And will the Moon soon be one too? As mentioned in my post Size – Solar System, Pluto lost its planetary designation over ten years ago. In 2006, the International Astronomical Union demoted Pluto from planet status to that of Kuiper belt object. Pluto didn’t meet all of their criteria to be called a planet. While it orbits the Sun without being the moon of another object, and it has enough gravity to form itself into a spherical shape, without having so much that it ignites a fusion reaction like a star — two of their criteria — it hasn’t cleared its orbital zone of most other bodies — their third criterion. There are a lot of other trans-Neptunian bodies out there, especially in the region called the Kuiper Belt. Pluto was relegated to the status of just another Kuiper Belt object (KBO). I agreed with them. Even though their criteria are incomplete and somewhat arbitrary, I think Pluto should be grouped with the other KBOs, rather than with the major planets. Rather than having a nearly circular orbit on or near the plane of the ecliptic, it has a very elliptical orbit canted at 17.16 degrees to the orbits of the planets. I think it should be called a minor (dwarf) planet, like Eris, another trans-Neptunian object, or Ceres, the largest asteroid belt object. Many people disagree with me, though. When the International Astronomical Union demoted Pluto, a great howl went up in defence of the little planetoid. Now there is a movement afoot to change the definition of planet so Pluto can regain its previous status. If they are successful, then there could be more than a hundred more planets added to the Solar System.
The key change the team is hoping to get approved is that cosmic bodies in our Solar System no longer need to be orbiting the Sun to be considered planets — they say we should be looking at their intrinsic physical properties, not their interactions with stars.
They want each body to be assessed on its own attributes, and not its relationship with other bodies. So what it orbits or how it does so would not come into it. Based on this, the Moon could become a planet, as could many of the moons of other planets. Here is their definition:
A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.
Put simply, anything massive enough to be round, but not as massive as a star. Anything up to and including what were formerly called brown dwarf stars (About 15-75 times as massive as Jupiter. See my post Size – Stars.) This would include a large and growing number of objects in the Solar System, and in other extrasolar systems, as well as any qualifying bodies that are not associated with any star. Those so-called “rogue” planets.
What do you think? Is it worth all that just to get Pluto its planetary status back?
rjb
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