How do planets—ours, and the others that stud the universe—actually form?
Our current, best knowledge posits that planets emerge from the formation of stars. Through a process like this:
A star and its planets form out of a collapsing cloud of dust and gas within a larger cloud called a nebula. As gravity pulls material in the collapsing cloud closer together, the center of the cloud gets more and more compressed and, in turn, gets hotter. This dense, hot core becomes the kernel of a new star.
Meanwhile, inherent motions within the collapsing cloud cause it to churn. As the cloud gets exceedingly compressed, much of the cloud begins rotating in the same direction. The rotating cloud eventually flattens into a disk that gets thinner as it spins, kind of like a spinning clump of dough flattening into the shape of a pizza. These "circumstellar" or "protoplanetary" disks, as astronomers call them, are the birthplaces of planets.
As a disk spins, the material within it travels around the star in the same direction. Eventually, the material in the disk will begin to stick together, somewhat like household dust sticking together to form dust bunnies. As these small clumps orbit within the disk, they sweep up surrounding material, growing bigger and bigger. The modest gravity of boulder-sized and larger chunks starts to pull in dust and other clumps. The bigger these conglomerates become, the more material they attract, and the bigger they get. Soon, the beginnings of planets—"planetesimals," as they are called—are taking shape.
Straightforward enough: cosmic pizzas, basically, with little pepperonis gradually taking shape. Here's a potential wrinkle in that process, though—one that takes the form of an adorable little orphan planet that astronomical renderers have decided is a perky shade of purple. That planet, hanging out some 80 light-years away from Earth, seems to have formed 12 million or so years ago—relatively recently in planet-formation terms. The planet, PSO J318.5-22, has six times the mass of Jupiter. It has properties that make it similar to the gas giants that are generally found orbiting young stars.
Except: The planet is free-floating. No host star. No known origin story. The Orphan Annie of the cosmos.
"We have never before seen an object free-floating in space that that looks like this," Michael Liu, of the Institute for Astronomy at the University of Hawaii at Manoa, told PhysOrg of the planet. "It has all the characteristics of young planets found around other stars, but it is drifting out there all alone."
The discovery paper for PSO J318.5-22—the paper terms the planet, delightfully, "The Extremely Red, Young L Dwarf"—is being published in Astrophysical Journal Letters. The paper was authored by Liu, along with 18 other co-authors.
PSO J318.5-22 isn't the first planet to be found without a host star. Astronomers have, in the past 10 years or so, discovered 1,000 or so of such extrasolar planets (or, more precisely, they have inferred their existence through observations of other cosmic bodies). Last year, scientists announced their discovery of CFBDSIR2149, believed to be a similarly rogue (or, less judgmentally, "homeless" or "orphan") planet. Such planets, scientists speculate, can form in one of two ways: They're either formed within solar systems according to the process above and then ejected—or they form independently in interstellar space.
But that ambiguity makes these seemingly homeless planets especially intriguing to astronomers: If it's the latter formation process, how exactly would these planets form? Can you have the chicken of the planet without the egg of a host star?
There have been only a few homeless planets that we've thus far directly imaged in the way the paper's authors have been able to image PSO J318.5-22. And while the planet is similar in mass and color and energy output to those planets, it differs in a significant way: Its mass is much, much lower than those planets'. It may even be, Liu and his colleagues say, one of the lowest-mass free-floating objects we've yet discovered. Perhaps the very lowest.
And that means that PSO J318.5-22, the little orphan planet, could be a great adoptee for scientists. "Planets found by direct imaging are incredibly hard to study," Niall Deacon, the paper's co-author, explained, "since they are right next to their much brighter host stars." A planet that's not orbiting a star, however, doesn't have that drawback. "It is going to provide a wonderful view into the inner workings of gas-giant planets like Jupiter shortly after their birth."
It also, in the meantime, serves as a reminder of how lonely and cold the cosmos can be. Here is a planet of unknown origin, of unknown lineage, in utter solitude. As Dr. Liu put it: "I had often wondered if such solitary objects exist, and now we know they do."