Trillions of Rogue Planets Could Be Careening Through Our Galaxy

A new estimate suggests the Milky Way contains more free-floating planets than stars. It’s a big claim, but an upcoming mission might actually prove it.

Rogue planets in our galaxy could number in the tens of billions and possibly even trillions, according to new research published in the Astronomical Journal. If confirmed, it means the Milky Way hosts more unbound, starless planets than it does stars. This estimate was developed in preparation for the Roman Galactic Exoplanet Survey (RGES), a five-year mission that’s scheduled to start in about five to six years.

For the survey, NASA’s Nancy Grace Roman Space Telescope—scheduled to launch in 2025—will continuously scan a narrow patch of sky in search of rogue planets. The Roman Space Telescope is predicted to detect and characterize hundreds of rogue planets, according to computer models described in the new paper, co-authored by astronomers Samson Johnson and Scott Gaudi from Ohio State University. This limited data sample will then enable astronomers to extrapolate and infer the total number of rogue planets across the entire Milky Way, which, as mentioned, is expected to be enormous.

To date, astronomers have found only a small handful of rogue planets, as they’re notoriously difficult to detect. To “actually get a complete picture, our best bet is something like Roman,” Johnson told Ohio State News, adding that this “is a totally new frontier.”

Not much is known about these starless planets, including the environment in which they’re born. These planets could form around stars and then get tossed out into interstellar space by powerful gravitational perturbations. It’s also possible that these planets formed without a parent star, emerging from dense clouds of gas and dust in a process similar to how stars are born. Data gathered by the Roman space telescope could test these theories.

Rogue planets are difficult to detect with conventional methods, but astronomers can sniff out signatures consistent with the presence of planet-sized objects careening through space, namely their ability to warp space-time. To that end, the RGES team will use a method called gravitational microlensing. NASA explains:

If a rogue planet aligns closely with a more distant star from our vantage point, the star’s light will bend as it travels through the curved space-time around the planet. The result is that the planet acts like a natural magnifying glass, amplifying light from the background star. Astronomers see the effect as a spike in the star’s brightness as the star and planet come into alignment. Measuring how the spike changes over time reveals clues to the rogue planet’s mass.

The incoming microlensing signal will be transient and very brief, lasting no longer than a few hours or days, but the Roman Space Telescope will be sensitive enough for the task. In fact, this space-based observatory, which will have its back to the Sun, will be 10 times more effective at detecting rogue planets than previous methods, such as ground-based telescopes, according to the paper.

For the planned microlensing survey, the Roman telescope will search a narrow strip of galaxy stretching from here to the center of the Milky Way—a distance of 24,000 light-years. The telescope will stare at the same patch of sky continuously for months in hopes of detecting the desired microlensing signature.

The technique should be sensitive enough to detect planets with masses as small as Mars, which is 10 times less massive than Earth. Planetary masses revealed by the microlensing technique, along with the rogue planet census, could improve our understanding of how these objects form.

As noted, the survey is expected to yield hundreds of rogue planets. If the Roman telescope finds far fewer or even none, astronomers will obviously have to reevaluate their thinking on the matter or their detection methodologies. If the upcoming survey meets our expectations, however, it means the total number of rogue planets in our galaxy is astronomically huge, pointing to these dark, free-floating objects as a ubiquitous fixture of the Milky Way.

And that’s a very spooky thought.