An analytical examination of the rotation movements of long-period comets shows that the comets’ aphelia – the point where they are at the farthest distance from the Sun – tends to sink close to either the already known ecliptic plane where planets are located or a newly found ’empty ecliptic.’
This has rather critical implications for models of the way comets initially formed in the Solar System.
Two Solar System Planes
In the Solar System, the planets and the majority of other bodies move in almost the same orbital plane, known as ecliptic, but there are exemptions such as comets.
Comets, more so long-period ones that take tens of thousands of years to complete an orbit, are not limited to the region near the ecliptic, but they are observed coming and going in different directions.
Models of Solar System formation indicates that even long-period comets initially formed close to the ecliptic and were later dispersed into the orbits we see today via gravitational interactions, most probably with the gas giant planets.
However, even with planetary scattering, the comet’s aphelion – the point where it is farthest from the Sun – should stay close to the ecliptic. Other external forces are needed in order to clarify the observed spread. The Solar System doesn’t exist in seclusion – the gravitational field of the Milky Way in which the Solar System is located also applies a small but not insignificant influence.
Arika Higuchi, an assistant professor at the University of Occupational and Environmental Health in Japan and previously a member of the NAOJ RISE Project, analyzed the impacts of the galactic gravity on long-period comets through analytical studies of the equations ruling orbital motion.
She showed that when the galactic gravity is considered, the aphelia of long-period comets are prone to collect around two planes: the already known ecliptic and around a second one – the empty ecliptic.
Cross-Checking the Findings
The ecliptic plane is inclined towards the disk of the Milky Way galaxy by around 60 degrees. The empty ecliptic is also inclined by 60 degrees but to the other side. Higuchi named this the ’empty ecliptic,’ based on mathematical classification and because it initially hosts no cosmic bodies, only later being filled with scattered comets.
Higuchi confirmed her findings by cross-checking with numerical computations performed in part on the PC Cluster at the Center for Computational Astrophysics of NAOJ.
Comparing the analytical and computational outcomes to the data for long-period comets reported in NASA‘s JPL Small Body Database showed that the scattering has two peaks, close to the ecliptic and empty ecliptic – just as she predicted. This is a great indication that the formation models are correct, and long-period comets took shape on the ecliptic.
Still, Higuchi cautions, “The sharp peaks are not exactly at the ecliptic or empty ecliptic planes, but near them. An investigation of the distribution of observed small bodies has to include many factors. Detailed examination of the distribution of long-period comets will be our future work. The all-sky survey project known as the Legacy Survey of Space and Time (LSST) will provide valuable information for this study.”