Not all planets orbit stars. There are worlds that go rogue; kicked out of their star system, they roam interstellar space without a stellar companion. In fact, our galaxy may be full of them. They are likely outnumbering stars in our galaxy 20 to one, according to new research. With the upcoming Nancy Grace Roman Telescope, astronomers predict we will discover an incredible 400 Earth-mass rogue planets.
“We estimate that our galaxy is home to 20 times more rogue planets than stars – trillions of worlds wandering alone,” David Bennett, a senior research scientist at NASA’s Goddard Space Flight Center, said in a statement. “This is the first measurement of the number of rogue planets in the galaxy that is sensitive to planets less massive than Earth.”
Astronomers estimate that low-mass rocky worlds might be the most common type of rogue planet. They are smaller so they are more easily kicked out from their star systems. Only one of these planets has been observed before. Now, the findings of a nine-year survey provide two new crucial findings.
The first is the second-ever observation of a terrestrial rogue planet, and the second is that it provides an estimate of how many the Roman Telescope will see when it is up and running. The team thinks once it launches in 2027, the telescope will be able to discover 400 new worlds smaller than our own.
Finding rogue planets is not easy. Being untethered from stars means they are dark. We usually discover planets using the transit method: when a planet passes in front of its star it creates a blip in light that we can see. Instead, astronomers have to wait for a fortuitous alignment. If a rogue planet moving through the galaxy passes in front of any star from our line of sight it will slightly boost the star's brightness. This is known as microlensing, a type of subtle gravitational lens effect.
Rogue planets discovered in this way can only be studied as they pass in front of stars. So they are a one-off. But the transit takes from a few hours to a day, which allows astronomers to study the distant world even if only a brief time frame.
“Microlensing is the only way we can find objects like low-mass free-floating planets and even primordial black holes,” said Professor Takahiro Sumi, lead author of the paper with a new estimate of our galaxy’s rogue planets. “It’s very exciting to use gravity to discover objects we could never hope to see directly.”
Roman will be hunting these planets by looking toward the center of our galaxy, where there are many stars, so more opportunities for a free-floating world to pass in front of one. If one is spotted, ground-based instruments such as Japan's PRIME (Prime-focus Infrared Microlensing Experiment) telescope will follow up the detection.
“Roman will be sensitive to even lower-mass rogue planets since it will observe from space,” explained Naoki Koshimoto, who led the paper announcing the detection of a second candidate terrestrial-mass rogue world. “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.”
The two studies will be published in upcoming issues of The Astronomical Journal.