Neptune May Have Eaten a Planet and Stolen Its Moon

jitendraharlalka noted a piece about the origins of Neptune. There is a theory now that it once ate a super-earth in the outer solar system, and kept its moon as some sort of macabre trophy to make sure that Mars and Venus didn't get any big ideas.

Re:What's that smell?

[Verteiron]

Nonsense, everyone knows Pluto was knocked out of its orbit around Neptune by the impact of an alien craft [wikipedia.org] traveling at extremely high velocity.

Some orbital dynamics

[Michael Woodhams]

Some may wonder what need there is for a third body at all - Triton wanders too close to Neptune, it gets captured, right?

The reason is conservation of energy: as Triton wanders near Neptune, it falls into Neptune's gravity well and accelerates, so it is going too fast to remain in orbit. Triton at infinity has more energy than Triton in orbit, so to get captured it has to lose energy, and that energy has to go somewhere.

With a few exceptions, three body interactions (e.g. Neptune, Superearth, Triton) are chaotic, and often end with one of the bodies being expelled and the remaining two left in orbit. The lightest body is the most likely to be expelled. This scenario has Superearth being expelled rather than Triton, which is somewhat unlikely but not impossible. (It is too long since I studied this for me to quantify 'most likely to be expelled'.)

It really doesn't seem to me that you need Superearth to explain Triton. The third body could very easily have been a normal Neptunian moon, which is now unobserved somewhere in the Oort cloud or expelled from the solar system entirely. (Could it be Pluto? This was thought of and rejected [nasa.gov] a long time ago.)

Disclaimer: All these comments are on the basis of reading the New Scientist summary, not the real paper.

Re:Worlds In Collision

[spacemandave]

Ugh. Every time one of these stories comes up, someone has to bring up Velikovsky. As someone who studies early solar system evolution, I've had the "pleasure" of talking with Velikovsky supporters on numerous occasions. What Velikovsky wrote about was wide-scale rearrangements of the architecture solar system WITHIN HISTORICAL TIMES, based on nutty interpretations of classical mythology. What the article here discusses is a hypothesis for the formation of Triton during an event called the Nice model that is thought to have happened about 3.9 billion years ago (based on dating of large lunar basins from Apollo samples). During this time, a much more massive precursor to the Kuiper belt fueled the migration of the outer four giant planets, disrupting stable reservoirs of small bodies throughout the solar system. Once the ancient Kuiper belt was depleted of mass, the migration stopped (so the "fuel" is gone, and therefore this process can only occur once in the lifetime of the solar system). Had planetary migration occurred within historical times, then we would currently be in the midst of a massive bombardment of comets and asteroids, and the Earth's oceans would currently reside in the atmosphere (along with perhaps some rock vapor clouds). The Nice model and Late Heavy Bombardment is backed up by observations of the structure of the Kuiper belt, observations of other solar systems around other stars, radioisotope dating of lunar rocks (in a variety of isotope systems, but most especially K-Ar, and U-Pb), observations of the structure of the asteroid belt, dynamical models based on plausible initial conditions for the early solar system (constrained by aforementioned observations), observations of zircon crystals found in ancient Earth rocks, cratering chronologies of the rocky planets, the Moon, and icy satellites. Basically it's a preponderance of evidence pointing toward plausible models for the early history of the solar system. Velikovsky has tortured interpretations of ancient literature. Who do you think is more likely to be closer to describing reality?

Re:Some orbital dynamics

[Michael Woodhams]

"...it's much more likely that the more massive object gets ejected and the smaller captured."

How does this work? My memory from a few lectures 20 years ago is the opposite, but clearly you're more reliable than I am. I thought it was an equipartition of energy thing - interactions will tend to divide the energy evenly between the objects, which means the lightest is the most likely to acquire escape velocity. Is it that ejecting the lightest object doesn't usually take away enough energy to leave the other two bound?

If you're trying to drop Superearth into Neptune, then it has to both get very low angular momentum and at the same time high energy (else Triton would not be bound to Neptune). This seems a very narrow target to hit. If you're arguing relative probabilities (it is more likely that the more massive object gets ejected) then you need to establish that the unlikelihood of impact is outweighed by the gain in likelihood of losing the larger rather than smaller object.

It had not occurred to me that the disappearance of the third body could be a two stage process: ejected from Neptune orbit, then secondarily ejected from the solar system by Saturn or Jupiter. What are the odds that an object ejected from Neptune orbit will eventually be ejected from the solar system? My gut feeling is that the odds are pretty good, that falling into a resonance with one of the giant planets or being ejected are the only long term options. (Where 'long term' I'd guess to be thousands or millions of years, not billions.)

Whether absorbed or ejected, this interaction with Superearth would tend to increase Neptune's orbit's eccentricity. How does the expected increase in eccentricity compare to the current eccentricity of Neptune's orbit?

My counter hypothesis is that the third body was a pre-existing but now lost Neptunian moon. Now that I think on it, equally plausible is that Triton was this original moon (originally in a regular prograde orbit) and an outside object came in, formed a 3 body system for a while, and then was ejected.