Until recently, astronomers have assumed that planets are only to be found obediently orbiting their stars. Not so, according to an international collaboration led by Hiroki Sumi of Osaka University. Apparently, not only are there planets roaming around the galaxy with no host star, but they may outnumber stars by two to one.
The astronomers used a technique called gravitational microlensing to find the wayward planets. Briefly, as a closer object (like a planet) passes in front of a distant bright star, the gravity of the closer object affects the light coming from the distant star. Thus far, ten Jupiter-sized rogue planets have been detected, but the scientists extrapolate that there may be as many as 400 billion star-less planets in our galaxy.
This artist's conception illustrates a Jupiter-like planet alone in the dark of space, floating freely without a parent star. Astronomers recently uncovered evidence for 10 such lone worlds, thought to have been "booted," or ejected, from developing solar systems.
Image credit: NASA/JPL-Caltech.
Where did these homeless planets come from? If current theories about planetary formation are true, namely that planets form from the dust swirling around proto-stars, then these orphan planets must have been flung out of their stellar systems, perhaps due to collisions. On the other hand, there may be a completely different mechanism for planet formation that does not involve orbiting any stars.
To be clear, just because a planet does not orbit a star does not mean that it can wander aimlessly about the galaxy. Like the stars in the Milky Way, such planets would eventually fall into stable orbits around the center of the galaxy.
For more details, check out Phil Plait’s Bad Astronomy blog.