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Figure 1. Dwarf planets Pluto and
Ceres. (c) Wikimedia
Is the Earth a Dwarf Planet?
IntroductionWith the discovery of over 1000 NEOs, it's getting hard to say Earth fulfills the third part of the definition of a planet, that it has cleared its orbit of debris. Same for Mars. To be ridiculously logical about it, this means that Earth and Mars ought to be demoted to dwarf planet status.
Actually, this article is just an excuse for showing some cool pictures of near-Earth objects. Happy Halloween!
Section 1: Definitions
Section 2:I don't think they will demote Earth and Mars to dwarf planets because of this. Instead, I think they will have to change the definition again soon. Perhaps the IAU will go with the degree of roundness or the surface gravity, which, depending on the number, might make Ceres, and possibly Pluto, eligible for planet status again. Ceres was considered a planet in the early 1800s, shortly after it was discovered.
Section 3: Getting Back in the ClubIt's pretty much impossible for Pluto to regain its planet status, unless you also let Ceres back into the club.4,5 If degree of roundness is used to set membership, then Pluto and Ceres will have to wait for photos from the New Horizons and Dawn probes, before their status can be determined.
Section 3: The Simplest FudgeOr, one could fudge this in the simplest way, and say that nothing below the diameter of Mercury is a planet. Probably they would want to set the definition about 10 km below the present diameter of Mercury, just in case Mercury is shrinking.6
There is some evidence for tectonic forces shaping the surface of Mercury at this time, besides the effects of meteor impacts. It is not clear yet what these forces are, or how they operate, only that they are not identical to Earth's plate tectonics.
Section 4: ConclusionsThis is just my Goofy prediction (Sorry, Pluto!). I think that once the New Horizons probe reports back with detailed pictures of the surface of Pluto, there will be tremendous pressure to let it back into the club of planets. I think the IAU will cave, and re-admit Pluto.
But I also think there is no way to re-admit Pluto, and keep Ceres out of the club. So I think the textboks after 2018 or 2020 will say the Solar System is a 10-planet system.
Epilog:NASA will maneuver Dawn away from Vesta, and over to Ceres, next year. We will get our first good look at Ceres in 2012. The New Horizons probe will fly past Pluto in 2015. What New Horizons will see could hold plenty of surprizes. I think the IAU will not reconsider the definitions of 'planet,' and 'dwarf planet,' until they have some more solid data on Pluto.
Pluto may have some tricks up its sleeve, but if the space probes come through, we will all be in for a treat.
Funny Pictures from Orbit