Cramped. Five of the six Kepler-11 exoplanets (all larger than Earth) orbit their star closer than Mercury orbits the sun.

A Disturbingly Weird Exoplanet System

Dick writes about Earth and planetary science for Science magazine.

Today's news that the Kepler spacecraft has turned up an alien solar system with five of its six known planets bunched tightly around their star had a familiar ring. In the past 15 years, astronomers have identified hundreds of solar systems, each bizarre in its own way. Yet this new system offers a unique way to learn more about how planets form and evolve.

"We keep hearing it over and over again: Other planetary systems aren't like our own," says exoplanet astronomer Debra Fischer of Yale University. But "this six-planet system is just incredible."

The find comes from NASA's Kepler telescope, launched in March 2010 to hunt for planets orbiting other stars. For months on end, it has been staring at 156,000 stars in the constellation Cygnus, searching for the tiniest fading of a star's light that signals a planet passing in front of it. Reporting online today in Nature, planetary dynamicist Jack Lissauer of NASA's Ames Research Center in Mountain View, California, and 39 Kepler co-authors announced that the effort paid off with the discovery of six planets orbiting a single star designated Kepler-11 2000 light-years away.

None of the planets is Earth-like or even as small as Earth. All fall between Earth and Uranus in mass, and three are clearly gas giants with thick atmospheres of hydrogen and helium like Uranus's. Most odd is that five of the six orbit much closer to their star than Mercury orbits the sun, whereas the sixth orbits closer to where Venus would be. "It is by far the most densely packed planetary system," Lissauer said during a press teleconference this week. "Why there are so many so close is a mystery."

But that mystery may eventually help astrophysicists understand how and where planets arise. Researchers have long debated how close in gas giant planets form from a flat disk of primordial gas and dust. One theory posits that they form far from their parent stars—about where Jupiter and Saturn are now—and move inward over time. Another says that they form in place. Researchers try to figure out which way real planets form by simulating planet formation in computer models.

The Kepler find dramatically shows how poorly the models are doing. "The usual planet formation model is severely challenged to explain the Kepler-11 system," says planetary dynamicist Peter Bodenheimer of the Lick Observatory at the University of California, Santa Cruz. Making all five Kepler-11 planets far out and moving them in "is so complicated it's unlikely," he says. But forming them about where they are now would require an unrealistic amount of solid particles to form a rocky core, he says. For now, Bodenheimer leans toward formation in place, if only because it is "a little bit less problematic."

Kepler-11's tightly bunched planets may have deepened the mystery, but researchers see a ray of hope in them as well. The combination of Kepler's extreme sensitivity and closely spaced planets gravitationally tugging on one another allows team members to learn more about the Kepler-11 planets than astronomers using other techniques have found out about other, less-populated systems. Those additional properties include a planet's mass, density, and proportion of rock and gas—just the sort of information modelers need to test their theories of planet formation convincingly.

Posted in Space