Is Jupiter a planet?

[Ags]

To qualify as a planet, a body must be round due to hydrostatic equilibrium, and it must have cleared its orbit of other debris. Jupiter satisfies the first criterion, but it has clouds of asteroids, the Trojans, preceding and following it in its orbit. These aren't small clouds of asteroids either, the number of Trojan objects rivals that of the asteroid belt. So, is Jupiter's orbit clear, or must we reclassify the big guy as a "dwarf planet"?

[badhex]

I'd have said that a dwarf planet does not have enough mass and therefore gravity to clear its orbit, whereas the Greeks and Trojans are hanging about in Jupiter's lagrange points *because* of its massive gravity, rather than it being a failure it clear its orbit of objects. 

Edited November 27, 2023 by badhex

[lunator]

Hi Agnes I think the IAU have a loophole on this one. All planets probably have material in their Lagrange  points, and if we compare the relative mass of the planet to the mass of the object Jupiter will be less 'dwarfish' than the others 😁

Cheers

Ian

[inFINNity Deck]

Funny, I happened to ask this same question last Friday to an astronomer who gave a talk about exoplanets. He just smiled and said that this still was a point of discussion, but that with "clearing" it was meant that all objects not being in Lagrange-points had to be cleared.

Even our rock already has two known Trojans: 2010 TK7 (300m diameter) and (614689) 2020 XL5 (1.2km diameter).

Nicolàs

[Ags]

Imagine a binary system consisting of a sunlike star and a red dwarf orbiting in the habitable zone... there could be a habitable planet caught in one (or both!) Of the lagrange points. The red dwarf would be a spectacular permanent morning or evening star!

[badhex]

That would be pretty cool, and given the size of the universe it probably exists, somewhere. I'd imagine that you'd have to have no other large bodies in the system in order to keep the planet from being pulled out of the lagrange point and/or ejected or smashed into, since it's only a relatively stable area to inhabit. 

[Paul M]

2 hours ago, Ags said:

Imagine a binary system consisting of a sunlike star and a red dwarf orbiting in the habitable zone... there could be a habitable planet caught in one (or both!) Of the lagrange points. The red dwarf would be a spectacular permanent morning or evening star!

There's a whole universe of ideas here!

I'd design the system with my planet in the Red Dwar's L2, I know, I know not stable enough, but this is artistic licence.. 😁

So the Red Dwarf would be permanently superimposed on the primary star. It would make for some interesting imagery.

I wouldn't want the planet's rotation tidaly locked either. No, a habitable planet with sun(star)rise and set. And butterflies, any alien world worth inhabiting would need butterflies.

 

[Mr Spock]

Jupiter could qualify as an unlit brown dwarf!

[badhex]

2 minutes ago, Mr Spock said:

Jupiter could qualify as an unlit brown dwarf!

Presumably an energy saving measure due to cost of living 😂

[Ags]

19 minutes ago, Mr Spock said:

Jupiter could qualify as an unlit brown dwarf!

A dwarf brown dwarf?

[Ags]

38 minutes ago, Paul M said:

There's a whole universe of ideas here!

I'd design the system with my planet in the Red Dwar's L2, I know, I know not stable enough, but this is artistic licence.. 😁

So the Red Dwarf would be permanently superimposed on the primary star. It would make for some interesting imagery.

I wouldn't want the planet's rotation tidaly locked either. No, a habitable planet with sun(star)rise and set. And butterflies, any alien world worth inhabiting would need butterflies.

 

I feel a science fiction novel coming on 😀

Because the habitable zone of a small red dwarf is so close to the star, I imagine we could squeeze a third habitable world in. Let's say it orbits just outside the red dwarf's habitable zone, so it would be a bit like Mars in our solar system if it only had the red dwarf's light and heat. Of course it would be in the sunlike star's habitable zone and it would be tidally locked to the red dwarf. One side of the planet would have a normal but slow day night cycle with the primary rising and setting as usual, while the side that is near to the secondary star would see the primary rise on blisteringly hot days with two suns, and then set on a freezing "red twilight" dominated by the glowering red dwarf.

