Beyond the Thirteenth Planet

Crescent of a Far World: Pluto from 18 000 km. Credit: NASA/JHUAPL/SwRI

In 2006, the International Astronomical Union classified Pluto—along with recently-discovered Eris and the asteroid Ceres—as a dwarf planet. Now, a decade later, as imagery from the New Horizons probe is still emerging, interest in the planet emeritus remains high. Controversy in the wake of the IAU’s decision lingers in the minds of some adults, even while high school seniors have only ever known Pluto as the little brother to the classical eight.

Pluto wasn’t the first member of the ex-planets club—or even the first ‘Ninth Planet.’

(And not the last. Shortly after this piece was drafted, evidence of a new Ninth Planet was announced by Mike Brown of Caltech, currently best known as the PI of the team that discovered Eris.)

In 1801, the asteroid (now also dwarf planet) Ceres was discovered by Giuseppe Piazzi. Ceres became the eighth planet, coming only twenty years after the discovery of Uranus. So the original Ninth Planet was Pallas, discovered in 1802.

Three more asteroids were discovered by 1845—Juno, Vesta and Astraea—and at the time, each was considered a planet. So when Neptune was finally discovered using mathematical predictions in 1846, it took its place as the Thirteenth Planet.

In the next few years, dozens more asteroids were discovered between Mars and Jupiter. As planets, the custom had been to create special symbols for each of them. But they were so numerous and minute, and so obviously different from the remaining eight, that this custom was given up. Since then, all asteroids have been given numbers in order of their discovery.

But even that wasn’t the first time we Earthlings decided something in the sky wasn’t a planet. The first was, of course, the Sun. It had been thought of as a planet—Greek for wanderer in the sky—for millennia before the revolution in astronomy ushered in by Copernicus, Tycho Brahe and Galileo.

This is great news for anyone who loves Pluto and loves astronomy. So far, there have only been three eras when the list of planets has changed, and each one came at the head of a remarkable new period of discovery. That doesn’t just mean that technology has improved from the time of Galileo’s spyglass to the New Horizons probe. More importantly, our understanding of what the cosmos is, and how we live in it, has been dramatically altered at each of these three points.

The first shift took place thanks in part to the introduction of the telescope: among other things, Galileo discovered the four major moons of Jupiter and the phases of Venus, both of which helped to discredit the old geocentric view of the Solar System.

Likewise, the discovery of the asteroid belt, Uranus and Neptune prompted a completely different view of the history of the Solar System. Not only has space been keeping secrets, but it has a past. Some 19th Century astronomers hypothesized that the asteroid belt was the remains of a destroyed planet, one that somehow broke up and left its bones behind like the ribs of a dinosaur eroding out of a rock face.

Even up until the time of Einstein, most people wanted to believe that the universe—our Solar System included—was an unchanging heaven. This idea goes deep into antiquity. Our Earth, the realm below the Moon, is a place of upheaval, discord and change—while the ‘fixed’ stars and the voyages of the planets proceed with serene orderliness.

In reality, the asteroid belt is made of up proto-planetary debris—the patchwork scraps left over after the planets formed. But even though the ‘destroyed planet’ hypothesis was discarded, the hypothesis itself was our first inkling that we could investigate the Solar System’s history.

It’s precisely this realization that allows us to ask and answer questions about planetary history, even more than the sophisticated technology we use to carry out our experiments.

Just as much as the Earth, our Solar System is a changing place. Once we started looking for it, we found evidence of change everywhere. The Moon’s geologic history is tied to that of the Earth. It’s now understood that the Moon formed during an off-center collision with a real-world “ex-planet,” a Mars-sized planetary object called Theia.

In the course of a single month following the impact, the remains of Theia were absorbed into the Earth, while the material ejected into Earth orbit coalesced into the body we now know as the Moon. A lot of evidence for this, as well as for the evocatively-named period of astronomical history known as the Late Heavy Bombardment, came from the Apollo lunar missions.

When Pluto was discovered, it was an oddball, just like the first few scattered asteroids discovered in the empty zone outside Mars’ orbit. Clyde Tombaugh, working alone through the freezing nights at the Lowell Observatory in the mountains above Flagstaff, Arizona, found Pluto as the conclusion to a search for an elusive planet—‘Planet X’—astronomers expected to find beyond Neptune, to explain anomalies in Uranus’ orbit.

Originally, Pluto was thought to be about Earth-size—easily a planet, and a clean break between the Solar System and the Galaxy beyond. But as observations continued, Pluto’s size estimates dwindled until astronomers realized it was scarcely more than a sixth the mass of Earth’s Moon. There were other peculiarities, too.

In contrast with the tidy nine-planet system taught in schools—and Pluto’s ‘last outpost’ image in science fiction—astronomers were increasingly convinced that there was more to find beyond Neptune. In 2005, the situation came to a head when it was announced that Mike Brown and his team discovered Eris, another object (in what’s now called the Kuiper Belt) now known to be almost exactly the same size as Pluto. And there are more—lots more.

In addition to Ceres, at least two other Kuiper Belt objects (Haumea and Makemake) qualify as dwarf planets. In reality there are probably dozens more, with estimates up to thousands of undiscovered bodies in the far reaches of the solar system.

That’s the exciting thing about Pluto: instead of the last planet before emptiness, it’s our first step into a new wilderness.