An artist’s illustration of 2003 UB313, whose discovery promises to revolutionize our understanding of just what a “planet” is. (credit: NASA/JPL-Caltech)

Copernicus smiled

by S. Alan Stern
Monday, September 12, 2005

A real revolution is afoot in planetary science. The first shot was fired in 1930, with the discovery of Pluto, but almost no one realized its import. The second and third shots came in the late 1970s, with the discovery of distant objects called Chiron and Charon, but again, few recognized what they would portend. Rapid-fire volleys began in the 1990s, as myriad discoveries of icy bodies hundreds to well over 1,000 kilometers across occurred in the Kuiper Belt, just beyond Neptune, became an observational reality. But it was only this year, with the recently announced discovery of 2003 UB313—a world larger than Pluto—that we have heard the equivalent of the American Revolution’s “shot heard round the world.”

When I was a boy in the 1960s, in college in the late 1970s, and in graduate school in the 1980s, we were taught that our solar system contains four rocky planets on the inside, four giant planets on the outside, and one spit of a planetary misfit called Pluto, moving in a markedly elliptical and oddly inclined orbit beyond Neptune. Like many people, I recall thinking: What an odd bird that lone Pluto is.

Today, however, we see a very different picture of our home solar system is emerging, one that reveals Pluto in context—as a nearby example representing what is almost certainly the most populous class of planet in our solar system: the “ice dwarfs.”

Consider that less than two percent of the Kuiper Belt has been thoroughly catalogued, yet over a thousand rogue worlds and worldlets have already been spotted there. Among just those bodies catalogued to date, we know that half a dozen (like Sedna and Quaoar) already rival—and in the case of the just-discovered 2003 UB313—exceed Pluto’s size. Moreover, most of these new worlds follow orbits that are as cockeyed as Pluto’s—some even more so.

Now we can see just how naive our 20th century perspectives were: Pluto is no misfit. Instead, once the advance of technology allowed us to probe deeply enough, it is becoming clear that Pluto was the advance harbinger of a populous new region of the solar system lying beyond the giant planets.

Modern simulations of planetary formation, performed by different research groups around the world, led to broad agreement that in the process of forming the giant planets, some hundreds to thousands of smaller worlds, ranging from a goodly fraction of Pluto’s size to at least Earth’s size, also formed. Most of these bodies were dwarf planets, like Pluto, with steeply declining populations at larger and larger sizes, so that only a few or few tens of bodies Earth’s size were formed. These simulations also show that most of these bodies were ejected from the giant planets region to much more distant orbits as Jupiter, Saturn, Uranus, and Neptune neared their current sizes and gravitationally cleared out their formation zones over four billion years ago.

Importantly, these numerical models are supported by some solid forensic clues that are scattered about the outer solar system, and which lead us to similar conclusions. One such clue is the fact that Pluto’s moon, Charon (itself half of Pluto’s size) seems to have been formed by a giant impact with a body nearly as large as Pluto itself. What is most important in this finding is that, in order to make such a collision probable, there must have been hundreds or more thousand-kilometer diameter bodies orbiting in the ancient outer solar system. A second clue comes in the form of Triton, a moon 2,700 kilometers in diameter that circles Neptune in a retrograde orbit, the hallmark of gravitational capture from a previous orbit around the Sun. Triton is compositionally much like Pluto, but a tad larger. Apparently, it is one of the “many Plutos” that once formed, and it seems to have escaped ejection by becoming caught in a long-lasting orbit around Neptune. Yet another clue comes from the polar tilts or Uranus (98 degrees) and Neptune (30 degrees). The only viable mechanism known to be able to generate such extreme tilting of these gargantuan (15 Earth mass-class) planets are off-center collisions with bodies of one to several Earth masses. Crucially, calculations also reveal that in order for both Uranus and Neptune to have had a high probability of suffering such collisions, as many as a few dozen such Earth-mass objects may have once orbited in their region of the solar system.

As a result of the modeling capability that modern computers give us, combined with the forensic observational clues just discussed, and now the discoveries of rivals and even successors to Pluto’s throne, we are slowly but surely coming to a conclusion both jarring and exciting: that our solar system formed not just the nine planets we were taught to name in school, but many dozens, if not hundreds of others as well!

A revolutionary aspect of this emerging, new paradigm is the dawning realization that the long-known eight rocky and giant planets, Mercury through Neptune, now seem to be the misfits. Indeed, from today’s 21st century perspective, the solar system seems likely to be dominated by a huge population of rock and ice planets ranging from dwarf sizes like Pluto to perhaps super-Earths. Most of these new worlds are expected to follow elliptical, highly-inclined orbits, like those of Pluto, Quaoar, Sedna, and UB313. Further still, of all the planets now expected to orbit within our sun’s grasp, most orbit between ten and a thousand times farther than do any of the planets we were taught about in school. It’s not at all your father’s solar system.

Less than two centuries ago it was discovered that all the stars one can see by eye, and their innumerable brethren seen by telescope, are distant suns, with numbers too great to count. Similarly, it was just under a century ago that our galaxy, the Milky Way, was realized to be but one of literally billions of galaxies. Both of these realizations, like the 16th century realization that the Sun (not Earth!) is the center of our solar system, jarred perceptions and changed textbooks in revolutionary ways. Just as jarring to us now is the newly emerging view that our solar system made, and is still littered with, very many distant planets, most of which are nothing like the familiar planets that orbit close to the Sun, like Earth. In a real sense, we are seeing a new chapter unfold in the revolution that Copernicus wrought when he displaced the Earth from the center of everything. Another slice of humble pie, anyone?

Alan Stern is a planetary scientist at the Southwest Research Institute and the Principal Investigator of NASA’s New Horizons mission to Pluto and the Kuiper Belt.

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