The set-up of our solar system, which is believed to be unusual as per the galactic planetary census, is said to have been markedly different billions of years ago from what it is now. A new study has attempted to explain how it evolved into its present state in terms of the journey Jupiter undertook in the early years – the first few million years – of the solar system’s existence.
The current layout of the solar system
Early in the history of our solar system, Jupiter is thought to have made a journey towards and away from the sun. This inward and outward movements might have played a great role in the shaping of the solar system. Before the formation of the smaller planets like Mercury, Venus, Earth and Mars, the inner solar system might have held in its midst super-Earths that have later disappeared because of Jupiter.
Konstantin Batygin, a Caltech planetary scientist, and Gregory Laughlin of UC Santa Cruz have described how this could have happened; the findings of their study entitled “Jupiter’s Decisive Role in the Inner Solar System’s Early Evolution” have been published in the journal Proceedings of the National Academy of Sciences (PNAS).
Illustration of planet Jupiter
The results of the research look promising: they might answer a number of pertinent questions about the current layout of the solar system and of our own planet. One of the unusual characteristics of the planets in the solar system is that they have relatively lower masses than planets orbiting around stars other than our sun.
“Our work suggests that Jupiter’s inward-outward migration could have destroyed a first generation of planets and set the stage for the formation of the mass-depleted terrestrial planets that our solar system has today,” says Batygin, an assistant professor of planetary science. “All of this fits beautifully with other recent developments in understanding how the solar system evolved, while filling in some gaps.”
The general appearance of those other systems near to our galaxy are strikingly different from ours. Most of them have one or more planets much bigger than the Earth orbiting closer to their suns than our own relatively small Mercury does to its sun, with few heavenly bodies beyond that region, while we might have a few debris only between Mercury and the sun, with no other planet.
“Indeed, it appears that the solar system today is not the common representative of the galactic planetary census. Instead we are something of an outlier,” says Batygin. “But there is no reason to think that the dominant mode of planet formation throughout the galaxy should not have occurred here. It is more likely that subsequent changes have altered its original makeup.”
So, it seems that our solar system once resembled the type of arrangement found elsewhere.
Explaining the ancient hectic journey of Jupiter, the authors of the new paper mentioned the Grand Tack scenario that posits that in the first few million years of the solar system, the heavenly bodies existed in a disk of gas and dust. At that time, Jupiter grew in size so much that it could exert a powerful gravitational force such that it created a gap in that disk. The sun would pull the disk gas towards itself, thereby attracting Jupiter inward like on a giant conveyor belt.
“Jupiter would have continued on that belt, eventually being dumped onto the sun if not for Saturn,” explains Batygin.
Meanwhile, Jupiter developed a relationship with the younger planet, Saturn. They were close to each other, involved in an interaction known as orbital resonance, where their orbital periods were rational: for example, in a 2:1 orbital resonance, Saturn would take the same amount of time to make two orbits around the sun as Jupiter would take to make one. This relationship would cause a certain gravitational influence to exist between the two planets.
“That resonance allowed the two planets to open up a mutual gap in the disk, and they started playing this game where they traded angular momentum and energy with one another, almost to a beat,” says Batygin.
The back and forth movement would have ultimately caused all of the gas floating in between the two to be pushed away. This would have then reversed the migration direction of the planets, propelling them outward in the solar system.
The image is taken from a new simulation done by the researchers. It illustrates the grand inward migration of Jupiter during the early years of the solar system. Jupiter’s orbit is shown as the thick white circle. The inward journey of Jupiter caused planetesimals to be driven into eccentric orbits (turquoise) overlapping the undisturbed part of the planetary disk (yellow), triggering a chain of collisions that would have eventually pushed interior planets into the sun. Photo credits: K. Batygin/Caltech.
In short, the scenario posits that the planets first migrated inwards to eventually change their courses dramatically.
Also, when Jupiter moved towards the sun on its conveyor belt, a gap in the disk is said to have been cleared till the Earth’s current orbit.
What about the empty hole in the inner solar system? The authors suggest that that region corresponds to the orbital neighbourhood that is home to super-Earths in other systems. Therefore, the scientists deem it plausible that this region could have been cleared out during the early years by a group of first-generation planets that ultimately died out.
The calculations of scientists Batygin and Laughlin show that the inward movement of Jupiter pulled all planetesimals (primordial planetary building blocks) that were found in its way into orbital resonances, bringing them closer to the sun. The close proximity with the latter would have then caused their orbits to turn elliptical.
“You cannot reduce the size of your orbit without paying a price, and that turns out to be increased ellipticity,” commented Batygin.
The new orbits would have then caused the planetesimals to pass through previously inaccessible regions of the disk, leading to a chain of collisions among the debris. The impacts thereof would have then broken them apart, sending them flying into the sun. If super-Earths were caught in this series of collisions, they would be pulled into the sun by an avalanche of planetesimals over a period of 20,000 years.
“It’s a very effective physical process,” says Batygin. “You only need a few Earth masses worth of material to drive tens of Earth masses worth of planets into the sun.”
Thereafter, fractions of the planetesimals carried by Jupiter would have returned to circular orbits, and would have been sufficient to make up the planets. Then, it would take millions of years for the former to come together to form the terrestrial planets that now exist in our solar system. These measurements seem to fit with the age of the Earth – the latter is said to have formed 100 to 200 million years after the sun came to be. Furthermore, the disk of hydrogen and helium would have disappeared by that time, such that Earth does not have a hydrogen atmosphere. This would also cause us to differ from other exoplanets.
Commenting on the formation of the Earth relating to these measurements, Batygin said:
“We formed from this volatile-depleted debris.”