Astronomers from Warsaw University in Poland have identified two “rogue planets” in our galaxy that do not orbit around a star. Unlike the vast majority of discovered planets, these rogue planets drift through space alone with no sun to shine on them.
Debate has raged in astronomical circles for years as to whether rogue planets could exist. Since they do not have a star to illuminate them, they are extremely difficult to find as they are almost always in the dark. However, a technique called gravitational microlensing allowed researchers to identify rogue planets by seeing when a planet comes between a distant star and the Earth. When this happens, the planet acts like a lens, distorting the light that we can see from that star when it reaches Earth. This indicates that a massive body like a planet is passing in front of the star, and the size of the body can be estimated from the size of the distortion.
The two discovered rogue planets are called OGLE-2017-BLG-0560 and OGLE-2012-BLG-1323, named after the Optical Gravitational Lensing Experiment (OGLE) survey that discovered them. The first planet detected, OGLE-2017-BLG-0560, was detected in April 2017 and could be between one and twenty times the mass of Jupiter. Finding evidence of this planet inspired the scientists to look back through their older research data for similar evidence, which is how they discovered the smaller OGLE-2012-BLG-1323, somewhere between Neptune and Earth in size, which was first captured in August 2012. The results were published in the preprint journal archive arXiv.
This makes the planets two of only a small handful of rogue planets that have been identified so far. The reason that the researchers cannot be sure of the exact size of each planet is that they cannot know how far the planets are from Earth. Therefore, the planets could be larger and further away or smaller and closer and produce the same gravitational effect on the light from distant stars.
Even though they are hard to spot, some astronomers predict that rogue planets are not rare in our galaxy. Neil DeGrasse Tyson speculated that there could be billions of rogue planets out in the cold of space, created in the chaotic births of solar systems and flung out into space.