Humans close to finding answers to origin of universe: Hawking
- Thread starter Deepak
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Chalnoth said:
In what respect?
I'm not sure by the way how much the discussion amongst the IAU is polluted by the prospect of having to classify and name tens of thousands of exoplanets which will be discovered over the next few decades. Given that we can be pretty certain that these discoveries are coming it would make sense to get the definition correct now. So we need a convention which will work in other solar systems, not just our own IMO.
Instead of looking at the size first and foremost, we would look at their behaviour. If they have the same orbital characteristics as the other planets, they might fit. Although we would still need a scale for size, from planet through planetoid to asteroid/comet.nutball said:
And planets can be further divided into gas giants, icy, and rocky planets.
Edit: we would extrapolate from the accretion of the initial disk at star formation, and say that everything in that disk that cumulates into a large body would be a planet, while everything that just gathered enough mass by gravity and ended up in some random orbit wouldn't be a planet.
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I seriously doubt that. Planets, as we know them, seem to follow a natural progression. Near the star you willl get rocky planets with little or no atmosphere, as most of the atmosphere has been blown away by the solar wind. Further away, you will get gas giants, in a region that is cold enough for them to form without being blown apart. Further than that, you will get relatively small icy bodies that are in a region that is too diffuse for all of the matter in one "ring" to coalesce into a single body, as happens with the nearer planets, and also too diffuse to ever reach sizes required to entrap the heavier gasses like Hydrogen. So you will get many relatively small objects that are relatively well-scattered.DiGuru said:
With that picture of planetary system formation, it seems obscenely unlikely that there are any planetary bodies beyond the start of the Kuiper belt. Because if there were, then the Kuiper belt would be further out.
Edit: This would lead to another decent definition: a planet and its moons comprise the only objects in an orbit in that planet's original orbital configuration. This would eliminate, for example, Pluto, because we believe it to be a Kuiper belt object that was deviated from its initial orbit. All of the 8 planets would satisfy this criterion, because they are all large enough to have scooped up everything in their original orbital path. Asteroids in the asteroid belt and Kuiper belt and Oort cloud objects would not satisfy these criteria because they share their orbits with a great number of other objects with very similar orbits.
Edit 2: By the way, I meant that I seriously doubt we'd find other planets in our own solar system, by the way. Obviously we'll find tons more planets in other systems.
Edit 3: Heh, just noticed that DiGuru had the same basic idea of a good definition. Oh, well, a bit late, I guess.
By the way, DiGuru, that is one wierd source you found for that Oort cloud information.
http://home.xtra.co.nz/hosts/Wingmakers/Contents.html
It seems to be a smattering of accurate scientific information and just plain off-the-wall mystic nonsense.
http://home.xtra.co.nz/hosts/Wingmakers/Contents.html
It seems to be a smattering of accurate scientific information and just plain off-the-wall mystic nonsense.
What do you define as "original" orbit? I think it's rapidly becoming apparent that the formation of planetary systems is a highly dynamic process, and very little matter ends up where it started out. The proto-planets migrate all over the place driven by tidal interactions with the proto-planetary disc. Later planetary collisions will lead to further mayhem. So a definition based on the configuration at some t(0) leads you to the problem of defining that t(0) and divining the evolutionary history of a planetary system. Neither of those are particularly easy.Chalnoth said:
Yes, I agree. But the Kuiper belt and Oort cloud are pretty unstable, due to the more or less random orbits of the bodies. They spread out over time, and so can slowly enlarge the area occupied inwards.Chalnoth said:
Sure, the large bodies would wipe a clean path over time, but there are enough new comets and asteroids slowly traveling inwards, that they could swamp the orbit of less massive planetoid bodies.
And they can enlarge smallish planets that are far out, just like they clump together to form larger and larger bodies on other trajectories.
In that sense, we cannot ever be sure if some bodies that might have classified as planets weren't diverted from their initial orbit by many collisions.
Absolutely. But none of the objects are likely to ever have an orbit with approximately the same characteristics (period, radius, eccentricity) of any of the planets. And even if one did, since the orbit would certainly not be exactly the same, it would interact with the planet either gravitationally, pushing it into some other orbit out of the system, or just collide with the planet. This would probably happen within a few thousand years at the longest.DiGuru said:
Well, that's why we'd look at composition in an attempt to determine the location during formation.In that sense, we cannot ever be sure if some bodies that might have classified as planets weren't diverted from their initial orbit by many collisions.
Well yes but a definition based on some reading of the tea-leaves of the history of a planetary system seems to me to be highly complex, error-prone and model dependent. Far better is a definition based on the current configuration, which is readily observable for our Solar system and will be in the mid-term future for exo-planets.
Not so much. For example, in our own solar system, the planets all have masses significantly larger than any other objects, and as such significant deviations from their initial orbits are highly unlikely. In the unlikely event that such a deviation does occur, the compositions of the known planets are extremely different from the compositions of the asteroids, Kuiper belt objects, and Oort cloud objects, and so the differentiation would still be easy.nutball said:
But, that is exactly what is making it a big problem to come up with a good definition of what makes something into a planet! Because, going from that, you have to make some really arbitrary rules and cut-off points to come up with a definition.nutball said:
In that case, it might be better and simpler just to state that there are very many bodies orbiting stars, and that we call a predefined set of them "planet", just because that's how we called them before we knew what we know now.
Ignorance is not considered a valid excuse in science.
If they call Pluto a planet, but make sure nothing else qualifies, it's no better than just making a list of bodies that are to be called planets. Which is very likely what they try to do.nutball said: