"Pluto is dead" - Mike Brown, It's official Options

[David]

I consider the (apparent) fact that I am able to discuss this planetary classification issue halfway intelligently to be positive proof that it is not in any sense a scientific question.

[karolp]

I consider the (apparent) fact that I am able to discuss this planetary classification issue halfway intelligently to be positive proof that it is not in any sense a scientific question.

And it won't really be until we find out what a planet REALLY means by exploring other planetary systems and seing what they are like and might be. I particularly find any criteria of circular orbits to be inappropriate and Sol-centered - there are "jupiters" and "neptunes" in eccentric orbits around other stars. But we shall not understand what it really means to "be a planet" until we see more planets around other stars. For now it is just a "distant flavour", not insight. But we ARE intelligent enough to tell a KBO from a planet. And until COROT tells as some more or we discover an "earth" in the Oort Cloud the case appears to be settled.

[Greg Hullender]

Something I thought sounded interesting in one of Mike Brown's papers was the idea that a "planet" in a really eccentric orbit is probably a separate condensation from the original nebula, not a condensation from the accertion disk of the star. Multiple stars do indeed tend to have very elliptical orbits, so a "planet" that condensed that way would likely have one too. He alluded to a "purist" view that insists all such bodies are "stars" not planets, although he didn't sign up to it.

The implied assumption (sounds reasonable, anyway) is that anything that does form from the accretion disk will unavoidably be in a circular orbit near the plane of the ecliptic.

[alan]

The web is being rearranged to accomodate the new definition: eightplanets.org and now redirects to nineplanets.org

[karolp]

Gas giants might get eccentric orbits from interacting with each other. It was suspected Uranus was coming close to Saturn in the past in its eccentric orbit which finally resulted in its odd tilt. Circular orbits are so "Copernican". Time to embrace the 21st c.

[Greg Hullender]

Uranus and Saturn are both in less eccentric orbits than Mars. I've never heard anyone suggest that they used to be in more elliptical orbits. I HAVE heard the suggestion that they both formed further in and have gradually drifted further out, but that would have happened when the Solar System was new.

[karolp]

Uranus and Saturn are both in less eccentric orbits than Mars. I've never heard anyone suggest that they used to be in more elliptical orbits. I HAVE heard the suggestion that they both formed further in and have gradually drifted further out, but that would have happened when the Solar System was new.

Well, it basically goes like this:

"Jupiter and Saturn start out at roughly 5 and 8 astronomical units; Uranus and Neptune begin much closer to the Sun than their current positions, at about 13 and 14 AU. They stay pretty comfortably in those positions for about 100,000 years. Then, quite suddenly, that 1:2 resonance is reached. Saturn and Jupiter don't change a lot initially, but the orbits of Uranus and Neptune go nuts. They get much more eccentric, so that their orbits cross; at times Uranus even gets very close to Saturn. After about a million years, the eccentricity dies down, and Uranus and Neptune are on their way out to more distant positions in the solar system, at the same time that Saturn begins to acquire its present orbit eccentricity."

The Whole Article

[ugordan]

"Jupiter and Saturn start out at roughly 5 and 8 astronomical units; Uranus and Neptune begin much closer to the Sun than their current positions, at about 13 and 14 AU.

I have a problem with that scenario. Namely, Saturn, Uranus and Neptune start closer in than they're now.

* Saturn : 8 AU --> 9.5 AU
* Uranus : 13 AU --> 19 AU
* Neptune : 14 AU --> 30 AU

All three are gaining momentum as they move into a higher orbit. Who's losing momentum here? You can't just get it out of nothing. There has to be a pretty large object that spirals inward as a consequence. I see Jupiter stayed pretty much where it is now so no luck there.

[Alan Stern]

Uranus and Saturn are both in less eccentric orbits than Mars. I've never heard anyone suggest that they used to be in more elliptical orbits. I HAVE heard the suggestion that they both formed further in and have gradually drifted further out, but that would have happened when the Solar System was new.

Actually, the simulations show U and N get up to e=0.3 or a bit higher en route from their former closer orbits to their current ones.
This occurs just after their strong scatterings by J or S and before dynamical friction on the outer disk damps the e back down.

[Greg Hullender]

Thanks, Alan. I knew the move had to be while there was still a lot of material in the accretion disk, but I had envisioned a gradual outward spiral. I hadn't considered a catastrophic interaction, but I guess I shouldn't be surprised.

How confident are astronomers/astrophysicsts of this simulation?

[Alan Stern]

Thanks, Alan. I knew the move had to be while there was still a lot of material in the accretion disk, but I had envisioned a gradual outward spiral. I hadn't considered a catastrophic interaction, but I guess I shouldn't be surprised.

How confident are astronomers/astrophysicsts of this simulation?

Its controversial that U and N originated in the J-S zone, but the simulations of that are convincing
that if this occured, the e's got pretty high. I myself am not convinced of the entire scenario.

-Alan

[Jyril]

Isn't it that according to the theories of planetary formation, Neptune should still be in the process of forming if it formed where it is now?

[Alan Stern]

Isn't it that according to the theories of planetary formation, Neptune should still be in the process of forming if it formed where it is now?

No. I think you're confusing the fact that in many old models, Neptune could not be made for form in
the age of the solar system. It has no significant feedstock to continue forming today in any real
sense, though technically it and all the planets--including Pluto--are gaining mass because things that
they run in to "stick" owing to their high gravity.

-Alan

[Guest_Myran_*]

Alan Stern wrote: I myself am not convinced of the entire scenario.

I am not entirely convinced of this scenario either.
One detail that makes me wonder are the fact that the disc where the planets formed should have been thinner at the outer edge. Yet Uranus have got 14 earth masses whereas Neptune got 17.
But there could of course have been a thicker belt of KBO's that Neptune gobbled up when it migrated outward and so gained more mass. The KBO's are after all just that, planet bulding blocs and if there was enough of them and we smashed them together we'd end up with something quite similar to Uranus or Neptune: A small rocky core surrounded by a vast and very deep ocean and perhaps even a similar atmosphere, at least it would have the methane.
If this scenario would be correct, then Chiron and Pluto simply are the leftovers which happened to survive with Pluto in resonance with Neptune and Chiron flipping back between Uranus and Saturn.

[Julius]

This Migrating Giant theory was clearly discussed on BBC sky at night programme.While N,U and S seem to be migrating outwards,is J supposed to be moving inwards towards the inner solar system?If I understood right,interactions with KBOS seems to be the trigger of this whole migration taking place with Jupiter being the final encounter resulting in KBOS being scattered into the interplanetary space.Would this explain the presence of Phoebe at Saturn??