Jupiter may be the largest planet in our Solar System (by far!), but that doesn’t mean it hasn’t traveled. Astronomers have long postulated that the gas giants, Jupiter and Saturn, formed far from the Sun, and traveled inward, to their current positions between Mars and Uranus. New computer simulations from researchers at Lund University in Sweden now provide some of the best evidence yet this theory is correct.
Jupiter is accompanied by two groups of asteroids, called Trojan asteroids, which travel around the Sun in the same orbit as the King of the Planets, just ahead and a little behind that massive world. One bizarre aspect of their behavior, found by the WISE spacecraft, is that the cluster of asteroids ahead of Jupiter is roughly 50 percent larger than the grouping of Trojans traveling behind the planet. This population density could best be explained, researchers concluded, if Jupiter formed far from the Sun and migrated inward, closer to our parent star.
“This is the first time we have proof that Jupiter was formed a long way from the sun and then migrated to its current orbit,” Simona Pirani, doctoral student in astronomy at Lund University, explains.
The Tale of an Ancient Journey
As the planets of the Solar System were forming from a cloud of gas and dust 4.5 billion years ago, Jupiter was no larger than the Earth is today. At that time, the simulations show, the primordial planet was four times further away from the Sun than it is in our modern age, orbiting at 3.1 billion kilometers (1.9 billion miles) from our stellar companion. Gases in the outer atmosphere would have nudged this world, just two to three million years old, toward the Sun.
The computer simulations suggested that the asymmetry in the number of Trojan asteroids could be explained if Jupiter formed in the outer solar system as a planet-sized asteroid of ice, and traveled closer to the Sun. During its 700,000 year migration, gravitational forces resulted in more asteroids gathering in front of it than the number that fell behind the world, simulations showed.
Both Venus and the Earth are also accompanied by a collection of asteroids which follow their respective planets in their journey around the Sun.
Lucy, I’m Home!
In 2021, NASA will launch Lucy, the first spacecraft ever sent to explore the Trojan asteroids of Jupiter. Researchers hope study of these bodies will unravel secrets about the formation of the Solar System, as well as the origins of organic material, and even life, on Earth. Trojans are thought to contain large quantities of carbon-rich materials, and water ice may be found laying under their dusty surface.
“The mission takes its name from the fossilized human ancestor (called ‘Lucy’ by her discoverers) whose skeleton provided unique insight into humanity’s evolution. Likewise, the Lucy mission will revolutionize our knowledge of planetary origins and the formation of the solar system,” NASA officials report on a web page dedicated to the mission.
Lucy will explore one asteroid in the main belt, orbiting between Mars and Jupiter, as well as six Trojan asteroids. These targets will include all three varieties of asteroids in the groups, dark-red P and D types similar to those found in the distant Kuiper Belt beyond Neptune, as well as the C-type, found in the outer bounds of the main belt.
“The Lucy mission is one of those rare moments where a single mission can have a major impact on our understanding of such fundamental questions,” said Keith Noll, Lucy project scientist from the Goddard Space Flight Center.
At the time of its ancient migration, Jupiter was not covered in gas as it is today. This likely means that the Trojan asteroids are composed of similar materials to those once found in the primordial core of Jupiter.
Asteroids hold a bevy of information about the solar system in which we live, and astronomers have been delighted by the secrets revealed from the study of one asteroid — Bennu, as it is explored by the OSIRIS-REx spacecraft. Our knowledge of asteroids, as well as the Solar System and ourselves, could be greatly enhanced when Lucy launches to the Trojan asteroids.
Jupiter may not be the only planet which migrated in this fashion, researchers suggest. It is also possible Saturn, as well as the ice giants of Uranus and Neptune, may have undertaken similar journeys in the distant past.