A good reason why Ceres should be regarded as a planet is because this would fit nicely with the Titius-Bode law - the observation that the semi-major axes of the planets correspond closely (though not perfectly) to the equation a = 4 + x, where x is the sequence 0, 3, 6, 12, 24… a = 10 is the semi-major axis of Earth, i.e., the astronomical unit. To my knowledge, this is purely an empirical observation - there is no theoretical explanation for why the orbits should behave that way. But as a matter of fact they do, and Ceres fits the sequence.
Sure, and that then would include Pluto and Eris?
But I thought it was the whole belt not just ceres? Oddly Vesta is brighter than Ceres.
Pluto would be there. And as for Ceres, sure, you could replace it with another body in the Belt, such as Pallas or Vesta - the “law” is only an approximation anyway, and the error (difference between actual semi-major axis and the one predicted by the “law”) isn’t much larger for them than it is for Ceres.
Arguably, it’s the whole asteroid belt, and not just Ceres or the other major inhabitants which comprise that entry in the Rule.
That’s what I thought.
The Titus-Bode fails miserably with respect to Neptune. It predicts, 38.8 AU, but Neptune’s semi-major axis is only 30 AU. It’s even worse with respect to Pluto, unless you just ignore Neptune, as Pluto’s semi-major axis is 39.5
The Titus-Bode law wasn’t the first attempt, either. Kepler tried a scheme based on Platonic solids and spheres nested inside of each other. But of course, being based on Platonic solids, that one couldn’t accommodate planets beyond Saturn at all.
I think it’s just that humans gonna human, and we’re so good at pattern-matching that we’ll find patterns even when there aren’t any. With all of the possible mathematical rules, there’s bound to be something that’ll work, especially if you allow dodges like “well, there’s nothing planet-sized there, but there should be, so we’ll count something in the right vicinity as a planet anyway”.
Which is why astronomers were looking for a planet and found the asteroids, isn’t it?
We can just ignore Pluto. It is smaller than people think.
But the point is that in order to make Titius-Bode work, we’d have to ignore Neptune (and count Pluto as the next planet after Uranus), and Neptune is pretty big. That’s why the discovery of Neptune was the primary argument refuting the law, which hasd previously been widely accepted by astronomers.
Right. Pluto is so small it proved nothing, but ignoring Neptune is an issue.
Pluto is kinda two large asteroids orbiting each other. That made it appear planet sized.
More or less. There was a group of astronomers who got together and decided to look for a planet between Mars and Jupiter. They’re usually called the Celestial Police. But someone else beat them to it and found Ceres before they did. They did find the second, third and fourth asteroids.
No it doesn’t. Pluto does present a very small disc in larger telescopes, but its moon does not make it look like a larger disc. Instead, its moon, Charon, was discovered because it periodically forms a bulge on one side of the disc and then the other.
There was, for some time, a belief that Pluto was actually very large, and only looked small for some reason (for instance, a highly-spherical glossy surface, so only a small portion of it reflected back towards us). This was back when it was still believed that it had a significant gravitational influence on other planets (the search for Pluto started with some supposed discrepancies with Neptune’s orbit, which turned out to be merely measurement errors). Neptune itself had been (successfully) detected in the same way, based on discrepancies in Uranus’s orbit, so it was a reasonable thing to try.
I wonder how much @DrDeth meant “appear” in the sense of “seems” due to its gravitational influence, not so much in the literal sense of “visual impression through a telescope”.
Before being found visually it was being searched for using all the tech of the day. Which includes calculations of second order effects like gravitational perturbations.
Now as @Chronos said just above, the idea that what was later found to be Pluto+Charon was causing meaningful perturbations to Neptune turned out to be factually wrong. But by happenstance it was a lot of what led folks to look with telescopes in the region of sky where Pluto+Charon was eventually literally seen & thereby found.
I haven’t read that anyone other than Tombaugh was actually looking for the planet at the time it was discovered. So I don’t know who these other folk would be.
As far as predictions, Lowell made two predictions and another guy at Harvard (Pickering) made a couple others. And it turned out that Pluto was found within about 5 degrees of one of Lowell’s predictions. However, Tombaugh did not use those predictions, but rather just made a systematic search.
At any rate, the reason Tombaugh discovered it, and did not find any of the other TNOs, was because Pluto is very much brighter than any other body that far out. By over two magnitudes, which is quite a bit. And he did keep looking for some years after finding Pluto.
Or that Pluto was small, but superdense. Once Charon was found Pluto’s mass could be measured directly (instead of inferred from its supposed effect on Neptune) and all the theories about why it could be massive but seem small were no longer needed
The total mass of the Asteroid Belt is about 4% of the Moon’s mass, so not much of a planet to satisfy Bode’s Law. Not sure, but it could have been more substantial in the past, until eons of Jupiter’s gravity thinning it out.
Both, actually. I thought Charon made Pluto appear brighter, but I guess that was wrong.
Yes, started as 4x earth mass, then earth mass, then got smaller and smaller, until it is currently kinda tiny.
How big could a TNO be without us (yet) knowing about it? Mars-sized? Earth-sized? Neptune-sized? Jupiter-sized? Bigger?