Earth Calling Planet 9 - Are you there?
It has long been suspected that the strange orbits of some distant objects beyond Neptune are evidence of an undiscovered planet, that would be more massive than Earth but not as massive as Neptune. We’ve never got direct evidence of this planet. And we still haven’t got that yet. But recent research carried out by a team headed by PhD student, Terry Long Phan, at the National Tsing Hua University in Taiwan, has highlighted something that may indicate Planet 9 does indeed exist.
Why is Planet 9 so hard to find?
Well, we don’t have definitive proof that it exists. But if it does, it would be so far from the Sun that any sunlight it might reflect back to Earth would be hard to detect without knowing where exactly to look.
One promising approach is to look beyond visible light. Rather than searching only for sunlight reflected by the planet, astronomers can look for the faint heat it might emit, using observations in the far-infrared part of the spectrum. This is the approach Phan and his colleagues have been investigating, analysing data from two space-based surveys taken more than two decades apart.
Traditional optical telescopes cannot detect extremely distant objects because sunlight reflecting off them is too faint. The team instead searched for thermal radiation in the far-infrared, since a planet generates its own internal heat. At distances of 500–700 AU, the infrared glow of a Planet 9 would be roughly 100 times brighter than its reflected visible light.
Phan’s team compared two far-infrared sky surveys separated by 23 years, NASA’s IRAS (1983) and Japan’s AKARI (2006–2007), looking for an object that had moved slightly across the sky. They filtered out stars and galaxies by expecting a very slow, steady motion consistent with a distant body orbiting the Sun that could be a candidate for the hypothetical Planet 9.
A quick note on how we originally discovered the planets
♬ We call ’em the wanderers
yeah the wanderers
they go around, around, around … the Sun ♬Some Ancient Greek, possibly
To the naked eye, and for us non-astronomers, it’s hard to tell a planet from a star. But ancient civilizations noticed that the bright bodies of Mercury, Venus, Mars, Jupiter and Saturn, seemed to wander independent of constellations. Stars and constellations of stars move together in a seemingly fixed formation. But planets look like they’re drifting through constellations on their own independent trajectory.
The ancient Babylonians first assigned names of their gods to the bright wandering lights in the sky. The ancient Greeks adopted and adapted this tradition, assigning names based on each planet’s physical appearance or behavior. Later, the Romans translated these Greek associations into their own pantheon. The word planet comes from the Greek phrase asteres planētai which translates literally to wandering stars. It derives from the planētes, wanderer.
These wanderers weren’t immediately understood to be other worlds. For most of our history astronomers held a geo-centric conception of our universe, with our planet being the only world, and it was orbited by bright celestial bodies. Around 150 AD, Ptolemy defined a complicated geo-centric model to explain the movements of the celestial bodies which included “epicycles” — small circles on top of larger circles — to explain why planets sometimes seemed to move backwards (retrograde motion) against the fixed stars. It wasn’t perfect, but it worked well enough for prediction and it dominated Western and Islamic astronomy for over a thousand years.
The first serious challenge came from Nicolaus Copernicus, a Polish canon and astronomer, who published De revolutionibus orbium coelestium in 1543. Copernicus proposed that the Sun, not the Earth, was at the centre of everything, and that Earth and the other planets orbited the Sun. This heliocentric model elegantly explained retrograde motion as a natural consequence of Earth overtaking the outer planets on its own faster orbit. But Copernicus still assumed perfectly circular orbits, so his model was not dramatically more accurate than Ptolemy’s, and so it was not immediately accepted.
In the early 17th century, Galileo Galilei using one of the first telescopes made a series of observations that strongly supported a heliocentric model. These included that Jupiter had four moons in its orbit, and that Venus appeared to have complete phases, waxing and waning from a thin crescent to a full circle, which were only explicable if it were orbiting the Sun. Galileo published his findings in Sidereus Nuncius in 1610. And we all know what happened next.
Around the same time, Johannes Kepler spent years trying to fit the extremely precise naked-eye observations of Mars made by Tycho Brahe into circular orbits and failed. He eventually discovered that the planets move in ellipses, with the Sun at the centre of their orbits. He published his first two laws in 1609 Astronomia Nova and the third in 1619 Harmonices Mundi. Kepler’s laws finally made heliocentrism quantitatively accurate; they could predict planetary positions better than any previous model.
The physics behind it all came from Isaac Newton, who in 1687 published his Philosophiæ Naturalis Principia Mathematica. Newton made the ground-breaking discovery that gravity is a universal force. By combining his laws of motion with gravity, Newton mathematically proved that a planet caught in the Sun’s gravitational tug must naturally follow an elliptical path, providing the mathematical proof that perfectly explained Kepler's elliptical orbits.
Uranus was the first planet to be discovered with a telescope. It was found by William Herschel on March 13, 1781. While surveying faint stars in the constellation Gemini, Herschel noticed an object that looked different—it appeared as a small disk rather than a point of light. He initially announced it as a comet, but after astronomers calculated its orbit over the following months, it became clear this was a new planet, roughly twice as far from the Sun as Saturn.
Other astronomers had noted it before, but assumed it was a star. It was only when Herschel noticed its strange appearance and motion that it was discovered to be a planet.
After Uranus was discovered, astronomers tracked its orbit closely. Over time, they noticed that Uranus was not following the path predicted by Newton’s law of gravity; it seemed to be pulled slightly off course. The existence of Neptune was calculated before it was observed.
