This magnificent banded-behemoth, like other monarchs, has a dedicated 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 distinct 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 at the Southwest Research Institute (SwRI) at 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 when it was still quite young and forming.
The duo of Trojan Asteroids studied by SwRI scientists carry the names of Petroclus and Menoetius. The duo are also goals 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 roughly 70 miles wide and orbit each other as they revolve round their planet together, both bound slavishly to their wandering enormous world. They are the only large binary known to exist among both heavy populations of Trojan Asteroids.
“The Trojans were probably captured through a dramatic period of dynamic instability when a skirmish between the Solar System’s giant planets–Jupiter, Saturn, Wildlife Removal, Uranus and Neptune–happened,” noted Dr. David Nesvorny at a September 10, 2018 SwRI Press Release. Dr. Nesvorny, who’s of the SwRI, is lead author of the paper describing this new study under the title: 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 outward, where they met up with a large ancient inhabitants of small bodies believed to be the ancestors of today’s Kuiper Belt Objects (KBOs), that dancing 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 remote region of perpetual twilight the Sun casts its weak flames from so far away that it hangs suspended from the sky like it were an especially big Star sailing through a dark celestial sea with myriad other celebrities. The dwarf planet Pluto is one of the largest known KBOs.
“Many small bodies of the primordial Kuiper Belt were sprinkled inwards, and some of those became trapped since Trojan Asteroids,” Dr. Nesvorny added. The set of gas-giants are much bigger than our Solar System’s duo of ice-giants, and they also sport much thicker gaseous envelopes. The smaller ice-giants are considered to have bigger solid cores enshrouded by thinner gaseous atmospheres compared to the ones that cloak both Jupiter and Saturn. Additionally, the gas-giant set may not even contain solid cores in any respect, but may be written entirely of fluids and gases.
The Jupiter Trojans are dark, and show featureless, red spectra. There’s not any strong evidence of the presence of water, or any other special compound, 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 screen densities (based on research of binaries or rotational light curves) that vary, and they are considered to have been gravitationally snared in their current 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 is termed a protoplanetary accretion disc . These rings encircle baby stars, and they feature the important ingredients from which an entourage of planets, as well as smaller objects, ultimately emerge.
Our Solar System, as well as other systems surrounding stars outside our Sun, evolve when a very dense and relatively small blob–tucked inside the undulating folds of a dark, frigid, giant molecular cloud–collapses gravitationally under its relentless and merciless gravitational pull. Such enormous, beautiful, and billowing clouds inhabit our Milky Way Galaxy in massive numbers, like they were beautiful floating phantoms swimming through the space between stars. These dark clouds act as the odd birthplace of infant stars.
Most of the collapsing blob collects at the center, and finally ignites as a consequence of nuclear-fusion responses –and a star is born. What remains of the gas and dust of the erstwhile blob becomes the protoplanetary accretion disc from a solar system forms. In the earliest phases, such accretion disks are both extremely massive and very hot, and they are able to linger around their young star (protostar) for as long as ten million years.
From the time a star like our Sun has reached the T Tauri period of its toddler years, the extremely hot, massive surrounding disc has grown both thinner and cooler. A T Tauri star can be compared to a human tot. These stellar toddlers are variable stars, and are very active in the tender age of a mere 10 million years. T Tauris are born with large diameters which are several times larger 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 leading toddler has reached this stage of its development, less volatile materials have begun to condense close to the middle of the swirling surrounding disc, thus forming extremely sticky and smoke-like motes of dust.
The little grains of dust finally 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 become a leading system’s primordial inhabitants of planetesimals, which are the building blocks of planets. What is left of a heavy population of planetesimals, following the era of planet-formation, can linger around their parent-stars for centuries following a mature system–such as our own Solar System–has shaped. Within 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 that display similar connections with bigger bodies. For example, there are Martian trojans and Neptune trojans. Additionally, the gas-giant planet Saturn has an entourage of trojan moons. 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 since the first Trojans were discovered near Jupiter’s orbit–and Jupiter also now 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 planet by residing 60 degrees ahead or behind it will be gravitationally snared if it’s close to certain factors (Lagrange Points). Lagrange, who based his forecast on a three-body problem, demonstrated that the gravitationally trapped body will librate slowly around the point of equilibrium in what he described as a horseshoe or tadpole orbit. These leading and trailing Lagrange Points are called the L4 and L5 Lagrange Points. The first asteroids to be recorded in Lagrange Points were discovered over a century later Lagrange had announced his hypothesis.
