Planet collisions may have rearranged crowded solar systems

Many worlds discovered by Kepler survived ruthless culling

Kepler 11

TIGHT FIT  The star Kepler 11 (illustrated) is home to six planets, five of which would fit inside the orbit of Mercury. Most solar systems might form with planets squished even closer together.

Tim Pyle/NASA

Sibling rivalries among planets can turn deadly. Families of worlds huddled close to their stars might destroy one another, leaving behind just one or two planets after the carnage. These family feuds probably erupt in planetary systems that form much differently than our own, though it’s possible our solar system suffered similar growing pains.

Many multiplanet systems discovered by the Kepler space telescope live on the edge of stability, astrophysicist Yanqin Wu and her student Bonan Pu report online February 19 at If the planets were any closer together, they would have already nudged each other off course and collided. Pu and Wu, both of the University of Toronto, propose that the planets in these systems are survivors from a more diverse population. Over time, planets that formed even closer together were whittled away until just one or two worlds remained.

The Kepler space telescope spent nearly four years monitoring over 150,000 stars for the silhouettes of planets passing across the face of their sun. “One of the most important things we learned,” says Anders Johansen, an astrophysicist at Lund University in Sweden, “is many systems are rich in planets.” Kepler 11, for example, hosts six worlds, five of which would fit inside the orbit of Mercury — though it’s not at risk for committing planeticide any time soon. These crowded neighborhoods are much different from our own solar system, where the four inner rocky planets have plenty of breathing room.

Pu and Wu ran computer simulations of planetary system evolution and found that when solar systems form, a retinue of planets bunched up close to their star might be the norm. “That makes the case of our sun even more puzzling,” Wu says.

Our solar system has a relatively large void between the sun and Mercury compared with the Kepler planets. “How come we are empty where other stars in the galaxy have lots of planets?” asks Wu. Perhaps Jupiter and Saturn interfered with planet-forming dust collecting in the inner solar system. Or maybe the Kepler worlds formed farther from their stars and wandered in later. But these are all speculations. “We’re still a little bit in a state of shock about having so many planets,” Johansen says of the nearly 1,900 known worlds in our galaxy. “We still don’t fully understand how to form them.”

Planetary scientist Kathryn Volk looks at it another way. “What if our solar system formed just like these Kepler systems?” she asks. She and Brett Gladman, both of the University of British Columbia in Vancouver, looked at what might happen if three or so Earth-sized planets had formed inside the orbit of Venus. Not only do they eventually destroy one another, but they leave behind a planet roughly the size of Mercury, Volk and Gladman report online February 23 at That could resolve a long-standing paradox of why our solar system’s innermost planet is so small.

It’s feasible that our solar system once harbored more planets. Researchers suspect that the moon formed after something the size of Mars smashed in to the Earth. “Apparently there was at least one extra Mars-sized body hanging around,” says Volk.

Wu agrees that it’s possible that our solar system was once more crowded, given the prevalence of compact systems, but emphasizes that many of these ideas are hard to prove. “You can always try a special condition,” she says, “but how do you know that’s the right one?” To answer these questions, researchers need to run more detailed simulations that investigate how planets weed each other out.

Editor’s note: This story was updated on March 4, 2015, to correct the spelling of Brett Gladman’s name in the second reference.

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