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NASA's New Horizons mission to Pluto, which has just begun six months of observations leading up to July's flyby, will shed new light on what are arguably the most common, least-understood objects in the solar system: the icy mini-worlds on the solar system's edge.
But what will the mission mean for the popular perception of Pluto? After scientists and the public get a look at the pictures sent back from 3 billion miles away, will Pluto turn into a planet again?
That question is intentionally facetious: Pluto is what it is, fascinatingly so, whether you call it a planet, a dwarf planet, a planetoid, plutoid or Kuiper Belt object. It's arguably all of the above, although I favor "dwarf planet" as the term that won't get you in trouble with anyone. It's even passed muster with the International Astronomical Union, which set up a controversial classification system just a few months after New Horizons was launched in 2006.
Pluto satisfies two of the IAU's three criteria for planethood: It's in orbit around the sun, and it's massive enough for gravity to crush it into a roundish shape and keep it that way (a condition known as "hydrostatic equilibrium"). The knock against Pluto is that it hasn't "cleared the neighborhood around its orbit" — the third criterion.
When New Horizons' pictures start coming back, we'll see that Pluto is clearly the master of its gravitational neighborhood (though its biggest moon, Charon, complicates matters). What's more, Pluto is expected to have most of the geophysical attributes associated with the bigger planets — including a thin atmosphere, weather patterns and potentially clouds, a mottled surface potentially dotted with ice volcanoes, more moons than Mercury, Venus, Earth and Mars put together, and maybe even faint rings.
Don't expect New Horizons' flyby to make supporters of the IAU's decision change their minds, or lead to a concerted effort to reverse the 2006 decision. Hardly anyone in the planetary science community wants to revisit the messy behind-the-scenes maneuvering that I describe in my book, "The Case for Pluto." But do expect a widening of perspective.
Even though the best images of Pluto we have today are little more than a blurry speck, there are ways to anticipate how New Horizons' images will strike you. First, take a look at Triton, Neptune's largest moon, as seen by NASA's Voyager spacecraft.
At a diameter of 1,680 miles (2,700 kilometers), Triton is only a couple of hundred miles wider than Pluto (1,430 miles, or 2,302 kilometers), and it has a similar density. The moon is thought to be an icy, Pluto-ish world that originated in the Kuiper Belt — the ring of icy material that extends well more than a billion miles beyond Neptune — and just happened to be drawn into Neptune's orbit.
If Triton were out on its own, would it be considered a planet? Just asking.
Next, stay tuned for the rendezvous that NASA's Dawn spacecraft is due to make in March with Ceres, a rocky dwarf planet that's also the biggest object in the main asteroid belt. When Ceres was discovered in 1801, it was considered a planet. But after lots of other objects were spotted at around the same distance, Ceres and other erstwhile planets were reclassified as "asteroids" — Latin for "starlike objects," because they looked like points of starlight.
The Hubble Space Telescope revealed that Ceres didn't look anything like a point of light, or like the traditional image of an asteroid. Hubble's fuzzy picture of Ceres looked a lot like a small telescope's fuzzy picture of Mars. Ceres' roundish shape has earned it a place on the IAU's roster of dwarf planets.
Ceres is a breed apart from Pluto, but just as interesting in its way: Dawn's scientists are hoping to detect geysers of water vapor spewing from its icy surface, and perhaps evidence of subsurface reservoirs of liquid water. Some think Ceres could be a "game-changer" for planetary science.
The public response to Dawn's pictures from Ceres could provide an early read on how well New Horizons' Pluto views will do. If Ceres' closeup has you changing your perspective, just wait until Pluto has its turn. And no matter where you stand on the planetary debate, sit back and enjoy the Year of the Dwarf Planets.