New estimates suggest that NASA’s future space telescope could detect more than 400 Earth-mass worlds lurking along the Milky Way and “realized” and therefore orbiting our galaxy.

Such orphaned worlds are believed to have started their life in a solar system-like planetary system, but at some point they were launched by a still unknown mechanism. Despite the common picture of planets clearly orbiting a star, new research suggests that such orphan starless worlds may be 20 to 1 more likely than stars in the Milky Way. That means there are about six times more disconnected worlds in our galaxy than planets orbiting their parent stars.

“We estimate that our galaxy has 20 times more rogue planets than stars — trillions of worlds roam alone,” study author and NASA senior scientist David Bennett said in a statement. “This is the first measurement of the number of rogue planets in the galaxy sensitive to planets smaller than Earth’s mass.”

Generally, planets outside our solar system, known as exoplanets, are defined by the effect they exert on their host stars. For example, an exoplanet may cause terrestrial observers to observe the dimming of its star as the planet’s path passes between the star and our planet. Or an exoplanet could affect such light by gravitationally pulling the glowing body through the wobble it creates in its host star’s orbit. But rogue planets are so far away from their host stars that they are difficult to detect.

One of the main goals of NASA’s Nancy Grace Telescope in Rome is to detect these rogues once the space-based vehicle becomes available. Initial estimates suggested that Roman could detect about 50 Earth-sized rogue planets due to launch in May 2027, but new discoveries have increased that number. Instead, they imply a number closer to 400. In fact, the same astronomers behind the discovery have already identified an Earth-size rogue planet as a candidate for Roman’s study.

Bennett and colleagues drew conclusions based on data collected during a nine-year astronomical study called Microlensing Observations in Astrophysics (MOA). Held at the Mount John University Observatory in New Zealand, the MOA searched for objects using a phenomenon first predicted by Einstein’s general theory of relativity called gravitational lensing – something Roman would also use to hunt bandits.

Two papers describing the team’s latest findings will appear in the next issue of The Astronomical Journal.

Hunt down rogue worlds with Einstein’s help

Einstein’s 1915 theory of general relativity predicts that objects with mass “deform” the fabric of space. Although this distortion works in three dimensions (four if time is taken into account), it can be viewed as analogous 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 indentation on the leaf. Similarly, the larger the space object, the stronger the distortion of space.

Also, when a very large object distorts space, it can also affect light emitted from other objects in the background, causing such light to be distorted as it traverses the cosmic footprint of the original object. Ultimately, this can create the effect of magnifying the background object, resulting in the phenomenon of gravitational lensing.

Microlensing is a variation of this concept that occurs when a smaller object, such as a planet or star, slides between the Earth and a background light source, such as a star or galaxy, and aligns with them almost perfectly. This causes ground machines to detect a burst of background object brightness, but not as extreme as gravity lensing effects. However, microlenses are useful for detecting rogue planets and other small objects that do not emit light and are therefore almost completely dark.

“Microlensing is the only way to find objects like free-floating low-mass planets and even primitive black holes,” said study author and Osaka University professor Takahiro Sumi. “Using gravity to detect objects we would never expect to see directly is very exciting.”

Find the world-like rogue and the catapult mechanism

Since the discovery of the first exoplanet around a Sun-like star in 1995, the exoplanet catalog has grown to include more than 5,000 of these intriguing objects. However, most of these planets are giant worlds orbiting close to their host stars.

Additionally, the recently discovered Earth-sized rogue planet is only the second time such a world has been found using microlensing. The team behind the discovery speculates that this means rogue worlds are typically smaller planets with dimensions similar to or smaller than Earth.

“We found that Earth-sized rogues were more common than larger ones,” said Sumi. “The difference in average mass of star-bound planets and free-floating planets is key to understanding planet formation mechanisms.”

The chaotic nature of planet formation may explain how rogues roam the galaxy alone. Less massive planets have a weaker gravity on their host stars, meaning that interactions in the forming system could more easily push them away, leaving them to wander through space alone.

Nancy Grace Roman joins the outlaw hunt

While microlensing events are useful for detecting rogue planets, observing these events in the vastness of space is still like observing a planetary needle in a cosmic haystack. And microlensing events caused by individual planets are incredibly rare.

This problem is complicated by the fact that lensing events are one-time deals, unlike the regular passage of a planet in front of its star or periodic oscillations caused by a rotating planet. When these planets pass the background star, they never return to the same region of space.

This is where the Nancy Grace Rome Telescope comes into play.

With its extremely wide field of view, the infrared space telescope can create a wider net to trap fraudsters. And it will have the vision needed to see Earth-sized orphan planets.

“As the novel will be observing from space, it will be susceptible to less massive rogue planets,” said study author and Osaka University associate professor Naoki Koshimoto. “The combination of Roman’s wide vision and keen vision will allow us to study the objects he finds in more detail than we can only with ground-based telescopes, which is an exciting possibility.”

Roman’s data will be combined with observations from the Japanese 1.8-metre Prime-focused Infrared Microlensing Experiment (PRIME) telescope at the South African Astronomical Observatory in Sutherland.

“A microlensing signal from a false planet can last from several hours to about a day, so astronomers will be able to make simultaneous observations with Roman and PRIME,” said Kosimoto.

This combination of data will allow astronomers to more precisely measure the masses of rambling orphan planets, helping them better determine what is rogueing them. Source