nullMercury marches across the sun in this
time-lapse photo taken during the
planet's 2003 transit. If anything orbits
the sun more closely than Mercury does it is too
small to see this way.

Image: Dominique Dierek

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?


As it turns out, trying to get a census of this area is tougher than you might think. Every effort has come up short. But our lack of finding anything there is not for lack of looking.

The search for a planet interior to Mercury is 400 years old, almost as old as the telescope itself. In 1611, less than two years after Galileo began examining the skies, German astronomer Christoph Scheiner spotted something silhouetted against the bright disk of the sun. He thought he might have found the seventh planet (Uranus and Neptune had not yet been discovered), but it was later shown to have been a sunspot. Many more mistaken observations followed.

In the 1850s the quest for an intra-Mercurial planet got a major boost when the French mathematician Urbain Le Verrier announced that Mercury appeared to be affected by just such a body. His detailed calculations indicated that the planet’s orbit slowly but steadily drifts. The only explanation conceivable at the time was that Mercury was being perturbed by the gravity of a smaller object orbiting even closer to the sun.

Many astronomers took up the hunt using the limited telescopes of the day. They even gave the putative new planet a name: Vulcan, after the Roman god of fire, fitting for a world whose surface temperature would be hot enough to melt lead and zinc.

As the decades advanced, telescopes got bigger and better able to spot small, faint objects. By the turn of the 20th century, any planet or planetoid inside Mercury’s orbit—even a small one just a few hundred miles across—should have been sighted. Trying to observe near the sun is difficult, but a planet is not exactly an easy thing to hide. Astronomers’ conviction that Vulcan existed began to weaken.

Then Albert Einstein seemingly put an end to the idea of Vulcan once and for all. To analyze Mercury’s orbit, Le Verrier had relied on Isaac Newton’s formulation of the laws of gravity. Brilliant though he was, Le Verrier didn’t know that gravity actually follows ever-so-slightly different rules. These rules wouldn’t be grasped for many decades, until Einstein formulated his general theory of relativity in 1915. The theory had implications for the way Mercury moves around the sun and, sure enough, Einstein calculated that relativity alone neatly explained the slow change of its orbit, without the need for an intra-Mercurial planet.

Story over? Not quite.