How Neptune may have captured Triton, Nature (May 11, 2006) Options

[volcanopele]

Neptune's capture of its moon Triton in a binary–planet gravitational encounter
http://www.nature.com/nature/journal/v441/...ature04792.html

More can also be read at:
http://news.yahoo.com/s/space/20060510/sc_...ecapturedtriton

This model suggests that at one point, Triton was part of a binary system, similar to that seen with Pluto and Charon. The pair made a close pass of Neptune, where Neptune's gravity disrupted the pair, and Triton was captured by Neptune.

[ljk4-1]

And the Space.com take on the story, complete with graphic:

Neptune Might Have Captured Triton

http://www.space.com/scienceastronomy/0605...ton_origin.html

Neptune's largest moon, Triton, was originally a member of a duo orbiting the
Sun but was kidnapped during a close encounter with Neptune, a new model
suggests.

[Guest_BruceMoomaw_*]

It's quite a logical theory -- a neat variation on the long-time suggestion that Triton made a close flyby of a big existing moon of Neptune which slowed down Triton into orbit around Neptune at the same time that it flipped the other moon into an escape trajectory. In this new version, it was a moon of Triton itself that served as the partner in Triton's gravitational trapeze act.

The trouble is that, at a certain point, this sort of speculation turns into a scientific Shaggy Dog Story -- what physical evidence can we possibly ever come up with to determine which of those two alternatives is correct (or if some other cause, such as gas friction from Triton brushing through Neptune's original surrounding nebula, is the correct one)?

[Rob Pinnegar]

Well, we may not be able to tell which of them is correct -- but we may very well be able to figure out which of them is more likely, from a statistical/dynamical standpoint. I haven't yet read the paper, but here are some thoughts, for what they're worth.

Triton's current 20-degree orbital tilt suggests that it probably approached Neptune from a direction that was pretty far out of Neptune's equatorial plane. If we assume that the orbits of Neptune's original large satellites were very close to this plane, this would make any close interactions (i.e. collisions or grazing near-misses) with other large satellites very unlikely. It is, of course, possible that Triton came barrelling down from Neptune's north polar direction and whammed into a "Titania". But an equatorial-plane pass is probably necessary to make this idea appealing statistically -- and an inbound-flight inclination of only a few degrees would dramatically decrease the likelihood of a collision or near-collision.

The binary-partner hypothesis, on the other hand, has no such requirement. It should be more efficient if the mutual orbital plane of Triton around its partner, and the plane of their centre of mass's hyperbolic trajectory around Neptune, are roughly aligned with each other. I'm guessing, though, that this requirement will turn out to be much, much less stringent than the one described above. Even if the angle between the two planes was, say, 60 degrees, it might still work more than well enough to do the job.

[Minor edits to correct grammar errors.]

[Guest_Richard Trigaux_*]

Another hypothesis could come from the recent discovery of a star having TWO accretion disks turning in opposite ways!! To make this possible, we must admit that the accretion disks are fed by different lumps or streams from the original nebula. In most of the cases, it would result into one disk, formed by the part of the matter which has enough energy to stay in orbit (the other part falling on the star or spiraling inwards). But there would be irregularities into the various ringlets, into composition, inclination, excentricity, dispertion, etc. so that each ringlet would give a different planet (independently of further processes such as fractionned distillation). In extreme cases, some planets would turn in opposite directions.

In the case of Neptune, we can imagine a process with two original nebulae, the largest in Neptune's orbit, and the smallest, say, on Pluto's orbit. When the two nebulae interact, they experience friction from their gasses, but they have too much energy to coalesce. The Pluto nebula lost a part of its mass to the larger neptune nebula, and the remainder forms the regular Pluto system and moons, on an orbit resonant with Neptune. In the neptune nebula, the havoc created by the collision of the two nebulae empties a large part of the accretion disk, forbidding the formation of a large number of large moons, like with the other giant planets. Only could form Triton, in a reverse orbit, two smaller inner moons on normal orbits, and the 5 millions kms away Nereid, not affected by the encounter.

