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.

Not alone: An artist's illustration of the idea that rocky, terrestrial worlds like the inner planets in our own solar system may be plentiful, and diverse, in the universe.

Fresh discoveries of new worlds around other stars – and the prospect of finding more in our own solar system – are giving the Milky Way an increasingly life-friendly look.

In the process, these and other finds are upending long-held notions of what a solar system should look like and where a livable planet is most likely to be found. Even the definition of a planet is up for grabs, despite a recent attempt among the world's astronomers to settle the issue.

"In our own solar system, there's a pretty clear distinction based on size," says David Morrison, the senior scientist at the National Aeronautics and Space Administration's Astrobiology Institute in Mountain View, Calif. "But as we look at other solar systems, there's no reason to think that these distinctions will survive. We'll probably see all kinds of planets in all kinds of configurations. That's what makes this so exciting."

Two sets of discoveries announced in recent days suggest that the Milky Way hosts many more sunlike solar systems than previously believed.

One team found a solar system that resembles our own, although it's a scaled-down version. The astronomers detected two planets orbiting a star nearly 5,000 light-years away. The star is only half as massive as the sun and the two planets are smaller than Jupiter and Saturn. But after adjusting for these differences, the astronomers found that the two planets orbit their star at roughly the same relative distance as Jupiter and Saturn orbit our sun. Their relative masses are similar. And the estimated temperatures of the new planets are similar to Jupiter's and Saturn's.

Led by Ohio State University astronomer Scott Gaudi, the team's results appear in the current issue of the journal Science.

Another team says it has found evidence of the formation of rocky planets around a significant number of sunlike stars up to 163 light-years away. Using NASA's Spitzer Space Telescope, the team found that at least 20 percent of the 328 stars it observed had dust disks displaying telltale signs that rocky planets were forming close to the host stars.

"The question is: How common are planetary systems like our own around sunlike stars in our galaxy?" says University of Arizona astronomer Michael Meyer, who discussed his team's findings at last weekend's annual meeting of the American Association for the Advancement of Science in Boston. Given the number of sun-like stars in the galaxy, the number of sunlike solar systems at various stages of development could be astronomical.

Having rocky planets at just the right distance from a star provides a footing for life to emerge, Dr. Meyer notes. But researchers also point to a Jekyll-Hyde role other Jupiter-style planets can play. Their gravity can shield inner planets from marauding asteroids or comets; but that gravity can also redirect other asteroids to collide with the same planets. Such collisions wipe out current inhabitants, but may also deposit water ice and other chemical building blocks for organic life onto a planet's surface.

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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