I’m not sure there’s a good answer to that. Different scientists are looking for different things. For some purposes, the classification of something as a planet doesn’t really matter. For young stars with protoplanetary disks, we might be looking at clumps that one day will become planets, but don’t meet any of the requirements to be called one - yet.
I’d say the most interesting characteristics of exoplanets we are currently looking for would be location in the habitable zone, rocky, and with an atmosphere. Other than that, just collecting data on thousands of different solar systems will really help us understand how they form, whether ours is in any way unique, how many earth-like planets there are, etc. Until very recently, we only had a sample size of one solar system, so pretty much all data we collect about extrasolar planets is very useful.
No, there have always been 8 planets. We just didn’t realize it until the 2000’s when we found that Pluto belonged to an entire class of previously undiscovered bodies.
And tomorrow we might find that there have always been 9 planets, because there is probably a Neptune sized object out in the Kuiper belt that we have not directly discovered. And if we do, it might turn out that it fits into a completely different category and we’ll need a new name.
That’s how science works. You must always be ready to change your mind if the evidence warrants it. Nothing is ‘proven’ - there are just varying levels of evidence for different propositions.
I don’t feel that has a lot to do with the (perhaps banal) point I was trying to make - which was that “rock” and “ice” aren’t necessarily conceptually different things - they’re just the solid phase of various minerals - so that the terms ‘rockball’ and ‘iceball’ (were we to adopt them) are pretty human-centric.
I completely agree on your point on clades and categories - it’s the same story as when we try to classify organisms or the colours of the visual spectrum.
Apropos of nothing much, this thread also seems like the right place to ask a question I have often wondered about, but never really seen discussed; Would you say that it’s true that the Solar system is perfoming (or has performed) a sort of fractionating process? - it seems to me that solar radiation and solar wind have somewhat differentiated the matter of the planets by some function of density and volatility.
“Planet” is an Anglicisation of the Greek word “planetos” (transliterated, obviously), which ancient Greek astronomers used to describe objects they could see in the night sky and which changed position in the night sky on a human timescale. “planetos” means “wanderers”.
Not all that much help, since it excludes the Earth and anything that can’t be seen from Earth with the naked eye and it includes the sun and the moon. It made a lot of sense to the ancient Greeks - it described what they could perceive as a distinct group of things different to other things - but it doesn’t make sense with modern knowledge.
I think the most useful approach is to;
Use more categories. Dwarf planet/very big asteroid, rocky planet, ice giant, gas giant and anything else that seems useful.
Ditch the idea of a more accurate classification being a “demotion”, along with the idea that it’s unique to Pluto (Ceres was initially classified as a planet and other objects similar to Pluto have never been classified as a planet).
Although I like the suggestion of just calling them all “spaceballs”
The single most important piece of data about a celestial object is its mass. Second most important is probably density, but you need the diameter to calculate that, so put them as a tie for #2. After that, it’s hard to say.
One thing to note about these discussions (this is approximately the 34,098th discussion of Pluto/“what is a planet” since they discovered the second TNO in 1992) is that hardly anyone notes that the IAU defiition of a planet is not a scientific one. Yes, it’s dressed up in scientific trappings, but the reason they came up with it is bureaucratic.
Anyway, since mass is the most important piece of info, I propose that the definition be based purely on that quantity:
A planet is a body of less mass than a brown dwarf and more than 10[sup]23[/sup] Kg.
It’s a very simple definition and has the advantage that it can be applied to any stellar system, or even freely floating objects between stars.
This is incoherent. I can see the argument that, in 1931, Pluto was a planet but Eris wasn’t, because Pluto was known at the time but Eris wasn’t. But what’s the argument that, in 1929, Pluto was a planet but Eris wasn’t? If you had told someone in 1929 that he definition of “planet” was “a thing that people in 1931 think is a planet”, why would they not have laughed at you?
On further thought, let’s add another cateogory. Get rid of the dwarf planet category and create another one called Almost Planets. These will be bodies with between 10[sup]22[/sup] and 10[sup]23[/sup] Kg. Almost Planets would include all the other large satellites plus Pluto and Eris.
Let’s see. The average American weighs 180 pounds. Ganymede has a surface gravity of .146g, so he or she would weigh 26 pounds there. If they were on Pluto, they would weigh 11 pounds. Somehow dtilque’s cutoff doesn’t sound too bad to me.
ETA: case for higher cutoff: Ganymede has only a trace atmosphere.
The presence or absence of an atmosphere is not an criterion I have for planethood. Not to say atmospheres are not important, but they’re a secondary characteristic and dependent on a number of factors besides the total amount of mass collected in one body.
As for the cutoff, I’m not married to 10[sup]23[/sup]. If you want to eliminate the three satellites, double it to 2×10[sup]23[/sup] and there’ll still be 8 planets. I’d be happy with either cutoff point.
The so-called “clearing its orbit” requirement is asinine. Jupiter hasn’t cleared its orbit; there are Trojan asteroids sharing it. Other planets have their version of Trojans. People seem to have a hard-on about what a “right” amount of planets should be. Then they make rules to exclude items that upset their sense of right. There is no right.
The “clearing the orbit” part of the IAU definition is widely misunderstood, which makes the definition bad from a public relations POV. Probably a better way of putting it would be “dominating the orbit”, but even that would sometimes be misunderstood (and lead to jokes about domination/submission) so still not the best. Which is why a simple definition, such as the one I suggested above based on total mass, would be better.
