Jupiter Trojan - Origin and Evolution

Origin and Evolution

Two main theories have emerged to explain the formation and evolution of the Trojans. The first suggests that the Trojans formed in the same part of the Solar System as Jupiter and entered their orbits while the planet was forming. The last stage of Jupiter's formation involved runaway growth of its mass through the accretion of large amounts of hydrogen and helium from the protoplanetary disk; during this growth, which lasted for only about 10,000 years, the mass of Jupiter increased by a factor of ten. The planetesimals that had approximately the same orbits as Jupiter were caught by the increased gravity of the planet. The capture mechanism was very efficient—about 50% of all remaining planetesimals were trapped. This hypothesis has two major problems: the number of trapped bodies exceeds the observed population of Trojans by four orders of magnitude, and the present Trojan asteroids have larger orbital inclinations than are predicted by the capture model. However, simulations of this scenario show that such a mode of formation also would inhibit the creation of similar Trojans around Saturn, and this has been borne out by observation: to date no Trojans have been found near Saturn.

The second theory, part of the Nice model, proposes that the Trojans were captured during planetary migration, which happened about 500–600 million years after the Solar System's formation. The migration was triggered by the passage of Jupiter and Saturn through the 1:2 mean motion resonance. During this period Uranus, Neptune and to some extent Saturn moved outward, while Jupiter moved slightly inward. Migrating giant planets destabilized the primordial Kuiper belt, throwing millions of objects into the inner Solar System. In addition, their combined gravitational influence would have quickly disturbed any pre-existing Trojans. Under this theory, the present Trojan population eventually accumulated from runaway Kuiper belt objects as Jupiter and Saturn moved away from the resonance.

The long-term future of the Trojans is open to question, as multiple weak resonances with Jupiter and Saturn cause them to behave chaotically over time. In addition, collisional shattering slowly depletes the Trojan population as fragments are ejected. Ejected Trojans could become temporary satellites of Jupiter or Jupiter-family comets. Simulations show that the orbits of up to 17% of Jupiter's Trojans are unstable over the age of the Solar System. Levison et al. believe that roughly 200 ejected Trojans greater than 1 km in diameter might be traveling the Solar System, with a few possibly on Earth-crossing orbits. Some of the escaped Trojans may become Jupiter-family comets as they approach the Sun and their surface ice begins evaporating.