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.
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.
The knowledge that objects of some kind could comfortably carve out a life between Mercury and the sun was enough to keep some astronomers wondering. Perhaps the problem was that people had been thinking too big. Instead of a planet Vulcan, maybe it made more sense to look for a whole bunch of Vulcanettes. Or, as scientists have since named the members of this hypothetical population of asteroid-like objects, vulcanoids.
To refocus the search, the most likely location of these objects had to be pinpointed. A big planet would have been relatively obvious, but perhaps smaller bodies could hide in the glare of the sun. Any object that strayed too close to the solar furnace would vaporize over the lifetime of the solar system, like a marshmallow held too close to a campfire. On the other hand, any object whose orbit took it too close to Mercury would be affected by that planet’s gravity. Over several million years, Mercury’s pull could boost such a body out of the hot zone or even steal enough energy from it to plunge it into the sun.
These limitations define a ring of space that starts about 6.5 million miles from the sun and extends out to just under 20 million miles—an area comprising about 1 quadrillion square miles. An object orbiting in that Goldilocks region could survive billions of years. But sitting securely in that stable zone between Mercury and the sun is not quite enough to guarantee a decent life span for a vulcanoid. There is also the matter of size.
Vulcanoids have a lower size limit, because very small things (think grains of dust) would be swept clean out of the innermost solar system by the wind of subatomic particles blowing off the sun’s surface. Even light itself exerts pressure, and anything smaller than a few hundred yards across would be long gone from the inner solar system by now. There is an upper size limit as well. The bigger the object, the brighter it would appear from Earth. Anything beefier than about 40 miles across would have been found by now. Astronomers don’t see such things, so they must not be there.
By the middle to late 20th century, these upper and lower bounds for both size and location were well defined. A new generation of astronomers could get serious about the search for vulcanoids—a search that has now heated up all over again.
The difficulty in hunting for vulcanoids, if they exist at all, is that they orbit so close to the sun. From our vantage point 93 million miles out, a vulcanoid would never wander more than 12 degrees from the sun in the sky, so it would be swallowed up by the glare. The only hope of finding one would be to observe it just after sunset or just before sunrise, when the sun is slightly below the horizon and the hypothetical vulcanoid is slightly above.
That is a very thin slice of time, mere minutes long, making any search extremely challenging. And the sky is bright enough at that moment to easily wash out the feeble light from the target. (Observations during total solar eclipses fare no better, for the same reason.) Looking near the horizon means peering out through miles of Earth’s turbulent, hazy, and sometimes polluted atmosphere, which would blur and dim the vulcanoid’s appearance even more.
Searching for vulcanoids is a Herculean task, but one that a few scientists have gladly taken on. Stern, now at the Southwest Research Institute, and his collaborator Dan Durda—both friends and colleagues of mine—have been peering carefully at the hot desert between the sun and Mercury for more than a decade. “I didn’t think it would be a 10- or 12-year quest,” Stern says wryly. “But we’re going to chase them down to the ground. We’re going to find them or eliminate the possibility that they’re there.”
Recognizing the difficulties imposed by atmospheric interference, Stern and Durda took the search in a new direction: up above most of Earth’s atmosphere. They built a special camera and in 2002 flew with it on an F-18 fighter jet at 49,000 feet, where the sky is much clearer. It was a valiant effort, but unfortunately at that height the sky is still too bright to find vulcanoids—even at twilight, when they tried.
Earth-orbiting spacecraft might seem the next obvious vantage point. However, even from 300 miles above the surface of our planet, the search would still be nearly impossible. In a space shuttle orbiting at five miles per second, for example, the period between sunset and the time any vulcanoids would dip below the rim of the Earth can be measured in seconds. Putting a dedicated spacecraft in orbit would be prohibitively expensive, as well. And so this approach was abandoned.
Space probes beyond Earth orbit, designed for other uses, have been tasked with the vulcanoid search. The Solar Dynamics Observatory (SDO), a NASA spacecraft launched in February to monitor the sun’s magnetic activity, should be able to spot any objects at the upper end of the size range. It has taken a preliminary look but found nothing, Stern says, narrowing the search to smaller bodies. Messenger, another NASA craft that will settle into orbit around Mercury in March 2011, has been scanning for vulcanoids too. So has the Solar Terrestrial Relations Observatory, or Stereo, a pair of satellites tagging along with Earth in its orbit around the sun—one just ahead of our planet, one just behind. Designed in part to examine the space around the sun for the effects of massive solar eruptions, Stereo is a good platform from which to search for the brighter end of the potential vulcanoid population. Stern, Durda, and a few colleagues have calculated that the twin satellites would be able to detect vulcanoids as small as 1.5 to 4 miles in diameter, but they haven’t found any so far. “These results are disappointing,” Durda says, “but we haven’t given up hope yet.”
The best hope may now rest on a new method of reaching the limits of our atmosphere. If airplanes are too low and satellites move too quickly to make an effective search tool, then how about a compromise? Enter suborbital rocket flights.
In the next few years, Virgin Galactic and other private companies will begin carrying passengers aboard small vehicles upward of 60 miles above Earth. Paying customers will see the arc of our planet’s rim and black skies and will experience three minutes of free fall before returning to the ground. Such a flight is nearly perfect for vulcanoid hunting. The dark skies at that height should allow even faint vulcanoids to be spotted. The three-minute window of opportunity may seem short, but it is long enough for a sensitive camera to find potential vulcanoids down to 0.6 mile in diameter—much better than Stereo, Messenger, or SDO can do. And the price tag of $200,000 per ticket is a bargain compared with the alternatives.
Stern and Durda certainly think so. They have tickets to ride, and they plan on using these flights to carry a specially designed camera to the edge of space. In a single flight their instrument should be able to observe up to a third of the volume of space where vulcanoids may exist, substantially increasing the odds of finding some of these objects. If they do find any vulcanoids, the astronomers will also be able to characterize this long-sought population: How many are there? How close to the sun do they orbit? What is their distribution in size?
There is another intriguing question lingering in all this: After coming up empty time and again, why do astronomers like Stern and Durda continue the search for vulcanoids? Many of their colleagues consider the whole project a bit quixotic. “Nobody wants to study something that doesn’t exist,” Stern admits.
But the solar system has surprised us before. Astronomers thought the space between Mars and Jupiter was empty until Giuseppe Piazzi (pdf) spotted Ceres, the first asteroid discovery, in 1801. Now it is estimated that there are millions of rocks orbiting there. Icy comets orbiting beyond Neptune were pure speculation until the first of these Kuiper belt objects turned up in the 1990s—and there may be millions of them, too. Both discoveries revealed a lot about how planetary systems form and evolve.
Also, astronomers are eternally curious. Absence of evidence, as Carl Sagan noted, is not evidence of absence. But even the finding of nothing in this cosmic desert would provide clues about the solar system’s behavior. If it turns out that one (and only one) orbital niche of the solar system is completely empty, that discovery would be important. Maybe the sun influences this region in ways we haven’t conceived. Perhaps it is harder to achieve stable orbits inside Mercury or more difficult to move objects there than originally thought. Such information could tell us a lot about possible planets around other stars as well.
There is also the simple thrill of pushing boundaries. “I love a frontier,” Stern says. “The idea that we could discover the remnants of an asteroid belt interior to Mercury’s orbit is scientifically seductive: a whole new class of objects in the solar system.”
And if he and Durda do find them? “I’m dying to name one of them Spock.”