These “lonely planets” represent an entirely new category of planets, and are perhaps more numerous in our galaxy than stars, scientists report Wednesday in the journal Nature.
"It gives us a good clue about how planet formation works. It suggests that there's a lot of violent encounters between planets near the end of the planet formation process," said David Bennett, astronomer at the University of Notre Dame.
Bennett and colleagues discovered 10 such planets, each probably the size of Jupiter, in a survey called the Microlensing Observations in Astrophysics, which used a 5.9-foot telescope in New Zealand to scan our galaxy, the Milky Way.
These planets are likely gaseous, and would not be hospitable to life.
Here’s how they may have formed: Generally speaking, a planet is born when a gas cloud around a star collapses into a disc shape. There are a few different theories about what happens next, but the leading idea is that the dust and the ice begin to stick together, growing into larger objects because of gravitational attraction, Bennett said. When those giant objects become about 10 times the mass of the Earth, they begin to pull in large amounts of hydrogen and helium gas, resulting in Jupiter-sized planets about 300 times the mass of the Earth, he said.
Astronomers haven’t seen these planets with a naked eye, but have detected them indirectly. When a planet passes in front of a distant star, that star brightens. That’s because the planet’s gravity warps the light from the star, as if magnifying it. This is called “gravitational microlensing.” Albert Einstein correctly predicted this effect in 1936.
Scientists can estimate the mass of a planet by looking at how long this brightening event lasts. For Jupiter-sized planets, it happens over the course of one or two days, which is what they observed in this study.
Previous explorations of distant planets found evidence of free planets also, but didn’t have a good estimate of how many there were or how big, said Takahiro Sumi, associate professor at Osaka University’s Department of Earth and Space Science and the lead author of the study.