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Meet one of the newest celestial bodies to be discovered: rogue planets, worlds that hurtle around the galaxy without any parent star. Caitlin Hofmeister explains how we found them, and where we think they might have come from.
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Caitlin Hofmeister: Earth just wouldn't be earth without the light of our star beaming down on us every day. But even though it's hard to imagine, that's what life is like for rogue planets; young, large planets that aren't tethered to any star.

These free agents may sound unusual, but astronomers believe there may actually be fifty percent more of them than so called normal, star-bound planets.

Since we've only just observed them in the past decade or so, there's still a lot we don't know about these planets.

Some of the first signs of them came in 2006 and 2007 when a team of astronomers from Japan and New Zealand was surveying the center of the Milky Way looking for brown dwarfs; small, cool, failed stars that weren't big enough to ignite fusion reactions in their cores.

Since brown dwarfs are so small and dim, scientists were scoping the mount in the same way they do for exoplanets, waiting for them to transit, or pass in front of a distant star in the background.

Over the course of their study, the team monitored 50 million stars in the center of the Milky Way. And in the end, they observed 474 objects transiting those stars.

But a handful of those objects were especially puzzling. Ten of the transits lasted less than 2 days, indicting the objects were much smaller than brown dwarfs. So the astronomers thought they were observing regular exoplanets, but then they couldn't find any evidence of nearby stars.

They started to suspect they were looking at rogue planets But since the objects were 10 to 20 thousand light years from us, they weren't able to get a very close look.

 But in 2012 another team found something a little closer to home. Only 80 light years from Earth, astronomers found what they affectionately call P S O J 3 1 8 point 5 dash 2 2 (PSOJ318.5-22)

It too turned up during a search for brown dwarfs, this time using the telescope called Pan-STARRS, which thanks too its super sensitive camera is really good at detecting the hard to spot infrared heat signatures of brown dwarfs.

And while difficult to spot, brown dwarfs are much larger than planets, and at just 6 times Jupiter's mass, PSOJ was clearly a planet, not a brown dwarf. And its infrared signature showed that it was much younger than a brown dwarf, probably only 12 million years old. And again this weird world appeared to be all on its own.

It's rare for such a large planet to orbit more than 10 astronomical units from its star, and PSOJ had no stellar neighbor in that range. Instead, it seemed to be happily orbiting the center of the Milky Way, like any other star.

So now that we're pretty sure we've observed rogue planets directly, what do we really know about them? Honestly, not much. At this point we could only guess about how they could have formed.

Right now a form of sibling rivalry seems most likely. The leading theory is called planet-planet scattering, where one gas giant kicks another out of the vicinity during the gravitational jostling that takes place as a star system is forming.

Another theory is that the rogue planet may have once orbited a star, but the star itself pushed it out of the way. When stars reach the end of their hydrogen burning lives, they begin expanding into red giants, and this process could eject planets from its system.

So, rogue planets were predicted to exist back in the 1990s, but we've only recently started observing them, now that we might have actually found a few. And scientists have calculated by extrapolating from the small samples they've explored so far that there might actually be billions of these orphan planets out there.

But don't feel bad for them. Rogue planets are young and independent, wandering around on their own, wild and free.

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