Relative to their tremendous host planet, every Jovian Trojan librates around one of Jupiter’s two secure Lagrange Points: L4 that is located 60 degrees ahead of Jupiter in its orbit, and L5 which is situated 60 degrees behind.
However, neither Barnard nor other astronomers understood its significance at the time. Indeed, Barnard mistakenly believed that he had detected 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 small object was an asteroid. The strange thing’s puzzling identity was eventually understood when its true orbit has been calculated in 1999.
The first reliable detection of a trojan happened 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. During the period 1906-1907 another duo of Jupiter Trojans were discovered by another German astronomer August Kopff (1882-1960). Hektor, like Achilles, belonged to the L4 inhabitants –traveling”ahead” of Jupiter in its orbit. By comparison, Patroclus became the first trojan known to live at the L5 Lagrangian Point situated”behind” its banded behemoth host planet.
The number of known Jupiter Trojans had climbed to just 14 by 1961. However, as the technologies used by astronomers continued to improve, the rate of discovery began to skyrocket. As of February 2014, 3,898 understood trojans had been discovered near the L4 stage, while 2,049 trojans had been 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 believed to consist of between 160-240,000 members, with diameters which are greater than 2 km and approximately 600,000 with diameters greater than one kilometer. If the L5 swarm consists of a comparable number of items, there are over 1 million Jupiter Trojans of 1 kilometer in size or larger. All of the objects that are brighter than absolute magnitude 9.0 are likely known. These numbers are remarkably similar to kindred asteroids dwelling from the Main Asteroid Belt between Mars and Jupiter. The complete mass of the Jupiter Trojans is calculated to be approximately 0.0001 the mass of our own planet. This is equivalent to one-fifth the mass of the denizens of this Main Asteroid Belt.
More recently, two studies now suggest that the members of both swarms mentioned above could be greatly overestimated. Really, the two new studies suggest that the real number of Jupiter Trojans may really be seven times less. The overestimate could be the result of the assumpton that Jupiter Trojans have a low albedo of only about 0.04, in contrast to small bodies which might have an average albedo as high as 0.12; a mistaken assumption regarding the distribution of Jupiter Trojans from the sky. In accordance with these more recent estimates, the total number of Jupiter Trojans with a diameter greater than two 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 small Jupiter Trojans are more reflective compared to bigger members of their kind.
The largest Jupiter Trojan is 624 Hektor, which has a mean diameter of 203 plus or minus 3.6 kilometers. There are only a few large Jupiter Trojans compared to the general population. The smaller the size, the larger the amount of Jupiter Trojans–there are many more smaller swarm members than bigger ones, and the amount of smaller trojans increases rapidly down to 84 kilometers. The increase in number of smaller trojans is much more extreme than at the Main Asteroid Belt.
Some Strange Things Happened Long Ago
A key problem with the new Solar System evolution model is determining exactly when the early 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 revealing small body formation indicate that these types of binaries are relics of that primeval era when pairs of little bodies could still form directly from the encompassing cloud of”pebbles” through our Solar System’s youth.
“Observations of the Kuiper Belt show that binaries like those were quite common in ancient times. Just some of them now exist inside 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 suggested in certain Solar System formation models, collisions inside the ancient small-body disk would have shaken up these relatively delicate and brittle binaries, thus leaving none to be snared in the Jupiter Trojan population. Earlier dynamical instabilities would have allowed more binaries to stay intact, thus increasing the probability that at least one might have been recorded in the Trojan population. The team developed some new models that demonstrate the existence of this Patroclus-Menoetius binary strongly suggests that there was an earlier instability.
This early dynamical instability model has significant consequences for the inner 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 approximately 4.56 billion years old. The impactors that excavated these large craters are less likely to have been hurled out from the outer domain of our Solar System. This suggests that they were formed by small-body relics left over from the early era of terrestrial planet formation.
This new study strengthens the significance of the population of Jupiter Trojan asteroids in shedding new light on the primeval history of the Solar System. Much more will likely be found about the Patroclus-Menoetius binary when NASA’s Lucy Mission, headed 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, as well as the Czech Science Foundation, funded this new study. Lucy is a Discovery course assignment that will address important key science questions about our Solar System. It is scheduled to launch in May 2021.