Most of the exoplanets we discover are in relatively tight orbits around their host stars, allowing us to track them as they move around them repeatedly. But we’ve also discovered a handful of planets through a phenomenon called microlensing. This happens when a planet passes across the line of sight between Earth and another star, creating a gravitational lens that distorts the star, causing it to briefly brighten.
The main thing about microlensing compared to other methods of finding planets is that lensing planets can be almost anywhere On the line between the star and the Earth. So, in many cases, these events are driven by rogue planets: those that are not part of any exosolar system, but that drift through interstellar space. Now, researchers have used microlensing and the Gaia space telescope’s accidental orientation to observe a Saturn-sized planet, the first planet found in an “Einstein desert” that may tell us something about the origins of rogue planets.
going rogue
Most of the planets we have identified orbit around the star and are formed from the disk of gas and dust that surrounded the star early in its history. We have photographed many of these disks and even observed some with evidence of planet formation within them. So how do you find a planet that isn’t bound to a star? There are two possible routes.
The first involves gravitational interactions, either between the system’s planets or due to an encounter between the exosolar system and a passing star. Under the right circumstances, these interactions can knock a planet out of its orbit and send it into interstellar space. As such, we should expect them to be like any normal planet, ranging in mass from small, rocky bodies to gas giants. An alternative method of forming rogue planets starts with the same process of gravitational collapse that forms a star – but in this case, the process literally runs out of gas. What’s left is likely to be a giant gas giant, possibly somewhere between Jupiter and a brown dwarf star in mass.
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