Cosmic nomads: NASA’s Roman Space Telescope could find 400 Earth-mass rogue planets

Illustration of the Roman Nancy Grace Space Telescope

Scientists from NASA and Osaka University suggest that rogue planets, which roam in space without ties to stars, far outnumber the planets in stellar orbit. The upcoming launch of NASA’s Nancy Grace Roman Space Telescope could lead to the discovery of hundreds of such rogue planets, adding critical insights into the mechanisms of planetary formation. Credit: NASA

New research by scientists at NASA and Japan’s Osaka University suggests that rogue planets, or worlds that wander through space without being tethered to a star, significantly outnumber planets that orbit stars. The findings indicate that NASA’s Nancy Grace Roman Space Telescope, scheduled to launch in May 2027, could find a staggering 400 Earth-mass rogue worlds. In fact, this new study has already identified one such candidate.

David Bennett, a senior research scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and co-author of two papers describing the findings, said: “We estimate that our galaxy is home to 20 times more rogue planets than trillion worlds roaming stars alone. This is the first measurement of the number of rogue planets in the galaxy that is sensitive to less massive planets than Earth.

The team’s findings come from a nine-year survey called Microlensing Observations in Astrophysics (MOA), conducted at the Mount John University Observatory in New Zealand. Microlensing events occur when an object such as a star or planet comes into nearly perfect alignment with an unrelated background star from our vantage point. Since anything with mass warps the fabric of spacetime, light from the distant star bends around the closer object as it passes by. The closer object acts as a natural lens, creating a brief spike in the brightness of background starlight that gives astronomers clues about the intermediate object that they can’t get any other way.

Rogue planet encrusted with ice and landmass

This concept art shows an ice-encrusted, Earth-mass rogue planet drifting through space on its own. Credit: NASA Goddard Space Flight Center

Microlensing is the only way we can find objects like low-mass free-floating planets and even primordial black holes, said Takahiro Sumi, a professor at Osaka University and lead author on the paper with a new estimate of our galaxy’s rogue planets. It’s very exciting to use gravity to discover objects that we could never hope to see directly.

The roughly Earth-mass rogue planet the team found marks the second discovery of its kind. The paper describing the discovery will appear in a future issue of The astronomical journal. A second paper, presenting a demographic analysis that concludes that rogue planets are six times more abundant than worlds orbiting stars in our galaxy, will be published in the same journal.

Pint-sized planets

In just a few decades, we’ve gone from wondering if the worlds in our solar system are the only ones in the cosmos to discover more than 5,300 planets outside our solar system. The vast majority of these new worlds are huge, extremely close to their host star, or both. In contrast, the team’s findings suggest that rogue planets tend to be small.

We found that Earth-sized thieves are more common than more massive ones, Sumi said. The difference between the mean masses of star-bound and free-floating planets is a key to understanding the mechanisms of planetary formation.

This animation illustrates the concept of gravitational microlensing with a rogue planet, a planet that doesn’t orbit a star. When the rogue planet appears to almost pass in front of a background source star, the source star’s light rays bend due to the warped space-time around it. This slightly changes the apparent position of the stars in the sky and can even produce multiple copies of them. Such changes signal the presence of planets to astronomers. Credit: NASAs Goddard Space Flight Center/CI Lab

Worldbuilding can be chaotic, with all the forming celestial bodies interacting gravitationally as they settle into their orbits. Light planetary weights aren’t bonded as strongly to their star, so some of these interactions end up throwing such worlds into space. Thus begins a solitary existence, hidden in the shadows among the stars.

In one of the first episodes of the original Star Trek series, the crew encounters one of these lonely planets in the middle of a so-called star desert. They were surprised to eventually find Gothos, the starless planet, habitable. While such a world might be plausible, the team points out that the newly detected rogue Earth probably doesn’t share many other characteristics with Earth beyond a similar mass.

Romans hunting for hidden worlds

Microlensing events that reveal lonely planets are extraordinarily rare, so one key to finding more of them is to cast a wider net. That’s just what Roman will be doing when it launches by May 2027.

Roman will also be sensitive to lower-mass rogue planets since it will be observing from space, said Naoki Koshimoto, who led the paper announcing the detection of an Earth-mass rogue world candidate. Now an assistant professor at Osaka University, he conducted this research at Goddard. The combination of the Romans’ wide field of view and sharp vision will allow us to study the objects he finds in more detail than we can using ground-based telescopes alone, which is an exciting prospect.

Previous best estimates, based on planets found orbiting stars, suggested that Roman would have spotted 50 Earth-mass rogue worlds. These new results suggest that he may actually find around 400, though we’ll have to wait until Roman starts scanning the skies for more certain predictions. Scientists will combine future Roman data with ground-based observations from facilities such as Japan’s Prime-focus Infrared Microlensing Experiment (PRIME) telescope, located at the South African Astronomical Observatory in Sutherland. This 1.8-metre telescope will build on the work of the MOA by conducting the first large-area microlensing survey in near-infrared light. It features four detectors from the Roman Detector Development Program, provided by NASA as part of an international agreement with JAXA extension (Japan Aerospace Exploration Agency).

Each microlensing event occurs only once, meaning astronomers can’t go back and repeat the observations once they’re done. But they are not instantaneous.

A microlensing signal from a rogue planet can take anywhere from a few hours to about a day, so astronomers will have the ability to make simultaneous observations with Roman and PRIME, Koshimoto said.

Seeing them from Earth and the Romans’ position a million miles away will help scientists measure the masses of rogue planets far more accurately than ever before, deepening our understanding of the worlds that grace our galaxy.

Grace Nancy Roman Space Telescope it is operated at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. It involves several collaborators, including the Jet Propulsion Laboratory and NASA’s Caltech/IPAC in Southern California, the Space Telescope Science Institute in Baltimore, and a scientific team made up of scientists from various research institutes. Its principal industrial partners are Ball Aerospace and Technologies Corporation in Boulder, Colorado; L3Harris Technologies in Melbourne, Florida; and Teledyne Scientific & Imaging in Thousand Oaks, California.

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