Not all planets orbit stars. There are some objects, known as rogue planets, that for some reason manage to escape the gravitational chains of their parent star and float freely around the galaxy. They are not easy to find, but thanks to a different approach, researchers have discovered two new ones.
The technique is known as gravitational microlensing and was made possible by the Optical Gravitational Lensing Experiment (OGLE). This survey has been going on for over two decades and is designed to detect the subtle variations in light from background stars as massive objects pass in front of them. The gravity of a planet-sized object can bend the light slightly for a few days but with the new objects, the effect was seen for less than half a day.
The team spotted the first object, OGLE-2017-BLG-0560, on April 16, 2017, and it was confirmed by three other observatories. The distance of the object is not known and that leads to some uncertainty about its mass, which could be anywhere between a few and 20 times the mass of Jupiter.
The discovery of this object, which had a very short microlensing signal, suggested that maybe similar events have been missed in previous analyses. The team discovered OGLE-2012-BLG-1323, an overlooked signal from August 2012. The rogue planet in question could have been as small as the Earth if it’s located in the nearby galactic disk, or as big as Neptune, if it’s further out in the galactic bulge.
As far as the researchers can tell, the planets are truly free-floating. If they are orbiting a star, they must be far away from it and it has to be quite small. Otherwise, the team would see the effects of the star. The research paper has been submitted for publication and it is now available on research repository arXiv.
In the paper, the team suggests that although only a few events like these are known, there could be a large population of small rogue planets. Otherwise, it would be quite a coincidence to have seen several of these events. They even think that there could be more Earth-mass rogue planets than stars in the Milky Way.
Several planetary system models have predicted that most rogue planets should be Earth or Super-Earth-sized, escaping the gravitational pull of their star more easily than gas giants. The Solar System might even have kicked some planets out. A gas giant could have been ejected by Jupiter billions of years ago.
[H/T: New Scientist]