Rogue is in vogue.
Planets without a star, known as rogue planets, may vastly outnumber star-orbiting worlds in the Milky Way, according to a pair of studies set to be published in The Astronomical Journal. Sans a sun, these rogues waywardly drift through the cosmos without end, and may number in the trillions.
"We estimate that our galaxy is home to 20 times more rogue planets than stars – trillions of worlds wandering alone," David Bannett, co-author of both studies and an astronomer 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."
The findings are the result of a nine year survey that used a phenomenon known as microlensing to detect distant worlds. In a nutshell, microlensing works through the immense gravity of a massive object that can distort light into creating natural lenses that magnify objects behind it.
"Microlensing is the only way we can find objects like low-mass free-floating planets and even primordial black holes," explained lead author of one of the studies Takahiro Sumi, an astronomer at Osaka University, in the statement.
Another intriguing insight the researchers discovered on these vagabond worlds is that they tend to be smaller, while homebody planets trend much larger, closer to their star, or both.
"We found that Earth-size rogues are more common than more massive ones," Sumi explained. "The difference in star-bound and free-floating planets' average masses holds a key to understanding planetary formation mechanisms."
The size disparity makes sense. Smaller planets forming around a star can easily get bullied by the gravity of larger ones in the same system. Over time, their orbits can become more distended by passing planetary giants, until the little one eventually breaks free from its star, going rogue.
Finding exoplanets — worlds beyond our solar system — is tough work, because they're quite literally outshined by the light of their stars. Fortunately, the researcher's work will stand to get a massive boost from NASA's upcoming observatory, the Roman Space Telescope, set to debut in 2027.
"Roman will be sensitive to even lower-mass rogue planets since it will observe from space," said Osaka University astronomer Naoki Koshimoto, co-author of one of the studies, in the statement.
"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."
Based on these most recent findings, they're optimistic that Roman will help them spot around 400 Earth-sized rogue planets, and are confident that they've already detected one such rogue of terrestrial-mass.
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