As good as we are getting at seeing young solar systems, there is so much we still don’t understand. 

One of our greatest frustrations is our inability to fully understand how our own solar system formed coupled with our inability to resolve solar systems like ours orbiting distant stars. This system, with four small rocky worlds near the Sun and four, possibly five, gas giants is so far unlike anything else we’ve ever seen. The distribution of objects we see, along with their chemistries and crater histories, can largely be explained by the 2005 Nice Model, which was developed by a large collaboration of researchers who met in Nice, France.

It states, to radically oversimplify, that the gas giants started out in positions much much closer to the Sun, and after some sort of unnamed instability occurred, the gravitational effects of Saturn and Jupiter were a driving force in flinging Uranus and Neptune to the outer solar system. This model works best if a fifth gas giant is added, but the model predicts that the fifth gas giant, or ninth planet, would have been radically flung out of our solar system.

But the model isn’t perfect. The timeline for when all of this moving around took place puts the planet flinging hundreds of millions of years after the gas disk of our early solar system would have been dissipated by the Sun’s heat. This is both inconsistent with the chemistry of Moon rocks and what we are seeing in other solar systems.

Enter Beibei Liu, Sean N. Raymond & Seth A. Jacobson. In a new paper in Nature, these three scientists present detailed models that ask: Could the Sun’s destruction of the gas leftover in the early solar system have been the instability that caused the solar system’s rearrangement?

This one change moves the rearranging of the solar system to the first few tens of millions of years and makes what we see here consistent with other solar systems. That is excellent.

It also has an interesting change involving a possible fifth gas giant. In the Nice model, and here I quote a release from Michigan State: …that extra planet was hammer-thrown from our solar system during the instability. 

If that’s true, Michael Brown’s ninth possible planet isn’t out there, unless it was stolen from another solar system. 

In this revised model, the potential extra planet is not required in the same way but also isn’t flung away in the same way. So, more work is needed, more observations are needed, and hopefully, we are within a couple of years of knowing if our solar system has one more gas giant and being able to use that information to revise our understanding of the solar system.

More Information

MSU press release

“Early Solar System instability triggered by dispersal of the gaseous disk,” Beibei Liu, Sean N. Raymond, and Seth A. Jacobson, 2022 April 27, Nature