Photograph by Stephen Alvarez, National Geographic

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The Milky Way, shown arcing over Madagascar, may have more planets than stars, a new study says.

Photograph by Stephen Alvarez, National Geographic

Alien Planets Outnumber Stars, Study Says

"Important" discovery: Jupiter-like runaways common in our galaxy.

If you look to the stars tonight, consider this: No matter how innumerable they may seem, there are far more planets than stars lurking out there in the darkness, a new study suggests.

The study uncovered a whole new class of worlds: Jupiter-like gas giants that have escaped the gravitational bonds of their parent stars and are freely roaming space.

What's more, "our results indicate that such planets are quite common," said study team member David Bennett, an astronomer at Notre Dame University in Indiana.

"There's a good chance that the closest free-floating planet is closer to Earth than the closest star."

Ohio State University astronomer Scott Gaudi added, "It's not surprising that free-floating planets are out there"—they've been predicted by planet-formation theories for years—"it's just how many of them that they're finding."

The findings, detailed in this week's issue of the journal Nature, indicate there are about two free-floating planets per star in our galaxy—and perhaps in other galaxies, too.

Scientists estimate there are about 200 billion stars in the Milky Way, so that means there could be at least 400 billion drifting planets in space. And that's not even counting the planets that orbit stars, or smaller, rocky free-floaters that can't yet be detected.

"These are just the ones that we found," study co-author Bennett said. "If we could see lower-mass planets, then presumably the

number would be even larger."

Planet Discovery Came in a Flash of Light

The team spotted ten runaway planets—with an average mass similar to Jupiter's—using a technique called gravitational microlensing.

Gravitational lensing takes advantage of the fact that large celestial objects such as stars or planets warp the fabric of space-time such that light rays passing nearby are bent.

One effect of this is that a star can appear to brighten temporarily as its light bends around a passing planet—an effect visible only if the planet passes directly in front of the star, as seen from a telescope.

Such planet-star alignments are rare and usually last less than two days.

Using a telescope with a nearly 6-foot (1.8-meter) lens at New Zealand's Mount John University Observatory, astronomers scanned more than a hundred million stars at the center of the Milky Way galaxy for two years in search of such alignments.

The survey found ten brief microlensing events, which the team says are evidence of planets of roughly Jupiter's mass. The precise distance of these planets from Earth is unknown but could range from 1,000 to 20,000 light-years—the distances scanned during the survey—Bennett said.

Rogue Planets Could Be Tamed

Each runaway planet is zipping through the galaxy at speeds of more than 450,000 miles an hour (200 kilometers a second). Even at these speeds, the rogue worlds could be corralled into orbit again, under the right conditions.

"A runaway planet can't be caught by a single star. It will have too much energy," Bennett said. But a binary star—two gravitationally locked stars that orbit each other—"could do it, and a star that already has at least one planet," he said.

"However, if a star with planets captures another one, then one of its existing planets must change its orbit, and this could make the system unstable. So, a capture might lead to another ejection."

New Planets Not Free-Floating After All?

The team said that it can't rule out the possibility that some of the planets are just orbiting their stars at very far distances and that the parent stars just don't show up in the data. But they say previous observations by other groups suggest that Jupiter-mass planets in such distant orbits are rare.

University of Heidelberg astronomer Joachim Wambsganss, who was not involved in the research, said more studies will be needed to confirm that the new planets are indeed drifters.

"Whether they're really freely floating planets or just in really wide orbits is not yet proven, I would say," said Wambsganss, who wrote an accompanying article in Nature about the discovery.

Planets Bullied Out of Orbit?

Ohio State's Gaudi called the results "important and exciting" and said they raise interesting new questions about so-called extrasolar planets, or "exoplanets"—worlds outside our solar system.

For instance, astronomers think free-floating planets can get kicked out of their star systems after being perturbed by the gravity of another passing star or of "bully" planets in the same system.

In the latter scenario, "the biggest bullies kick out the smaller guys," explained Gaudi, who wasn't part of the study.

"In our solar system, Jupiter is the biggest bully," he said. But bigger gas giants have been detected in other star systems, and it's perhaps such "super Jupiters" that sent the newfound rogue planets packing.

Life on the Run?

Just because rogue planets orbit no life-giving star, they're not necessarily lifeless—particularly rocky ones—the University of Heidelberg's Wambsganss said. (See "Earth-Size 'Lone Wolf' Planets May Host Life.")

"If you think about free-floating planets, there's no nearby star that can produce heat and energy ... but even in our solar system, there are [externally frigid worlds] that have hot cores," he said.

"So it's not entirely impossible that free-floating planets have hot cores, cold surfaces, and an intermediate layer, where water could exist in some fluid form. This is only speculation, of course."