Is Jupiter a planet?

[Stu]

5 hours ago, badhex said:

I admit I was very sad at Pluto's demotion. I saw a cool (if you like that kind of thing) graph somewhere before that plots planetary mass against combined mass of all the other stuff in its orbit, and basically it does show a significant line in the sand between what we now deem to be planets vs dwarf planets.

Regarding lagrange points I am pretty sure they are included in the definition - mainly because anything caught there is due to, not in spite of, the planet's gravity. 

I could be wrong about this but that's how I understood it. 

I have just looked up the definition and confess that I still find the last point to be ill defined and ambiguous. Not least because there seem to be two of them….

1. A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

Or

1. It must orbit a star (in our cosmic neighborhood, the Sun).
2. It must be big enough to have enough gravity to force it into a spherical shape.
3. It must be big enough that its gravity has cleared away any other objects of a similar size near its orbit around the Sun.

I think I know what they are trying to say, but the last point just seems so sloppy for something that is key to saying which is a planet and which isn’t. There are many objects which fulfil the first two criteria, so the last point is the vital one really.

What does neighbourhood mean? Is that just clearing the same orbit for a certain distance away either side? Could there be another Jupiter sized planet on the opposite side of its orbit and they are both planets? Or does it mean a range of orbits inside and outside of the planet?

It also says of a similar size. How similar? Not as similar as our Moon for instance but what, half the size? What does it mean?

I know I’m being Mr Pedant from Pedantic street, Pedantsville, but this is supposed to be a scientific definition, no? My main point is that it should be unambiguous if and when they come across more solar system bodies in future which need classifying, and I’m not sure it is.

I’ve often thought that the Galilean Moons for instance are effectively planets. They were born out of the same dust disk as Jupiter, rather than the rest which are effectively captured asteroids as far as I’m aware.

Please feel free to put right any misunderstandings I have, happy to be corrected 👍👍

[Mr Spock]

A few dwarf planets 

[Ags]

I learned today that all planets except Saturn and Mercury have trojans. I suppose Mercury is too close to the Sun, while Saturn's L4 and L5 are unstable, I guess because of Jupiter's influence.

Apparently the mass ratio for a nice stable L4/L5 is 1:100:10,000, with the ratios going up with every additional confounding body. So Jupiter is the only body in the solar system that gets close to holding up it's side of a Trojan triad.

Edited November 27, 2023 by Ags

[Mr Spock]

Yep, the Earth definitely needs a lot of Trojans... 

[Zermelo]

18 minutes ago, Mr Spock said:

A few dwarf planets 

 

Apparently 2007OR10 goes by the name of "Gonggong", which I had thought to be a giant panda loaned by China to Huish Episcopi Zoo.

[globular]

If aliens ever pop by to say hello I suspect our first argument with them will be over this. 
"What do you mean you're on the 3rd of 8 planets? Can't you count you fools there are way more than 8."

[Ags]

Wackypedia tells me that the Neptunian trojans are probably even more numerous than the Jovian trojans - possibly by an order of magnitude.

Edited November 27, 2023 by Ags

[badhex]

1 hour ago, Stu said:

I have just looked up the definition and confess that I still find the last point to be ill defined and ambiguous. Not least because there seem to be two of them….

1. A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

Or

1. It must orbit a star (in our cosmic neighborhood, the Sun).
2. It must be big enough to have enough gravity to force it into a spherical shape.
3. It must be big enough that its gravity has cleared away any other objects of a similar size near its orbit around the Sun.

I think I know what they are trying to say, but the last point just seems so sloppy for something that is key to saying which is a planet and which isn’t. There are many objects which fulfil the first two criteria, so the last point is the vital one really.

What does neighbourhood mean? Is that just clearing the same orbit for a certain distance away either side? Could there be another Jupiter sized planet on the opposite side of its orbit and they are both planets? Or does it mean a range of orbits inside and outside of the planet?

It also says of a similar size. How similar? Not as similar as our Moon for instance but what, half the size? What does it mean?

I know I’m being Mr Pedant from Pedantic street, Pedantsville, but this is supposed to be a scientific definition, no? My main point is that it should be unambiguous if and when they come across more solar system bodies in future which need classifying, and I’m not sure it is.

I’ve often thought that the Galilean Moons for instance are effectively planets. They were born out of the same dust disk as Jupiter, rather than the rest which are effectively captured asteroids as far as I’m aware.

