Solar System Packs its planets like Sardines

If you thrill at the discovery of new exoplanets, hold tight. A sextuplet alien solar system has been glimpsed in exquisite detail, revealing six planets of varying mass, five of which are packed closer to one another than in any planetary system seen before.

"We think this is the biggest thing in exoplanets since the discovery of 51 Pegasi b, the first exoplanet, back in 1995," Jack Lissauer of NASA's Ames Research Center in Moffett Field, California, told reporters earlier this week.

The six newly found planets orbit a star dubbed Kepler-11, which sits some 2000 light years from Earth. They were glimpsed by NASA's Kepler telescope, which has been staring at the same patch of sky since its launch in March 2009.

The Kepler telescope captured periodic dips in Kepler-11's brightness, created when planets pass between the star and Earth. These "transits" allow astronomers to measure a planet's size.

But Kepler-11's inner five planets – which all orbit closer to their host than Mercury does the sun – are close enough to one another to exert gravitational tugs that continually alter the length of time it takes for each planet to orbit the star. These timing variations allowed Lissauer and a team of colleagues to estimate the planets' masses, which range from 2 to 14 times the mass of the Earth (shown in the bottom row of this image).

Planetary puzzle

The researchers then used this information to estimate the density of the innermost planets and found that all are less dense than the Earth. Some may have massive hydrogen atmospheres, they say, while others may contain significant amounts of water.

Future observations should pin down the planets' densities, which could help astronomers discern whether they formed close to or far from their present locations. Either scenario could present a challenge for planet formation models, says Phil Armitage of the University of Colorado, Boulder.

These models suggest the region close to Kepler-11 might have been hot enough to keep ice vaporised and blow away nearby gas, preventing the growing planets from capturing as much gas and ice as they seem to have.

Conversely, if the planets formed farther out, models suggest they would have exerted strong tugs on their neighbours as they migrated inwards, sending the six planets into orbits on differing planes. Yet Kepler-11's planets all seem to orbit in a single plane. "That certainly makes for a puzzle about how the system was set up," Armitage says.

Testing laboratory

Solving that puzzle could refine our understanding of how planets form. "It's a pretty big deal," says Kristen Menou of Columbia University in New York. "I think this will be one of the best laboratories for testing planetary formation theories."

The discovery of the Kepler-11 planets comes against a backdrop of recent exoplanet excitement.

Researchers just announced a fresh haul of data on exoplanets, including 54 planet candidates in the habitable zones of their stars, also captured by the Kepler telescope. And last September, the first alien planet capable of hosting life on its surface was glimpsed.

Although its existence has yet to be confirmed by further data, scientists have been using climate models to figure out whether its climate is life friendly. An even bigger prize would be the discovery of an Earth twin – a planet the size and temperature of our own.