There Could be Trillions of Rogue Planets Wandering the Milky Way

A pair of new studies set to be published in The Astronomical Journal examine new discoveries in the field of rogue planets, which are free-floating exoplanets that drift through space unbound by the gravitational tug of a star. They can form within their own solar system and get ejected, or they can form independently, as well. The first study examines only the second discovery of an Earth-mass rogue planet—the first being discovered in September 2020—while the second study examines the potential number of rogue planets that could exist in our Milky Way Galaxy.

“We estimate that our galaxy is home to 20 times more rogue planets than stars – trillions of worlds wandering alone,” said Dr. David Bennett, who is a senior research scientist at NASA’s Goddard Space Flight Center, and a co-author on both studies. “This is the first measurement of the number of rogue planets in the galaxy that is sensitive to planets less massive than Earth.”

This research builds upon a previous hypothesis that the number of rogue planets could be greater than the number of stars within our galaxy, and is the result of a nine-year survey called MOA (Microlensing Observations in Astrophysics), which was carried out at the Mount John University Observatory in New Zealand. Current estimates state at least 100 billion stars exist in the Milky Way Galaxy, so this latest research means there could be at least 2 trillion rogue planets randomly floating throughout our tiny corner of the universe.

The Earth-mass rogue was found with the microlensing method like the first discovery. This detection method involves using a gravitationally strong object, traditionally a star, to act as a gravitational lens that bends any light traveling from behind it towards Earth, which astronomers can then observe using these warped light waves. In the case of rogue planets, the planet uses its own gravitational field to be observed when passing in front of a distant star.

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

These two studies are preludes to what the Nancy Grace Roman Space Telescope (previously named the Wide-Field Infrared Survey Telescope, or WFIRST) will accomplish, which currently has a contracted launch date of October 2026 and a commitment launch date of May 2027. Shortened as Roman, this NASA-built infrared space telescope will be located at the Sun-Earth L2 Lagrange Point, which is located on the opposite side of the Earth from the Sun.

Artist illustration of the Roman Space Telescope. (Credit: NASA)

Once there, Roman will spend its currently planned 5-year mission trying to answer some of the universe’s most profound questions, specifically pertaining to dark matter and dark energy, the theory of general relativity, and searching for exoplanets using the microlensing method. The second study’s results imply there could be up to 400 Earth-mass rogue planets just within our Milky Way.

“Roman will be sensitive to even lower-mass rogue planets since it will observe from space,” said Dr. Naoki Koshimoto, who is lead author of the first study but conducted the research at Goddard and is now an assistant professor at Osaka University. “The combination of Roman’s wide view and sharp vision will allow us to study the objects it finds in more detail than we can do using only ground-based telescopes, which is a thrilling prospect.”

Given there are very few confirmed rogue planets, the knowledge that our galaxy could contain up to 2 trillion rogue planets and the findings from the upcoming Roman mission could provide greater insights into these intriguing worlds, including the formation and evolution of planets and planetary systems.

What new discoveries will astronomers make about rogue planets in the coming years and decades? Only time will tell, and this is why we science!

As always, keep doing science & keep looking up!