If it forms like a star but looks like a planet...

Ars Technica » Scientific Method 2012-11-15

You may have seen headlines earlier this week about a planet found wandering alone, without a star to orbit. Although that aspect of the new planetary find seems to have grabbed the headlines, it's actually one of the least interesting features of the new find. The body in question awkwardly straddles the border between near-stars called brown dwarfs and giant super-Jupiter class planets, and its features suggest it may have formed through a process identical to one that creates stars. The alternative—that it was ejected from a system of planets that formed orbiting a star—would suggest interstellar space is teeming with rogue planets.

Rogue planets, those that aren't gravitationally bound to a star, have been identified previously through surveys looking for a phenomenon called "microlensing." This occurs when the planet acts as a gravitational lens, briefly brightening a star in the background as the planet passes between the star and Earth. A survey of microlensing objects identified a number of possible planets that mapped to points in space where there were no stars in the neighborhood.

But in this case, the authors managed to directly image one relatively nearby (less than 140 light years away). It happened while they were looking for something else: brown dwarfs. These bodies are large enough to have ignited deuterium fusion (over 13 times the mass of Jupiter), but not large enough to have kicked off sustained fusion of hydrogen. Anything below the brown dwarf cutoff is considered a planet (with the largest of these gas giants typically termed "super Jupiters").

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