17November2018

‘Exciting’ find: Possible planets without orbits

So many lonely planets with no star to guide them. Are these planets without orbits? 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.

Free-floating planets may be more common in our Galaxy than stars.NASA/JPL-Caltech/R. HurtTo 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 Nature

Astronomers have found 10 potential planets as massive as Jupiter wandering through a slice of the Milky Way galaxy, following either very wide orbits or no orbit at all. And scientists think they are more common than the stars.

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.”

These mysterious bodies, apparently gaseous balls like the largest planets in our solar system, may help scientists understand how planets form.

“They’re finding evidence for a lot of pretty big planets,” said Alan Boss of the Carnegie Institution of Washington, who wasn’t involved in the research.

If they orbit stars, their sheer number suggests every star in the galaxy has one or two of them, “which is astounding” because that’s five or 10 times the number of stars scientists had thought harbored such gas-giant planets, he said.

And if instead they are wandering free, that “would be really stunning” because it’s hard to explain how they formed, he said.

If that’s the case, it would give a boost to some theories that say planets can be thrown out of orbit during formation, said Lisa Kaltenegger of the Harvard-Smithsonian Center for Astrophysics, another outside expert.

Other scientists have reported free-wandering objects in star-forming regions of the cosmos, but the newfound objects appear to be different, said one author of the new study, physicist David Bennett of the University of Notre Dame.

Bennett and colleagues from Japan, New Zealand and elsewhere report the finding in Thursday’s issue of the journal Nature. They didn’t observe the objects directly. Instead, they used the fact that massive objects bend the light of distant stars with their gravity, just as a lens does. So they looked extensively for such “microlensing” events.

They found 10, each caused by one of the newfound objects. They calculated each object has about the mass of Jupiter, and estimated how common such objects are. They also found no sign of a star near these bodies, at least not within 10 times the distance from Earth to the sun. (For comparison, within our solar system that would basically rule out an orbit closer than Saturn’s.)

So the newfound objects either orbit a star more distant than that, or they don’t orbit a star at all, the researchers concluded. They drew on other data to determine most of the objects don’t orbit a star.

Scientists believe planets are formed when disks of dust that orbit stars form clumps, so that these clumps — the planets — remain in orbit. Maybe the newfound objects started out that way, but then got tossed out of orbit or into distant orbits by the gravitational tugs of larger planets, the researchers suggest.

The work suggests that such a tossing-out process is quite common, Bennett said.

Boss said maybe the bodies formed around a pair of stars instead, one of which supplied the gravitational tug. But even that would take some explaining to produce an object without an orbit, he said. Or maybe they somehow formed outside of any orbit. So the theoretical challenge in explaining the existence of such bodies is “exciting,” he said.

Boss said he suspects most of these are in a distant orbit, and that maybe they even formed at that great distance rather than being tossed outward from a closer orbit.

Kaltenegger also said the new results can’t rule out the possibility that these possible planets are in orbit, and that they may only have the mass of Saturn, about a third of Jupiter’s.

But if they aren’t orbiting a star, she noted, they don’t fit the official definition of a planet — at least not the definition applied to objects in our own solar system.

All in all, Boss said, the new work is “pretty exciting in telling what is out there in the night sky… Lots of theories will grow in this environment.”

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 bodies. 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.”

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9 Responses to "‘Exciting’ find: Possible planets without orbits"

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