Astronomers have reason to believe that a massive object is waiting to be discovered in the outer reaches of the solar system—the so-called Planet Nine. A recent search for this proposed planet produced a null result, but the case is far from closed.
Planet Nine, if it’s out there, is doing a stellar job of staying invisible. A team of researchers led by Sigurd Naess, a researcher at the Institute of Theoretical Astrophysics at the University of Oslo in Norway, was the latest to attempt to spot it, using the Atacama Cosmology Telescope in Chile. “No significant detections [were] found,” they write in their new paper, published in The Astrophysical Journal.
That a ninth planet exists in our solar system is a distinct possibility, as scientists Konstantin Batygin and Michael Brown cogently argued in 2016. Astronomers have an inkling to its presence owing to the peculiar movements of objects in the distant Kuiper Belt. To date, over 30 of these objects have been identified, each exhibiting exaggerated and inclined orbits as well as clustering behavior.
To explain this, scientists have invoked the presence of a rather large object—the proposed Planet Nine. But for this to work, the planet must be reasonably big and heavy. Astronomers figure that the planet is between five and 10 times the size of Earth and located as far as 200 au to 800 au (18.6 billion to 74.5 billion miles) from the Sun. Large, or at least large-ish, objects do exist in the region beyond Neptune—objects like Pluto, Eris, Haumea, and Makemake—so the idea isn’t totally bananas.
It might seem like we should’ve spotted something so big a long time ago, but our failure to detect the object is not altogether surprising, given the distances involved and the likely dim nature of the planet. If it’s out there, Planet Nine is reflecting very little sunlight and emitting scant amounts of radiation. In previous surveys, astronomers failed to detect it with the Wide-field Infrared Explorer, leading to rival theories that attempt to explain the Kuiper Belt anomalies, such as a bowling-ball-sized black hole in the Oort Cloud or a massive ring of debris in the outer solar system.
For the latest search, the team used the Atacama Cosmology Telescope, which scans space at millimeter wavelengths. ACT is primarily used for studying cosmic microwave background radiation produced by the Big Bang, but the telescope’s sensitivity lends itself to this kind of needle-in-a-haystack search. Arthur Kosowsky, an astronomer at the University of Pittsburgh and a co-author of the study, said ACT was up for the task owing to two unique characteristics.
“We have sufficient sensitivity to microwave-wavelength radiation to possibly detect Planet Nine’s thermal emission, and we have observed a wide swath of the sky where Planet Nine might be located,” he explained in an email. “ACT is the only current experiment which meets these two criteria.”
The team took observations at 98 GHz, 150 GHz, and 229 GHz as they searched for an object at distances between 300 and 2,000 au (1 au is the average distance from Earth to the Sun). For an object five times bigger than Earth, the scientists expected to see something between 325 and 625 au, and for objects 10 times bigger than Earth, they expected to see something between 425 and 775 au.
The astronomers scanned 87% of the sky accessible from the southern hemisphere, gathering data from 2013 to 2019. Various techniques were used to process the data, including a computational “binning and stacking,” a method that “might uncover faint sources but at the expense of losing positional information,” according to a press release from the Center for Astrophysics | Harvard & Smithsonian.
No one knows where Planet Nine sits in the sky, if it’s really there, but it must reside within certain ranges to explain its gravitational influence. At the same time, the hypothetical planet is moving, and in specific possible directions.
“Since we don’t know which position or direction of motion, we performed a computational search over all 100 million possibilities,” said Kosowsky. “For each one of these possibilities, we averaged together all of the data we collected along one specific path to see if we can pick out a faint source moving along that path.”
Naess and his colleagues found a few promising paths, but this came with a catch. By doing this 100 million times, “you are bound to find some paths which look like an object is there, but really that is just due to random noise in the data,” Kosowsky explained.
As the astronomers write, their scan produced around 38,000 “raw candidates,” of which roughly 3,500 could be categorized as something approximating Planet Nine. Perhaps discouragingly, “none” of these candidate signals “could be confirmed,” and “there were no statistically significant detections,” the Center for Astrophysics release said. The team is 95% confident that the new survey excludes a solar system object inside the surveyed area with the stated search properties.
Still, the team listed the 10 strongest candidate signals for possible follow-ups. But even then, “our 10 brightest average signals looked just like we would expect from random noise,” said Kosowsky.
This latest survey can hardly be described as exhaustive, as the research eliminates about 17% of the total possible orbits in which Planet Nine might be moving if it’s a 5-Earth mass object, and 9% of the possible orbits for a planet with 10 Earth masses. “There are still a lot of places Planet Nine can be lurking,” Kosowsky added.
With new telescopes poised to come online in the near future, including the Vera Rubin Observatory in Chile, astronomers may finally have the tools required to detect something so faint and distant. The truth about Planet Nine can only hide for so long.
“I’m confident that if Planet Nine is out there, astronomers with new telescopes will nab it over the next five years,” said Kosowsky.