Free-floating planets — planets moving through interstellar space without stars — may not be unusual. If solar systems in their epoch of formation go through chaotic periods when the orbits of their giant planets are affected by dynamical instability, then ejecting a gas giant from the system entirely is a plausible outcome. David Nesvorny (SwRI) has been studying the possibilities for such ejections in our Solar System, using computer simulations of the era when the system was no more than 600 million years old. Clues from the Kuiper Belt and the lunar cratering record had already suggested a scattering of giant planets and smaller bodies then.
An ejected planet makes sense. Studies of giant planets interacting with the protoplanetary disk show that they tend to migrate and wind up in a configuration where pairs of neighboring planets are locked in a mean motion resonance. Such a resonance occurs when two planets exert a regular, periodic gravitational influence on each other (there is a 2:3 resonance, for example, between Pluto and Neptune, with Pluto completing 2 solar orbits for every 3 of Neptune). Current work suggests that these resonant systems then become dynamically unstable once the gas of the protoplanetary disk disappears. Just how this happens is what the new work is all about.
From Nesvorny’s paper:
To stretch to the present, more relaxed state, the outer solar system most likely underwent a violent phase when planets scattered off of each other and acquired eccentric orbits… The system was subsequently stabilized by damping the excess orbital energy into the transplanetary disk, whose remains survived to this time in the Kuiper belt. Finally, as evidenced by dynamical structures observed in the present Kuiper belt, planets radially migrated to their current orbits by scattering planetesimals…
The scattering outlined here suggests that Jupiter moved inward in its orbit and scattered smaller bodies both outward and inward, some to take up residence in the Kuiper Belt, others to cause impacts on the inner planets and Earth’s Moon. The problem: A slow change to Jupiter’s orbit based on interaction with small bodies would have thoroughly disrupted the inner system.
A faster change of orbit due to interactions with Uranus and Neptune would have caused both the latter two planets to be ejected from the system. At this point Nesvorny added to the model an additional giant planet, working with an initial state where all the giant planets reached resonant orbits in the protoplanetary disk within a range of some 15 AU from the Sun. As the gas disk dispersed, Uranus and Neptune would have been scattered by the gas giants, reaching their current orbits and in turn scattering the planetesimals in that region into the Kuiper Belt.
The final consequence is the ejection of the fifth gas giant. An additional planet between Saturn and the ice giants — a world that was ultimately ejected from the system entirely — leaves us with a simulation that models the four giant planets we see today. Nesvorny’s simulations put a range of different masses for the planetesimal disk into play, with a total of 6000 scattering simulations following each system for 100 million years, when the planetesimal disk was depleted and planetary migration at an end. And it turns out that you are roughly ten times more likely to wind up with an analog to our Solar System if you start with five giant planets rather than four.
This scenario solves a variety of problems. Understanding how Uranus and Neptune formed is difficult because at their present distances of roughly 20 and 30 AU, accretion would have required too long a timescale. Nesvorny points out that the ice giants form readily at 15 AU or less, and the five planet resonant system his work discovered accounts for their movement outward. The results shift depending on whether one assumes an initial 3:2 resonance between Jupiter and Saturn or a 2:1 resonance, with the latter pushing the outer ice giant to a problematic 18-20 AU for formation. This makes the 3:2 resonance the most likely, but the scientist notes the need for more work on the question.
In the meantime, we’re left with the vision of an even more interesting early Solar System than we thought, and the possibility that the ejection of that fifth giant planet may be what spared the inner system — and our Earth — from complete disruption. We’re also given another look at the processes that produce dark, interstellar wanderers, planets with no star to light them, as Nesvorny notes:
“The possibility that the solar system had more than four giant planets initially, and ejected some, appears to be conceivable in view of the recent discovery of a large number of free-floating planets in interstellar space, indicating the planet ejection process could be a common occurrence.”
The paper is Nesvorny, “Young Solar System’s Fifth Giant Planet?” Preprint available.