This artist's conception shows a Jupiter-like planet alone in the dark of space, floating freely without a parent star. Astronomers recently uncovered evidence for 10 such lone worlds, thought to have been "booted," or ejected, from developing solar systems.
By Senior writer
updated 5/18/2011 1:19:59 PM ET 2011-05-18T17:19:59

Astronomers have discovered a whole new class of alien planet: a vast population of Jupiter-mass worlds that float through space without any discernible host star, a new study finds.

While some of these exoplanets could potentially be orbiting a star from very far away, the majority of them most likely have no parent star at all, scientists say.

And these strange worlds aren't mere statistical anomalies. They likely outnumber "normal" alien planets with obvious parent stars by at least 50 percent, and they're nearly twice as common in our galaxy as main-sequence stars, according to the new study. [Photos: The Strangest Alien Planets]

Astronomers have long predicted the existence of free-flying "rogue alien planets." But their apparent huge numbers may surprise many researchers, and could force some to rethink how the planets came to be.

"Previous observations of bound planets tell us only about planets which are surviving in orbits now," said study lead author Takahiro Sumi, of Osaka University in Japan. "However, [these] findings inform us how many planets have formed and scattered out."

Alien worlds under gravitational lens
Sumi and his colleagues made the find using a method called gravitational microlensing, which watches what happens when a massive object passes in front of a star from our perspective on Earth. The nearby object bends and magnifies the light from the distant star, acting like a lens.

This produces a "light curve" — a brightening and fading of the faraway star's light over time — whose characteristics tell astronomers a lot about the foreground object's size. In many cases, this nearby body is a star; if it has any orbiting planets, these can generate secondary light curves, alerting researchers to their presence.

Before the current study, astronomers had used the gravitational microlensing technique to discover a dozen or so of the nearly 550 known alien planets. (NASA's Kepler mission has detected 1,235 candidate planets by a different method, but they still need to be confirmed by follow-up observations.)

Sumi and his team looked at two years' worth of data from a telescope in New Zealand, which was monitoring 50 million Milky Way stars for microlensing events. They identified 474 such events, including 10 that lasted less than two days.

The short duration of these 10 events indicated that the foreground object in each case was not a star but a planet roughly the mass of Jupiter. And the signals from their parent stars were nowhere to be found.

Independent observations from a telescope in Chile backed up the finds. Either these 10 planets orbit very far from their host stars — more than 10 times the Earth-sun distance — or they have no host stars at all, researchers said. [ Infographic: A Sky Full of Alien Planets ]

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Common throughout the galaxy
Gravitational microlensing events are rare, because they require the precise alignment of a background star, a massive foreground object and Earth. So the discovery of 10 short-duration events in two years suggests a huge population of these unbound or distantly orbiting Jupiter-mass exoplanets throughout the galaxy, researchers said.

Sumi and his team calculated, in fact, that these planets are probably almost twice as common in our own Milky Way as main-sequence stars. And they likely outnumber "normal" planets with known host stars by more than 50 percent.

Other studies have established that it's probably pretty rare for huge planets to orbit more than 10 Earth-sun distances from a parent star. So the research team argues that most of the Jupiter-mass planets — at least 75 percent of them — are likely true "rogues," floating through space unbound to a star.

Theory predicts that such rogues should exist throughout the galaxy, and other researchers have found evidence of unbound objects that may indeed be orphan planets. But those worlds were much bigger, from three to 10 times Jupiter's mass, and there's a lot of uncertainty in the measurements.

Many of the previously detected objects could actually be "failed stars" known as brown dwarfs, Sumi said.

Sumi and his colleagues report their results in the May 19 issue of the journal Nature.

Rethinking planetary formation theories
The newly discovered rogue planets may have formed close to a host star, then been ejected from their solar systems by the gravitational influence of a huge neighbor planet, researchers said. Indeed, such planet-planet interactions are thought to be responsible for the odd, extremely close-in orbits of the giant alien planets known as "hot Jupiters."

But the abundance of the seemingly starless worlds may force astronomers to rethink some of their ideas about planet formation, according to Sumi.

The "current most recognized planetary formation theory (core accretion model) cannot create so many giant planets," Sumi told in an email interview. "So we need a different theory to create [so] many giant planets, such [as the] gravitational instability model."

In the core accretion model, dust coalesces to form a solid core, which later accretes gas around it, creating a planet. The gravitational instability model invokes the rapid collapse of gas, with a core forming later due to sedimentation.

The new study should inspire much follow-up research. One of the next steps could involve training more instruments on the microlensing alien planets, further monitoring them for any signs of a parent star. Such work, which may take years, could eventually reveal how many of these worlds actually do have parent stars, and how many are true rogues.

"The implications of this discovery are profound," astronomer Joachim Wambsganss, of Heidelberg University, wrote in an accompanying essay in the journal Nature. "We have a first glimpse of a new population of planetary-mass objects in our galaxy. Now we need to explore their proper­ties, distribution, dynamic states and history."

You can follow senior writer Mike Wall on Twitter: @michaeldwall. Follow for the latest in space science and exploration news on Twitter @Spacedotcom and on Facebook.

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Photos: Month in Space: January 2014

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    Stars, galaxies and nebulas dot the skies over the European Southern Observatory's La Silla Paranal Observatory in Chile, in a picture released on Jan. 7. This image also shows three of the four movable units that feed light into the Very Large Telescope Interferometer, the world's most advanced optical instrument. Combining to form one larger telescope, they are greater than the sum of their parts: They reveal details that would otherwise be visible only through a telescope as large as the distance between them. (Y. Beletsky / ESO) Back to slideshow navigation
  2. A balloon's view

    Cameras captured the Grandville High School RoboDawgs' balloon floating through Earth's upper atmosphere during its ascent on Dec. 28, 2013. The Grandville RoboDawgs’ first winter balloon launch reached an estimated altitude of 130,000 feet, or about 25 miles, according to coaches Mike Evele and Doug Hepfer. It skyrocketed past the team’s previous 100,000-feet record set in June. The RoboDawgs started with just one robotics team in 1998, but they've grown to support more than 30 teams at public schools in Grandville, Mich. (Kyle Moroney / AP) Back to slideshow navigation
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    Russian cosmonauts Oleg Kotov, right, and Sergey Ryazanskiy perform maintenance on the International Space Station on Jan. 27. During the six-hour, eight-minute spacewalk, Kotov and Ryazanskiy completed the installation of a pair of high-fidelity cameras that experienced connectivity issues during a Dec. 27 spacewalk. The cosmonauts also retrieved scientific gear outside the station's Russian segment. (NASA) Back to slideshow navigation
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    More about SpaceShipTwo on PhotoBlog (Virgin Galactic) Back to slideshow navigation
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    Slideshow: The Year in Space (Brian Peterson / The Bismarck Tribune via AP) Back to slideshow navigation
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