Five of the new planets are closer to their parent star than Mercury is to the sun. The sixth world lies farther out, within a region that would fit inside the orbit of Venus.
"This is the most closely packed known planetary system," said study co-author Jonathan Fortney, an astronomer with the Lick Observatory at the University of California, Santa Cruz.
The planets are relatively small—ranging from roughly 2 to 4.5 times Earth's radius—but they are also surprisingly lightweight, indicating they are made mostly of gases. Based on their densities, four of the planets appear to have thick atmospheres of hydrogen and helium.
The two planets closest to the star have higher densities, suggesting these bodies have atmospheres of mostly water, with just a thin skin of hydrogen and helium.
Finding so many planets around the same star and being able to calculate their properties is a scientific boon, Fortney said. Like paleontologists studying related dinosaur species, astronomers can look at multiple worlds that were born together to get a better understanding of planetary transformations.
"We can do comparative science and ... we can think about how the evolution of the planets has diverged over time," he said.
New Planets Found Via Orbital Dance
The Kepler space telescope was designed to look for Earth-size planets that transit—or pass in front of—their host stars, as seen from Earth.
"We're basically targeting 100,000 to 150,000 stars next to the constellation Cygnus," said Fortney, who is a member of the Kepler science team. "Kepler just stares at that patch of sky unblinking for four years."
With enough time, astronomers can tease out periodic dips in the light from stars as planets pass. (Find out how you can help hunt for planets using Kepler.)
Fortney and colleagues found six signature dips in light from a star called Kepler-11, about 2,000 light-years away. The star is almost the same size, temperature, and brightness as our sun: "It's very much like a solar twin," Fortney said.
At such a distance, though, the star is very dim, making it hard for astronomers to use other planet-hunting techniques to verify the find. Instead the Kepler team confirmed the discoveries using a method called transit timing variations, or TTV.
"Think about one planet that transits," Fortney said. "If its orbital period is ten days, every ten days it will pass in front of the parent star. But if multiple planets transit, they'll affect each other [via gravity]. A planet may transit early or late."
Similar calculations of planets affecting each other's orbital timing are what allowed French and German astronomers to find Neptune in 1846.
The outermost planet in our solar system, Neptune was barely visible to telescopes of the time, and those who could see it thought it was a star. But astronomers knew that Uranus wasn't orbiting as it should, based on laws of physical motion.
Uranus's odd orbit led mathematician Urbain Joseph Le Verrier to predict Neptune's position and mass to account for the discrepancies.
In the case of the Kepler-11 star, the gravity-driven choreography of dips in starlight is almost certainly the product of multiple planets, the team concluded. Further studies, detailed this week in the journal Nature, allowed astronomers to calculate with significant accuracy the masses, positions, sizes, and densities of five of the new worlds.
The sixth planet is far enough away from its siblings that it doesn't affect their orbital dance. Instead astronomers had to run through a suite of calculations to be sure the planet exists. While the team is confident it's there, they weren't able to tease out as many details about the outlying world.
"We know the radius is about 3.6 times that of Earth," Fortney said, "and it's probably less than 30 Earth masses."
"Super Earth" Among New Planets?
The new planetary sextet is remarkably similar to another tight-knit set of worlds found around the sunlike star HD 10180, which was announced last fall. That star lies a mere 127 light-years from Earth.
A team of scientists based in Europe found five Neptune-like worlds close to the star and a larger sixth planet farther out.
They also saw a faint signature of a much smaller world very close to the star that may be a "super Earth"—a rocky planet much larger than our own.
One key difference between the two newfound systems is that "the 'packing' is much more pronounced in the new Kepler system," HD 10180 study leader Christophe Lovis, an astronomer with the Observatory of Geneva in Switzerland, said in an email.
"At first glance I was very surprised that such a system could be dynamically stable on the long term," he said. "Planet masses in the Kepler system are lower than in HD 10180 ... which probably makes it possible to have an even more compact configuration."
Lovis and colleagues had found the planets around HD 10180 using the High Accuracy Radial velocity Planet Searcher, or HARPS, instrument in Chile. The radial-velocity method measures wobbles in starlight caused by the gravitational tugs of orbiting bodies.
"The fact that HARPS and Kepler, using different techniques, find similar things is reassuring in itself and shows how complementary both approaches can be."
Lovis notes, however, that he'd prefer to have the planetary properties of the Kepler-11 system confirmed by other techniques.
"In this particular case, radial velocities would be helpful to measure the masses, probably more precisely than is feasible with TTV," he said. The problem is that the Kepler-11 star "is almost too faint for precise radial velocity followup and would require a lot of telescope time.
"This will remain the main contrast between the Kepler survey and a typical radial velocity survey like the HARPS one: Kepler looks at tens of thousands of distant stars, whereas HARPS looks at a few hundred nearby, bright stars."
Even More New Planets Waiting for Discovery?
For now astronomers aren't sure whether the Kepler-11 system hosts any rocky Earthlike worlds like the one circling HD 10180.
"It's possible that there are smaller planets that we haven't been able to see yet," said Fortney, co-author of the new Kepler study.
"With a few more years of data, one may emerge. It's also possible that TTV will show us there's another planet in the system that doesn't transit. ... "
What the existing data do reveal is that the closely orbiting group around Kepler-11 most likely formed very quickly.
"The way we think that big gas giants like Jupiter form is that first a protocore of ice and rock forms that's like ten [times Earth's mass]. Through its gravity, the core pulls tremendous amounts of gases on top"—notably hydrogen—Fortney said.
In the disk of planet-forming materials, free hydrogen gas lasts just five million years or so before stellar wind—charged particles flowing from the host star—blows it all away. That means the gassy planets around Kepler-11 must have grown up quickly for them to be full of hydrogen.
The Kepler-11 system is also a good case study for what happens to gas planets that move in close to their stars. Based on their sizes and orbital dynamics, the Kepler planets formed farther from the star and migrated inward, the team thinks.
The two mini-Neptunes closest to the star may once have had thicker hydrogen-helium atmospheres like their siblings, but as the two nearer worlds cuddled close to the star, its powerful radiation began to strip away the outer layers.
Looking at the differences between the two sets of planets can help astronomers understand exactly how such worlds lose mass over time.
"In the long term, I think we're going to find that multiplanet systems are common," Fortney added. "With transits limited to seeing edge-on systems, we're always going to be finding the minimum number of planets—a system could have an alignment so that we measure three planets when really there are five."
But overall, he said, "when you see one planet, you're probably going to find another."