theorists who study planetary formation and dynamics will be eager to jump on the data immediately, said exoplanet expert Sara Seager of MIT.

"Kepler is making people’s dreams come true. It sounds kind of corny, but it’s really true," she said. "It’s changing exoplanet science as we know it."

*Images: NASA/Tim Pyle

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