A new study suggests that the number of planets that are not in orbit around a star or any other body is much higher than the planets that are.
Those planets with no ties to stars or comparable bodies are called “starless planets”, “rogue planets” or “lone planets”. Without having to follow an orbit around a star, they simply revolve around the center of the galaxy.
The results of the new study imply that NASA’s Nancy Grace Roman Space Telescope, scheduled to launch into space in May 2027, could find a staggering 400 Earth-mass rogue worlds. In fact, a possible planet of that class has already been identified in this new study.
The study was carried out by scientists from NASA (the US space agency) and the University of Osaka in Japan.
The team led by David Bennett (NASA) and Takahiro Sumi (Osaka University) estimates that in our galaxy there are 20 times more rogue planets than stars, that is: trillions (millions of millions) of worlds roaming the galaxy alone.
This is the first estimate of the number of rogue planets in the galaxy that takes into account worlds less massive than Earth.
The team’s findings come from a nine-year survey of the cosmos called MOA (Microlensing Observations in Astrophysics) at the Mount John Astronomical Observatory in New Zealand.
Gravitational lensing is the phenomenon that occurs when a supermassive astronomical object in space, for example a cluster of galaxies, crosses, from Earth’s visual perspective, in front of a galaxy or other luminous object that shines much further away in the background. The strong gravitational pull of the supermassive object bends light rays from the distant object behind it, magnifying the image like an optical lens. That allows astronomers to see the most distant object with much higher resolution than would be possible without gravitational lensing.
The gravitational microlensing phenomenon follows the same principle but on a much smaller scale. It occurs when a body, such as a star or a planet, aligns almost perfectly with a star that is unrelated to the body but located just behind it from Earth’s visual perspective. Since anything with mass warps the fabric of space-time, even a little, light from the distant star bends around the closest object to Earth as it passes close to it. The closest object acts as a natural lens, creating a brief spike in the brightness of the light from the background star that gives astronomers clues about the object in between that they couldn’t get any other way.
“Microlensing is the only way to find objects like low-mass rogue planets,” Sumi says.
The Earth-mass planet the team has found is the second of its kind to be discovered.
An artist’s recreation of a rogue planet of a rocky type like Earth and a mass similar to its own but frozen and plunged into a cold perpetual night, due to the absence of a nearby star. (Image: NASA’s Goddard Space Flight Center)
In just a few decades, we have gone from knowing only the planets in our solar system to discovering more than 5,300 planets outside of it. The vast majority of these worlds captured outside the solar system are huge, are very close to their star, or have both characteristics, because such features favor detection with the means currently available to us. In contrast, the results of the new study suggest that rogue planets with masses similar to Earth’s are more common than high-mass rogue planets.
In one of the first episodes of the original Star Trek series, the crew of the ship Enterprise finds one of these lonely planets in the middle of a supposed stellar desert (an area of the cosmos devoid of stars). The crew are surprised to discover that Gothos, the starless planet, is habitable. Although a habitable rogue planet may be plausible, the authors of the new study stress that the newly detected rogue planet likely doesn’t share many other characteristics with Earth beyond a similar mass.
Microlensing events that reveal rogue planets are extraordinarily rare, so one way to find far more than we can now is to use much more powerful and specific observing instruments than are currently available. We will have such an instrument in May 2027, when, if all goes according to plan, the Nancy Grace Roman Space Telescope will be launched into space.
Previous best estimates, based on planets orbiting stars, suggested that Roman would detect 50 starless planets with masses similar to Earth’s. The results of the new study suggest that it could actually find about 400, although we will have to wait until Roman starts scanning the sky to make more accurate predictions. (Fountain: NCYT by Amazings)