Decked out in colorful clown-like bands and pockmarked by whirling crimson 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 move as it wends its way around our Sun. The Jovian Trojan Asteroids are a large 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) in San Antonio, Texas, announced their new findings revealing the true nature 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 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 studied by SwRI scientists carry the fabled names of Petroclus and Menoetius. The duo are also targets of NASA’a upcoming Lucy mission that aims 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 circle around their planet together, both bound slavishly to their wandering enormous world. They are the only large binary known to exist among the two 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–occurred,” noted Dr. David Nesvorny in 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 title: Evidence for Very Early Migration of the Solar System Planets from the Patroclus-Menoetius Binary Jupiter Trojan, published 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 primeval population of small bodies believed 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 frozen 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 as if it were just an especially large Star sailing through a dark celestial sea with myriad other stars. The dwarf planet Pluto is one of the largest known KBOs.
“Many small bodies of this primordial Kuiper Belt were scattered inwards, and a few of those 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 pair of gas-giants are much larger than our Solar System’s duo of ice-giants, and they also sport much thicker gaseous envelopes. 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 pair may not even contain solid cores at all, but may be composed entirely of gases and liquids.
The Jupiter Trojans are dark, and show featureless, reddish spectra. There is no strong evidence of the presence of water, or any other specific compound, on their surfaces based on their spectra. However, 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 studies of binaries or rotational light curves) that vary, and they are thought to have been gravitationally snared into their current orbits during the early stages of our Solar System’s evolution–or, perhaps, slightly later, during the period of the migration of the giant planets.
All stars, our own Sun included, are born surrounded by a whirling, swirling disk of gas and dust, which is termed a protoplanetary accretion disk. These rings encircle baby stars, and they contain 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 beyond our Sun, evolve when an extremely 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, as if they were lovely floating phantoms swimming through the space between stars. These dark clouds serve as the strange birthplace of infant stars.
Most of the collapsing blob collects at the center, and ultimately ignites as a result of nuclear-fusion reactions–and a star is born. What remains of the gas and dust 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 youthful star (protostar) for as long as ten million years.
By the time a star like our Sun has reached the T Tauri stage of its toddler years, the extremely hot, massive surrounding disk 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 extremely active at the tender age of a mere 10 million years. T Tauris are born with large diameters that are several times greater than the diameter of our Sun today. However, T Tauris are 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 started to condense close to 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 become 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 era 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 relationships with larger bodies. For example, there are Martian trojans and Neptune trojans. 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 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 planet 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, 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 after Lagrange had announced his hypothesis. Those associated with Jupiter were the first to be observed.
Relative to their enormous host planet, each Jovian Trojan librates around one of Jupiter’s two stable Lagrange Points: L4 that is located 60 degrees ahead of Jupiter in its orbit, and L5 which is situated 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. 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 this time. Barnard alternatively entertained the possibility that this tiny object was an asteroid. The strange object’s puzzling identity was finally understood when its true orbit was 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 the Sun-Jupiter system. The object 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). The newly discovered pair were named after the Trojan War heroes 624 Hektor and 617 Patroclus. Hektor, like Achilles, belonged to the L4 population–traveling “ahead” of Jupiter in its orbit. In contrast, Patroclus became the first trojan known to dwell at the L5 Lagrangian Point situated “behind” its banded behemoth host planet.
The number of known Jupiter Trojans had risen to only 14 by 1961. However, as the technology used by astronomers continued to improve, the rate of discovery began to skyrocket. By January 2000, a total of 257 Jupiter Trojans had been discovered, and by May 2003, the number had ballooned to 1,600! As of February 2014, 3,898 known trojans had been discovered near the L4 point, while 2,049 trojans had been detected at the L5 point.
Estimates of the total number of Jupiter Trojans are based on deep surveys of limited regions of the sky. The L4 swarm is believed 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 that are brighter than absolute magnitude 9.0 are probably known. These numbers are remarkably similar to 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 own planet. This is equivalent 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 above may be greatly overestimated. Indeed, 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 contrast to small bodies that may have an average albedo as high as 0.12; a mistaken assumption concerning 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 in the L4 and L5 swarms, respectively. These numbers could be reduced by a factor of 2 if small 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 few large Jupiter Trojans when compared to the general population. The smaller the size, the greater the number of Jupiter Trojans–there are many more smaller swarm members than larger ones, and the number of smaller trojans increases rapidly down to 84 kilometers. The increase in number of smaller trojans is much more extreme than in the Main Asteroid Belt.
Some Strange Things Happened Long Ago
A key issue with the new Solar System evolution model is determining exactly when the ancient shake-up occurred. In this new study, the SwRI team of planetary scientists demonstrate that the very existence of the Patroclus-Menoetius duo strongly suggests that the dynamic 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 showing small body formation indicate that these types of binaries are relics of that primeval era when pairs of small bodies could still form directly from the encircling cloud of “pebbles” during our Solar System’s youth.
“Observations of today’s Kuiper Belt show that binaries like these were quite common in ancient times. Only 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 some Solar System formation models, collisions within the ancient small-body disk would have shaken up these relatively delicate and fragile binaries, thus leaving none to be snared in the Jupiter Trojan population. Earlier dynamical instabilities would have permitted more binaries to remain 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 that the existence of the Patroclus-Menoetius binary strongly suggests that there had been an earlier instability.
This early 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 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 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 primeval history of our Solar System. Much more will likely be discovered 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 mission 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 class mission that will address important key science questions about our Solar System. It is scheduled to launch in May 2021.