Where Is Planet Nine? Its Hiding Places Are Running Out

From observing other stars, we know the possibility of such an as-yet-undiscovered world isn⁘t too far-fetched (figuratively speaking). In 2000, astronomers published a paper showing that an exoplanet might exist around the star HD 163296 at a distance of nearly 50 billion kilometers, over 10 times Neptune⁘s distance from the sun (not-so-humblebrag: I was an author on that paper), much farther out than models at the time suggested. Indeed, several planets were later found orbiting that star at great distances. So a priori, it⁘s possible a planet might exist in the cold, dark depths of our own solar system. It could even be quite large, the size of an ice giant like Neptune .

Somewhat tongue-in-cheek, they gave this hypothetical planet the moniker Planet Nine (or P9), a bit of fun-poking at astronomers still unhappy that the International Astronomical Union demoted Pluto⁘s planetary status in 2006 . And since I brought it up, I⁘ll add that I don⁘t think ⁘planet⁘ is something we need to define; in fact, I don⁘t think it can be defined . It⁘s a concept, not a definition, like ⁘red⁘ or ⁘continent.⁘ The edges around such words are fuzzy, and we shouldn⁘t allow our thinking about them to be overly constrained by arbitrary definitions.

Anyway, over time, more indirect evidence for P9⁘s existence came to light , notably a slight tilt of the sun⁘s spin axis, none of which was conclusive, but all curious enough to keep astronomers searching for a culprit.

Still, all the indirect evidence in the world (or worlds) doesn⁘t add up to a single direct observation of the planet. So the game was afoot.

Many telescopes take sky surveys, wide-angle image campaigns that map large swaths of the heavens to look for what astronomer call ⁘ transients ⁘: objects that change brightness or position over time. These include exploding stars, black holes fitfully gobbling down matter, asteroids and, potentially, Planet Nine.

Brown and Batygin have led the charge to sift through the search data. Along with their colleague Matthew Holman they published their latest results in the April 2024 issue of the Astronomical Journal .

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