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Trillions of rogue planets, 20x more than stars, are wandering alone in our galaxy

In an astonishing revelation, researchers from NASA and Japan’s Osaka University have uncovered data suggesting the rogue planets – those solitary wanderers unhinged from any star – significantly outnumber the approximately hundred billion planets which orbit stars. 

The findings indicate that NASA’s Nancy Grace Roman Space Telescope, slated for launch by May 2027, could discover as many as 400 rogue planets similar in mass to Earth. The data has already singled out a likely candidate for this category.

David Bennett is a senior research scientist at NASA’s Goddard Space Flight Center and co-author of two papers detailing these findings. 

“We estimate that our galaxy is home to 20 times more rogue planets than stars – trillions of worlds wandering alone. This is the first measurement of the number of rogue planets in the galaxy that is sensitive to planets less massive than Earth,” said Bennett.

Microlensing events 

The groundbreaking findings originated from a nine-year MOA (Microlensing Observations in Astrophysics) survey conducted at New Zealand’s Mount John University Observatory

Microlensing events provide valuable information about the objects in our cosmos. These phenomena occur when a star or planet aligns almost perfectly with an unrelated background star from our viewpoint. 

The mass of the nearer object warps space-time, causing light from the distant star to bend around the closer object. The intervening object acts like a lens, briefly intensifying the brightness of the background star’s light. This method gleans crucial insights that other methods can’t provide.

Takahiro Sumi is a professor at Osaka University and lead author of the paper estimating our galaxy’s rogue planets. He elaborated on the significance of microlensing.

“Microlensing is the only way we can find objects like low-mass free-floating planets and even primordial black holes. It’s very exciting to use gravity to discover objects we could never hope to see directly.”

The MOA survey has enabled the second-ever discovery of an Earth-mass rogue planet. The team’s finding, along with a demographic analysis suggesting rogue planets outnumber star-orbiting planets by sixfold, will soon be published in The Astronomical Journal.

Rogue planets tend to be smaller

In the past few decades, our understanding of the cosmos has dramatically evolved. From pondering our solar system’s solitude, we’ve discovered over 5,300 planets beyond it. Contrary to the trend, which reveals most of these newfound worlds are either massive or in close proximity to their host star, the team’s study suggests rogue planets lean towards a smaller size.

Professor Sumi explained the prevalence of pint-sized planets: “We found that Earth-size rogues are more common than more massive ones. The difference in star-bound and free-floating planets’ average masses holds a key to understanding planetary formation mechanisms.”

Planetary formation is a chaotic process. Young celestial bodies, bound by gravity, jostle for space in their orbits. The less massive planets, not as strongly tethered to their stars, often find themselves catapulted into space. From there, they begin a solitary, shadowy existence.

Despite their mysterious nature, not all rogue planets are inhospitable. Sci-fi enthusiasts might recall an episode of the original Star Trek series where the crew stumbles upon Gothos. It was a lone habitable planet in a star desert. However, despite sharing mass characteristics, it’s essential to note that our newly discovered “rogue Earth” is likely starkly different from Earth.

The Roman Space Telescope 

Discovering more rogue planets depends on expanding our field of observation – a task perfectly suited to the Roman Space Telescope. 

Naoki Koshimoto is now an assistant professor at Osaka University and lead author of the paper. He announced the candidate terrestrial-mass rogue world by saying, “Roman will be sensitive to even lower-mass rogue planets since it will observe from space. 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.”

Previously, estimates suggested Roman would uncover around 50 terrestrial-mass rogue worlds. The new research revises this expectation to approximately 400. However, more accurate predictions will have to wait until Roman begins its stellar sweep. 

Roman’s data will be supplemented by ground-based observations from facilities like Japan’s PRIME (Prime-focus Infrared Microlensing Experiment) telescope at the South African Astronomical Observatory.

Each microlensing event is a one-off, and cannot be revisited. However, they do last from a few hours to a day, allowing time for coordinated observations with Roman and PRIME. 

“Seeing them from both Earth and Roman’s location a million miles away will help scientists measure the masses of rogue planets much more accurately than ever before, deepening our understanding of the worlds that grace our galaxy,” said Koshimoto.

More about rogue planets

Rogue planets, also known as interstellar planets, nomad planets, or orphan planets, are celestial bodies that have the characteristics of planets but do not orbit a star. Instead, they traverse the vast expanse of interstellar space independently. Although scientists first proposed the concept in the 20th century, only in recent years has the technology become available to detect and study these elusive objects.

Rogue planet formation

The primary mechanism for the formation of rogue planets relates to the volatile dynamics within young star systems. These systems often exhibit chaotic gravitational interactions between developing planets.

The unstable conditions can eject a planet from the system, sending it into interstellar space. In some cases, rogue planets may form directly from the collapse of interstellar gas clouds, similar to how stars form but on a smaller scale.

Detection and observation

Detecting rogue planets presents a significant challenge due to their inherent darkness. They emit no light of their own, and they do not transit a host star to create observable patterns. Nevertheless, astronomers have developed two primary methods for identifying these celestial wanderers: gravitational microlensing and direct imaging.

Gravitational microlensing occurs when the gravity of a massive object, such as a rogue planet, bends and magnifies the light of a distant star. This creates a noticeable brightening effect, alerting astronomers to the presence of an intervening object.

Direct imaging, on the other hand, relies on sophisticated telescopes to observe the weak infrared radiation that rogue planets emit as they cool. The introduction of the James Webb Space Telescope in 2021 greatly enhanced our capacity for direct imaging.


Rogue planets vary widely in their characteristics. Many are similar to conventional planets, with masses ranging from Earth-like to Jupiter-like sizes. However, some border on the line between large gas giant planets and brown dwarfs, objects which are too massive to be considered planets but not massive enough to initiate nuclear fusion in their cores like stars.

Despite their solitary existence, rogue planets may possess atmospheres and even maintain internal heat from their formation. There is ongoing debate about the potential for these bodies to support life, but it remains largely speculative due to the lack of energy input from a star.


Rogue planets contribute to our understanding of planetary formation and the dynamic processes within young star systems. They challenge traditional definitions of planets and blur the lines between different categories of celestial objects.

Furthermore, they hold implications for astrobiology. If rogue planets can host conditions suitable for life, it expands the range of environments in which we might find extraterrestrial organisms. This broadens the scope of our search for life beyond our solar system.

In summary, rogue planets are fascinating objects that traverse the interstellar void independent of a host star. Their detection and study push the limits of our astronomical technology and knowledge, offering unique insights into planetary formation, dynamics, and the potential for life in unexpected places. As technology continues to evolve, our understanding of these mysterious objects will undoubtedly expand.


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