Rogue Planets: 'Billions of Jupiters' on the Loose in the Milky Way

The bitterly fought Pluto wars of a few years back showed that even the experts disagree on what is and what isn’t a planet. One thing there’s no quarrel about, of course: a planet is, by definition, something that orbits a star.

Except, it turns out, when it isn’t. Writing in the latest issue of Nature, a team of astronomers is reporting the discovery of ten objects roughly the size of Jupiter that seem to be on the loose, roaming the galaxy untethered to any star. And while ten seems like an insignificant number in a galaxy packed with 200 billion or more stars, the search was an extremely limited one. Unless the observers happened to be absurdly lucky, there could actually be a lot more of these rogue Jupiters — perhaps twice as many stars as there are in the Milky Way. See the top 50 space moments since Sputnik.

“The implications of this discovery,” writes Joachim Wambsganss, of the Center for Astronomy at the University of Heidelberg in an accompanying Nature commentary, “are profound.”

They aren’t, however, completely unexpected. As the known number of more conventional exoplanets — that is, those that actually do orbit stars — has grown to more than 500 in recent years, astronomers have begun to realize that our own well-behaved Solar System isn’t necessarily typical. The eight planets orbit the Sun in nearly circular orbits, all moving in the same direction as the Sun rotates. But plenty of alien worlds orbit their stars in eccentric, somewhat egg-shaped orbits and surprising numbers move around their stars in highly tilted orbits as well. (Non-planet Pluto inscribes just such an inclined and elliptical path.) Some planets even orbit backwards. See pictures of Saturn.

That suggests that sometime in the past, close gravitational encounters with other planets flung them out of their previously conventional orbits. Theorists have predicted for years that such close encounters might also fling planets out into interstellar space.

“There’s nothing physically mysterious about this,” says David Stevenson, a Caltech astronomer who has been working on rogue-planet scenarios for more than a decade. “It’s a perfectly natural outcome.” See pictures of five nations’ space programs.

Still, he says, it’s exciting to see something that had been purely theoretical backed up by hard evidence. It’s also notable that these planets were discovered with a technique that sounds like something out of Star Trek. As Einstein explained nearly a century ago, the gravity of a massive object such as a star or a planet warps the spacetime that surrounds it. If a ray of light passes close by, the light will be diverted, just as though it were passing through a giant lens. In the 1930′s, Einstein realized that if one star passed in front of another, the closer star could act as a “gravitational lens,” focusing the light of the more distant star and making it look brighter. We’d never actually see such an effect, conceded Einstein. All stars orbit the center of the galaxy at slightly different speeds, meaning that they are in constant motion relative to one another; still, the exact lineup needed to make true lensing occur would happen only very rarely, Einstein believed.

For once, though, the great man was wrong. The first gravitational lens was spotted in 1979 and they’ve become a mainstay of astronomical research ever since. This has aided in the search for exoplanets, as astronomers scan the skies for double flashes — one caused by a foreground star moving in front of a background star, and one following soon after, caused by the orbiting planet. Read more

Video below:

The movie begins by showing the busy, central region of our Milky Way galaxy, where the planets were found with a ground-based telescope. It then zooms in on a star that brightens. This brightening is due to the passage of an unseen, free-floating planet (and has been exaggerated here). As a planet just happens to cross in front of a more distant star, its gravity causes the starlight to warp, and this warping resulted in an overall brightening of the star as seen by the telescope. In this effect, called gravitational microlensing, the planet’s gravity plays the role of a magnifying lens.

The next part of the animation shows a zoomed in view of what the microlensing of a star would look like if it could be seen at much higher resolution. The blue dot represents the planet, but has been enlarged to make it easy to see. The main star is the brightest dot in the center, shown amidst other smaller, red and yellow stars. As the planet passes by, its gravity causes light from the stars to split into multiple, mirrored and reversed images. When the planet is directly in front of the main star, that star’s multiple images are stretched into arcs. The overall result is a temporary brightening of the star.

Astronomers refer to the circular shape that can be seen as the planet passes by the stars as an Einstein Ring. When a planet is directly in front of star, it will cause the starlight to bend into a full Einstein Ring. When the planet is near stars, it will cause the star images to either appear deflected away from the ring, or inverted and reversed within the ring.

The duration of the microlensing event will reveal the rough mass of the passing body. Jupiter-mass objects will cause a star to brighten more quickly, over just a day or two. A passing star would cause a more distant star to brighten over a period of weeks.

The overall density of stars, as well as the brightness of their inverted images within the Einstein ring, have been exaggerated in this animation to help show the effects of the gravitational lensing. It is very rare for one passing planet to distort the light from multiple stars at once.

The movie ends with an artist’s conception of a free-floating, Jupiter-mass world.

The gravitational microlensing shown is based on simulation data from M. Freeman (University of Auckland, New Zealand).