The Long Piece about Defining "Planet"
"Why's Pluto not a planet any more?" ... it means 20 other things would be too, and 100 later on ... "But what's wrong with 100 planets?"
100 planets isn't really a) teachable, especially to young children, b) able to be memorized, and most importantly, c) reflective of the classes of objects we see now.
Those classes of solar system objects, with their rough distances to the Sun in "astronomical units" (in units of the Earth's average distance to the Sun), are pretty widely accepted among astronomers to be:
Note that this class of "Trans-Neptunian Objects" is brand new. We discovered the first TNO, Pluto, in 1930, but didn't discover another until 1992. Sedna is also new. Note also that we strongly suspect there's something else between the TNOs and the Oort Cloud besides Sedna. We just can't see it yet. It's too dim.
Well, OK. Knowing that, what do you think the definition of "planet" should be? You could pick out all the round-ish objects from there, but Alan Taylor showed that was a truckload. (And that was a few years ago now. And also, what counts as round, exactly? and how do you prove it? do you have to see 20x20px? 50x50?)
You could say "anything bigger than X" but what's X? and why? "Anything bigger than Pluto" leads to the 100+ planet problem in 100 years when we can see the whole Oort cloud. "As big as Mercury", maybe - but why Mercury? Just because it was known a long time? Seems a bit arbitrary. Plus, we don't really KNOW exactly how big the TNOs are right now, not unless they have moons. Long story.
Even the IAU definition - "round balls that have 'cleared their neighborhoods'" - is a bit iffy. Place Earth where Pluto is, and it's not a planet any more, by that definition. Earth could not have kicked out all those TNOs from that neighborhood by now. They're too far apart.
Other definitions - "those planets known to the ancients", "whatever a particular generation thinks of them" - don't really help us think about this modern version of the solar system.
And any of those doesn't really capture the really interesting part about Pluto. It's just one of many objects a lot like it: cold, icy, never very big, and in these chaotic or resonant screwed-up orbits that all intersect and play off each other and Neptune.
Pluto's not even clearly the most interesting TNO, necessarily; maybe not even the 2nd most interesting one. Eris is bigger, and in a more odd orbit. And Haumea's got a really wacky shape & spin. Pluto's interesting because it was first, it's relatively close & visible, it's a binary (Charon's nearly as big as it), and we have a spacecraft going there.
In the end, I think I agree with Alan Taylor, and astronomers Neil deGrasse Tyson and Mike Brown. What's interesting about the Solar System is its varieties, not our definitions. There's just a big pile of stuff out there that we can break up into interesting chunks, is all. Drawing a thick line between Mercury and Eris, or between Pluto and Makemake, because of our own history, seems awfully arbitrary.
Worse, it cuts off fascinating areas of the Solar System from our childrens' - and therefore most of the population's - education. Most people don't know how fascinating the giants' moons, Ceres, the big TNOs, the Oort Cloud, and the little whizzing asteroid and TNO bits everywhere are. I'd even argue that in some ways, things like Titan, Haumea, Sedna, and Triton are more fascinating than some of the planets half the world has memorized by name.
100 planets isn't really a) teachable, especially to young children, b) able to be memorized, and most importantly, c) reflective of the classes of objects we see now.
Those classes of solar system objects, with their rough distances to the Sun in "astronomical units" (in units of the Earth's average distance to the Sun), are pretty widely accepted among astronomers to be:
- the Sun
- 0.3-1.7 AU - the 4 inner terrestrial/rocky planets (+ The Moon and Phobos/Deimos, two captured asteroids),
- 1.7-4.5 AU - the asteroid belt
- 4.5-15 AU - the 2 gas giants + their moons + their assorted captured rocks and iceballs,
- 15-32 AU - the 2 ice giants + their moons + their assorted captured rocks and iceballs (and apparent captured "Trans-Neptunian Object", Triton),
- 32-75+ AU - the cold-as-hell, mostly icy, not-rocky and smallish "Trans-Neptunian Objects" (Pluto, Eris, Haumea, Makemake, etc) with the Kuiper belt,
- whatever the hell Sedna is (can be out to 975AU) and a few others are,
- 2000-50000 AU - the Oort Cloud, source of a lot of comets (mostly unseen as of yet),
- comets/asteroids/trojans/NEOs/centaurs and all the miscellaneous stuff that floats around across and between the various areas
Note that this class of "Trans-Neptunian Objects" is brand new. We discovered the first TNO, Pluto, in 1930, but didn't discover another until 1992. Sedna is also new. Note also that we strongly suspect there's something else between the TNOs and the Oort Cloud besides Sedna. We just can't see it yet. It's too dim.
Well, OK. Knowing that, what do you think the definition of "planet" should be? You could pick out all the round-ish objects from there, but Alan Taylor showed that was a truckload. (And that was a few years ago now. And also, what counts as round, exactly? and how do you prove it? do you have to see 20x20px? 50x50?)
You could say "anything bigger than X" but what's X? and why? "Anything bigger than Pluto" leads to the 100+ planet problem in 100 years when we can see the whole Oort cloud. "As big as Mercury", maybe - but why Mercury? Just because it was known a long time? Seems a bit arbitrary. Plus, we don't really KNOW exactly how big the TNOs are right now, not unless they have moons. Long story.
Even the IAU definition - "round balls that have 'cleared their neighborhoods'" - is a bit iffy. Place Earth where Pluto is, and it's not a planet any more, by that definition. Earth could not have kicked out all those TNOs from that neighborhood by now. They're too far apart.
Other definitions - "those planets known to the ancients", "whatever a particular generation thinks of them" - don't really help us think about this modern version of the solar system.
And any of those doesn't really capture the really interesting part about Pluto. It's just one of many objects a lot like it: cold, icy, never very big, and in these chaotic or resonant screwed-up orbits that all intersect and play off each other and Neptune.
Pluto's not even clearly the most interesting TNO, necessarily; maybe not even the 2nd most interesting one. Eris is bigger, and in a more odd orbit. And Haumea's got a really wacky shape & spin. Pluto's interesting because it was first, it's relatively close & visible, it's a binary (Charon's nearly as big as it), and we have a spacecraft going there.
In the end, I think I agree with Alan Taylor, and astronomers Neil deGrasse Tyson and Mike Brown. What's interesting about the Solar System is its varieties, not our definitions. There's just a big pile of stuff out there that we can break up into interesting chunks, is all. Drawing a thick line between Mercury and Eris, or between Pluto and Makemake, because of our own history, seems awfully arbitrary.
Worse, it cuts off fascinating areas of the Solar System from our childrens' - and therefore most of the population's - education. Most people don't know how fascinating the giants' moons, Ceres, the big TNOs, the Oort Cloud, and the little whizzing asteroid and TNO bits everywhere are. I'd even argue that in some ways, things like Titan, Haumea, Sedna, and Triton are more fascinating than some of the planets half the world has memorized by name.