Cosmic Loners: Orphan Stars and Planets Wander the Galaxy

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David A. Aguilar (CfA)

An artist's conception of a runaway planet zooming through interstellar space. A new paper suggests planets can blaze through space entirely by themselves, ripped free of their parent star by the power of the black hole

The Milky Way contains all kinds of stars — young ones, old ones, red ones, blue ones, yellowish ones, white ones, ranging in size from much smaller than our sun to tens or even hundreds of times bigger. All told there are at least a hundred billion stars in our home galaxy — and perhaps as many as 300 billion. But a few members of this vast stellar population have a special claim to cosmic fame. They're racing away from the core of the Milky Way at speeds of up to 1.5 million m.p.h. (2.4 million km/h), on a trajectory that will ultimately take them out into intergalactic space.

None of this is entirely surprising to astronomers, or at least, not any more. These so-called hypervelocity stars were predicted back in 1988; the first one was actually discovered in 2005, and by now 21 such stellar speedsters have been observed and confirmed. But a new paper slated for publication in Monthly Notices of the Royal Astronomical Society suggests that hypervelocity stars may not be the lone wolves they seem. Some of them may be taking planets along with them on their perambulations. And sometimes the planets themselves may go rogue, ripping free and fishtailing off on their own.

The reason for all this galactic gadding about: the giant black hole, weighing in at more than 4 million times the mass of our sun, which lurks at the core of the Milky Way. Anything that ventures close to the core orbits the dark monster at high speed. If the "anything" happens to be a double-star system (two stars orbiting each other, which is a common thing in the Milky Way) and if the pair gets a bit too close, theory suggests that one member can fall in while the second star is flung out into deep space — kind of like the "whip" in a roller derby. (Our solar system is located a comfortable 30,000 light-years from the galactic center, so we're at no risk of being either swallowed up or hurled to oblivion.)

The gravitational breakup of a pair of stellar twins explains how hypervelocity stars came to be, but when Dartmouth astrophysicist Gary Wegner, Dartmouth grad student Idan Ginsburg and his advisor, Harvard astrophysicist Avi Loeb, began looking at the phenomenon more deeply, they began to realize that not only is it entirely plausible for the stars to be accompanied by one or more planets, but also it wouldn't be all that difficult to prove the existence of such free-range solar systems.

The Kepler space telescope and some Earth-based observatories have now spotted more than 2,000 so-called exoplanets orbiting distant stars. So big a head count in the brief time scientists have been searching a relatively small patch of sky suggests that there should be billions upon billions of exoplanets out there, making it entirely reasonable to conclude that at least some of them orbit hypervelocity stars. The planets themselves would be far too small and dim to see, but that's not how exoplanets are typically detected anyway. Instead, they're spotted either through the gravitational wobble they cause in their parent star or, a bit more easily, by a slight dimming in light every time the orbiting planet passes in front of the star.

It's this second technique that the Kepler space probe, the ground-based MEarth Project and other observing programs use, and it doesn't even take a superpowerful instrument for them to make their discoveries, says Ginsburg. A telescope on the ground with a mirror just 13 ft. (4 m) across could do it, a relative shrimp compared with the 26- to 33-ft. (8 to 10 m) telescopes astronomers now favor.

Ginsburg and his colleagues don't expect to spot any planets orbiting hypervelocity stars just yet, in part because 21 such wandering bodies are an awfully small sample group. What's more, a planet would have to be aligned just right for the light dimming to be observable. "Still," says Ginsburg, "we think it's worth looking. And if observers don't find any in this first round, they can wait for more hypervelocity stars to turn up." If such planets do exist, the odds would be exceedingly slim they'd contain life: they'd have to be in very tight orbits to avoid having been stripped away by the black hole, making things too hot for biology to get started — though planets around dim hypervelocity stars might find things a bit more comfortable.

Least comfortable — and certainly loneliest — of all would be planets that were blazing through space entirely by themselves, ripped free of their parent star by the same whipcrack power of the black hole and flung free on their own. There'd be nothing, in theory at least, to prevent this from happening, and the authors of the Monthly Notices paper even went so far as to estimate just how fast the planets might be traveling: a blazing 30 million m.p.h. (48 million km/h) — or close to 5% of the speed of light — thanks mostly to their smaller mass.

This is one case, of course, in which theory bumps up against the limits of technology. With no star to jostle or dim, the planets would be entirely undetectable — at least given the limits of current telescopes. It's enough, perhaps, to know that those planets may be out there, and to contemplate not just how solitary their travels are, but also how much more they'll be when they reach the end of the Milky Way and sail off into the ocean of true intergalactic space.