why we don't consider pluto as a planet?
Answers
Pluto was discovered because Neptune’s orbit showed an anomaly that indicated that there was something else out there. So they looked and eventually Clyde Tombaugh discovered Pluto.
Right from the start, Pluto was a weird one. Its orbit was not on the plane of the solar system, and it crept inside Neptune’s orbit from time to time. As we got better telescopes in the 1970s, we found Pluto’s moon Charon. And from that it was possible to estimate Pluto’s mass with some accuracy. It was a lot smaller than expected. So, astronomers started to look for other things out there. Ceres was already known, but it was stuck in the asteroid field and thus sort of ignored.
In the mid 1990s, astronomers began to discover a lot of new things out there, in the outer solar system. They were planet-like and small, like Ceres, but they had weird orbits, like Pluto. By the mid-2000s, they had discovered Eris, Sedna and Quaoar.
Astronomers started to realise that they needed to do something about it. To be honest, Pluto behaved a lot more like Sedna, Eris (which was even more massive than Pluto), Quaoar, Haumea and Makemake … wait a minute? Had we discovered more of these things?
Well, anyway, either they would have to introduce a new concept for these new things discovered, or the solar system would have 17 planets – hey, where did Orcus and Salacia come from? And what the heck is a planet anyway?
So astronomers started to discuss what a planet actually was, in order to classify the 19 planet-like things – 19? 19! Now joined by Ixion and Varuna, and 2002TC302, I mean 20 things… oh gods, this is really getting out of hand.
Eventually the International Astronomical Union (IAU) agreed on the following definition of a planet:
1. It must orbit the Sun.
2. It must have enough mass to reach hydrostatic equilibrium – i.e. its gravity makes it spherical.
3. It must have cleared the neighbourhood of its orbit.
Those that fit all three criteria would be planets, and those that fit only the first two would be dwarf planets or possibly minor planets.
Right, so there were nine planets and a dozen-plus dwarf planets. And everyone sighed with relief.
Until someone said, “you know, Pluto behaves a lot more like those dwarf planets than a planet. And one of the dwarf planets is even bigger than Pluto. So if Eris is not a planet, then clearly Pluto can’t be either.”
So the debate ran wild for a while, and eventually IAU not only agreed on the definition above, but also that Pluto did not satisfy the third criterion and thus was a dwarf planet. Because it has a weird orbit that even crosses Neptune’s orbit, and really can’t be said to have cleared its neighbourhood, Pluto didn’t fit in.
Pluto actually doesn’t care, but orbits the Sun as it always has. But among the hairless apes on the third planet that really like to categorise things, the debate still rages – mostly among those on the home continent of Clyde Tombaugh, because they have no longer discovered a planet.