Edited November 27, 2023 by Ags

[IB20]

Does that mean Earth isn’t a planet due to all the crud in its orbit?

[Stu]

I never liked that particular definition of a planet; it just seemed ambiguous and a way of demoting Pluto. I don’t think ‘clearing its orbit’ was defined very well either as Jupiter was always a clear example of this not being the case (no mention of Lagrange points in the definition as far as I’m aware??)

From the New Horizons mission I reckon Pluto is a pretty credible planet; it has five moons of its own, one of which is big enough to be roughly spherical so I think they got that call wrong. Doesn’t really matter though, I doubt Pluto loses much sleep over whatever we say about it 🤪.

 

[The60mmKid]

I wonder if anyone bothered to ask Jupiter.

Typical.

[Swoop1]

1 hour ago, Paul M said:

There's a whole universe of ideas here!

I'd design the system with my planet in the Red Dwar's L2, I know, I know not stable enough, but this is artistic licence.. 😁

So the Red Dwarf would be permanently superimposed on the primary star. It would make for some interesting imagery.

I wouldn't want the planet's rotation tidaly locked either. No, a habitable planet with sun(star)rise and set. And butterflies, any alien world worth inhabiting would need butterflies.

 

Paul,

I will quite happily go with your planet design proposal as long as Slartibartfast subs to do the fjords.

[badhex]

2 hours ago, Stu said:

I never liked that particular definition of a planet; it just seemed ambiguous and a way of demoting Pluto. I don’t think ‘clearing its orbit’ was defined very well either as Jupiter was always a clear example of this not being the case (no mention of Lagrange points in the definition as far as I’m aware??)

From the New Horizons mission I reckon Pluto is a pretty credible planet; it has five moons of its own, one of which is big enough to be roughly spherical so I think they got that call wrong. Doesn’t really matter though, I doubt Pluto loses much sleep over whatever we say about it 🤪.

 

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. 

Edited November 27, 2023 by badhex
extraneous not!

[Simone_DB]

1 hour ago, Ags said:

I feel a science fiction novel coming on 😀

Because the habitable zone of a small red dwarf is so close to the star, I imagine we could squeeze a third habitable world in. Let's say it orbits just outside the red dwarf's habitable zone, so it would be a bit like Mars in our solar system if it only had the red dwarf's light and heat. Of course it would be in the sunlike star's habitable zone and it would be tidally locked to the red dwarf. One side of the planet would have a normal but slow day night cycle with the primary rising and setting as usual, while the side that is near to the secondary star would see the primary rise on blisteringly hot days with two suns, and then set on a freezing "red twilight" dominated by the glowering red dwarf.

You made think about the sci-fi novel "the three body problem" by Liu Cixin, that I've read a few months ago. One cool depiction of the story was (SPOILERS !) about this inhabitated planet in a pseudo-stable orbit somewhere around Alpha Cen or another star of the triple system. People from there were biologically adapted to recurrent extreme conditions, which included periods near one of those stars, during which they completely dehydrated and rendered to some kind of "hibernated" dusty carpets, and epochs, at the contrary, when they were far from all the stars and had to cope with long glaciations. Very cool! I think he won both Nebula and Hugo awards.

[Mandy D]

@badhex I would say it is pretty clear that anything trapped at L4 and L5 are not obstructing the planet's orbit as they orbit the star with the planet. Hence, the planet's orbit is 'clear'. i.e. it cannot crash into bodies at those points as they have the same average angular velocity as the planet. Hence, I believe the modern definition of a planet is correct or, at least, acceptable.

[badhex]

18 minutes ago, Mandy D said:

@badhex I would say it is pretty clear that anything trapped at L4 and L5 are not obstructing the planet's orbit as they orbit the star with the planet. Hence, the planet's orbit is 'clear'. i.e. it cannot crash into bodies at those points as they have the same average angular velocity as the planet. Hence, I believe the modern definition of a planet is correct or, at least, acceptable.

