Planets pack tightly in the Milky Way

By Andrew Grant

Web edition: March 6, 2013
Print edition: April 6, 2013; Vol.183 #7 (p. 12)

Planetary systems in our galaxy are packed to the brim, according to a new study — throw in another orb and all hell will break loose. The study, posted February 28 at arXiv.org, argues that planets around other stars share an evolutionary history similar to that of the solar system’s eight planets.

“This study supports results that have been building for a long time,” says Jack Lissauer, a space scientist at NASA Ames Research Center in Moffett Field, Calif., who was not involved in the study. In 2011, he discovered Kepler-11, a star with planets so tightly packed around it that five of them have smaller orbits than Mercury’s.

About 25 years ago, astrophysicists examining the solar system’s planets realized that their orbits teeter on the edge of instability. Add another world, and the eight planets would start pulling each other into new, unstable orbits; some would ultimately collide or get tossed out into interstellar space.

In other words, our solar system is filled to capacity.

Scientists believe this state of affairs is the result of a game of planetary pinball that occurred soon after the sun formed 4.5 billion years ago. Swirling dust and gas coalesced into many miniature planets that were so close together that they constantly grappled for position. After countless collisions, migrations and ejections, only the eight planets remained, spaced just far enough apart to leave each other alone but close enough together that no other planets would fit.

So when astronomers started discovering planets around other stars in the mid 1990s, scientists wondered whether faraway planetary systems had any vacancies. Astronomers Julia Fang and Jean-Luc Margot at UCLA decided to test the idea with the help of recent exoplanet discoveries by NASA’s Kepler space telescope.

Fang and Margot simulated millions of solar systems and then adjusted the spacing between planets based on the actual orbits of worlds detected by Kepler. The average spacing between neighboring planets in the simulated systems turned out to be very similar to the spacing among the eight planets around our sun.

The researchers then tested each simulated star system’s stability by tacking on a hypothetical extra planet and running the simulation forward in time by hundreds of millions of years. Around many stars, the planets jostled until multiple worlds collided or a giant planet flung a smaller one out of the system. Fang and Margot concluded that at least one-third of three-planet systems and 45 percent of four-planet systems are crammed.

“Our work illustrates something fundamental about the formation and evolution of planetary systems,” Margot says.

Fang warns that most of the planets that Kepler found orbit their stars closely, so the results may change when Kepler finds planets farther out. And Lissauer notes that other telescopes have found a handful of giant planets in long orbits around their stars; those systems may have room for additional worlds, he says.

Despite this caveat, Margot says these early results give perspective to the thousands of likely planets discovered over the last two decades. Astronomers have found plenty of oddballs, such as Jupiter-sized gas giants baking in orbits shorter than one Earth day and planets made primarily of diamond. But some deep-seated characteristics appear to hold true across a wide variety of planetary populations.