Scientists have known for a long time that there are “wandering” planets, attached to no star, that wander alone through the Galaxy. We have so far discovered a few, but a recent study suggests that there may be more than expected.
Putting your finger on an exoplanet is not an easy task. Planets do not emit any light, making them hard to detect. Also to find them, you must rely on indirect means. There are two main methods for this. First, that of the radial velocity, on one side, which allows astronomers to detect the gravitational effect of the planet on its star. The method of transit, on the other, which allows us to spot minute variations in brightness of a star when a planet is passing in front of it, in our field of vision.
Detecting Orphaned Planets
These two methods are very interesting. They enabled scientists to identify more than 3,900 exoplanets. But as you can see, each implies the presence of a star. To detect orphan planets, astronomers rely on a third option: the gravitational micro-lens. The gravitational force of the planet here inflects the light coming from behind it. In other words, the deviated light betrays the presence of a body between the observer and the object in the background. And sometimes it’s a planet.
In total, about twenty of these worlds have been detected to date. But could there be more? To find out, astronomers from the University of Leiden in the Netherlands conducted mathematical simulations using the Cluster of Trapeze, a group of young stars nestled in the heart of the Orion Nebula. A typical nursery in the Milky Way, and therefore a good example on which we could extrapolate results.
16.5 billion wandering planets
Five hundred Sun-like stars have been modeled with four, five, or six planets each. We had a total of 2,522 planets analyzed, with masses of about three times that of Earth up to about 130 times the mass of Jupiter. In evaluating their movements and evolution, 16.5% (357) of these planets detached themselves from their host star within 11 million years of their formation. Some were captured by other systems, but most (282 planets) found themselves completely alone, roaming freely in the galaxy.
By extrapolating this figure to the entire Milky Way, astronomers estimate that there could be at least 16.5 billion wandering planets in our Galaxy alone. One of those lonely worlds could even have visited our system not so long ago. A recent study suggests that Uranus, the penultimate planet in our system, was hit by a huge object about 3.5 billion years ago. This collision would explain today its axis of rotation different from that of the other planets.