Pluto’s orbit is surprisingly unstable

(ORDO NEWS) — In 1930, astronomer Clyde Tombaugh, working at the Lowell Observatory in Flagstaff, Arizona, discovered the legendary “Planet Nine” (or “Planet X”). The existence of this body was predicted earlier on the basis of perturbations in the orbit of Uranus and Neptune.

After receiving more than 1000 proposals from around the world and debate among the staff of the observatory, the newly discovered object was named Pluto – this name was proposed by a schoolgirl from Oxford (Venice Burney).

Since then, Pluto has been the subject of much research and debate about its name, and on July 14, 2015, it was first visited by the New Horizons mission.

One thing was clear from the very beginning – the nature of Pluto’s orbit is extremely eccentric and inclined. Pluto’s orbit is relatively stable on longer timescales, but subject to chaotic perturbations and changes on shorter timescales, according to new research.

The study was conducted by Dr. Renu Malhotra, Louise Foucard Marshall Professor of Science at the Lunar and Planetary Laboratory (LPL) at the University of Arizona, and Takashi Ito, Associate Professor at the Planetary Research Center (PERC) at the Chiba Institute of Technology and the Center for Computational Astrophysics at the National Astronomical Observatory of Japan (NAOJ). An article describing their results recently appeared in the journal Proceedings of the National Academy of Sciences.

Pluto’s orbit is radically different from the orbits of the planets, which move in nearly circular orbits around the sun near its equator (the ecliptic).

In contrast, Pluto takes 248 years to complete one revolution around the Sun, and moves in a highly elliptical orbit inclined 17° to the plane of the solar system’s ecliptic. The eccentricity of its orbit also means that during each period, Pluto spends 20 years in an orbit closer to the Sun than Neptune.

The nature of Pluto’s orbit is an ongoing mystery that astronomers learned about soon after its discovery. Since then, many attempts have been made to model the past and future of its orbit, revealing a surprising property that protects Pluto from colliding with Neptune. As Dr. Malhotra said, this is an orbital resonance condition, known as “mean motion resonance”:

“This condition ensures that at the time when Pluto is at the same heliocentric distance from Neptune, its longitude differs by almost 90 degrees from Neptune’s longitude”.

Later, another special property of Pluto’s orbit was discovered: Pluto comes to perihelion in a place located much higher than the plane of Neptune’s orbit; this is another type of orbital resonance known as “vZLK oscillations.”

This abbreviation stands for von Zeipel, Lidov and Kozai, who studied this phenomenon as part of the “three-body problem”.

This problem is to take the initial positions and velocities of three massive objects and decide on their subsequent motion in accordance with Newton’s three laws of motion and his theory of universal gravitation, for which there is no general solution. As Dr. Malhotra added:

“In the late 1980s, with the advent of more powerful computers, numerical simulations revealed a third special property – Pluto’s orbit is technically chaotic, that is, small deviations in initial conditions lead to exponential divergence of orbital solutions over tens of millions of years.

However, this chaos is limited. In Numerical simulations have found that the two special properties of Pluto’s orbit mentioned above persist over multi-year timescales, making its orbit surprisingly stable despite the chaos indicators.”

These results are likely to have significant implications for future studies of the outer solar system and its orbital dynamics. Dr. Malhotra believes that with further study, astronomers will be able to learn more about the migration history of the giant planets and how they eventually settled in their current orbits.

It may also lead to the discovery of a new dynamical mechanism that will explain the origin of the orbit of Pluto and other bodies with a high orbital inclination.

This will be especially useful for astronomers studying the dynamics of the solar system. As Dr. Malhotra noted, researchers in the field began to suspect that evidence that could shed light on the evolution of Pluto’s orbit could be obliterated by the instability and chaotic nature of this very orbital mechanics. As Dr. Malhotra summarized:

“I think that our work provides new hope for establishing a connection between the modern dynamics of the solar system and the historical dynamics of the solar system.”

The origin of orbital inclinations of minor planets throughout the Solar System including trans-Neptunian objects is a major unsolved problem; perhaps our work will draw more attention to it.”

“Another point that our study highlights is the value of simple approximations to a complex problem: i.e. reducing the 21 parameters to one in the three-body equation opened the door to the underlying dynamical mechanisms that affect the very interesting but difficult to understand orbital Pluto dynamics.


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