NASA JPL-Caltech | 2011 May 18
Astronomers, including a NASA-funded team member, have discovered a new class of Jupiter-sized planets floating alone in the dark of space, away from the light of a star. The team believes these lone worlds were probably ejected from developing planetary systems.
The discovery is based on a joint Japan-New Zealand survey that scanned the center of the Milky Way galaxy during 2006 and 2007, revealing evidence for up to 10 free-floating planets roughly the mass of Jupiter. The isolated orbs, also known as orphan planets, are difficult to spot, and had gone undetected until now. The newfound planets are located at an average approximate distance of 10,000 to 20,000 light-years from Earth.
"Although free-floating planets have been predicted, they finally have been detected, holding major implications for planetary formation and evolution models," said Mario Perez, exoplanet program scientist at NASA Headquarters in Washington.
The discovery indicates there are many more free-floating Jupiter-mass planets that can't be seen. The team estimates there are about twice as many of them as stars. In addition, these worlds are thought to be at least as common as planets that orbit stars. This would add up to hundreds of billions of lone planets in our Milky Way galaxy alone.
"Our survey is like a population census," said David Bennett, a NASA and National Science Foundation-funded co-author of the study from the University of Notre Dame in South Bend, Ind. "We sampled a portion of the galaxy, and based on these data, can estimate overall numbers in the galaxy."
The study, led by Takahiro Sumi from Osaka University in Japan, appears in the May 19 issue of the journal Nature.
The survey is not sensitive to planets smaller than Jupiter and Saturn, but theories suggest lower-mass planets like Earth should be ejected from their stars more often. As a result, they are thought to be more common than free-floating Jupiters.
Previous observations spotted a handful of free-floating, planet-like objects within star-forming clusters, with masses three times that of Jupiter. But scientists suspect the gaseous bodies form more like stars than planets. These small, dim orbs, called brown dwarfs, grow from collapsing balls of gas and dust, but lack the mass to ignite their nuclear fuel and shine with starlight. It is thought the smallest brown dwarfs are approximately the size of large planets.
On the other hand, it is likely that some planets are ejected from their early, turbulent solar systems, due to close gravitational encounters with other planets or stars. Without a star to circle, these planets would move through the galaxy as our sun and other stars do, in stable orbits around the galaxy's center. The discovery of 10 free-floating Jupiters supports the ejection scenario, though it's possible both mechanisms are at play.
"If free-floating planets formed like stars, then we would have expected to see only one or two of them in our survey instead of 10," Bennett said. "Our results suggest that planetary systems often become unstable, with planets being kicked out from their places of birth."
The observations cannot rule out the possibility that some of these planets may have very distant orbits around stars, but other research indicates Jupiter-mass planets in such distant orbits are rare.
The survey, the Microlensing Observations in Astrophysics (MOA), is named in part after a giant wingless, extinct bird family from New Zealand called the moa. A 5.9-foot (1.8-meter) telescope at Mount John University Observatory in New Zealand is used to regularly scan the copious stars at the center of our galaxy for gravitational microlensing events. These occur when something, such as a star or planet, passes in front of another, more distant star. The passing body's gravity warps the light of the background star, causing it to magnify and brighten. Heftier passing bodies, like massive stars, will warp the light of the background star to a greater extent, resulting in brightening events that can last weeks. Small planet-size bodies will cause less of a distortion, and brighten a star for only a few days or less.
A second microlensing survey group, the Optical Gravitational Lensing Experiment (OGLE), contributed to this discovery using a 4.2-foot (1.3 meter) telescope in Chile. The OGLE group also observed many of the same events, and their observations independently confirmed the analysis of the MOA group.
'Homeless' Planets May Be Common in Our Galaxy
Science NOW | Jon Cartwright | 2011 May 18
Our galaxy could be teeming with "homeless" planets, wandering the cosmos far from the solar systems of their birth, astronomers have found. The study could help clear up a long-running debate of whether free-floating planets really exist, and how common they are.
