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NOT NINTH OF NINE, BUT FIRST OF MANY

May 16, 2010 12:37 am

NEW YORK

It's an issue that will affect how we spend billions of dollars, and how future decades of exploration will unfold. We're not just talking about poor little Pluto. We're talking about a convergence of developments that will widen our view of the solar system and what lies beyond:

Just last month, President Barack Obama announced that the first destination beyond Earth orbit for human space flight would not be the moon, but an asteroid--one of the solar system's "minor planets." If that goal holds true, billions of dollars will be spent over the next 15 years to send astronauts to what's officially considered a non-planet.

During the past 15 years, more than 450 planets have been detected beyond our solar system, and hundreds more are expected to be found, thanks to NASA's recently launched Kepler mission. Some of those planets will be Earth-like worlds orbiting sun-like stars, in circumstances that could support life as we know it. And some of the planets already discovered do not fit the definition of planethood as currently described.

Over the next decade, more than a billion dollars will be spent on telescopes with acronym-heavy names such as Pan-STARRS and LSST--telescopes that could spot tens of thousands of previously unseen ice dwarfs on the rim of our solar system. In the first week after the LSST begins operation, "we will see more data from this telescope than all the telescopes in humanity up to that point," said software billionaire Charles Simonyi, one of the project's backers.

NASA's New Horizons probe has just passed the halfway point in its nine-year journey to Pluto. Another spacecraft, called Dawn, is well on its way to Ceres, the largest asteroid as well as the smallest known dwarf planet. Five years from now, the probes will give us our first close-up looks at two very different examples of a newly designated type of celestial object: dwarf planets.

MATCHING EXPECTATIONS?

What will we see in 2015? Astronomers expect Dawn's cameras to see Ceres as a clay-covered world, perhaps with patches of bright frost and underground reservoirs of water. New Horizons is expected to confirm the existence of Pluto's atmosphere and ice clouds, with frost and perhaps even ice volcanoes visible on the surface. Scientists aren't sure how closely the reality will match the expectations--but they are sure that Ceres and Pluto will loom big and round in the pictures sent back by their robotic visitors.

Roundness--that's the defining characteristic for dwarf planets. The International Astronomical Union currently distinguishes between dwarf planets and the eight larger worlds in our solar system that have "cleared out the neighborhood of their orbits." That's a useful distinction for some astronomers, to be sure. But it shouldn't be used as a reason to cross Pluto and the other dwarf planets off the list. Dwarf planets should be included on the full roster of planets--just as dwarf stars (like our sun) are counted as stars, and just as dwarf galaxies (like the Magellanic Clouds) are counted as galaxies.

Why does the issue matter? One line of reasoning simply appeals to common sense: Based on the IAU's definition, a world as big as Earth would not be considered a planet if it were put in Pluto's place. Any definition that would exclude Earth as a planet has to be viewed with skepticism. But to my mind, the more important line of reasoning relates to the future discoveries I've just mentioned.

FROZEN LEFTOVERS

To put those discoveries in their proper perspective, we need a wider view of the cosmos, not a narrower view. We need to think of asteroids not merely as cosmic junk piles, but as deep-space steppingstones that can kill us or sustain us on the final frontier. We need to think of worlds beyond our solar system not merely as points on a star chart, but as potential laboratories for the study of life in the universe.

We need to think of Pluto and its brethren not merely as frozen leftovers, but as true members of the solar system's wider clan. Never again can Pluto be the ninth planet. Or the littlest planet. Or the most distant planet. But does that make Pluto a non-planet? No way. Even before Pluto was discovered, the solar system was divided into two classes of planets: the rocky worlds like Earth, and the gas giants beyond. Pluto has pointed the way to the solar system's third great class of planets, no less important than the other two.

Pluto isn't the ninth of nine; it's the first of many. The discoveries ahead will shift our view of the universe the way Galileo Galilei and Nicolaus Copernicus shifted it four centuries ago. In the 17th century, the world came to understand that Earth was not the center of the universe. In the 21st century, we will come to understand that Earth provides just one template for the way the cosmos builds planets--and not even the most common template.

MISFIT PLANETS

Alan Stern, the principal investigator for the New Horizons mission to Pluto, says there could be many thousands of icy worlds out on the solar system's rim--and hundreds of them could well qualify as dwarf planets. Over the long run, that will almost certainly change the way we look at our own place in the universe. And what's so bad about that?

"The original view, until 10 or 15 years ago, was that we had four Earth-like terrestrial planets, four gas giants, and the misfit Pluto. But the new view is four terrestrial planets, four gas giants, and hundreds of Plutos," Stern says. "It's jarring, because Pluto's no longer the misfit. It's the Earth-like planets that are the misfits."