Across the Milky Way, more planet Earths?

Two new discoveries suggest that sunlike solar systems like ours – with life-hosting planets – are more common than previously thought.

from the February 20, 2008 edition

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The hunt for planets outside our solar system may be driven in large part by a search for solar systems like ours. But along the way, astronomers are finding an intriguing array of systems that don't fit the "classic" mold.

Over the past 12 years, astronomers have discovered more than 250 extrasolar planets, notes Debra Fischer, a planet-hunting astronomer at San Francisco State University. "The amazing thing about them is their diversity," she says.

"If you put a bunch of schoolchildren in a room with some clay and ask them to build models of solar systems, you might find that they would create a whole range of things. That's what we see nature doing. There's only one rule nature has to follow, and that's gravity. As long as planets are in stable orbits, we can get anything."

Yet even as astronomers look for similarities and differences between our solar system and the new ones they find, the view of our own patch of the galaxy is undergoing a revolution, says Alan Stern, lead scientist for New Horizons, a Pluto fly-by mission, and associate administrator for NASA's science missions directorate.

The classic, place-mat view of our solar system – four inner planets, the gas giants, then Pluto – "is very myopic," he says. With the discovery of the Kuiper Belt of icy objects beyond Neptune and current speculation about the Oort Cloud, a dispersed cloud of icy planetary debris even farther afield, he argues that what was once dubbed the outer solar system is actually the middle solar system.

Indeed, Dr. Stern says, the objects in outer regions like the Kuiper belt may prove to be the most common planetary objects in the galaxy.

Moreover, several planets – Mercury, Uranus, and Neptune, for instance – are now thought to have formed in locations other than where humans see them today, replacing the old notion that planets formed in their current orbits.

This scattering, along with other observations, leads Stern to suspect that as telescopes get bigger and more sensitive, astronomers may well find Mars-sized objects or larger in the deepest reaches of the Kuiper Belt or beyond. Even at those distances, conditions below an icy crust could host simple forms of life.

"If you like change," he says, "then hold on to your hat."

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