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 fabulous family. This magnificent banded-behemoth, like other monarchs, has a devoted retinue of followers accompanying its every movement as it wends its way around our Sun. The Jovian Trojan Asteroids are a huge group of rocky followers who share their planet’s orbit, and write two distinct stable groups–one group that travels ahead of the world 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 odd 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 rather young and forming.
The Trojan Asteroids are named for heroes appearing in the classic Greek epic poems, the Iliad and the Odyssey, both attributed to Homer. The duo of Trojan Asteroids analyzed by SwRI scientists carry the fabled names of Petroclus and Menoetius. The duo are also targets of NASA’a upcoming Lucy assignment that aims 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 circle around their planet together, both bound slavishly to their wandering enormous world. They’re the only large binary known to exist among the two heavy populations of Trojan Asteroids.
“The Trojans were likely captured through a dramatic period of dynamic instability when a skirmish between the Solar System’s giant planets–Jupiter, Saturn, Uranus and Neptune–occurred,” 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 outward, where they met up with a large ancient inhabitants of small bodies considered to be the ancestors of today’s 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 remote region of perpetual twilight the Sun casts its feeble fires from so far away that it hangs suspended in the sky as if it were just an especially large Star sailing through a dark celestial sea with myriad other celebrities. The dwarf planet Pluto is among the largest known KBOs.
“Many tiny bodies of the primordial Kuiper Belt were scattered inwards, and a few of them became trapped since Trojan Asteroids,” Dr. Nesvorny added. The smaller ice-giants are considered to have bigger solid cores enshrouded by thinner gaseous atmospheres compared to those that cloak both Jupiter and Saturn. Additionally, 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’s not any strong evidence of the presence of water, or any other special chemical, on their surfaces based on their spectra. But many planetary scientists propose they’re encased in tholins, which are organic polymers formed by our Sun’s radiation. The Jupiter Trojans display densities (based on studies of binaries or rotational light curves) that change, and they are considered to have been gravitationally snared into 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 dust and gas, 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, in addition to other systems surrounding stars outside our Sun, evolve when an extremely dense and relatively compact blob–tucked inside 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, as if 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 finally ignites as a consequence of nuclear-fusion reactions–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 first phases, such accretion disks are both extremely massive and very hot, and they are able to linger around their young celebrity (protostar) for as long as ten million years.
From the time a star like our Sun has attained the T Tauri stage of its toddler years, the extremely hot, massive surrounding disk has grown both cooler and thinner. These stellar toddlers are variable stars, and are extremely active at 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. Unlike human tots, T Tauris psychologist as they develop. By the time a stellar 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 exceptionally sticky and smoke-like motes of dust.
The small 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’s 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 formed. In our own Solar System, comets and asteroids are remnants of the primordial planetesimals.
The term “trojan” has come to be used more generally to refer to other small Solar System bodies that display similar connections with larger bodies. In addition, the gas-giant planet Saturn has an entourage of trojan moons. Indeed, NASA has recently announced the discovery of an Earth trojan! The expression Trojan Asteroid itself is commonly understood to specifically refer to the Jupiter Trojans since the first Trojans were discovered close to Jupiter’s orbit–and Jupiter also now has by far the most known Trojans.
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’ll be gravitationally snared if it’s close to certain points (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 captured in Lagrange Points were discovered more than a century later Lagrange had declared his hypothesis.
Relative to their tremendous host planet, every Jovian Trojan librates around among Jupiter’s two stable Lagrange Points: L4 that’s located 60 degrees ahead of Jupiter in its orbit, and L5 which is located 60 degrees behind.
The American astronomer E.E. Barnard (1857-1923) conducted the first recorded observation of a trojan, (12126) 1999 RM11 (identified as A904 RD at the time of its discovery), in 1904. But, neither Barnard nor other astronomers understood its significance at the time. Indeed, Barnard mistakenly believed that he had noticed that the then-recently found Saturnian mini-moon Phoebe, which was a mere two arc-minutes away from the sky at this time. Barnard alternatively entertained the possibility that this tiny object was an asteroid. The strange object’s puzzling identity was eventually understood when its true orbit was 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 the Sun-Jupiter system. The thing eventually 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). Hektor, such as Achilles, belonged to the L4 inhabitants –traveling”ahead” of Jupiter in its orbit. In contrast, Patroclus became the first trojan known to live at the L5 Lagrangian Point situated”behind” its banded behemoth host world.
The amount of known Jupiter Trojans had climbed to only 14 by 1961. However, as the technology 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 had been discovered at the L5 point.
Estimates of the total number of Jupiter Trojans are based on deep surveys of restricted regions 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 1 kilometer. If the L5 swarm contains a comparable number of objects, 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 probably known. These numbers are remarkably like kindred asteroids dwelling in the Main Asteroid Belt between Mars and Jupiter. The total mass of the Jupiter Trojans is calculated to be approximately 0.0001 the mass of our planet. This is equivalent to one-fifth the bulk of the denizens of the Main Asteroid Belt.
More recently, two studies now suggest that the members of both swarms mentioned previously could be greatly overestimated. Indeed, the two new studies indicate that the true number of Jupiter Trojans may really be seven times less. The overestimate might be the effect of the assumpton that all Jupiter Trojans have a low albedo of only about 0.04, in contrast to small bodies that may have a normal albedo as high as 0.12; a mistaken assumption regarding the distribution of Jupiter Trojans from the skies. 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 little Jupiter Trojans are more reflective than larger members of the kind.
The most significant Jupiter Trojan is 624 Hektor, which has a mean diameter of 203 plus or minus 3.6 kilometers. There are few big Jupiter Trojans when compared to the overall 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 a lot 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 energetic instability among the quartet of gaseous giant planets must have occurred within the first 100 million years of our then-young Solar System’s evolution
Some recent models revealing small body formation indicate that these kinds of binaries are relics of that ancient era when pairs of little bodies could still form straight from the encircling cloud of”pebbles” through our Solar System’s youth.
“Observations of today’s Kuiper Belt show that binaries like these were fairly common in ancient times. Just a few of them now exist within the orbit of Neptune. The question is how to translate the survivors,” study coauthor Dr. William Bottke explained in the September 10, 2018 SwRI Press Release. Dr. Bottke is manager of SwRI’s Space Research Department.
If this primeval instability was delayed by many hundreds of millions of years, as suggested in some Solar System formation models, collisions within the early small-body disk could have shaken up these comparatively delicate and fragile binaries, thus leaving none to be snared in the Jupiter Trojan inhabitants. Earlier dynamical instabilities could have permitted more binaries to stay intact, thus increasing the probability that at least one would have been captured in the Trojan population. The team developed some new models that demonstrate the presence of the Patroclus-Menoetius binary strongly suggests that there was an earlier instability.
This ancient dynamical instability model has important consequences for the inner rocky terrestrial planets, especially in regard to the ancient excavation of large impact craters on Earth’s Moon, Mercury, and Mars that apparently were formed by the crashing impacts of smaller objects about 4 billion years back. 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 the 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 significance of the populace of Jupiter Trojan asteroids in shedding new light on the primeval history of our Solar System. Much more will probably be found about the Patroclus-Menoetius binary when NASA’s Lucy Mission, headed by SwRI planetary scientist and study coauthor Dr. Hal Levison, polls the duo in 2033. This will culminate a 12-year mission 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 course assignment that will address important key science questions about our Solar System. It is scheduled to launch in May 2021. Animal Removal loves this kind of stuff.