Pluto: ten years a dwarf

Talk about being on the outer. Pluto. It was the last planet to be discovered and only because it was thought a ninth planet had to exist to explain some odd happenings around Uranus and Neptune. That was in 1930. And Clyde Tombaugh was the lucky astronomer.

It turns out Tombaugh was very lucky, indeed. Those odd happenings only appeared to be odd because scientists had yet to determine the correct mass of Neptune. But he was looking in the right spot and found Pluto. And it was more or less assumed Pluto had to be a planet because no other objects were known to exist at that distance from the sun.

But people did often wonder about its planetary status. Then about 60 years later, with the discovery of the Kuiper belt, things started to go pear-shaped for poor old Pluto.

"In 1992, when a second object was observed at [a similar] distance, and then many more, Pluto was no longer seen as something special that made it a planet," says Dr. Hermann Böhnhardt at the Max Planck Institute for Solar System Research.

The discovery of the Kuiper belt really was the death knell for Planet Pluto. The Kuiper belt is home to thousands of objects, and some are larger than Pluto.

"There were a number of objects observed to be the same size as Pluto and that resulted in this discussion about whether Pluto can be a planet or not, and if it's the ninth one do we keep counting to 100?!" says Dr Ralf Jaumann at the Institute for Planetary Research at the German Aerospace Center. "So we needed to really think about what makes a planet and what is a dwarf planet."

Pluto degraded

Out of that came a decision from the International Astronomical Union (IAU) to degrade Pluto to the status of dwarf planet. Pluto was just narrowly spared an ignominious fate as an asteroid - no offense intended, of course, to any asteroids, some of whom moved up the greasy pole.

"Ceres [once an asteroid] and Eris, another object in the Kuiper belt beyond Pluto, fulfilled the criteria and were, if you will, promoted," says Böhnhardt. "But from a scientific point of view - for me and many colleagues - this reclassification made no difference, because Pluto was no longer considered a planet since the discovery of the Kuiper belt."

Essentially, though, the IAU's criteria for dwarf planets says the shape of the object is dominated by hydrostatic forces, making them "nearly round."

So what was the point?

These small objects may be small but they loom large in the evolution of our solar system. They are described as "primitive" objects because they have undergone the least change - for instance through collisions with other objects - since the formation of the planetary system. And because planets essentially consist of such small objects, and the materials that derive from them, small objects can tell us a lot about the larger planets, such as Earth.

And what is there to learn about Earth? Well, for one, we don't even know where all the water came from.

So did Pluto's reclassification as a dwarf planet - a small object - redirect science priorities and inspire new research in this area? Not quite.

"There is more research in this area but not as a result of Pluto's reclassification," says Jaumann. "There is more research because we have better telescopes now and better spacecraft for investigating smaller bodies."

An old dawn

One such spacecraft is Dawn. The Dawn mission is headed by NASA. It is currently investigating Ceres, and before that it visited Vesta, an asteroid.

Researchers hope Dawn will let them pop back to the first few millions of years after the planets were formed. The most recent data about Ceres' gravity has even allowed them to look inside the dwarf planet.

However, the mission was not spawned by the new dwarf planets. Although it had been canceled and reinstated a few times, Dawn was first up for consideration in 2001.

"So it was not triggered by the renaming of Pluto. The trigger was we are able to do it now," says Jaumann.

New Horizons

Similarly, the New Horizons mission to Pluto launched in January 2006 - eight months before the IAU's decision to reclassify the small object.

Its encounter with Pluto ended in January of this year. But it's not over yet. There are still hoards of New Horizons data to sift through. And let's not forget, Pluto is in the Kuiper belt, an area vastly unknown.

Scientists say there could be several hundred thousand objects bigger than 30 kilometers (20 miles) across, and we have yet to find them, these primitive things from the beginnings of our planetary system. There will be future missions.

You have a "spectrum of bodies" in our planetary system from "small dust grains to gravel-sized objects," says Böhnhardt, and then you have these small objects of a few meters, kilometers or a few thousand kilometers, and a few number of planets.

"But they are the outcome of the same process and that's how I see them," he says.

"We've visited all the planets with space missions now, and you can get a first glimpse of planetary properties around other stars from Earth-based observations, so [probing small objects] will hopefully give us a complete picture of how things are and how things have evolved. But we're not there yet, we still can't explain planetary formation," says Böhnhardt.

So then what of the tenth anniversary of the IAU's degrading of Pluto, and classification of dwarf planets? Was it any use?

"It's been useful," says Böhnhardt, "and it is used a lot. The classification of the dwarf planet is less specific than the one for a planet, so the IAU could have given this a bit more emphasis, but it is like it is for the moment."