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Astronomers have identified a compelling new candidate for the elusive "Planet Nine," analyzing decades-old infrared data that may finally provide evidence of a massive world lurking in our solar system's distant frontier.
Key Takeaways
- Researchers have identified a promising Planet Nine candidate by comparing infrared data from satellites operating 23 years apart.
- The potential planet was spotted as a faint, moving point of light in archival data from IRAS (1983) and AKARI (2006-2011) satellites.
- If confirmed, this object would orbit approximately 500-700 astronomical units from the Sun, far beyond Neptune.
- The candidate was discovered by Terry Phan and colleagues at National Tsing Hua University in Taiwan.
- Scientists remain cautious, as further observations are needed to confirm whether this is truly Planet Nine.
- This discovery could potentially fill the "super-Earth" gap in our solar system.
- The object's orbit appears to be highly tilted, which differs from some earlier Planet Nine predictions.
The Hunt for a Distant World
The search for a ninth planet in our solar system has taken an exciting turn with astronomers reporting the discovery of a promising candidate situated far beyond Neptune. This potential discovery comes from analyzing archival satellite data, potentially offering humanity's first glimpse of the long-speculated Planet Nine.
The concept of Planet Nine first gained serious scientific attention in 2016 when astronomers Konstantin Batygin and Mike Brown from Caltech presented a compelling hypothesis based on the unusual orbital patterns of distant objects in the Kuiper Belt. Their observations suggested these objects were being gravitationally "herded" by a large, unseen body-potentially a planet several times Earth's mass orbiting far beyond Neptune.
Now, after years of searching, researchers led by Terry Phan, a doctoral student at National Tsing Hua University in Taiwan, have identified what they describe as "one good candidate" for this mysterious world. The discovery has generated considerable excitement, with Phan expressing that finding this candidate has "driven us significantly" in their research efforts.
Clever Use of Decades-Old Infrared Data
What makes this discovery particularly fascinating is the methodology employed. Rather than searching for reflected sunlight-which becomes extremely faint at such vast distances-the researchers looked for the planet's intrinsic infrared glow. They compared data from two infrared space telescopes launched decades apart: the Infrared Astronomical Satellite (IRAS) from 1983 and the AKARI satellite, which operated from 2006 to 2011.
The 23-year gap between these surveys proved crucial. At the extreme distances where Planet Nine is theorized to exist-between 500 and 700 astronomical units from the Sun-any planet would move very slowly across the sky. This time difference was just enough to potentially capture the object's subtle movement between observations.
Starting with approximately 2 million objects in their combined datasets, the team methodically filtered out known celestial bodies, sources that didn't move over time, and those with excessive noise. After rigorous selection criteria, they narrowed the list to 13 potential candidates. Visual inspection of these candidates ultimately yielded just one promising object that appeared in both surveys with matching colors and brightness, but in slightly different positions-exactly what you'd expect from a distant, slow-moving planet.
Characteristics of the Mysterious Candidate
If confirmed, this object would represent a significant addition to our understanding of the solar system. Current estimates suggest the planet could have a mass 4.4 to 8.4 times that of Earth, placing it in the category of "super-Earths" or mini-Neptunes-planet types common around other stars but missing from our solar system.
The candidate's orbit is particularly intriguing. It appears to be located approximately 500-700 astronomical units from the Sun, with some estimates suggesting it could take between 10,000 and 20,000 years to complete a single orbit. For perspective, Neptune, our most distant known planet, orbits at just 31 astronomical units and completes an orbit in about 165 years.
Perhaps most surprising is the object's orbital tilt. While earlier Planet Nine models predicted an inclination of around 16±5 degrees, this candidate appears to have a much more extreme tilt. This discrepancy has led some astronomers, including Mike Brown (one of the original Planet Nine theorists), to suggest that if this infrared signal does represent a planet, it might be a different object than the originally hypothesized Planet Nine.
Scientific Caution and Next Steps
Despite the excitement, the astronomical community remains appropriately cautious. The concept of Planet Nine has been controversial since its inception, with some studies suggesting the unusual orbital patterns of distant Kuiper Belt objects could be explained by observational bias or other physical phenomena rather than a hidden planet.
Gary Bernstein, an astronomer at the University of Pennsylvania, captures the challenge perfectly: "It is pretty amazing to think that something as big as Neptune could be sitting out there and no one would have ever noticed it. But if you put it far enough away, it gets fainter and fainter very fast."
The next crucial step will be additional observations to confirm the object's existence and determine its precise orbit. If verified, this discovery would represent the first substantial planetary finding in our solar system in nearly two centuries and could significantly reshape our understanding of the solar system's architecture.
The findings were shared on the preprint server arXiv on April 24, 2025, and have been accepted for publication in the Publications of the Astronomical Society of Australia, though they have not yet completed peer review.
While this infrared candidate represents the most promising lead yet in the search for Planet Nine, astronomers will need additional observations across multiple wavelengths to confirm whether this faint, distant object truly represents a new planet in our cosmic neighborhood.