"The results are convincing enough that I suspect this paper will be cited for years to come as the best evidence of free-floating planets," says Dimitri Veras, an astrophysicist at the University of Cambridge in the United Kingdom, who was not involved with the study.
Over the past 2 decades, astronomers have identified more than 500 planetlike objects outside of our solar system. Most of these "exoplanets" orbit stars. The few that don't could be either free-floating planets or stars themselves; astronomers aren't sure, because their mass is so uncertain. Anything less massive than about 13 Jupiters is generally considered a planet, while anything between about 13 and 80 Jupiters is a small star known as a brown dwarf.
Astrophysicist Takahiro Sumi of Osaka University in Japan and colleagues—who form the Microlensing Observations in Astrophysics (MOA) and the Optical Gravitational Lensing Experiment (OGLE) collaborations—now appear to have figured out what is what. In a paper published online today in Nature, the researchers list 10 objects in our galaxy that are very likely to be free-floating planets. What's more, they claim that in our galaxy, free-floaters are probably so populous that they outnumber stars.
Most exoplanets have been discovered using one of two techniques: either seeing a star blink as a planet passes in front of it, or seeing a star wobble because of the shifting pull of a planet's gravity. Searching for planets that have no stars, however, requires a different approach: microlensing. In this technique, astronomers use a planet's gravity to bend light like a magnifying lens. As the planet passes in front of a distant star—one it isn't orbiting—it gives itself away by magnifying the star's light. Generally speaking, the shorter the magnifying time as the planet crosses the star, the smaller the planet.
Over 2 years, Sumi and others in the MOA collaboration monitored 50 million stars in our galaxy using the 1.8 meter MOA-II telescope at New Zealand's Mount John Observatory and the 1.3 meter Warsaw University Telescope at Chile's Las Campanas Observatory. They found 474 incidents of microlensing, just 10 of which were brief enough to be planets of around Jupiter's size. For each of those 10 planets, the researchers couldn't find any trace of a parent star within 1.5 billion kilometers—about the distance Saturn orbits around our sun. Although planets do orbit at greater distances, it is exceedingly rare for Jupiter-sized ones, and so the 10 planets were very likely to be free-floaters. The data were backed up by the OGLE collaboration.
The other 464 microlensing events were due to bigger objects--live stars, dead stars, and brown dwarfs. But because longer microlensing events like these are easier to spot, they skew the statistics. Taking that bias into account, the researchers estimated that there are nearly two free-floaters for every star in our galaxy.
"It is not surprising that there are such free-floating planets, as people have been expecting [they] exist," says Sumi. "But it is surprising that they are so common."
Perhaps the biggest question arising from the discovery is where free-floaters come from. One option proposed by astronomers was that they formed in the same way as stars—by using gravity to suck up nearby material. Yet that process is unlikely to have formed so many small objects, says Sumi. Instead, he and his colleagues think it's more likely that the free-floaters started out in planetary systems but were slingshotted away during a chaotic orbit. The question of formation is an important one, not least because—according to some—life on Earth could have originated from a free-floater that crashed into our solar system billions of years ago.
"These results suggest that violent, dynamical events are quite common in the history of planetary systems," says Sascha Quanz, an astronomer at the Swiss Federal Institute of Technology in Zurich, who was not involved with the research. "So, forming planets is one thing, but keeping them is another."
So many lonely planets with no star to guide them
Nature News | Nadia Drake | 2011 May 18
Our Galaxy may be full of worlds without a sun to call their own.
Scattered about the Milky Way are floating, Jupiter-mass objects, which are likely to be planets wandering around the Galaxy's core instead of orbiting host stars. But these planets aren't rare occurrences in the interstellar sea: the drifters might be nearly twice as numerous as the most common stars.
"This is an amazing result, and if it's right, the implications for planet formation are profound," says astronomer Debra Fischer at Yale University in New Haven, Connecticut.
To find the wanderers, scientists turned their telescopes towards the Galactic Bulge surrounding the centre of the Milky Way. Using a technique called gravitational microlensing, they detected 10 Jupiter-mass planets wandering far from light-giving stars. Then they estimated the total number of such rogue planets, based on detection efficiency, microlensing-event probability and the relative rate of lensing caused by stars or planets. They concluded that there could be as many as 400 billion of these wandering planets, far outnumbering main-sequence stars such as our Sun. Their work is published today in Nature1.
