Space & Innovation

The Hunt for Rogue Planets Is On: Photos

Learn more about the strange new class of planets that wander alone in the galaxy.

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As we've found more and more planets outside the solar system in recent years -- more than 2,000 candidates in all -- a strange new class of world has emerged. These are ones that appear to be flying all alone in space without a host star to orbit. The reasons why they became this way are still poorly understood. Perhaps a star flung them out of their native solar system, or did they form out there alone in interstellar space?

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Here are some of the recent discoveries of "rogue planets" made by astronomers. In some cases, these planets are so large that they border on what are known as "brown dwarfs" -- huge planets that are not quite massive enough to sustain nuclear fusion in their cores, a requirement to be called a "star." It's still a little fuzzy as to what constitutes a planet and what constitutes a star at these masses. Also, sometimes the mass of the object is not known precisely. So there are different perceptions out there of how "planetary" these objects are.

CFBDSIR 2149-0403 is so close to Earth that scientists were able to analyze its atmosphere, suggesting that the atmosphere may possess methane and water vapor.

The object appears to be associated with young stars known as the AB Doradus Moving Group

, which are just a few tens of millions of years old. At the time of discovery (2012), this was the first planet that was identified as part of a group of young stars.

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If the object is indeed a part of that group, scientists estimate the object is about 4 to 7 times the mass of Jupiter and roughly 50 million to 120 millon years old. The object was found as part of a project hunting for brown dwarfs.

WISE J085510.83-071442.5, which may actually be a brown dwarf, was identified using NASA's Wide-field Infrared Survey Explorer (WISE) and Spitzer Space Telescope. It's roughly 7.2 light-years away from the solar system, or just past the nearest star system (Alpha Centauri, 4 light-years away.) WISE spotted the object in part because the object

was moving so fast relative to the background stars

.

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Astronomers are uncertain about the size of this object, which could be anywhere from three to 10 masses the size of Jupiter. If it's near the smaller end of the scale, this would make the object more like a planet. Researchers noted, however, that brown dwarfs are more common -- making this the more likely scenario.

PSO J318.5-22 is about six times the mass of Jupiter and is moving in sync with a group of young stars, Beta Pictoris, which was formed about 12 million years ago. The object was found while scientists were engaged in a search for brown dwarfs. The object appeared redder in infrared wavelengths than any known brown dwarfs, making it distinct to the discovering team. Its youth also attracted attention.

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"We have never before seen an object free-floating in space that that looks like this. It has all the characteristics of young planets found around other stars, but it is drifting out there all alone. I had often wondered if such solitary objects exist, and now we know they do,” said lead author Dr Michael Liu, from the Institute for Astronomy at the University of Hawaii at Manoa,

in a statement

.

The free-floating planet MOA-2011-BLG-262 is up to four times the mass of Jupiter. What makes it stand out, however, is the planet may also have a moon with it. Scientists found the possible moon during a

microlensing event in 2011

, which occurs when a distant object amplifies reflected starlight from a brief, closer object passing in front of it.

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Unfortunately for science, such an event cannot be duplicated, so it would be very difficult to figure out if this moon discovery was for real. The researchers cautioned that the possible moon could actually have been an exoplanet from the star behind it, for example.

The most recent discovery, from earlier this month, is 2MASS J1119–1137. In a press release, scientists called it one of the youngest and brightest "free-floating, planet-like objects" close to the sun. The research team found out that the planet is moving in sync with a young group of stars that is about 10 million years old, a group known as the TW Hydrae Association. This makes the newly found planet only half the age of a brighter lone object known as PSO J318.5−22, discovered in 2013.

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"Discovering free-floating planet analogs like 2MASS J1119–1137 and PSO J318.5−22 offers a great opportunity to study the nature of giant planets outside the Solar System," lead author Kendra Kellogg, a graduate physics and astronomy student at Western University in Canada,

in a press release

. She added that these objects are “much easier to scrutinize than planets orbiting around other stars. Objects like 2MASS J1119–1137 are drifting in space all alone and our observations are not overwhelmed by the brightness of a host star next door.”