Nature / NASA / Ames / JPL-Caltech / T. Pyle

This illustration, appearing on the cover of the journal Nature, shows the six planets of the Kepler-11 system as they might have looked up close during a triple transit observed on Aug. 26, 2010. The Kepler probe couldn't produce direct imagery of the planets, but it could detect the dip in starlight caused by the transit.

Astronomers behind NASA's $600 million Kepler mission say they’ve detected a star system that packs six planets inside a space that would fit within the orbit of Venus in our own solar system. It’s the marquee event for this week’s “big reveal” from the most sensitive planet-hunting probe ever launched.

It's also a huge puzzle for planetary scientists.

The worlds around a star now known as Kepler-11 rank as the "most densely packed planetary system known," said Jack Lissauer, a researcher at NASA's Ames Research Center. Lissauer is one of the principal authors of a study about the Kepler-11 system, published in this week's issue of the journal Nature.

The six planets range in size from two to four times as wide as Earth, with orbital periods that go from 10 to 118 Earth days. The lightest of the planets is only about twice as massive than Earth. But unlike the close-in "super-Earth" that was reported by the Kepler team last month, none of these worlds is anywhere near as dense as our own planet. If the Kepler team's figures are right, ice or gas must make up a significant proportion of their mass.

"It is clear that such planets need not resemble the earth in any way, which adds to our incredible planetary diversity," astronomer Jonathan Fortney, a member of the research team from the University of California at Santa Cruz, told journalists. "If connections to our solar system can be drawn at all, the low-mass planets in the Kepler-11 system appear to be more like small Neptunes than massive Earths."

Such low-mass, low-density planets are "truly astounding," Alan Boss, a planetary scientist at the Carnegie Institution for Science, told me in an e-mail.

"No theorist would have been nutty (or brave) enough to have claimed that a planetary system like this one might exist," he said. "Yet it does."

Lissauer said he rated the findingas the "biggest thing in exoplanets" since the 1995 discovery of 51 Pegasi, the first extrasolar planet detected around a sunlike star. Yale astronomer Debra Fischer, a planet-hunting pioneer who was not involved in the Kepler-11 study, agreed. "With five low-mass planets in the system, this discovery is as momentous as 51 Peg was in 1995," she said today during a NASA news briefing.

"Kepler is actually reaching the milestone discoveries faster than certainly I anticipated," Fischer said. "Kepler has blown the lid off of everything that we know about extrasolar planets, and this week to me feels very different than last week did."

15 confirmed planets, 1,200 candidates
Today's announcement boosts the Kepler mission's count of confirmed planets to 15, less than two years after the van-sized spacecraft was launched from Cape Canaveral. From its Earth-trailing orbit, Kepler has been focusing on an area of the sky in the constellations Cygnus and Lyra, staring at more than 150,000 stars simultaneously. The probe's telescope documents the tiny dips in starlight that could signal the passage of a planet across a star's disk.

NASA / Nature

This graphic compares the sizes of Jupiter and Earth with the nine Kepler planets that were previously confirmed (above the line) as well as the six planets reported today (below the line). The term "RE" indicates the radius of the planet in terms of the radius of Earth.

Eventually Kepler should find thousands of new planets beyond our solar system, adding to a list that now amounts to more than 500. Just this week, the science team listed a fresh batch of 1,200 planetary candidates, including more than 50 planets that appear to be in the "habitable zones" of their respective star systems.

Not all of those candidates will be confirmed as planets. Some will turn out to be eclipsing binary stars or other phenomena that can mimic the dimming caused by a planet. But in time, astronomers expect to find hundreds of Earthlike planets in Earthlike orbits around sunlike stars. Kepler's census should shed new light on an age-old question: How prevalent are the conditions that could give rise to life in the universe?

If you were looking for life as we know it, you wouldn't look in the Kepler-11 system, which is about 2,000 light-years from Earth. Five of the planets orbit closer to their parent star than Mercury does around our own sun — much closer than the "habitable zone" where life as we know it could exist.

Finding so many worlds in one faraway planetary system is notable, but not unprecedented. Last year, astronomers using a different detection method found a star system that has at least five and perhaps as many as seven planets. Lissauer said the key distinction here is that the planets are so close to each other, circling in a disk that is proportionally flatter than a vinyl record.

"It's just totally unexpected to have this much material, to be able to get a planetary system where planets can be this close to another, that there can be so many of them, that they can be so flat," he said. "It's really a sense of extremes there."

The other curious thing about Kepler-11 is that the planets are close to the mass of Earth, but must be structured like ice giants or gas giants to account for their low density. "How in the world they formed is going to be a headache for theorists for some time to come," Boss told me.

NASA / Nature

This chart graphically shows the estimates of the new planets' radius and mass, as well as how they would fit in among the solar system's planets and other worlds that have been discovered (Kepler-10b and CoRoT-7b). The newly discovered planets (shown in blue) are more similar in composition to Uranus and Neptune than to Earth and Venus.

UC-Santa Cruz's Fortney said the findings suggest that the composition of planets up to 10 times as massive as our own can be "extremely variable."

Lissauer said it's "more likely than not" that astronomers will be detecting more of these mini-Neptunes than super-Earths. But that doesn't mean the search for alien Earths will be fruitless, as demonstrated by the Kepler mission's latest batch of planet candidates.

"There could very well be as many true Earth analogs as people have previously suspected," Lissauer told me. "Over the next two, three, four years, Kepler will be weighing in on that. It just takes a long time for us to collect enough data to address that question directly."

To untangle the complicated orbital dynamics of the Kepler-11 system, scientists tracked the apparent planets' movements for months. Usually, astronomers can confirm the existence of distant planets by checking for slight gravitational wobbles in the star that they orbit. But in Kepler-11's case, the planets were too small and the star was too far away to use that method. Instead, the Kepler team worked out the timing of the planets' movements so precisely that they could figure out their masses using those calculations alone.

"The timing of the transits is not perfectly periodic, and that is the signature of the planets gravitationally interacting," Daniel Fabrycky, a postdoctoral researcher at UC-Santa Cruz, explained in a news release. "By developing a model of the orbital dynamics, we worked out the masses of the planets and verified that the system can be stable on long time scales of millions of years."

The same method should come in handy as Kepler gathers more data on potential alien Earths. "We expect to be doing a lot of those analyses," Fabrycky said. 

In addition to Lissauer, Fabrycky and Fortney, the authors of the Nature paper, "A Closely Packed System of Low-Mass, Low-Density Planets Transiting Kepler-11," include Eric B. Ford, William J. Borucki, Francois Fressin, Geoffrey W. Marcy, Jerome A. Orosz, Jason F. Rowe, Guillermo Torres, William F. Welsh, Natalie M. Batalha, Stephen T. Bryson, Lars A. Buchhave, Douglas A. Caldwell, Joshua A. Carter, David Charbonneau, Jessie L. Christiansen, William D. Cochran, Jean-Michel Desert, Edward W. Dunham, Michael N. Fanelli, Thomas N. Gautier III, John C. Geary, Ronald L. Gilliland, Michael R. Haas, Jennifer R. Hall, Matthew J. Holman, David G. Koch, David W. Latham, Eric Lopez, Sean McCauliff, Neil Miller, Robert C. Morehead, Elisa V. Quintana, Darin Ragozzine, Dimitar Sasselov, Donald R. Short and Jason H. Steffen.

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