How many planets are there in our solar system anyway?

A. Feild / STScI / NASA / ESA

An artist's conception shows the dwarf planet Haumea and its two moons, Hi'iaka and Namaka.

So just how many planets are there in our solar system anyway? Eight? Nine? Thirteen? Or thousands? Far from settling the question, the "Great Planet Debate" has revealed just how complex and interesting the question is.

The planethood question got more interesting this week with the naming of yet another dwarf planet, Haumea. It's traditional to name planets after mythological deities - and Haumea, the Hawaiian goddess of childbirth and fertility, follows that formula.

The football-shaped world was found by Caltech astronomer Michael Brown just after Christmas 2004 (which prompted its initial, unofficial nickname: "Santa"). Haumea's discovery was shrouded in a scientific controversy that Brown recaps in his Weblog. At the time, controversy surrounded its planetary status as well, because it added to a growing class of objects in the same general class as Pluto. Astronomers surmised that hundreds of Pluto-scale objects may lurk on the icy rim of the solar system's disk, known as the Kuiper Belt.

The controversy came to a head in 2005 when Brown's team found the object now known as Eris - a world like Pluto, only bigger and farther out. All this led the International Astronomical Union to agonize over where to draw the line on planethood. In 2006, the IAU came up with a definition aimed at putting the solar system's eight biggest planets in one class, and Pluto in a different class with Eris and other dwarf planets or "plutoids."

The Great Planet Debate has been simmering ever since. In August, astronomers held a teach-in on the subject at Johns Hopkins University's Applied Physics Laboratory, which is the base of operations for NASA's New Horizons mission to Pluto. One of the purposes of the meeting was to see how teachers were handling the planethood question.

Scientist (and parent) sees 'teaching moment'
The education angle literally hit home for planetary scientist Alan Stern - and not just because he's the principal scientific investigator for New Horizons.

"My own son was told by a teacher that an answer was wrong on a test about Pluto," Stern told me last week. According to the test, the "right" answer for the number of planets in the solar system was eight - but Stern said that August's installment of the Great Planet Debate proved that the question was still up for grabs, even among educators.

"It was clear at the end of the two and a half days that there was no consensus," he said. "We're in transition. I think that's a teaching moment."

Stern has long argued that the IAU's definition of planethood provided more confusion than clarification. "There's a lot of unhappiness with the IAU's solution," he said. "I didn't hear anybody say, 'Oh, I think it's the cat's meow.'"

He maintains that it's wrong to think about the solar system as if there were a sharp division between eight planets and everything else. Even dwarf planets are still planets - and in Stern's mind, they may be more representative of the planetary spectrum than the eight biggies.

"It's the most populous class of planets in the solar system," Stern said. "Pluto's no longer the misfit."

There's something about Haumea
The fact that the IAU is giving names to dwarf planets - Pluto, Eris, Ceres, Makemake and now Haumea - shouldn't make a difference in the debate, Stern said. In fact, it totally makes sense. "From our perspective, these are planets. They deserve names," he said.

Stern has often compared the definition of planets with the definition of rivers: Sure, there might be six great rivers in the world ... or are there 14? In any case, that doesn't mean you have to set the Maquoketa River or thousands of other streams apart as "dwarf rivers." Every river, great or small, has its own special appeal - and it's the same with planets.

In fact, Haumea may be one of the most endearing little planets out there: Caltech's Brown has said it's his "favorite object in the solar system," in part because of its fast, end-over-end spin, the elongated shape it has as a result, and also because of its tightly orbiting satellites (which have been named Hi'iaka and Namaka, after two of the goddess Haumea's children). Brown said additional bits of ice and rock were apparently struck off Haumea in a cosmic collision long ago and are now circling the sun in their own orbits.

New Horizons gets a transplant ... and Twitter!
Oodles of such oddities may well come to light when the New Horizons spacecraft makes its way through the Kuiper Belt, starting in seven years. Last week, the probe underwent a successful "brain transplant" that upgraded the onboard software. It's now more than a billion miles from Earth, flying toward Pluto at a rate of about a million miles a day.

You can keep up with the mission's progress via Twitter or Facebook. (In the wake of Phoenix Mars Lander's Twitter success, it seems as if every space mission nowadays is getting into social networking.)

New Horizons' team will be checking out the spacecraft's instruments over the next couple of months, and then put the probe back to sleep for another months-long nap. The first "dress rehearsal" for the Pluto flyby will be conducted next year, but there's still a long way to go before showtime in 2015.

Will the planethood debate be settled by that time? Stern won't be surprised if it isn't. After all, it took decades for scientists to settle the controversy over continental drift - and some are still going back and forth over the implications of climate change and evolutionary biology.

"This is not atypical," Stern said. "It's just one of the most visible topics on the table right now."

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