Two mathematicians, Urbain Le Verrier in France and John Couch Adams in England, independently concluded that the gravitational pull of an unknown planet beyond Uranus must be causing these deviations. Le Verrier calculated where this unseen planet should be and sent his prediction to the Berlin Observatory. On the night of September 23, 1846, astronomer Johann Galle pointed his telescope at the predicted spot and found Neptune within about an hour. As the French astronomer François Arago put it, Le Verrier had discovered a planet “with the point of his pen”.
Astronomers still thought there was something disturbing the orbits of Uranus and Neptune. So the hunt continued. When Pluto was discovered by Clyde Tombaugh in 1930, it was initially thought it could be the elusive disturberer. Eventually astronomers realised Pluto was far too small to be causing the orbital disturbances.
Then in 1989, NASA’s Voyager 2 spacecraft flew past Neptune and measured its exact mass. It turned out scientists had over-estimated Neptune’s mass by about 0.5%. When NASA plugged the correct, slightly lighter mass into their computers, the “orbital disturbances” of Uranus completely vanished.
And in 2006 Pluto was reclassified as a dwarf planet.
So there’s no disturberer lurking in the outer edges of our solar system? Not so fast…
In 2014, astronomers, Chad Trujillo and Scott Sheppard, noticed that a newly discovered object, 2012 VP113, and a dwarf planet named Sedna (discovered in 2003) together with other objects in the region, shared weirdly similar aligned orbits. Their landmark paper prompted astronomer, Mike Brown, and mathematician, Konstantin Batygin, working at Caltech, to use computer simulations and physics to figure out what could cause these disturbed orbits. At first, they were highly skeptical and actively tried to prove that a hidden planet wasn’t the answer. In 2016, after every other explanation failed, they introduced their Planet Nine hypothesis to the world.
But what did Phan’s team find?
When comparing the 1983 IRAS data to the 2006/7 AKARI data, looking to match up the signals, the team filtered out stars and galaxies, looking for the slow, steady motion that would be consistent with a distant planet orbiting the Sun. From millions of signals, they narrowed the list to 13 candidate pairs, then to a single best candidate.
The one surviving candidate showed these characteristics:
- Motion: It moved approximately 47.5 arcminutes over 23 years—about 1.5 times the width of the full Moon.
- Heat signature: Its infrared color temperature suggests a surface temperature of roughly 40–50 K, consistent with a cold, distant planet.
- Estimated properties: Mass of roughly 7–17 Earth masses, with a semi-major axis estimated between 280–700 AU.
So is this Planet 9??
The team and outside experts emphasized that this is a candidate, and not a confirmed planet.
So far they have only two data points, one from 1983 and one from 2006, which is not enough to determine a celestial body’s orbit. Two positions could fit countless possible trajectories, and the signal could still be noise or a coincidental pairing of unrelated sources.
And the motion of the candidate does not match the expected trajectory of Brown & Batygin’s hypothesized Planet Nine.
Why is a Planet 9 important anyway?
Well, if a ninth planet isn’t the cause of some unusual movements, then it would be important to identify just what is. And if it is a planet, let’s find it.
The more we understand about the movement and placement of different objects in our solar system, the better we can predict its future and the better we will understand other solar systems, presumably.
So what’s next?
The team plans follow-up observations using the Dark Energy Camera (DECam) on the 4-metre Blanco Telescope in Chile. If the object is real and bright enough, DECam should be able to re-detect it and track its motion over time, which would allow astronomers to calculate its orbit and confirm whether it is a new planet.
Wait, the mystery deepens…
As hinted earlier, Mike Brown, when looking closely at the orbital path suggested by Phan’s data, noted that the candidate appears to be on a highly inclined, mildly retrograde orbit almost entirely perpendicular to the rest of the solar system. Because of this bizarre geometry, it wouldn’t actually have the gravitational capability to cluster those distant space rocks together. So Brown was insistent that the object hinted at in Phan’s findings was not Planet Nine.
But then just a few months later, the existence of a Planet 9 suddenly seemed much less certain. Using the Subaru Telescope, astronomers discovered a brand-new, deeply isolated outer solar system object nicknamed “Ammonite” (officially 2023 KQ14). It was actually first observed in 2023. But when news of Ammonite’s completely misaligned, rogue orbit emerged, it blindsided proponents of the Planet 9 hypothesis, because its trajectory actively weakens the original “orbital clustering” data that made astronomers suspect a Planet 9 existed in the first place.
Aside from this, skeptics of Phan’s work continue to argue that cross-referencing decades-old data from two entirely different satellites is highly prone to false positives. The two faint dots could easily just be background noise or a coincidental alignment of stationary cosmic glitches.
But I really hope it’s fully investigated. Who knows what we might find?
A Dark New Mystery?
So is there a disturberer at the outer edges of our solar system? Or are the outer edges themselves disturbing?
If follow-up observations by the DECam do end up confirming that Phan’s candidate is real, it might not be the Planet Nine we’ve been mathematically tracking for a decade. Instead, it could be a completely new, unexpected type of celestial body altogether.
Our eyes, pens and the infrared sensors have brought us ever closer to understanding our solar system, but for now, its deep, dark edges remain a mystery. The hunt for the wanderer, or wanderers, continues. And are they what is disturbing or is it the region they are wandering through that disturbs the path they take? So many secrets yet to be revealed…
