Monday, November 29, 2010

The Fight for Dwarf Planets Continues

Check out this excellent interview with Dr. Alan Stern, Principal Investigator of NASA's New Horizons mission to Pluto, by Space.com here:

http://www.space.com/scienceastronomy/pluto-planet-debate-stern-qanda-101129.html

I want to commend Space.com for presenting both sides of the Pluto debate rather than just one. Stern's argument makes sense because it is based on what an object is rather than where it is. Gravitational dominance is addressed if we class these small planets as "dwarf planets." It means they are planets but not large enough to gravitationally dominate their orbits.  Arguments that "we cannot have too many planets" because kids won't be able to memorize them or because then the term planet won't be "special" have no scientific merit. We might as well limit Jupiter to four moons instead of 63 for the same reason. The same is true for arguments that there are only two kinds of planets, terrestrials and jovians. Just because an object is not a terrestrial or a jovian doesn't make it not a planet--it makes it a different kind of planet. We very well may discover objects in other solar systems that require adding a fourth, fifth, etc. category of planets. As for eccentric orbits, many giant exoplanets, some with several Jupiter masses, have orbits far more eccentric than that of Pluto. There are at least two cases of systems with two giant planets orbiting their stars in a 3:2 resonance just like Neptune and Pluto. If these objects are not planets, what are they?

Similarly, spherical moons of planets are compositionally akin to planets themselves, which is why Stern and others have proposed calling them "satellite planets." If we ever land rovers on Triton and Pluto, the challenges and circumstances will be very similar in spite of the fact that one orbits a planet and the other orbits the Sun directly.

Brown's argument that there are only a few astronomers left who view dwarf planets as planets and that they are all on the New Horizons mission was a cheap shot and is blatantly untrue. How many of the 300 professional astronomers who signed the petition rejecting the IAU decision are on the New Horizons mission? The answer is very few. New Horizons is already fully funded and unaffected by whatever resolutions the IAU passes. In fact, the Dawn mission to Ceres and Vesta, launched to a dwarf planet and an asteroid, receives no less credibility than any planetary mission. The reality is many astronomers know that the IAU decision was politically motivated and done surreptitiously in a way that violated the group's own bylaws. Rather than accept it, most planetary scientists are simply ignoring it. Most of the 424 IAU members who voted on the 2006 resolution are not planetary scientists but other types of astronomers. Why should they and not those who study planets be the ones who determine what a planet is?

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