More to Come

How many more dwarf planets will be added to the list? Theoretically, there could be scores, or hundreds, or thousands. Critics who say the classification scheme is too broad point to the Saturnian moon Mimas—which is only 250 miles (400 kilometers) wide, and mostly round. That is, if you don't count the big bull's-eye crater that makes it look like the Death Star from the Star Wars movies.

If Mimas were in its own orbit, it might be considered a dwarf planet. But what's the harm in that?

The critics complain that there might be thousands of 250-mile-wide objects in the Kuiper Belt, and that the list of planets might spin out of control. But as long as a distinction is made between the eight dominant planets and the lesser worlds, what's the harm in that?

Practically speaking, even the dwarf-planet registry will likely expand slowly. The first measure of dwarf-planetary status, based on brightness, is relatively easy to calculate. If newly discovered objects have an absolute magnitude that is dimmer than, say, Charon, they won't automatically be added to the dwarf-planet list.11 In order to add those dimmer objects, astronomers would have to present evidence of hydrostatic equilibrium—for example, direct observations of the objects by next-generation telescopes.

Given the right kind of exposure, the myriad of planetary bodies could easily become as well-known to children as, say, the myriad of dinosaur species. There are already enough such worlds for a respectable set of trading cards. The five— or fifty, or five hundred—dwarf planets should become as much a part of the full set as the solar system's eight trump cards.

And Alan Stern has suggested that if we hang around the solar system long enough, the dwarf planets may become the only game in town. Billions of years from now, when the sun puffs itself up into a blazing red giant, the ice dwarfs would lie in what Stern calls a "Delayed Gratification

Habitable Zone." In such a scenario, Earth and the other inner planets would be turned into cinders at best, while worlds as far out as 50 AU might just offer a temporary foothold for life.

"When the sun is a red giant, the ice worlds of our solar system will melt and become ocean oases for tens to several hundreds of millions of years," Stern has said. "Our solar system will then harbor not one world with surface oceans, as it does now, but hundreds."

Other scientists aren't as optimistic about the prospects for beachfront property on Pluto. Once the sun enters its red giant phase, the entire solar system would be thrown into chaos, and it's not clear whether any region would be stable enough for long enough to sustain life. "The idea of organicrich distant bodies getting baked by a red giant star is an intriguing one, and could provide very interesting if shortlived habitats for life," said Donald Brownlee, an astronomer from the University of Washington who has delved into the prospects for life elsewhere in the universe. "But I am glad that our sun has a good margin of time left."12

In the meantime, astronomers will be using increasingly powerful telescopes to explore the wide realm beyond the Kuiper Belt—the mysterious region of the solar system where Sedna was discovered in 2004. Sedna may be the first object found on an astronomical frontier where an even bigger Planet X still waits to be discovered.

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