Haha, I was trying to say the same thing, and realised upon re-reading I had said they *are not* included in the definition when in fact I mean they *are*. As you said and per my original post:

2 hours ago, badhex said:

because anything caught there is due to, not in spite of, the planet's gravity.

I've definitely read somewhere that the IAU essentially didn't specifically define planets that well because basically everybody knew what a planet was until Eris came along, which, given how similar it is to pluto, prompted the question of whether Pluto was indeed a planet after all; if Pluto was, then Eris must be - if Eris was not, then Pluto could not be either. Once more had been done to come up with a definition along the lines of this total-mass-of-satellites-in-the-same-orbit-vs-planet-ratio-thing (which I'm probably remembering incorrectly as evidenced by my highly technical description), and not including satellites trapped in L4 and L5 thanks to the body's gravity, it's pretty clear that there is a definable thing that has either happened or not happened, i.e. has the body cleared its neighbourhood or not, by the above criteria.? If yes, it's a planet, if not, it's not.

Edited November 27, 2023 by badhex

[badhex]

I could have just looked up clearing the neighbourhood on wikipedia instead of vaguely stabbing in the dark of my memory. I should have also added for clarity that as well as clearing the neighbourhood, it would need to also satisfy the previous two criteria of being in hydrostatic equilibrium and orbiting the sun, of course, but obviously we were only really debating the Trojans etc.

https://en.wikipedia.org/wiki/Clearing_the_neighbourhood

 

[Mandy D]

@badhex well, it's made for an interesting discussion. Yes, it has to meet all three criteria to be classified as a planet, but as you said, we were only discussing trojans at L4, L5. I'm perfectly happy for Pluto to be 'demoted' on this basis, but perhaps a little sad. I suppose the other thing up for debate, now, will be the satellites of the gas giants with Jupiter being expected to have 500 - 1000 or more. What happens when we get to that point? I'm fully expecting the 'planet' definition will be applied to bodies orbiting such large masses. I honestly cannot see anything that makes more sense. We really cannot go on adding pebbles to the collections of moons. There was similar debate around asteroids after the first few were found, so this is all following a rather predictable pattern.

[Paul M]

7 minutes ago, Mandy D said:

I'm perfectly happy for Pluto to be 'demoted' on this basis, but perhaps a little sad.

Yes, it's a cold,  almost heatless reality.

What saddens me most is that Pluto had already been demoted before I tracked it down for myself.

By the time I had a scope big enough, its declination and distance had put it beyond visual observation.

So I've got a few grainy images to call my own.

[badhex]

28 minutes ago, Mandy D said:

@badhex well, it's made for an interesting discussion. Yes, it has to meet all three criteria to be classified as a planet, but as you said, we were only discussing trojans at L4, L5. I'm perfectly happy for Pluto to be 'demoted' on this basis, but perhaps a little sad. I suppose the other thing up for debate, now, will be the satellites of the gas giants with Jupiter being expected to have 500 - 1000 or more. What happens when we get to that point? I'm fully expecting the 'planet' definition will be applied to bodies orbiting such large masses. I honestly cannot see anything that makes more sense. We really cannot go on adding pebbles to the collections of moons. There was similar debate around asteroids after the first few were found, so this is all following a rather predictable pattern.

Oh yes, no doubt - it really is sad. I was quite adamant at the time that I would continue to call and think of it as a planet. That said though, since then I have sort of changed my thoughts on the matter in that Pluto is really bloody awesome regardless, so it doesn't really make it 'less than' in my eyes.

Just after the change, one of my favourite geeky songwriters, Jonathan Coulton, released this tune written about Pluto from the perspective of Charon, which sort of sums it up:

 

[Mandy D]

58 minutes ago, Paul M said:

Pluto had already been demoted before I tracked it down for myself.

By the time I had a scope big enough, its declination and distance had put it beyond visual observation.

 

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.

Edited November 27, 2023 by Mandy D

[bingevader]

10 hours ago, lunator said:

All parents probably have material in their Lagrange  points

That's getting a bit personal, isn't it?

As a parent, I try to keep all my points clean and tidy, thank you very much!