Science textbooks may be in for another revision. Our solar system shrank from nine planets to eight after the International Astronomical Union demoted Pluto in 2006. But there may yet be another world lurking beyond Neptune — and astronomers may have just found it.
In 2016, astronomers proposed that our solar system could harbor a stealthy ninth planet based on a strange clustering of orbits they detected among distant Kuiper Belt objects. They suggested the unseen world may be gravitationally tugging on those objects with the heft of five to 10 Earth masses. So far, there’s no direct evidence that Planet Nine exists.
Is this the one?
A new study, however, identifies a possible candidate. A separate team of astronomers compared 1983 data from the Infrared Astronomical Satellite (IRAS) with 2006 data from Japan’s AKARI mission in search of faint sources that moved slightly over the intervening 23 years — the kind of slow motion you’d expect from a massive, faraway planet.
“Observing in the far-infrared is advantageous because a distant planet would be extremely faint at optical wavelengths but may emit detectable thermal radiation in the infrared,” says Terry Long Phan, an astronomy graduate student at National Tsing Hua University in Taiwan, who led the study. By eliminating stationary objects and any pairs that moved too much or too little to be the theorized world, they winnowed down from 2 million objects to just 13 possible pairs. “Most of these turned out to be false positives, typically caused by high background noise or potential stationary sources that were not detected but present in the image,” Phan says. “However, we identified one promising candidate that is consistent with the expected properties of Planet Nine.”
But the mystery isn’t solved yet. Tomo Goto, a professor at National Tsing Hua University who co-authored the paper, cautions, “The two detections from IRAS and AKARI alone are insufficient to determine the orbit of our candidate.” The team hopes to better pin down the object’s motion using the Dark Energy Camera. “Only with a well-constrained orbit can we confirm whether our candidate is indeed Planet Nine,” Goto says.
Probably not
Mike Brown, an astronomer at Caltech who originally proposed Planet Nine’s existence with his colleague Konstantin Batygin, says the candidate’s motion suggests it’s on a highly inclined and mildly retrograde orbit, nearly perpendicular to the solar system’s plane. With such an orbit, the possible planet could not explain the unusual clustering of Kuiper Belt objects that led to the prediction of Planet Nine.
“Whether it is real or not, it is 100 percent NOT Planet Nine,” Brown says. “Planet Nine is a prediction based on inferred gravitational effects in the Kuiper Belt region, and this object, even if real, would not cause these effects. So if it is real it is not Planet Nine,” and in fact, “probably disproves the existence of Planet Nine.”
So what else might it be? Brown guesses it could be either “noise” (like the static that can muddle a television or radio transmission) in the data or an astrophysical transient — an event in space that can flare up and fade away.
Whether or not this new candidate turns out to be Planet Nine, the mystery may not last much longer. The Legacy Survey of Space and Time (LSST) survey by the upcoming Vera C. Rubin Observatory will transform our ability to detect distant, slow-moving objects in the outer solar system.
“There is a very good chance that this survey will find it, and, even if not, will firm up the evidence that it exists,” Brown says.
The search is surprisingly difficult, considering we’ve found more than 5,800 planets around other stars.
“Personally, I feel it is unacceptable that we do not even fully understand our solar system, while 100 billion planetary systems are in our Milky Way galaxy alone, and 100 billion such galaxies in the universe,” Phan says. “It is my belief that before we try to understand stars, galaxies, or the universe, we must first come to understand our own home: the solar system.”
For now, astronomers will continue searching for clues hidden in the dark outskirts of our solar system. To some, it’s not a question of if they’ll find Planet Nine but when.
“The solar system makes no sense without it,” Brown says.
