When most people think of “planets,” they probably imagine warm, rocky spheres, blanketed in air, and neatly circling some brightly shining star.
In fact, it’s possible most planets don’t belong to a star. These rogue, free-floating planets—“FFPs,” in scientific parlance—chart lonely paths through the vast dark of interstellar space. Cold, dark, and alone.
We know hardly anything about FFPs. It’s possible they’re the wandering survivors of the self-destruction of young star systems. Maybe the star failed. Maybe planets orbited too close together and threw off the whole system’s gravitational clockwork.
“They are our lost children,” Seth Shostak, an astronomer with the California-based SETI Institute, told The Daily Beast. They are possible Earth-like planets that don’t share Earth’s good fortune.
If FFPs are like Earth, they might even harbor life—tough, dark-loving microbes or other simple organisms that could really expand our notion of what the term “life” means.
The study of FFPs starts with one simple step—finding and counting them. To that end, a team led by Iain McDonald, an astronomer at the University of Manchester in the United Kingdom, tapped data from a now-defunct NASA space telescope.
The astronomers described their study in the August issue of Monthly Notices of the Royal Astronomical Society, a science journal.
After closely analyzing the data, McDonald’s team came to a surprising conclusion. Free-floating rogue planets might be more numerous than we previously thought. That could have profound implications.
The main aim in studying FFPs is to “better understand processes that govern formation and early evolution of planetary systems,” Radosław Poleski, an astronomer at the University of Warsaw and a member of McDonald’s team, told The Daily Beast. “This is one of the most important questions in astronomy.”
Astronomers for decades have believed the galaxy contains a fair number of FFPs. But it wasn’t until 2011 that a joint Japanese-American survey actually spotted the first rogue planets. The astronomers used a technique called “microlensing” that takes advantage of the weird way that light and gravity interact.
It works like this: Point a telescope at some corner of the galaxy. Register any flashes of light. While there are a lot of things in the galaxy that emit or reflect light—stars, for one—certain flashes could point to a nearby FPP.
Lacking a parent star of its own in whose light it might bathe, the FPP is dark. But when it passes between some other bright object and the telescope, it can bend the light passing between the two, causing what looks to the telescope like a flicker.
The 2011 microlensing survey confirmed the first FFP sightings. But the survey was imprecise. The astronomers in that effort predicted that, if you pointed a telescope at the center of the Milky Way and observed a four-degree arc of space, you’d probably find at least one rogue planet. You might even find as many as six.
McDonald’s team aimed for a more precise count. The raw data came from a 2016 scan of a four-degree arc of the galactic core by NASA’s Kepler space telescope, which launched into orbit in 2009. The space agency decommissioned the aging Kepler, a smaller cousin of the ailing Hubble space telescope, three years ago.
The 2016 scan captured 22 flashes of light. McDonald’s team scrutinized each flash.
It wasn’t easy. “A bespoke reduction process was necessary due to the poor resolution and complex systematics of [the Kepler space telescope],” the astronomers wrote.
They concluded that 17 flashes were “stellar flares.” Sunspots, basically. One flash was a new “bound” planet orbiting a star. Four flashes were FFPs passing across a light source, each a cold planet on some lonely journey.
Four FFPs falls at the high end of the potential population of rogue planets that the 2011 survey projected. In other words, it looks like there are actually a lot of cold, dark, star-less planets wandering across our galaxy.
The implications are exciting. “It’s quite possible that the majority of the planets in the universe are untethered to stars,” Shostak said. “And while they have very little external sources of energy, such planets—at least the size of Earth or more—are great Thermos bottles. They will remain warm on the inside for billions of years. Which is to say that they could have long-lasting biology under their skins.”
In other words, it’s possible there’s life—or evidence of extinct life—deep inside some of those billions of cold, wandering planets. “If there are indeed billions of free-floating planets in the Milky Way galaxy, we only need a one-billion-to-one chance to happen for one of them to have life,” McDonald told The Daily Beast. “Personally, I wouldn’t bet against that.”
The next step is a better, more comprehensive survey. The Kepler telescope is out of commission, but new instruments are in development. NASA is optimizing its new Nancy Grace Roman Space Telescope for exactly the kind of long-distance surveys that FFP hunters rely on.
Roman is slated to launch in the next few years. With its superior technology, the new telescope should be able to easily differentiate between stellar flares and passing FFPs, Douglas Vakoch, president of the San Francisco science-advocacy group Messaging Extraterrestrial Intelligence, told The Daily Beast.
Poleski said he’d prefer to pair the Roman telescope with the European Space Agency’s own Euclid telescope, which is scheduled for a 2022 launch.
Pointing both telescopes at the same swath of space could give astronomers even better data on possible FFPs. Poleski said they might even be able to tell how big a rogue planet is as it flits past some distant star.
As a bonus, the widening search for rogue planets could benefit other galactic inquiries. The same data astronomers accrue while scanning for FFPs could also help them study asteroids, quasars, and supernovas.
It all begins with a survey of the Milky Way’s “lost children.”
