About a decade ago, astronomers realized that something was seemingly amiss in the outer solar system. Various icy bodies beyond the orbit of Pluto were seen moving in strange, hard-to-explain patterns; hard to explain, that is, unless their orbits were being sculpted by an unseen world farther out from the sun, midway in size between Earth and Neptune. The possibility of a so-called "Planet Nine" lurking undiscovered at the fringe of our knowledge thrilled scientists and the public alike, and soon multiple intensive searches were underway.
Ten years on, those searches have failed to find their elusive quarry, in large part because even a big planet has plenty of places to hide in the vastness of our solar system's cold, dark hinterlands. Planet Nine remains, so far, just a hypothesis. But the circumstantial evidence supporting its existence has not gone away. And in 2025 a revolutionary telescope, the Vera C. Rubin Observatory, will open its almost all-seeing eye on the sky to settle decisively whether this possible world really exists or was instead just a phantomatic fluke of orbital dynamics and overeager imaginations.
Our top story, from journalist Robin George Andrews, lays out all the latest developments in this still-ongoing search, and explains why, despite a decade of null results, the chances have never been better for settling the strange case of Planet Nine. —Lee Billings
