Planet Nine? Scientists uncover new evidence of a massive planet hidden in our solar system

Toronto Star

01-05-2025

A new look through decades-old data has unveiled some of the most compelling evidence yet of a hidden ninth planet in the far reaches of our solar system, far beyond the bounds of Neptune and the dwarf planet Pluto.
The findings hint at an enormous astral body more massive than Neptune, located hundreds of times further from the sun than Earth.
Astronomers have been hunting for the elusive planet ever since it was proposed in 2016 as a cause for the unusual clustering of objects in the Kuiper Belt, a doughnut-shaped disc of icy objects off the orbit of Neptune.
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At the time, this so-called 'Planet Nine' was believed to be 10 times more massive than Earth, with an orbit so far out that a single trip around the sun would take around 10,000 to 20,000 years. But astronomers have struggled to capture strong visual evidence of the planet — until now.
'The moment I found a truly promising candidate after months of unsuccessful attempts, I was so excited that I couldn't wait until the next day to share the result with my adviser,' Terry Phan, the lead author of the study and an astronomer with Taiwan's National Tsing Hua University, wrote over email.
'I have a dream to become a 'planet hunter,' and that will become true if our candidate is exactly a planet.'
The team's paper has been accepted for publication in peer-reviewed journal Publications of the Astronomical Society of Australia. The preprint was posted last week on arXiv, an open-access archive.
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At its great distance, Planet Nine would reflect only a hint of sunlight — far too dim for observation. But its faint thermal radiation might be visible to infrared sensors.
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Thus, Phan's team of international astronomers scoured through two archived, far-infrared all-sky surveys from space telescopes, taken 23 years apart, in search of any hint of the planet.
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Remarkably, out of around two million objects, 13 potential candidates for Planet Nine were discovered. Each were 500 to 700 times further out than the distance from Earth to the sun, and measured seven to 17 times the mass of the Earth.
The researchers found each candidate by comparing the two sky-surveys, looking for objects found by the first scan that appeared to have moved by the time the second scan took place. This distance was relatively tiny, given the colossal length of time it would take Planet Nine to orbit the sun.
After rigorous analysis, including visual inspection, 'most of these turned out to be false positives,' Phan said.
'However, we identified one promising candidate that is consistent with the expected properties of Planet Nine.'
Is this thing really Planet Nine?
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Phan admits his team can't determine the orbit of their candidate through the two sky-surveys alone: 'Only with a well-constrained orbit can we confirm whether our candidate is indeed Planet Nine,' he said.
'To constrain its trajectory and verify its nature, follow-up observations are essential.'
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In this regard, he said his team is working on an observation proposal for the Dark Energy Camera to hopefully better view the potential planet and more-accurately calculate its orbit.
Meanwhile, some astronomers are skeptical of Phan's results. Mike Brown, an astronomer at the California Institute of Technology who first proposed Planet Nine nearly a decade ago, told Science that the team's candidate appears to lie on an orbit far outside of his original prediction.
This 'doesn't mean it's not there, but it means it's not Planet Nine,' said Brown, who is not affiliated with the project. 'I don't think this planet would have any of the effects on the Solar System that we think we're seeing.'
Regardless, Phan remains optimistic about his results.
'I always believe that there are a lot of mysterious objects hiding in the dark regions of the solar system,' he said. 'Before trying to understand stars, galaxies or the Universe, we must come to understand our own home: the solar system.'