Our simulated Marses grow way too big. They’re typically about the mass of Earth, about nine times more massive than the real Mars.

To date there are two viable solutions to the small Mars problem. I’ve written about both before (because I’ve worked on both). The first is the Grand Tack model, which proposes that the inner Solar System was sculpted by Jupiter’s migration. The second is the Low-mass Asteroid belt model, which instead proposes that there was never much rocky stuff between Earth and Jupiter’s orbits in the first place.

Today I’ll present a new (in some ways simpler) solution: the Early Instability model. The scientific paper on the model was led by Matt Clement at the University of Oklahoma and it was just published in Icarus (download it here).

This new solution piggybacks on a different Solar System story. It started in 2005 when a new idea came out, called the Nice model (you can tell it’s a big deal because it has its own Wikipedia page). The key new idea was that the Solar System’s giant planets underwent an instability. Not a super-strong instability as is characteristic of exoplanet systems, but an instability nonetheless.

This animation shows the idea. The giant planets probably formed on a more compact orbital configuration with an outer disk of planetary leftovers called planetesimals. After some time, an instability in the giant planets’ orbits was triggered, causing their orbits to rapidly spread out and change shape. The outer disk of planetesimals was completely cleared out, and one extra ice giant may even have been kicked out of the Solar System (Planet Nine anyone?).