We may have found the first direct evidence of a ninth major planet in the Solar System

Suggestions of a ninth planet in our Solar System have been rumbling on for years now.

Some astronomers argue that the clustering and orbital tilt of a group of trans-Neptunian objects indicate gravitational shepherding by an unknown planet.

This hypothetical Planet 9, probably similar to Neptune, has been calculated to be on an elliptical orbit coming to around 300 Astronomical Units (1 AU is the Earth–Sun distance).

However, despite several surveys, no such remote new planet has been spotted, and the claim remains controversial.

The problem with finding Planet 9

This lack of observational evidence isn’t itself a damning outcome.

The predicted orbit for Planet 9 is at least 10 times further from the Sun than Neptune, so the planet would reflect precious little light back to Earth.

There’s also a large degree of uncertainty around the proposed planet’s orbital parameters and therefore exactly where in the sky searches should be targeted.

Even if it is out there, Planet 9 would be easy to miss.

Why can't we see it already?

The problem with trying to detect Planet 9 by its reflected light comes down to some simple physics.

To spot it at visible wavelengths, the light would have had to travel all the way out from the Sun, reflect off the planet and then travel back to the Earth.

If a roughly Neptune-sized planet were 10 times further away than Neptune, it would appear 10,000 times fainter.

But the planet’s emitted thermal radiation only has to make a one-way journey.

Thus, at infrared wavelengths, Planet 9 would only be roughly 100 times fainter, so it makes sense to search for Planet 9 using space-based infrared telescopes.

Looking for movement in the sky

And this is exactly what Terry Long Phan, at National Tsing Hua University in Taiwan, tried for his PhD.

Supported by a broad team of colleagues in Taiwan, Japan and Australia, Phan analysed data from two far-infrared all-sky surveys: IRAS and AKARI.

IRAS (the Infrared Astronomical Satellite) was launched in 1983, and AKARI (which means light in Japanese) in 2006, so their observations were 23 years apart.

At over 300 AU, any Planet 9 would have such a slow orbital speed that it would appear stationary in the datasets from either of these observatories, but it would have shifted slightly between their missions.

Phan used software to sift through all the infrared sources detected by these space telescopes and pick out only those that had shifted across the sky by roughly the expected amount.

This automated process found 13 candidate pairs of sources, each of which Phan then checked manually.

After this verification process, only one candidate pair remained.

This infrared source, matching what would be expected of a roughly Neptune-sized planet in the far outer reaches of the Solar System, had moved 47.5 arcminutes in those 23 years.

That's roughly one and a half times the width of the full Moon.

Have we found Planet 9?

It’s very early days, but if this pair of faint pinpricks does in fact represent a single, slow-moving object in our Solar System, we may have just got our first observational evidence of Planet 9.

However, knowing just two positions isn’t enough to pin down the exact orbit of this object.

Phan says follow-up observations are required, potentially using the widefield, very sensitive DECam (Dark Energy Camera) on the Victor M Blanco Telescope in Chile to add another family member to our Solar System.

Lewis Dartnell was reading A Search for Planet Nine with IRAS and AKARI Data by Terry Long Phan et al. Read it online at: arxiv.org/abs/2504.17288.

This article appeared in the June 2025 issue of BBC Sky at Night Magazine