Comment by Symmetry | Hacker News Reader

Comment by Symmetry | Hacker News Reader

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It's in the nature of science that sometimes things that were previously believed to be true turn out to be false. The notion that there are nine planets matched what astronomers knew for many decades but with better measurements turned out be be an unsupportable idea.

For centuries there were just 6 planets. Then people discovered Uranus and there were 7. Then people discovered Ceres and there were 8. Then people discovered lots of other main belt asteroids and there were 7 again. Then Neptune and 8 again. Then Pluto and 9. And now we've found all these other big bodies in the outer system and we decided to demote Pluto rather than add a half dozen new planets, just like we did with Ceres.

I don't think anybody particularly objected to the demotion of Ceres since it hadn't been around that long when it happened. But I can only imagine the furor that must have arisen when people tried to claim that Uranus was a planet despite the number of planets being fixed since the time of Homer. Well, Ok, there was the bit when they turned Earth into a planet and we all know about the people like Giordano Bruno literally burned at the stake over that.

So this demotion of Pluto is just another event in the long, long history of scientists revising what counts as a planet. I understand that it's uncomfortable to have to unlearn things you learned as a kid but science is all about changing our view of the world.

A great { essay / blog post } about this constant churn:

- [Ontological remodeling | Meaningness](https://meaningness.com/eggplant/remodeling)

My favorite bit:

> Jupiter doesn’t go around the sun, and therefore is not a planet by the 2006 definition.

> Don’t believe me? In Newtonian mechanics, two bodies orbit their barycenter, or center of mass. If they have equal masses, the barycenter is the midpoint between them. If one is heavier than the other, the barycenter is closer to it. If one has much greater mass than the other, their common barycenter is located within the larger body, and the smaller object goes around that point. Only then is the smaller body said to orbit the larger one. Otherwise, the two form a binary system.

> Jupiter is ludicrously heavy: it has 2.5 times the mass of everything else in the solar system combined, apart from the sun. The sun is much heavier still—but the barycenter of their mutual orbit is outside it. Jupiter and the sun are a binary system. Their barycenter is, to be fair, quite close to the sun, and informally it may be reasonable to say Jupiter goes around it. But in terms of the formal definition, it doesn’t, so by the IAU criteria, Jupiter is not a planet.

Well that depends on how you define what is inside versus outside the sun. The sun doesn't have a clear outer edge, it just becomes gradually less dense the further out you go. Earth and Jupiter are inside the heliosphere. These definitions are just arbitrary.

Imagine a less convenient example, e.g. a binary star system where the two stars are of roughly equal or similar mass (and the two stars aren't [somehow] 'right next to each other'). We'd say or write that the two are orbiting each other and that the center of their orbits is outside both.

If Jupiter was more massive then that would also be the case, i.e. the center of its orbit would be obviously outside the sun. Unless you're next going to argue that the entire universe is arguably 'inside the sun' because " it just becomes gradually less dense the further out you go".

Once you reach the heliopause it's pretty clear that you're completely outside the sun.

Are you claiming that it's perfectly reasonable to consider the entire solar system basically as 'inside the sun'?

That's a very non-standard usage.

I'm not sure why you would say that A isn't orbiting B if the barycenter of the orbit isn't inside B? We normally talk about Jupiter orbiting the Sun or Charon orbiting Pluto despite this not being true. I can see why someone might want that to be the definition for aesthetic reasons but I'm not seeing any evidence that it's the accepted definition. Wikipedia certainly says "near or within" instead of "within" in the most relevant section and I couldn't find anything clearer or contradicting that.

> I'm not sure why you would say that A isn't orbiting B if the barycenter of the orbit isn't inside B?

If A and B were roughly the same mass then you'd say they're orbiting each other. We don't tho say or write that about Jupiter in my experience.

Going further back, the moon was considered to be a planet, in orbit around the Earth, just as all planets were thought to be.

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