Decked out in colorful clown-like bands and pockmarked by turning reddish storms, our Solar System’s largest planet, Jupiter, is truly the planetary monarch of our Sun’s fantastic family. This magnificent banded-behemoth, like other monarchs, has a devoted retinue of followers accompanying its every move as it wends its way around our Sun. The Jovian Trojan Asteroids are a huge group of rocky followers that share their planet’s orbit, and compose two different stable groups–one group that travels ahead of the planet in its orbit, while the other trails it from behind. In September 2018, planetary scientists in the Southwest Research Institute (SwRI) in San Antonio, Texas, announced their new findings revealing the true character of an unusual and delightful duo of Jupiter Trojans. Their new study points to an ancient planetary shake-up and consequent rearrangement of our Solar System as it was still rather young and forming.
The duo of Trojan Asteroids studied by SwRI scientists take the fabled names of Petroclus and Menoetius. The duo are also targets of NASA’a upcoming Lucy assignment that intends to explore the rocky followers of our Solar System’s largest planet.
Petroclus and Menoetius are both approximately 70 miles wide and orbit each other as they revolve round their planet together, both bound slavishly for their drifting enormous world. They’re the only large binary known to exist among both heavy populations of Trojan Asteroids.
“The Trojans were likely captured during a dramatic period of dynamic instability when a skirmish between the Solar System’s giant planets–Jupiter, Saturn, Uranus and Neptune–happened,” noted Dr. David Nesvorny at a September 10, 2018 SwRI Press Release. Dr. Nesvorny, who is of the SwRI, is lead author of the paper describing this new study under the name: Evidence for Very Early Migration of the Solar System Planets from the Patroclus-Menoetius Binary Jupiter Trojan, printed in the journal Nature Astronomy.
This ancient planetary rearrangement of our Solar System pushed the duo of ice-giants Uranus and Neptune external, where they met up with a large primeval population of small bodies considered to be the ancestors of the Kuiper Belt Objects (KBOs), that dance around our Star in our Solar System’s outer limits. The Kuiper Belt is the distant, frigid home of a suspended multitude of comet nuclei, dwarf planets, and tiny icy tidbits. In this distant region of perpetual twilight the Sun casts its weak fires from so far away that it hangs suspended in the sky like it were just an especially big Star sailing through a dark celestial sea with myriad other stars. The dwarf planet Pluto is among the largest known KBOs.
“Many tiny bodies of the primordial Kuiper Belt were scattered inwards, and some of them became trapped as Trojan Asteroids,” Dr. Nesvorny added. Jupiter and the beautiful “Lord of the Rings”, Saturn, are gas-giants. In contrast, their outer Solar System neighbors, Uranus and Neptune, are ice-giants. The smaller ice-giants are thought to have larger solid cores enshrouded by thinner gaseous atmospheres than those that cloak both Jupiter and Saturn. Also, the gas-giant set may not even contain strong cores at all, but may be composed entirely of gases and liquids.
The Jupiter Trojans are dark, and show featureless, reddish spectra. There is not any strong evidence of the presence of water, or some other special chemical, on their surfaces based on their spectra. But many planetary scientists propose that they are encased in tholins, which are organic polymers formed by our Sun’s radiation. The Jupiter Trojans display densities (based on research of binaries or rotational light curves) that change, and they are thought to have been gravitationally snared in their present orbits during the early phases of our Solar System’s evolution–or, possibly, slightly later, during the period of the migration of the giant planets.
All stars, our own Sun included, are born surrounded by a spinning, swirling disk of gas and dust, which can be termed a protoplanetary accretion disc . These rings encircle baby celebrities, and they contain the critical ingredients from which an entourage of planets, in addition to smaller objects, ultimately emerge.
Our Solar System, in addition to other systems surrounding stars beyond our Sun, evolve when a very dense and relatively small blob–tucked within the undulating folds of a dark, frigid, giant molecular cloud–collapses gravitationally under its own relentless and merciless gravitational pull. Such enormous, beautiful, and billowing clouds inhabit our Milky Way Galaxy in large numbers, like they were lovely floating phantoms swimming through the space between stars. These dark clouds act as the strange birthplace of infant stars.
