Most of the exoplanets we've discovered have been in relatively tight orbits around their host stars, allowing us to track them as they repeatedly loop around them. But we've also discovered a handful of planets through a phenomenon that's called microlensing. This occurs when a planet passes between the line of sight between Earth and another star, creating a gravitational lens that distorts the star, causing it to briefly brighten.
The key thing about microlensing compared to other methods of finding planets is that the lensing planet can be nearly anywhere on the line between the star and Earth. So, in many cases, these events are driven by what are called rogue planets: those that aren't part of any exosolar system at all, but drift through interstellar space. Now, researchers have used microlensing and the fortuitous orientation of the Gaia space telescope to spot a Saturn-sized planet that's the first found in what's called the "Einstein desert," which may be telling us something about the origin of rogue planets.
Going rogue
Most of the planets we've identified are in orbit around stars and formed from the disks of gas and dust that surrounded the star early in its history. We've imaged many of these disks and even seen some with evidence of planets forming within them. So how do you get a planet that's not bound to any stars? There are two possible routes.
