In A Nutshell
Our galaxy, the Milky Way, contains anywhere from 200 to 400 billion stars. Astronomers are finding that a large number of these are like our Sun, with planets in orbit around them. Most people, asked to imagine what an alien planet may be like, would see it whizzing around a parent star. For decades, that was the consensus of astronomy. Recent research, however, suggests that rogue planets flying around interstellar space, free from any stars, outnumber the number of stars in our galaxy by 100,000 to 1. Even if every star in the Milky Way had as many planets as are in our solar system, there would still be 12,500 rogue planets for every planet in an orbit.
The Whole Bushel
Scientists have been discovering extrasolar planets—that is, planets outside of our solar system, at a good rate. There are 974 that we know about so far, orbiting 744 stars. A rogue planet is defined as a body with planetary mass that doesn’t orbit a star. Only four specific candidates exist for that title that we know about, but scientists have been able to deduce that there must be many more.
Rogue planets are hard to observe because our current detection methods for extrasolar planets rely on observing the effects they have on their parent star. A planet can pass in front of it during orbit and dim it, or make it wobble with its gravitational pull, both of which we can observe with a telescope. Rogue planets, being without any source of radiation, are much harder to find. They do produce a small amount of infrared radiation, which has allowed us to detect the ones we have so far.
Scientists used to think that all rogue planets were formed during the creation of a typical solar system and ejected out into space, left to orbit the center of the galaxy like stars. This would account for billions of planets that are out there, but recent observations suggest that rogue planets could form independent of a star. Stars form in massive dust clouds, but if a small cluster of dust breaks off, it will form what scientists term a “globulette.” These globulettes are still pretty big, 50 times as wide as the distance between Neptune and the Sun, but not big enough to produce a star. Instead, the dust will condense into a planet, destined to catapult alone through the galaxy.
Could a rogue planet support life? Possibly. There may be subterranean oceans at a suitable temperature, according to astrobiologists. As on Earth, life could evolve around thermal vents, in an ecosystem that requires no sunlight. There’s also the possibility that a rogue planet could pass a lot closer to Earth than those planets around another star, so a rogue planet may be the first extrasolar visit we make as a species.