Many astronomers remain convinced that they will one day locate the mysterious Planet Nine, the hypothetical body thought to be hiding far beyond Neptune. “Every time we take a picture,” says astronomer Surhud More, “there is a possibility that Planet Nine exists in the shot.” Yet, despite all the circumstantial evidence for its existence, no telescope has been able to spot it.
The first evidence for Planet Nine transpired in 2014, when astronomers discovered a handful of mini ice-worlds beyond the Kuiper belt following similar paths, writes Charlie Wood for Quanta Magazine. According to Scott Sheppard, an astronomer at the Carnegie Institution for Science, “if things are in the same orbit, then something’s pushing them.” Astronomical calculations predict that the light coming back from Planet Nine would appear more than 1 million times weaker, creating a “brick wall” to any sightings of it, according to astronomer Kevin Luhman. Teams led by Michael Brown at the California Institute of Technology, and Sheppard are searching for the planet with the Subaru telescope in Hawaii. Subaru’s wide field of view can sour a potential search area the size of 4,000 full moons. Still, factors like light pollution in the Milky Way, or the glare of a bright star can keep the planet hidden.
Some scientists are suggesting backup plans like searching for the heat glow emitted by the planet, which could be detected by current telescopes in Antarctica and Chile. Others wonder if signs of Planet Nine might lie buried in today’s data sets by just slightly affecting the paths of known planets. As astrophysicist Matthew Holman puts it, “if you put a planet in” the outer solar system, “the fit would be better.”