Here’s the thing about planets: nobody’s quite sure how to define them. The ancient Greeks named them, deciding they were “wandering stars,” which we later realized wasn’t accurate at all. Then we thought we might be able to define them by their size, but Pluto ruined that for everyone. Lately, they’ve been giving the name to bodies that are round and large enough to clear out any objects in their neighborhood. Notice how that definition doesn’t require orbiting a sun?
That’s because of a recently-named phenomenon called Nomad Planets -- large round bodies that are free floating in the universe with no star system to call home. Science has had a vague idea about them for years (in the past they’ve been referred to as rogue or orphan planets), assuming they were a rarity -- about 500 that they could sort of observe. Just another strange but possible outcome from the formation of a solar system.
But in March this year scientists at Stanford did a little math and determined their numbers probably aren’t in the hundreds but more like the hundreds of thousands. It was a revelation that, says the paper’s lead astrophysicist Louis Strigari, “changes how we view the formation of stars and planets.”
That’s because astrophysicists speculate the formation of rogues is tied directly into the creation of a solar system. When a cloud of gas and particles gets spinning fast enough it will eventually collapse under its own weight. The result of that is a group of planets orbiting a sun. But not all of those planets are in a well behaved, stable orbit. Some are wobbly. They bump into their fellow planets or get pushed around by the orbits of others and eventually get slingshot out of the solar system. Without a sun they’re left cold and dark. Telescopes can’t see them, so it’s no wonder that Strigari and his colleagues are excited about the result of their research.
The More We Know The Less We Know
The Nomad revelation is part of a continuing problem in the world of astronomy. We’re constantly learning new and interesting things about the universe. But as we learn more we realize how little we actually know. This is causing a bit of a disconnect between the astrophysicists who theorize and imagine all the possibilities in the universe and the astronomers who do the actual observing.
So, while Strigari says that their paper was an attempt to “lay the gauntlet down” to astronomers, hoping they’ll design an observational experiment that attempts to prove them right (or wrong), the astronomers see the nomads as just another phenomenon to add to their long list of things in the universe worth looking at.
According to John Johnson, an astronomer at Caltech whose research focuses on detection and characterization of exoplanets, telescope time is so limited and expensive the only option is for them to dedicate it to objects they can easily see. “It’s not to say that these [Nomads] are not interesting. They’re pretty amazing. We have a way to explain them but we can’t observe them because we don’t know their nature. The skill of being an astronomer is applying a filter. We as observers spend a lot of time looking at the cost of telescope time and the benefit of what we choose to observe.”
Even SETI scientists are forced to make the same difficult choices. While there’s a possibility that these Nomads have the makings of life on their cold surface (or actual life at their warm centers), SETI’s eye on the sky is focused on the planets NASA’s Kepler telescope discovers -- the ones they can easily see. “You have to ask yourself if it’s significant,” says Seth Shostak, Senior Astronomer at the SETI Institute. “If you really want to go out on a limb you could say they’re unlikely to have intelligent life...unless somebody uses them to hitch a ride. Just to escape from the solar system you have to go 10 miles a second. These things are moving pretty fast.”
At the moment, that means the results of the Stanford study is more likely to change our ideas about the universe then it is to change our actual observation of it. According to Dimitar Sasselov, a professor of Astronomy at Harvard and Director of the university’s Origins of Life Initiative, “there’s a lot that needs to be done in the next few years to understand planets. Nomads are the big example. If you look at our solar system and try to decide what a planet is and then you go to the exoplanets and try to put that into the new understanding you see that you have revise what you knew before. And now Nomad planets just compound the mystery. It’s more of the drudge work to figure out the diversity of what planets are.”
But just because we can’t easily look at Nomads right now doesn’t mean we’ll never see them. Tomorrow, we’ll look at how astrophysicists went about “seeing” the rogue planets to begin with -- and how new, extremely powerful telescopes might push the cold, fast moving planets up the list of objects in the universe astronomers choose to get a look at.