Astronomers got their first inklings that these objects existed in the from computer simulations they performed in the 1970s. But it was not until 2000 that the first rogue planet was actually observed. Since then, astronomers have unintentionally found about 50 of these rogue planets. Most are very much like the planets we’re familiar with, have all the characteristics of planets here in our own solar system, they’re just not orbiting a star. Others, however raise questions about the very processes through which stars and planets can form. All of them, the familiar and the peculiar, challenge astronomers’ definitions of what exactly a planet is.
University of Hawaii in Honolulu’s Michael Liu, thinks it’s time we stopped finding and studying rogue planets by accident. He wants to start doing a systematic search for them. “A census of rogues,” says Liu, “is the only way we are going to fully understand the extent of what’s out there in the Milky Way.”
Liu and his colleagues first spotted rogue planet PSO J318.5-22 in 2010; but, wanting to confirm their finding first, they didn’t report the finding until 2013. The team wasn’t looking for rogue planets at the time. They were using the Maui-based Pan-STARRS 1 telescope, looking for failed stars called brown dwarfs. Brown dwarfs appear to start their lives in the same manner that stars do when clump of gas breaks free from a very large cloud of cold, dense gas. The break-away clump collapses, and begins pulling material into a swirling disk around it. At the disk’s center is a baby star, a brown dwarf.
But two traits distinguish brown dwarfs from other stars: mass and the absence of nuclear fusion. Even small stars, are still massive, at least 80 times the mass of Jupiter, and just to give you some scale: keep in mind that Jupiter, the most massive planet in our solar system, is 318 times the mass of Earth. It’s also a standard of measurement used by astronomers to describe the mass of large planets.
Theoretical calculations about how stars work, predict that objects must be at least 80 Jupiter masses to fuse hydrogen nuclei (protons) into helium. This is the process that allows stars to shine. Brown dwarfs on the other hand are not so lucky. They are smaller, anywhere between 13 and 80 Jupiter masses, and therefore not dense enough to fuse hydrogen. But that doesn’t mean they don’t burn. They do sometimes become big and hot enough to fuse deuterium nuclei (a proton plus a neutron) with protons or other nuclei.
Then there are the spheres that form into masses of less than about 13 Jupiter masses. They aren’t large or dense enough to fuse any kind of atomic nuclei, and so they are defined by some astronomers as planets, but without a star to orbit, and so they are: rogue planets.