New predictions suggest that an upcoming NASA space telescope could find more than 400 Earth worlds hidden throughout the Milky Way that have “gone awry” and are therefore just wandering around our galaxy.
Such orphan worlds are thought to begin their lives in a planetary system similar to the Solar System, but are trampled at some point by an as-yet-unknown system. Despite the familiar image of planets gracefully orbiting a star, new research suggests that worlds without such orphan stars outnumber stars in the Milky Way by 20 to 1. This suggests that there are six times more unbound worlds in our galaxy than planets orbiting their parent stars.
“We estimate that there are 20 times more rogue planets than stars in our galaxy — trillions of worlds wandering alone,” study author and senior NASA scientist David Bennett said. said in a statement. “This is the first measure of the number of rogue planets in the galaxy that are sensitive to planets less massive than Earth.”
Related: ‘Trojan planet’ finds first evidence of co-orbiting worlds
Normally, planets outside our solar system, known as exoplanets, are discovered by the impact they have on their host stars. For example, an exoplanet can be witnessed by Earth-based observers as its star dims because the planet’s path takes it between the star and our planet. Alternatively, an exoplanet can influence such light by creating a motion in its host star’s orbit as it gravitationally pulls the luminous body. But the fact that rogue planets are so far from their host stars makes them difficult to detect.
One of the main goals of NASA’s Nancy Grace Roman Telescope is to find these rogues when the space-based instrument comes online. Previous estimates suggested that Roman, due to launch in May 2027, would be able to find about 50 Earth-sized rogue planets – but the new findings have raised that number. Instead, they indicate a number closer to 400. In fact, the same astronomers behind the discoveries have already identified an Earth-sized rogue planet for Roman to investigate.
Bennett and colleagues drew their conclusions with data collected during a nine-year astronomical survey called Microlensing Observations in Astrophysics (MOA). At New Zealand’s Mount John University Observatory, MOA searched for objects with the help of a phenomenon first predicted by Einstein’s theory of general relativity – gravitational lensing – something the Romans would also use to hunt down rogues.
Two papers describing the team’s latest findings will be published in a future issue of The Astronomical Journal.
Hunts rogue worlds with Einstein’s help
Einstein’s 1915 theory of general relativity predicts that massive objects “distort” the very structure of space. While this warping works in three dimensions (four if you factor in time), it can be thought of as similar to the 2D effect of placing balls of different masses on a stretched rubber sheet. The greater the mass of the ball, the deeper the sheet. Similarly, the more massive the cosmic object, the stronger the warp in space.
Also, when a very large object bends space, it also affects the light emitted by other objects sitting in the background, bending such light as it passes through the cosmic imprint of the original object. This will ultimately create a magnifying effect on the background object, thereby leading to a phenomenon called gravitational lensing.
Microlensing is a variation on this concept that occurs when a small object, such as a planet or star, slips between Earth and a background light source, such as a star or galaxy. This causes Earth-based machines to detect a spike in the brightness of the background object, but it is not as intense as the effects of gravitational lensing. However, microlensing is useful for detecting rogue planets and other small objects that do not emit light and are thus almost completely dark.
“Microlensing is the only way to detect objects such as low-mass free-floating planets and primordial black holes,” said Takahiro Sumi, researcher and Osaka University professor. “Using gravity to detect objects that we would never be able to see directly is very exciting.”
Discovers an Earth-like rogue and an ejection mechanism
Since the discovery of the first exoplanet orbiting a Sun-like star in 1995, the exoplanet catalog contains more than 5,000 intriguing objects. However, most of these planets are giant worlds orbiting near their host star.
Also, the newly discovered roughly Earth-sized rogue planet marks the second time such a world has been discovered using microlensing. The team behind the discovery suggests that rogue worlds are typically Earth-like and small planets.
“We found that Earth-sized rogues are more common than larger ones,” Zumi said. “Difference in average mass of star-bound and free-flowing planets holds key to understanding planet formation mechanisms.”
The random nature of planet formation may explain how rogues come to roam the galaxy alone. Low-mass planets have less of a gravitational pull on their host stars, which means it’s easier for interactions in a formation system to articulate them, leaving them alone to wander through the universe.
Nancy Grace Roman joins the hunt for the thug
Although microlensing events are useful for detecting rogue planets, viewing these events across vast expanses of space is still like finding a planetary needle in a cosmic haystack. Microlensing events caused by single planets are incredibly rare.
This problem is compounded by the fact that lensing events are one-time transactions, unlike the regular transit of a planet across the face of a star or the periodic motions caused by a rotating planet. When these planets pass a background star, they do not return to the same region of space again.
That’s where the Nancy Grace Roman Telescope comes in.
With an unusually wide field of view, an infrared space telescope can cast a wide net around rogues. And it would have enough vision to see Earth-sized orphan planets.
“As observed from space, Roman will be sensitive to even low-mass rogue planets,” said study author Naoki Koshimoto, assistant professor at Osaka University. “The combination of Roman’s wide field of view and sharp vision will allow us to study the objects it detects in much more detail than we can with Earth-based telescopes, which is an exciting prospect.”
Data from ROMAN will be combined with observations from Japan’s 1.8m Prime-Focus Infrared Microlensing Experiment (PRIME) telescope, located at the South African Astronomical Observatory in Sutherland.
“A microlensing signal from a rogue planet can take anywhere from a few hours to a day, so astronomers will have the opportunity to make simultaneous observations with Roman and Prime,” Koshimoto said.
The combination of this information should allow astronomers to more precisely measure the masses of wandering orphan planets, helping them better determine what drove them rogue in the first place.