Two planets which do not orbit stars have been spotted by astronomers.

Two rare free-floating planets, which do not orbit stars, have been spotted by astronomers. 
According to Motherboard, one of the planets "was detected on April 16, 2017, and confirmed through follow-up observations from several other observatories." 
The exact size of it has not been determined due to its unknown proximity to Earth, though it is believed to be "anywhere between one to 20 times the mass of Jupiter."
Meanwhile, the second planet, whose discovery was prompted by combing through previous observations, could potentially be somewhere between Earth and Neptune in scale, notes Motherboard. 
Popular Mechanics reports, the existence of free-floating planets was long doubted by scientists because of the lack of light from a star to signal their presence. 
However, an approach called gravitational microlensing was developed where a planetary body can be detected by its ability to curve a star's light as from the point of view of Earth.
In fact, the recently published study about the rogue pair states: "Gravitational microlensing is the only method that enables one to find Earth-mass free-floating planets."
As for how such planets come into being, the paper notes they can sometimes be "formed by the fragmentation of gas clouds, in a way similar to that in which stars form."