Unexpected bounty
Study author Takahiro Sumi, an astrophysicist at Osaka University in Japan, says the deduced number of homeless exoplanets surprised him. "The existence of free-floating planets has been predicted by planetary formation theory, but nobody knew how many there are," he says.
And because current theories of planet formation hold that lower-mass planets are more readily flung from developing planetary systems than are higher-mass planets, there could be a huge number of lighter planets on the loose. "They might be littering the Galaxy," says Fischer.
Sumi and scientists from the Microlensing Observations in Astrophysics (MOA) and Optical Gravitational Lensing Experiment (OGLE) collaborations used gravitational microlensing to detect the planets. Microlensing involves measuring changes in the brightness of distant, background stars as a passing planet's gravity bends and magnifies the starlight. As a result, the star brightens and fades in a pattern distinct from random twinkling, and the duration of brightening indicates the mass of the magnifying object.
Gregory Laughlin, an astronomer at the University of California at Santa Cruz, says the authors have done a good job of ruling out other possible explanations for the light-distorting objects. But he adds that it's difficult to speculate about the number of unbound, lower-mass planets on the basis of the wandering Jupiters, because that assumes that they were formed by a similar mechanism to planets in our neighbourhood. "I think we might be seeing a different formation mechanism here, something more similar to that of a tiny star than a giant planet," he says. "But that's just a hypothesis."
Life on the road
Planetary scientist David Stevenson at the California Institute of Technology in Pasadena has considered how the temperatures on ejected planets might compare with those on star-bound bodies2. If Jupiter were kicked out of the Solar System, its surface temperature would drop by only about 15 kelvin, he says – although it would still be unsuitable for supporting life. However, "when you eject a planet that is quite massive, it could have carried along an orbiting body", Stevenson adds. "And that might be a more attractive possibility for life."
Unbound Earth-mass planets might still be capable of carrying liquid water, Stevenson says, even in the frozen reaches of interstellar space – as long as they have a heat-trapping hydrogen atmosphere. "That can bring the surface temperature up to 300 kelvin [about 27 °C]," he says. "And then you can have oceans."
Study author David Bennett, an astrophysicist at the University of Notre Dame in Indiana, agrees that life could exist on these wandering worlds. He says that the next steps in the search include confirming the absence of host stars and looking through new data for the footprints of smaller, Saturn- or Neptune-mass planets.
In the future, drifting Earth-mass planets could be detected using NASA's planned Wide-Field Infrared Survey Telescope (WFIRST), a space-based telescope capable of resolving the more rapid bright blips associated with lower-mass objects. "Detecting Earth-mass unbound planets?" says Scott Gaudi, an astrophysicist at the Ohio State University in Columbus. "That would be very interesting."
- Unbound or distant planetary mass population detected by gravitational microlensing - MOA Collaboration, OGLE Collaboration
- Nature 473 349 (19 May 2011) DOI: 10.1038/nature10092
- Life-sustaining planets in interstellar space? - DJ Stevenson
- Nature 400 32 (01 July 1999) DOI: 10.1038/21811
The galaxy may swarm with billions of wandering planets
Discover Blogs | Bad Astronomy | 2011 May 18
Lone Planets “More Common Than Stars”
Universe Today | Jason Major | 2011 May 18
Exoplanets without a star: galaxy teems with lonely Jupiters
ars technica | Christopher Dombrowski | 2011 May 18
Lonely Planets Populate the Cosmos
Discovery News | Irene Klotz | 2011 May 18
Astronomer Bennett's team discovers new class of planets
University of Notre Dame | via EurekAlert | 2011 May 18
Lonely Rogue Worlds Surprisingly Outnumber Planets with Suns
Space.com | Mike Wall | 2011 May 18
'Lonely planets' may outnumber star-hosted worlds
New Scientist | MacGregor Campbell | 2011 May 18
