A 9th planet in our solar system might be found — and no, it's not Pluto

Independent

03-05-2025

There have been questions about a mysterious ninth planet in our solar system for nearly a decade.
Pluto was unseated as number nine in 2006. Now, a group of international researchers say they may have found a candidate — although nothing is certain.
'I felt very excited,' Terry Long Phan, an astronomy graduate student at Taiwan's National Tsing Hua University, told Science this week. 'It's motivated us a lot.'
Phan was the lead author of the findings in a study that has been accepted for publication in the journal Publications of the Astronomical Society of Australia. The research was initially posted to the preprint server arXiv.
The researchers used surveys of the sky from two infrared space telescopes that were launched in 1983 and 2006. They theorized that a potential planet's long orbit would enable them to see it move across the sky. They found 13 pairs of dots that could be explained by a moving planet that resembled Planet Nine. One set of dots had matching colors and brightness.
But, the announcement has been met with some skepticism.
Mike Brown, an astronomer at Caltech who was part of the team responsible for the hypothesis in 2016, told the publication he isn't convinced that the infrared dots Phan and his team identified out of old, infrared satellite data is a ninth planet. His calculations suggested the body would be on a much greater tilt than the solar system's plane and would orbit in a different direction from the known planets.
This difference "doesn't mean it's not there, but it means it's not Planet Nine," Brown told Science. "I don't think this planet would have any of the effects on the solar system that we think we're seeing."
But, if the researchers are right, their planet would disprove the original planet. They would make each other's orbits unstable and could not exist together, he explained.
'It's kind of fun that a paper that purports to find a candidate for Planet Nine is really finding something that would basically say that we were wrong the entire time,' he noted.
However, skepticism doesn't mean there's nothing there.
The argument for the planet has to do with the Kuiper Belt: a region of icy debris far beyond Neptune's orbit. The hypothetical planet would explain some strange orbits of the objects there.
Theoretically, the Neptune-sized planet Brown and his colleague Konstantin Batygin found would orbit about 20 to 30 times farther from the sun than Neptune, taking between 10,000 and 20,000 Earth years to make one full orbit around our star.
"Although we were initially quite skeptical that this planet could exist, as we continued to investigate its orbit and what it would mean for the outer solar system, we become increasingly convinced that it is out there," Batygin, an assistant professor of planetary science, said in 2016. "For the first time in over 150 years, there is solid evidence that the solar system's planetary census is incomplete."
No observational evidence for Planet Nine has been found, although this is not the first time a candidate has been identified in infrared data. The last time was in 2021.
Some researchers even posit that evidence pointing toward the existence of an undiscovered ninth planet may actually indicate our ideas of gravity are incorrect.
the Vera C. Rubin Observatory on Chile's Cerro Pachón mountain.
'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,' Gary Bernstein, an astronomer at the University of Pennsylvania, told Science. 'But if you put it far enough away, it gets fainter and fainter very fast.'