All the planets in the Solar System orbit close to a flat plane, as do most asteroids. Comets, have a much wider distribution, but astronomers debate whether this is random or if there is a pattern to the cometary distributions. A new theory proposes that comets align with a second plane, a product of the Milky Way's gravitational field.
Observations of proto-star systems and astrophysical models agree that the planets originated in a disk nearly aligned with the Sun’s equator. Since then, Jupiter has acted as a planetary sheepdog, its mighty gravity keeping its smaller counterparts on what is known as the ecliptic. Cometary orbits are much more varied, a product of encounters with objects with enough gravity to change their course. However, Dr Arika Higuchi of the Japanese University of Occupational and Environmental Health argues such disruption should leave a comet’s aphelion, or furthest point from the Sun, close to the ecliptic.
Yet many comets we see have aphelia nowhere near the ecliptic. Like many before her, Higuchi concluded there are too many comets with non-ecliptic aphelia to be explained by random forces. Higuchi noted that the Milky Way’s gravitational field exerts a force within the Solar System – small compared to that of the Sun and larger planets but omnipresent and possibly capable of influencing cometary orbits. This would particularly apply to those that spend most of their time remote from other forces.
In the Astronomical Journal, Higuchi reveals the presence of what she calls a “second ecliptic” caused by the misalignment between the Solar System’s main ecliptic and the Milky Way’s disk. This plane is at an angle of 120 degrees compared to the main ecliptic plane.
Initially, Higuchi assumes, the second ecliptic would have been empty. However, her modeling shows over time that disrupted comets would start to congregate. Even a passing star can disrupt a comet's orbit, and over billions of years many have, Higuchi notes. However, without some more lasting influence, what we see should be effectively random, aside from those still on their intiial, ecliptic-aligned orbits.
NASA’s Small Body Database reveals there is indeed two clusters of cometary aphelia, as Higuchi’s work predicts. Some have previously wondered if such a cluster could be an indicator of some unknown object, an even more distant counterpart of Planet X. However, Higuchi argues, the second peak is more consistent with the influence of the galactic gravitaitonal field. Nevertheless, the match is imperfect.
“The sharp peaks are not exactly at the ecliptic or [second] ecliptic planes, but near them,” Higuchi said in a statement. Among comets from the outer edges of the Solar System, approximately equal numbers lie at the peak near the ecliptic and near the second ecliptic, with a much smaller number scattered randomly through the remaining space.
Higuchi believes some other factor has pulled the bulk of cometary aphelia slightly away from either plane and intends to keep searching for what this might be.