The majority of the collapsing blob collects at the middle, and ultimately ignites as a consequence of nuclear-fusion reactions–and a star is born. What remains of the dust and gas of the erstwhile blob becomes the protoplanetary accretion disk from a solar system forms. In the earliest phases, such accretion disks are both extremely massive and very hot, and they can linger around their young celebrity (protostar) for as long as ten million years.
From the time a star like our Sun has reached the T Tauri stage of its toddler years, the hot, massive surrounding disk has grown both cooler and thinner. A T Tauri star can be compared to a human tot. These stellar toddlers are variable stars, and are extremely active in the tender age of a mere 10 million years. T Tauris are born with large diameters which are several times greater than the diameter of our Sun today. But, T Taurisare in the act of shrinking. Unlike human tots, T Tauris shrink as they grow up. By the time a stellar toddler has reached this stage of its development, less volatile materials have begun to condense near the center of the swirling encircling disk, thus forming extremely sticky and smoke-like motes of dust. These “sticky” particles of dust contain crystalline silicates.
The little grains of dust eventually collide in the crowded disk environment, and glue themselves to one another, thus creating ever larger and larger objects–from pebble-size, to mountain-size, to asteroid-and-comet-size, to moon-size, to planet-size. These growing objects turned into a stellar system’s primordial population of planetesimals, which are the building blocks of planets. What is left of a heavy population of planetesimals, following the age of planet-formation, can linger around their parent-stars for billions of years after a mature system–such as our own Solar System–has shaped. In our own Solar System, comets and asteroids are remnants of the primordial planetesimals.
The term “trojan” has come to be used more commonly to refer to other small Solar System bodies which display similar relationships with bigger bodies. For example, there are Martian trojans and Neptune trojans. Indeed, NASA has just announced the discovery of an Earth trojan! The term Trojan Asteroid itself is commonly understood to specifically refer to the Jupiter Trojans because the first Trojans were discovered close to Jupiter’s orbit–and Jupiter also currently has by far the most known Trojans.
History Of The Hunt
In 1772, the Italian-French mathematician Joseph-Louis Lagrange (1736-1813) predicted that a small body sharing an orbit with a world by residing 60 degrees ahead or behind it will be gravitationally snared if it is close to certain points (Lagrange Points). Lagrange, who based his prediction on a three-body problem, revealed that the gravitationally trapped body will librate slowly around the point of balance in what he described as a horseshoe or tadpole orbit. The first asteroids to be recorded in Lagrange Points were discovered over a century after Lagrange had declared his hypothesis.
Relative to their tremendous host planet, each Jovian Trojan librates around among Jupiter’s two stable Lagrange Points: L4 that’s situated 60 degrees ahead of Jupiter in its orbit, and L5 that’s situated 60 degrees behind.
However, neither Barnard nor other astronomers understood its significance at the time. Indeed, Barnard wrongly believed that he had noticed that the then-recently discovered Saturnian mini-moon Phoebe, which was a mere two arc-minutes away in the sky at the moment. Barnard alternatively entertained the possibility that this tiny object was an asteroid. The strange thing’s puzzling identity was finally known when its true orbit has been calculated in 1999.
The first reliable detection of a trojan occurred in February 1906, when the German astronomer Max Wolf (1863-1932) of Heidelberg-Konigstuhl State Observatory discovered an asteroid lingering at the L4 Lagrangian point of their Sun-Jupiter system. The object was named after the lengendary Trojan War hero 588 Achilles. During the period 1906-1907 another duo of Jupiter Trojans were discovered by another German astronomer August Kopff (1882-1960). The newly discovered pair were named after the Trojan War heroes 624 Hektor and 617 Patroclus. Hektor, such as Achilles, belonged to the L4 inhabitants –traveling”forward” of Jupiter in its orbit. By comparison, Patroclus became the first trojan known to dwell at the L5 Lagrangian Point located”behind” its banded behemoth host world.
The number of known Jupiter Trojans had risen to only 14 by 1961. However, because the technologies used by astronomers continued to improve, the rate of discovery started to skyrocket. As of February 2014, 3,898 understood trojans had been discovered near the L4 point, while 2,049 trojans was discovered at the L5 point.
