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| Space on Feb 18, 2010

Alan Stern: ‘A Chihuahua is still a dog, and Pluto is still a planet’

“Our whole view of the solar system was turned upside in the 1990s when we discovered this plethora of Pluto-like planets. The fact is that it’s our larger Earth that’s the misfit,” says Stern.

Alan Stern: If an august body of scientists got together and all voted unanimously that the sky is now green, that wouldn’t make it so, because voting is irrelevant in science.

Planetary scientist Alan Stern is principal investigator for the New Horizons space mission now on route to Pluto. Dr. Stern disagrees with the International Astronomical Union, a body of 10,000 professional astronomers, who voted in 2006 to demote Pluto from planet status.

Alan Stern: When I was a kid – and I think most of your listeners, as children learn this – the solar system consisted of four rocky, terrestrial planets, four gas giants, and that little misfit Pluto. But with the advance of technology, we’ve discovered actually that Pluto is just the first and the brightest of a large population of small planets called dwarf planets that have been discovered since the 1990s. In fact it’s thought that there may be as many as a thousand Pluto-like planets in the solar system. So in fact, the whole equation has been turned upside down.

Stern believes strongly that these small bodies should be called planets.

Alan Stern: In fact, Pluto, and its cohorts, are planets. They have all the attributes of planets. Let me give you some examples. They have cores. They have geology. They have seasons and atmospheres. They have clouds. They have polar caps in many cases. They have moons. And I can’t think of a single distinguishing characteristic that would set apart Pluto and other things that you’d call a planet, other than its size. So I like to say, a Chihuahua is still a dog.

Dr. Stern talked more about the New Horizons mission, which he described as “a small, robotic space craft that’s making its way across the entire expanse of the solar system to explore Pluto and the Kuiper belt in 2015.”

Alan Stern:
We launched New Horizons to explore Pluto and the Kuiper belt in January of 2006. It was the fastest spacecraft ever launched, and it has been making a beeline across the solar system for Pluto. The spacecraft is very healthy, but it still has five and a half years to go until we begin the Pluto encounter. It’s a very large solar system. So we travel about a million kilometers per day. And it takes about 3,500 days to reach Pluto.

Stern invoked the name of the famous astronomer Galileo when explaining why he thinks Pluto should retain its planet status.

Alan Stern: Science works through a process of successive approximation. Our whole view of the solar system, its architecture, was turned upside in the 1990s when we discovered this plethora of Pluto-like planets. The fact is that it’s our larger Earth that’s the misfit. Well, that’s jarring. And for some people, they just couldn’t handle it. Some scientists said, well, that’s too many planets. I don’t think I can remember all of the names of those, so we just have to stop. We have to find some way of limiting the number of planets. I don’t like that way of approaching science, and many don’t.

Stern speculated that a similar situation could have arisen during Galileo’s time.

Alan Stern: This is the 400th year of Galileo’s early telescope. I imagine that there must have been at the time someone in the early 17th century said, ‘You know, all those uncountable stars the new telescopes can see. We’re not going to be able to deal with those. So we won’t call those stars at all.” But in fact they are stars. And in fact, Pluto, and its cohorts, are planets. They have all the attributes of planets.