So the collision of two protoplanetary nebulae, rather than of already formed systems, can explain all the strange properties of the Neptune-Pluto system, without the impossibilities of a direct interaction of two already formed bodies. Especially if Pluto had encountered Neptune, was ejected by it, in a counterpart of the insertion of Triton, it would not have a complex moons system. But if this moon system formed later, there is no impossibility. And the friction between two nebulae is far enough to explain the braking required for the interaction, in more of course of their gravitationnal attraction.

[Rob Pinnegar]

This is at least the second time you've mentioned this discovery of a star with multiple accretion disks, Richard. Where did you find out about this? I haven't seen it in the news, though admittedly I haven't been looking for it either.

[alan]

I believet this is what he is referring to

Still-Forming Solar System May Have Planets Orbiting Star in Opposite Directions
Astronomers studying a disk of material circling a still-forming star inside our Galaxy have found a tantalizing result -- the inner part of the disk is orbiting the protostar in the opposite direction from the outer part of the disk.

http://www.spaceref.com/news/viewpr.html?pid=18998

[Guest_Richard Trigaux_*]

Thanks, Alan, you saved me: I don't remember where I saw this, maybe in a paper science review. I think your info is the good one, although my source said things in other way.


I read your link, it refers to its original source, but I have however a little bit of a concern: the publication of this info is due to appear in the Astrophysical Journal, edition April 1...


I wrote to the site which published the info.

Edited the next day Wednesday 12: I received a reply by Dave Finley, from the National radio Astronomy Observatory, who confirmed that id is not an April fool, but a real result. They are not responsible of the publication date... End of suspicion.

[Planet X]

My buddy, Michael C. Emmert, at Gravity Simulator discussed the issue of Neptune capturing Triton awhile back. His discussion can be found here. Later!

J P

[tedstryk]

Great News! HST mapped Triton with ACS in 2005. I was scared this would never happen.

Proposal 10422:
Resolving Changes on Triton's Surface: Imaging over Triton's full longitudinal range.
HST Proposal 10422
James Bauer
Jet Propulsion Laboratory
Cycle: 13
Category: SOLAR SYSTEM
Proposal type: GO
Status: completed

HST Proposal Information:
about this proposal
about other proposals by this PI

Proposal Abstract
We have found, from ground-based photometry, evidence of possible seasonal surface changes on Triton which may be brought about by geophysical activity and which have transpired since as recently as 1995. We propose to confirm and characterize these changes using the HST ACS instrument to image Triton at UV, B, V, I and Methane-band wavelengths over as much of its surface as visible from near Earth. By so doing, we will determine the resurfacing rates and set model constraints on activity and surface temperature as well as composition. Such constraints have profound implications for our understanding of Triton's evolution as well as the history of other outer solar system bodies, including outer solar system satellites which may undergo similar geophysical processes or have similar composition

[djellison]

http://archive.eso.org/preview/preview/pre...1F1Q/jipa/ascii

Lots of HST stuff w.r.t. Triton on there. Obviously it's just going to be spectral work really - it's too small/distant to resolve anything I'd have thought.

[um3k]

http://archive.eso.org/preview/preview/pre...1F1Q/jipa/ascii

Lots of HST stuff w.r.t. Triton on there. Obviously it's just going to be spectral work really - it's too small/distant to resolve anything I'd have thought.

Don't forget that Triton is larger and (somewhat) closer to Earth than Pluto, and the HST mapped Pluto!

[djellison]

Good call - perhaps a handfull of pixels

Then again - even one pixel is better than nothing

Doug

[tedstryk]

Flynn et. al did some good mapping work with the FOC in 1995, which is of similar quality to HST's Pluto mapping. It is true that the resolution will be poor, but it may be good enough to track things such as changes to the shape and extent of the polar cap.

Here are some of Flynn's results.



Here is an image I constructed using data from the raw images in this set that has been used in a number of places (it is an artificial mosaic, but heck, it looks cool).