Research thinks it has unlocked the secret behind Pluto’s strangely unstable orbit
In 1930, astronomer Clyde Tombaugh discovered the legendary “Ninth Planet” (or “Planet X”) while working at Lowell Observatory in Flagstaff, Arizona. The existence of this body had previously been predicted based on disturbances in the orbit of Uranus and Neptune.
After receiving more than 1,000 suggestions from around the world and some debate among Observatory staff, this new object was named Pluto – which was proposed by a young Oxford schoolgirl (Veneto Burney).
Since that time Pluto has been the subject of considerable study, naming controversy, and was first visited on July 14, 2015 by the New Horizons assignment.
One thing that’s been clear from the start is the nature of Pluto’s orbit, which is very eccentric and tilted. According to new searchPluto’s orbit is relatively stable on longer timescales but is subject to chaotic disturbances and changes on shorter timescales.
The research was conducted by Renu MalhotraProfessor of Scientific Research Louise Foucar Marshall at the University of Arizona Lunar and planetary laboratory (LPL) and Takashi Itōassociate professor at the Chiba Institute of Technology Planetary Exploration Research Center (PERC) and the National Astronomical Observatory of Japan (NAOJ) Computational Astrophysics Center.
The article describing their findings recently appeared in the Proceedings of the National Academy of Sciences.
To break it down, Pluto’s orbit is radically different from those of the planets, which follow nearly circular orbits around the Sun near its equator, projecting outward (i.e. the ecliptic).
In contrast, Pluto takes 248 years to complete a single orbit around the Sun and follows a highly elliptical orbit inclined 17° to the ecliptic plane of the solar system.
The eccentric nature of its orbit also means that Pluto spends 20 years during each period orbiting closer to the Sun than Neptune.
The nature of Pluto’s orbit is an enduring mystery and something that astronomers became aware of very soon after its discovery. Since then, multiple efforts have been made to simulate the past and future of its orbit, which revealed a surprising property that protects Pluto from colliding with Neptune.
As Malhotra told Universe Today via email, this is the condition of orbital resonance known as “mean motion resonance”:
“This condition ensures that by the time Pluto is at the same heliocentric distance as Neptune, its longitude is nearly 90 degrees from that of Neptune. Later, another peculiar property of Pluto’s orbit was discovered: Pluto arrives at perihelion at a location well above the plane of Neptune’s orbit; this is a different type of orbital resonance known as a “vZLK wobble”.
This abbreviation refers to von Zeipel, Lidov and Kozai, who studied this phenomenon within the framework of the “three-body problem”.
This problem involves taking the initial positions and velocities of three massive objects (then extended to include particles) and solving their subsequent motion according to Newton’s Three Laws of Motion and his Universal gravitation theory – for which there is no general solution.
As Malhotra added: “In the late 1980s, with the availability of more powerful computers, numerical simulations revealed a third peculiar property, namely that Pluto’s orbit is technically chaotic, that is- that is, small deviations from initial conditions lead to exponential divergence of orbital solutions. over tens of millions of years.
“However, this chaos is limited. It has been found in numerical simulations that the two special properties of Pluto’s orbit mentioned above persist over giga-year timescales, making its orbit remarkably stable, despite the indicators of chaos.”
For their study, Malhotra and Ito performed numerical simulations of Pluto’s orbit up to five billion years into the solar system’s future.
In particular, they hoped to address unresolved questions about the particular orbits of Pluto and other Pluto-sized objects (aka Plutinos). These questions have been addressed by research conducted over the past decades, such as the “planetary migration theory”, but only to a point.
In this hypothesis, Pluto was pulled into its present mean motion resonance by Neptune, which migrated during the early history of the solar system.
A major prediction of this theory is that other Trans-Neptunian Objects (TNOs) would share the same resonance condition, which has since been verified with the discovery of a large number of plutinos.
This discovery also led to a wider acceptance of planetary migration theory.
But as Malhotra explained: “Pluto’s orbital inclination is closely related to its vZLK wobble. So we thought that if we could better understand the conditions of Pluto’s vZLK wobble, we might be able to solve the mystery of its tilt. We started by investigating the individual role of the other giant planets (Jupiter, Saturn and Uranus) in Pluto’s orbit.”
To do this, Malhotra and Ito performed computer simulations in which they simulated Pluto’s orbital evolution up to 5 billion years ago, including eight different combinations of giant planet disturbances. These many-body simulations included interactions with:
- Neptune (—NP)
- Uranus and Neptune (–UNP)
- Saturn and Neptune (-S-NP)
- Jupiter and Neptune (D–NP)
- Saturn, Uranus and Neptune (-SUNP)
- Jupiter, Uranus and Neptune (J-UNP)
- Jupiter, Saturn and Neptune (JS-NP)
- Jupiter, Saturn, Uranus and Neptune (JSUNP)
“We found no subset of the three inner giant planets to recover Pluto’s vZLK wobble; all three – Jupiter, Saturn, and Uranus – were needed,” Malhotra said. “But what about those planets that [are] essential to Pluto’s vZLK oscillation?”
Malhotra added. “There are 21 parameters needed to represent the gravitational forces of Jupiter, Saturn, and Uranus on Pluto. It’s a prohibitively large parameter space to explore.”
To simplify these calculations, Malhotra and Ito grouped them into a single parameter by introducing some simplifications. This included representing each planet with a circular ring of uniform density, a total mass equal to that of the planet, and a ring radius equal to the average distance of the planet from the Sun (aka semi-major axis).
As Malhotra pointed out, this gave a single parameter representing the effect of Jupiter, Saturn and Uranus (J2), which was equivalent to the effect of an “Oblate Sun”.
“[W]We discovered a fortuitous arrangement of the masses and orbits of the giant planets that delimits a narrow range of the J2 parameter in which the vZLK oscillation of Pluto is possible, a kind of “Goldilocks zone”, “he said. she declared.
“This result indicates that in the era of planetary migration in [the] In the history of the solar system, the conditions of trans-Neptunian objects have changed in such a way as to promote many of them – including Pluto – into the vZLK oscillation state. It is likely that Pluto’s tilt originated during this dynamic evolution.”
These findings will likely have important implications for future studies of the outer solar system and its orbital dynamics.
With further study, Malhotra believes astronomers will learn more about the giant planets’ migration history and how they eventually settled into their current orbits. It could also lead to the discovery of a new dynamical mechanism that would explain the origins of the orbit of Pluto and other bodies at high orbital inclinations.
This will be especially useful to astronomers who are dedicated to studying the dynamics of the solar system. As Malhorta noted, researchers in this field were beginning to suspect that evidence that might shed light on Pluto’s orbital evolution might have been erased by the instabilities and chaotic nature of those same orbital mechanisms.
As Malhotra summarized: “I think our work raises new hope for linking the current dynamics of the solar system with the historical dynamics of the solar system. The origin of the orbital inclinations of the minor planets throughout the system solar – including the NWT – presents a major unresolved problem; perhaps our work will draw more attention to it.
“Another point that our study highlights is the value of simple (r) approximations for a complicated problem: that is, merging 21 parameters into a single parameter opened the door to accessing dynamical mechanisms essentials affecting the very interesting but difficult to understand orbital dynamics of Pluto and Plutinos.”