The path of true love never runs smooth, it is said. Especially on Valentine’s Day. And for young planets, that path turns out to be an inward-moving annulus. 

Impression of a hot Jupiter closer to the Sun. Credit: NASA

A simulation by scientists in France and USA appears to show that Jupiter once strayed to flirt with the inner Solar System, before being “jilted” and sent back to its present-day position.

The effect of this was to form the inner planets, according to the theory, which comes up with mass ratios for Earth and Mars similar to that observed today and which, remarkably, also accurately depicts the Asteroid Belt. If the findings are borne out it will mean that our Solar System has more in common with exosolar systems, in terms of the way planets migrate. In other words, our Solar System wasn’t always as stable as we thought. Early in our Solar System’s formation, the nascent, Gas Giants were bedded snugly inside a protoplanetary disk, like newly-laid eggs in a love nest. However, these eggs had ‘legs’ and wanted to move.

Disk-driven migration meant that the young Jupiter and Saturn lost angular momentum as they were dragged down by gas in the Solar System disk. Like a slowing romance they spiralled ever-further apart, and towards the Sun. According to the simulation, Jupiter could only have moved inwards before Saturn approached its final mass. Like an immature lover, Saturn had let Jupiter slip away. The two planets just needed some time apart.

Jupiter stopped at around 1.5 astronomical units (AU – one AU being the distance between the Earth and Sun). This is the same distance from the Sun that Mars is today. By this stage the inner planets hadn’t formed and Jupiter’s effect was to truncate the inner disk to 1 AU.

But what effect would this have on the Asteroid Belt (lying between 2 and 3.5 AU in the present-day Solar System)? Surely a giant planet’s migration would cause chaos? It seems that inward migration shepherded “S-type” material (lacking so-called volatiles such as nitrogen, water, carbon dioxide, ammonia and methane) to the inner disk. But about 14-percent was scattered outwards up to 3 AU. Jupiter started to migrate outwards again.

The migration meant that objects were scattered again (some of it originating from beyond Neptune), to be captured by the Belt. Much of it stayed there due to stable resonances with a now-settled Jupiter. Both “S-type” volatile-poor, and “C-type” volatile-rich objects populated the Asteroid Belt in a similar distribution to what is observed today.

All this took a few hundred thousand years in the simulation, whereas the subsequent formation of the inner planets took a further 30 million years. So the Asteroid Belt would have been in place before the inner planets formed. The scientists conclude that the Gas Giant migration makes our Solar System more like exoplanetary systems than was realised.

A lot of these systems harbour so-called “hot Jupiters’ – gas giants orbiting extremely close to their parent star. Why didn’t this happen in our case? It seems that a mature, fully-formed Saturn won Jupiter back by pulling the planet into a stable, 2:3 gravitational resonance, now that the disk had cleared. In a perfect Valentines’ Day ending, they lived happily ever after.

The study was carried out by: Kevin J. Walsh and Alessandro Morbidelli from the University of Nice, (Walsh is also affiliated with the Department of Space Studies, Southwest Research Institute); Sean N. Raymond from the University of Bordeaux, and the French National Centre of Scientific Research (CNRS); David O’Brien of the Planetary Science Institute in Tucson, Arizona; Avi M. Mandell of NASA Goddard Space Flight Center.