Most of the exoplanets we discovered were in relatively narrow orbits around their host stars, allowing us to track them as they repeatedly orbited them. But we've also discovered several planets thanks to a phenomenon 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 become brighter.
The key point about microlensing compared to other planet-finding methods is that the lensing planet can be almost anywhere on the line between the star and the Earth. So, in many cases, these events are caused by so-called rogue planets: those that are not part of any exosolar system at all, but are drifting in interstellar space. Now researchers have used microlensing and the lucky orientation of the Gaia Space Telescope to discover a Saturn-sized planet, the first discovered in the so-called “Einstein Desert,” that may tell us something about the origins of rogue planets.
Becomes a fraud
Most of the planets we have identified orbit stars and formed from disks of gas and dust that surrounded the star early in its history. We've taken pictures of many of these disks and even seen evidence of planets forming within them in some. So how do you get a planet that is not associated with any star? There are two possible routes.
The first involves gravitational interactions either between the planets of the system or as a result of an encounter between an exosolar system and a passing star. Under certain circumstances, these interactions can knock a planet out of its orbit and send it into interstellar space. So we should expect them to be like any typical planet, from small rocky bodies to gas giants. An alternative method of creating a rogue planet starts with the same gravitational collapse process that creates a star, but in this case the process literally runs out of gas. What remains is likely to be a large gas giant, perhaps with a mass somewhere between Jupiter and a brown dwarf.
