The solar system is a crowded place. Everywhere we look there’s something zipping past: a handful of planets, a million asteroids, a trillion comets, countless bits of fluff and dust. With a big enough telescope and adequate time and patience, there is almost nowhere you can fix your eyes without seeing something.
Almost nowhere.
There is one puzzling region in our solar system that appears to be empty, even though it should easily be able to support thousands of objects in stable orbits. It is not far away; situated inside Mercury’s orbit, it is much closer to Earth than Jupiter ever gets. It is not poorly lit; the nearby sun blazes with fierce intensity. Nor is it a particularly small region, measuring millions of miles across. And yet no resident planet, asteroid, or what-have-you has been seen there.
A few determined astronomers—including Alan Stern, until recently the associate administrator for NASA’s Science Mission Directorate—believe the emptiness may be an illusion. Objects that formed in that inner zone during the early days of the solar system could still survive there billions of years later. Comets or asteroids shifted by the planets’ gravity could wander into this area, only to find themselves permanently entrapped by the sun’s intense pull. New images of Mercury show it to have been mercilessly pummeled by small objects, implying that the space between it and the sun once was, and potentially still is, occupied by as-yet-unseen bodies. Above all, every single other stable zone in the solar system is occupied. Why should there be one glaring exception?
