Pluto's retinue of moons entices the team as well. Charon, first spotted in 1978, is so bulky compared with its parent—about one-tenth the mass of Pluto—that scientists regard the Pluto-Charon system as a double planet. The other moons orbit this pair: icy shards called Styx, Nix, Kerberos, and Hydra. Here's the nifty part: Those four moons take almost precisely three, four, five, and six times longer to orbit Pluto, respectively, than Charon does. It's cold clockwork on a satellite scale.
To the SwRI team, this waltz points to a long-ago cataclysm. Some other object from the Kuiper Belt—Stern thinks it was about 600 miles wide—slammed into Pluto to spawn Charon and the other whirling bits. Gravitational kicks and ongoing collisions with smaller debris then shifted the orbits, over time, into the synchrony we see today. If true, that scenario suggests Pluto has more moons; Stern wouldn’t be surprised if he saw at least 10.
Crash remnants could be spinning in New Horizons’ path, so an onboard telescope will peer ahead next spring and summer. If the approaching spacecraft has to avoid plowing into smaller fragments, it would be a rather more graceful version of the famous asteroid-dodging sequence in Star Wars—fitting for a mission led by a jet jockey.
Then there's Pluto's orbit, which gave Pluto its oddball reputation ever since Clyde Tombaugh identified it as a planet in 1930. Pluto's languid 248-year-long path tilts way out of the flattened plane in which the other planets orbit the sun. It also swoops near and far, lasso-style. (For 20 years in every orbit, Pluto is closer to the sun than Neptune is.) To SwRI theorist Hal Levison, that orbit is a thing of beauty. It's his portal to an outer solar system full of objects on similar orbits. Those motions, he says, preserve traces of a wilder past.
The major clue came from a curious bond between Pluto and Neptune. As Neptune orbits the sun three times, Pluto orbits exactly twice—in such a way that the bodies never approach one another. This stable connection, called a "resonance," doesn't arise by chance. The planets must nudge one another via gravity and migrate into those orbital paths until the lengths of their orbits form a neat ratio. Then gravity keeps them there. This happened to Pluto's moons, too—and astronomers see the same thing around other stars, where some exoplanets orbit in syncopated lockstep.
In our solar system, Neptune took the lead. It's so much bigger than Pluto that it dictated how and when Pluto moved. In the 1980s other scientists proposed that Neptune formed closer to the sun and gradually moved outward, pushing Pluto into the off-kilter loop we now see.
The reaction to this idea was tepid until the 1990s, when telescopes started spotting other objects in the Kuiper Belt. "Pluto is not the lone weird thing," Levison says. Indeed, there are "plutinos," which share the same 2–to–3 orbital bond with Neptune, and "twotinos," which orbit the sun once for each two of Neptune's years. They all got swept into those paths by Neptune's outward march.
This new vision, an outer solar system sculpted by the gravity of Neptune, has taken hold. "Neptune controls everything now," Levison says. But he and his colleagues didn't stop there. They used Pluto and the rest of the Kuiper Belt to map out a grander, much more startling picture of our entire solar system's youth.
All four giant planets, they claim, crowded closer together in their orbits in the early age of our solar system about 4.5 billion years ago. They gravitationally tugged on one another, along with swarms of smaller bodies, for hundreds of millions of years. Then, in a spasm set off by an unstable connection between Jupiter and Saturn, the planets migrated violently. Neptune and Uranus might have swapped places. Jupiter's gravity might have even cast a fifth giant planet out of the solar system. As Neptune raced outward and the other planets shifted, they flung asteroids and comets into an icy, rocky blizzard.
Levison came up with the provocative idea, and many astronomers think it's plausible. Clues are everywhere, he says. Chief among them are the orbits of Pluto's brethren in the Kuiper Belt. They career every which way, but their paths—when mapped mathematically—trace back to an epoch of wandering planets. Mercury and our moon hold other clues: big craters whose ages and patterns suggest a massive storm of comets and asteroids set off by the moving planets, in a pulse lasting 100 million years or so.
If Pluto and Charon are frozen relics, their surfaces may preserve records of this intense bombardment, too. Charting their roles in the chaotic history of the outer solar system is one more mystery New Horizons may solve.
As for Pluto's demotion, Levison isn't concerned. "Clyde Tombaugh discovered the Kuiper Belt," he told me, his eyes locked on mine. "That's a helluva lot more interesting than the ninth planet. Pluto is the granddaddy of the most populated region in the solar system, with the most to tell us about our history. It must not mind."
Stern and his team hope to steer New Horizons toward another, much smaller object in the Kuiper Belt after they bid farewell to Pluto. But for now, the Pluto system has Stern's full attention. It's probably the only time we'll go there in the next half-century, and he's not going to muck it up. He knows how excited we all will be.
Years ago Stern asked his colleagues to write predictions for what we'll discover at Pluto and seal them in manila envelopes, to be opened after July 2015. Stern doesn't need to unseal his prediction; he recalls it word for word. "What we will find," he wrote, "is something wonderful."
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