Please feel free to put right any misunderstandings I have, happy to be corrected 👍👍

I agree, it does feel a bit 'loose' for a scientific definition. I think part of the reason for this is that nature tends to gives us spectrums rather than hard stops. To the questions (btw I am absolutely not an expert!):

- The neighbourhood means approximately the same non-resonant orbit. I suspect the size of the neighbourhood depends on the mass of the body. With sufficient gravity in play, anything nearby in its own orbit will either be ejected from the orbit or accreted, or maybe turned into a satellite of the body (or sucked into a lagrange point, which counts as cleared). Not sure if you could have two Jupiters opposite each other, but if they were, they'd be in each other's L3 lagrange points, so technically cleared, I guess? 

- I am not sure about this similar size thing. In general, the objects being cleared are dramatically smaller than the body itself even when combined, and this is where the hard definition lies: the planetary discriminant µ, which is the ratio of the mass of the body vs the combined mass of all the neighbourhood objects is significantly large (>100:1) for all planets except Pluto and Eris and the other dwarf planets.

I found the graphs I was on about which makes it very obvious when you plot the planetary discriminant vs various other properties like mass or diameter, or just on their own even:

[Paul M]

3 hours ago, Mandy D said:

I believe I was told my physics teacher in school that an 8 inch telescope was required for visual observation of it at the time. I now have a 12 inch and it is still not big enough, today. At least you have imaged it, whereas for me it remains on my list, having imaged all the planets in our solar system.

When I finally got my first proper scope, the Fullerscope's 6.25 Newt, about 1983, Pluto was closer than Neptune and relatively accessible in the area between Arcturus and Spica (had to check back on that, my memory isn't that good!). But it was still mag 13.7.

I remember sweeping for it, forlornly, in the knowledge that I will have seen it's location if not its faint light. 

It's nearly hopeless now. Rarely is my southern sky good enough to even image it.

Anyway, Pluto Schmuto, I've since bagged my second TNO, Makemake earlier this year.

 

[Ags]

13 minutes ago, Paul M said:

Makemake

Impressive!

[Paul M]

43 minutes ago, Ags said:

Impressive!

It sounds impressive but, it don't look impressive. Thanks to @Xilman for pointing me in that direction. 

It's an interesting area for those, like me, who's deep sky images always look like a child has thrown a tantrum in the art room 🤣

 

 

[badhex]

I did a bit of digging on whether a planet could inhabit another planet's (well, planet/star) lagrange points and it turns out yes, with caveats. L1, L2 and L3 are not stable enough but L4 and L5 are both theoretically possible, and now we may even have proof:

https://earthsky.org/space/2-planets-share-an-orbit-photo-pds-70/#:~:text=July 23%2C 2023-,Can 2 planets share an orbit%3F,orbit appears to be true!

Edited November 28, 2023 by badhex

[Mandy D]

@badhex it is well known that L4 and L5 are stable, whereas the other three are not. JWST is at the Sun-Earth L2 point beyond the Moon's orbit and has to orbit that point to avoid falling into the Sun's's gravitational well. This is known as a halo orbit.

[badhex]

14 minutes ago, Mandy D said:

@badhex it is well known that L4 and L5 are stable, whereas the other three are not. JWST is at the Sun-Earth L2 point beyond the Moon's orbit and has to orbit that point to avoid falling into the Sun's's gravitational well. This is known as a halo orbit.

Yes, understood, but typically we talk about maintaining objects with relatively small masses in lagrange points - the question from Stu was specifically whether two planets of a similar size could share an orbit. I did not think L3 would be stable enough, but it turns out that L4 and L5 can in theory support a planet sized object sharing an orbit with another planet sized object.

[Mandy D]

@badhex Sorry, for getting off track. Yes, it is the fact that L4, L5 are stable Lagange points that allows planetry mass bodies to exist there. L1, L2, L3 are like trying to balance a pencil on it's point.

[globular]

24 minutes ago, badhex said:

L4 and L5 can in theory support a planet sized object sharing an orbit

Planet sized maybe - but it can't be called a planet.... because it hasn't 'cleared it's neighbourhood' - being held in the other 'planets' lagrange point means it isn't dominating the neighbourhood.

Edited November 28, 2023 by globular

[Franklin]

I was brought up to believe that Jupiter was "King" of the planets and after the Sun it dominates the solar system. I also read that if Jupiter was any larger it might even have "switched on" and become a star itself, making a binary system with the Sun.

