Pluto Is About to Make a Stunning Comeback

What are astronomy's most intriguing puzzles?
Feb. 5 2014 10:34 AM

Pluto Wins

It may not be a planet, but Pluto explains the whole solar system.

Pluto!

Illustration by Charlie Powell

It's time to stop throwing pity parties for Pluto. The beloved not-quite-planet is about to become the star of our solar system. I'll go out on a limb and predict that in July 2015, the up-close photos of Pluto we'll get from a NASA spacecraft will be the most popular astronomical images of a generation. Screensavers, posters, live TV, Twitpics, you name it—Pluto will be everywhere. After all, we've been wondering what it looks like for decades.

But even now—and this is what most people don't realize about Pluto—we owe that little iceball a ton of respect. It led the way to a brand-new view of what our solar system is really made of. Here's the answer in advance: Almost all of the worlds circling our sun are like Pluto. There are thousands of them. Pluto's orbit, which used to seem weird, also showed us the violent history of how the big planets—the ones that get all the glory—got to where they are now. Its idiosyncrasies gave us the first, and best, clues about the history of our little nook of the Milky Way.

In other words, Pluto rules, regardless of what we humans deign to call it.

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Formerly our ninth planet, Pluto was downgraded to "dwarf planet" in 2006 by some stuffy folks at the International Astronomical Union. A lot of astronomers disagreed, and Plutophiles howled. But the deal was done, and Pluto became a trendy verb. "I've been Plutoed" is a terrible thing to hear at the office. Even diminutive Tom Cruise got into it. A character in his Mission: Impossible—Ghost Protocol movie whined to him about a codename: "Why am I Pluto? It's not even a planet anymore."

All this angst over something that, to be charitable, is a smudge of light. No missions have flown past Pluto, so we're limited by what we see in telescopes. The object, with a surface area the size of Russia, is so far away—currently about 3 billion miles, more than 30 times the distance between Earth and the sun—that even the best telescopes show Pluto as an indistinct disk with light and dark patches. It has a gossamer atmosphere, frosts made of nitrogen and methane and carbon monoxide, and a whopping moon called Charon (pronounced by some astronomers with a soft "Sh" and others with a hard "K" sound). The Hubble Space Telescope has spotted other flecks of light circling Pluto: four more moons. Most everyone thinks Pluto has a bunch of other companions.

Beyond that, Pluto has remained mysterious. That will all change in 17 months.

140204_MOU_PlutoNewHorizons
Artist’s concept of the New Horizons spacecraft as it approaches Pluto and its largest moon, Charon, in July 2015.

Illustration courtesy Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute (JHUAPL/SwRI)

On July 14, 2015, the New Horizons spacecraft will whip past Pluto at more than 30,000 miles per hour after a flight lasting almost a decade. It carries what mission director Alan Stern calls "the best first-reconnaissance set of instruments for a planet, ever." We've had some amazing first looks at the four giant planets over the decades: majestic Jupiter and its bizarre moons, Saturn's ethereal rings, and the ocean-hued Uranus and Neptune. If you're old enough to remember the transformations of those bodies from blobs to fully realized worlds by the Voyager missions, you know the potential impact of Pluto's debut.

Not long ago I visited Stern at his home base, the Southwest Research Institute in Boulder, Colo. SwRI is Obsession Central for Pluto, and Stern is at the core of it. He's an intense man with a compact build, a flier of high-altitude jets, and a former official at NASA. He can tell you, down to the minute, what the spacecraft and its observing tools will do during the zippy encounter. His nimble team has practiced the entire event many times. And he bristles, rightfully so, when any story (including one of mine, I must confess) refers to Pluto as an "oddball" or a "misfit."

"Pluto was the harbinger of knowledge that the solar system has thousands of planets, and nearly all of them are tiny," Stern says. Pluto and those other icy worlds inhabit a vast realm of the solar system called the Kuiper Belt. New Horizons will pay the first visit to such a world.

The biggest question is about Pluto's surface: Is it alive or dead? Pluto's craters, ridges, valleys, and other landforms may have been frozen and locked in place for eons. The sculpted ices of nitrogen, methane, and carbon monoxide would look eerie and beautiful, but it's less interesting scientifically if Pluto is stuck that way. There's a chance—despite surface temperatures of about minus-400 degrees Fahrenheit—that some internal source of heat could make Pluto's surface ooze, flow, and perhaps erupt with icy geysers.

The largest moon of Neptune, called Triton, ebbs and shifts in such ways, creating fabulously exotic terrains. Scientists think Triton is one of Pluto's cousins, captured by Neptune from the Kuiper Belt billions of years ago. Comparing the two worlds, so unlike anything else we've seen up close, will consume scientists for years.

Pluto's tenuous atmosphere is another focus for New Horizons. From Earth, it's a nearly imperceptible wisp around Pluto's margins. The light-and-dark patterns seen by Hubble are the barest hint that the atmosphere and the surface interact as the planet's climate changes. Pluto's oval orbit takes it so far away (up to about 50 times the distance from Earth to the sun) that the atmosphere may freeze out as bright crystals onto a surface tinged red by methane. The spacecraft also may see clouds and fog: Plutonian meteorology, driven by the planet's all-natural global warming and cooling.

Surface of Pluto
This is the most detailed view to date of the entire surface of Pluto, as constructed from multiple NASA Hubble Space Telescope photographs taken from 2002 to 2003.

Image by NASA

The barely-there atmosphere has kept us from knowing exactly how big Pluto is. Marc Buie, Stern's colleague at SwRI, wrote his Ph.D. thesis on Pluto as a last-minute swap 30 years ago, and the planet's elusive size has fascinated him since. "We're finally going to know the diameter of Pluto!" he told me in an urgent voice that only an obsessed scientist could conjure. For the record, Buie's published estimate is 2,306 kilometers (1,433 miles) in diameter. (In contrast, Earth's moon is 3,475 kilometers wide.) But Buie confided: "I think it's closer to 2,320 kilometers." Stay tuned!

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