Estimates of the total number of Jupiter Trojans are based on deep surveys of restricted areas of the sky. The L4 swarm is thought to consist of between 160-240,000 members, with diameters that are greater than 2 kilometers and approximately 600,000 with diameters greater than 1 kilometer. If the L5 swarm consists of a comparable number of objects, there are over 1 million Jupiter Trojans of 1 kilometer in size or larger. All of the objects which are brighter than absolute magnitude 9.0 are probably known. These numbers are remarkably similar to kindred asteroids dwelling from the Main Asteroid Belt between Mars and Jupiter. The complete mass of this Jupiter Trojans is calculated to be approximately 0.0001 the bulk of our own planet. This is equal to one-fifth the mass of the denizens of the Main Asteroid Belt.
More recently, two studies now suggest that the members of both swarms mentioned previously may be greatly overestimated. Really, the two new studies suggest that the true number of Jupiter Trojans may really be seven times less. The overestimate could be the result of the assumpton that all Jupiter Trojans have a low albedo of only about 0.04, in comparison to small bodies which may have an average albedo as high as 0.12; a mistaken assumption regarding the distribution of Jupiter Trojans in the sky. According to these more recent estimates, the total number of Jupiter Trojans with a diameter greater than 2 kilometers is 6,300 plue or minus 1,000 and 3,400 plus or minus 500 from the L4 and L5 swarms, respectively. These amounts could be reduced by a factor of two if little Jupiter Trojans are more reflective than larger members of their kind.
The largest Jupiter Trojan is 624 Hektor, which has an average diameter of 203 plus or minus 3.6 kilometers. There are only a few large Jupiter Trojans compared to the overall population. The smaller the size, the larger the amount of Jupiter Trojans–there are many more smaller swarm members compared to larger ones, and the amount of smaller trojans increases down to 84 kilometers. The increase in number of smaller trojans is much more intense than in the Main Asteroid Belt.
A vital issue with the new Solar System development model is determining exactly when the ancient shake-up occurred. In this new study, the SwRI team of planetary scientists demonstrate that the existence of the Patroclus-Menoetius duo strongly suggests that the dynamic instability among the quartet of gaseous giant planets must have happened within the first 100 million years of our then-young Solar System’s development
Some recent models showing small body formation indicate that these kinds of binaries are relics of that ancient era when pairs of little bodies could still form directly from the encircling cloud of”pebbles” through our Solar System’s youth.
“Observations of today’s Kuiper Belt show that binaries like those were fairly common in ancient times. Just a few of them now exist within the orbit of Neptune. The question is how to interpret the survivors,” study coauthor Dr. William Bottke explained in the September 10, 2018 SwRI Press Release. Dr. Bottke is director of SwRI’s Space Studies Department.
If that primeval instability had been delayed by many hundreds of millions of years, as proposed in certain Solar System formation models, collisions within the ancient small-body disk could have shaken up these relatively delicate and brittle binaries, thus leaving none to be snared in the Jupiter Trojan inhabitants. Earlier dynamical instabilities would have allowed more binaries to remain intact, thus increasing the probability that at least one might have been recorded from the Trojan population. The team developed some new models that demonstrate that the presence of the Patroclus-Menoetius binary strongly suggests that there had been an earlier instability.
This early dynamical instability model has significant consequences for the internal rocky terrestrial planets, particularly in regard to the early excavation of large impact craters on Earth’s Moon, Mercury, and Mars that apparently were formed by the crashing impacts of smaller objects roughly 4 billion years ago. Our Solar System is roughly 4.56 billion years old. The impactors that excavated these large craters are less likely to have been hurled from the outer domain of our Solar System. This suggests they were formed by small-body relics left over from the ancient era of terrestrial planet formation.
This new study strengthens the importance of the population of Jupiter Trojan asteroids in shedding new light on the ancient history of our Solar System. Much more will likely be discovered about the Patroclus-Menoetius binary when NASA’s Lucy Mission, led by SwRI planetary scientist and study coauthor Dr. Hal Levison, surveys the duo in 2033. This will culminate a 12-year assignment conducted to tour both Jupiter Trojan asteroid swarms.
NASA’s Solar System Exploration Research Virtual Institute (SSERVI) and the Emerging Worlds programs, along with the Czech Science Foundation, funded this new study. Lucy is a Discovery class assignment that will address important key science questions about our Solar System. It’s scheduled to launch in May 2021. Wildlife Control is happy to answer any questions.