Planet Nine is still missing
Astronomers searching for our Solar System’s elusive Planet Nine, a theoretical world that may lurk deep in a cloud of icy rocks far beyond the orbit of Neptune, have come up short once again. In a recent study, researchers pored over six years of telescope data in an attempt to identify potential signs of Planet
Nine in the southern sky. Captured with the Atacama Cosmology Telescope (ACT) in Chile between 2013 and 2019, the observations covered about 87 per cent of the sky visible from the Southern Hemisphere.
While the team identified more than 3,000 candidate light sources located between 400 and 800 astronomical units (AU) away – that’s 400 to 800 times the distance between Earth and the Sun – none of those candidates could be confirmed as planets. However, the fruitless search doesn’t disprove the theoretical planet’s existence; it merely narrows down where that planet may be lurking and what its properties could be, the researchers said. Ultimately, the study covers between just 10 and 20 per cent of the planet’s possible locations in the sky.
Astronomers first began searching for Planet Nine in 2016, noticing that six rocky objects beyond the orbit of Neptune clustered in a strange way, with the most distant points in their
orbits located much farther from the Sun than the nearest points of their orbits. The team calculated that the gravitational pull of an unseen planet measuring five to ten times the size of Earth could explain the eccentricity in those rocks’ orbits. Half a decade later, many teams have tried and failed to detect that theoretical world. The biggest hurdle in the hunt for Planet Nine is the sheer distance involved. While Pluto orbits between 30 and 50 AU from the Sun, the authors of the 2016 study estimated that Planet Nine could be anywhere between 400 and 800 AU away – so far away, in fact, that sunlight may not reach the planet at all.
That means there’s little hope of detecting the cold, dark Planet Nine with standard visible-light telescopes. Instead astronomers turn to the likes of the ACT, which can search the cosmos in millimetre wavelengths, a short form of radio waves that come close to infrared radiation. Millimetre telescopes are often used to peer into dim, freezing gas clouds where new stars form, because such clouds don’t absorb millimetre light. While this survey failed to turn up any compelling evidence, new millimetre telescope facilities, such as the Simons Observatory that’s currently under construction in Chile’s Atacama Desert, will continue the search with even more sensitive telescopes.