[badhex]

1 hour ago, Mandy D said:

@badhex Sorry, for getting off track. Yes, it is the fact that L4, L5 are stable Lagange points that allows planetry mass bodies to exist there. L1, L2, L3 are like trying to balance a pencil on it's point.

No worries - yes, and now I come to think - the whole Planet X theory was disproven years ago, which is essentially what we're talking about. I don't know why I didn't think of that. 

[badhex]

1 hour ago, globular said:

Planet sized maybe - but it can't be called a planet.... because it hasn't 'cleared it's neighbourhood' - being held in the other 'planets' lagrange point means it isn't dominating the neighbourhood.

True enough. Though, if one were playing devil's advocate and say, both planets were equal in mass, then they would both be holding each other in their Lagrange points, ie planet A would be in planet B's L4 and planet B would be in planet A's L5. In that potentially unlikely scenario, we would not be able to say which planet was dominant 😅

[badhex]

2 hours ago, Franklin said:

I was brought up to believe that Jupiter was "King" of the planets and after the Sun it dominates the solar system. I also read that if Jupiter was any larger it might even have "switched on" and become a star itself, making a binary system with the Sun.

I think it needs to be at least 90 times more massive (which is not necessarily that much more massive in comparison with the most massive stars) in order to begin nuclear fusion of hydrogen and become a true star, but it only needs to be 13 times more massive to be able to fuse deuterium and become a brown dwarf - which is a weird grey, or brown, area between gas giants and true stars.

Edited November 28, 2023 by badhex

[badhex]

Can anyone tell I'm really looking forward to studying Astrobiology next year? 😂

[globular]

5 minutes ago, badhex said:

we would not be able to say which planet was dominant

Under current 'rules' I think neither would be a planet.  But more likely we'd come up with some new rules that fit what we 'want' if such a system existed.

Rogue planets and exoplanets don't need to fulfil the 'cleared their neighbourhood' rule, so why solar system ones?  But then I guess you could argue that we can't measure things that well for other systems.

As I hinted at earlier - it would be interesting to see what an alien race (or rather us but with without the historical baggage of our gradual technological improvements and, most importantly, our local bias) would come up with as a definition.

[badhex]

51 minutes ago, globular said:

Under current 'rules' I think neither would be a planet.  But more likely we'd come up with some new rules that fit what we 'want' if such a system existed.

Rogue planets and exoplanets don't need to fulfil the 'cleared their neighbourhood' rule, so why solar system ones?  But then I guess you could argue that we can't measure things that well for other systems.

As I hinted at earlier - it would be interesting to see what an alien race (or rather us but with without the historical baggage of our gradual technological improvements and, most importantly, our local bias) would come up with as a definition.

Yes, I mean we weren't even that hard and fast on what constituted a planet until 2006, and as the excellent discussion on here proves, not everyone agrees that it was the correct decision. I guess we have to start somewhere, hopefully it will evolve as we learn more and become less of a loose definition. Like with brown dwarfs, there is a lot of grey area to tighten up. 

One more thing that didn't note earlier: apparently there had been a motion to make dwarf planets a subcategory of Planet which members of the IAU voted against. I'm not sure why I was not accepted but that really does seem like an odd choice. 

[Mandy D]

3 hours ago, Franklin said:

I was brought up to believe that Jupiter was "King" of the planets and after the Sun it dominates the solar system. I also read that if Jupiter was any larger it might even have "switched on" and become a star itself, making a binary system with the Sun.

Yes, Jupiter so dominates the planetary side of the Solar System, that it's barycentre with the Sun is outside of the Sun's body. It is the only one to do that. So, do we have to declassify Jupiter as a planet, as it is not technically orbiting the Sun, rather the pair orbit a common point? 😁

[MalcolmP]

53 minutes ago, Mandy D said:

as it is not technically orbiting the Sun, rather the pair orbit a common point? 😁

Well, actually, the earth and sun combo rotate about a barycenter, but because of the mass difference that is so very near the sun center. that only a pedant would say that, so I'll get my coat >>  : )  !

Re: Jupiter ignition. When I were a lad it was said (Eddington ?) that Jupiter was just on the edge of being massive enough to become a star.
Time went on and Hoyle et al revised that to Jupiter needing to be (I think) 11 times more massive. Now it seems that figure could be anywhere in the range 13 - 90x thanks @badhex  very interesting.