Building Earths in Tandem: A New Theory for Planet Formation

Tokuhiro Nimura and Toshikazu Ebisuzaki propose a new way of explaining how Earth-sized planets like ours form. Their model builds on earlier “tandem planet formation” theory, which describes planet growth at two special regions of a star’s disk: the inner edge and the outer edge of what’s known as the MRI-suppressed region. MRI, or magneto-rotational instability, normally stirs up gas in the disk, but in certain places this turbulence weakens. These calm zones become natural traps for solid particles, making them good spots for building planets. The authors focus on how rocky planets can grow at the inner MRI edge and gas giants at the outer edge, providing a possible explanation for why the solar system looks the way it does.

Background and Challenges in Earlier Models

The introduction of the paper places this work in context. Older models of planet formation assumed planets grew slowly where they were, without migrating much. But that raised two big problems: the growth rates were too slow to build giant planets in time, and there was no clear explanation for the “gap” between rocky worlds like Earth and gas giants like Jupiter. Other ideas, like the Grand Tack model (where Jupiter and Saturn migrate inward and then outward), solve some of these issues, but rely on large planetary movements. Nimura and Ebisuzaki instead argue that natural particle concentrations at MRI boundaries can explain both the fast growth and the rocky/gas planet divide.

How the Model Works

In the methods section, the authors lay out their assumptions for how disks evolve and how particles behave. They model how tiny grains collide, grow into “pebbles,” and then drift toward the star. At the inner MRI boundary, gas pressure prevents further inward drift, causing pebbles to pile up. Gravity eventually pulls these dense layers together into planetesimals, which then grow into Earth-mass planets. Importantly, once a planet reaches a certain size, gravitational torques from the gas disk force it to migrate outward, leaving space behind for another planet to form. This cycle repeats until the solid material is used up.

Results of the Simulations

The results of their calculations show how this process plays out under different disk conditions. In their most solar system–like case, the model naturally produced two planets about the mass of Earth, closely resembling Earth and Venus. The smaller planets, like Mars and Mercury, could arise from leftover material or special locations such as Lagrange points. Interestingly, the model also shows that depending on disk parameters, multiple Earth-mass planets could form, or even larger “super-Earths” and hot Jupiters, echoing what astronomers see in other planetary systems.

Linking the Model to Earth and Other Planets

In the discussion, Nimura and Ebisuzaki go further by connecting their model to Earth’s composition and history. Because the inner MRI region is a hot, reducing environment, it would naturally form planets with compositions like enstatite chondrites, matching Earth’s unusual chemistry. They also argue that volatile elements like water likely arrived later through asteroid impacts, rather than being present from the beginning. The paper even touches on the origins of Mars and Mercury, suggesting they might represent smaller “depletion planets” or have formed at different boundaries in the disk.

Broader Implications and Future Work

Finally, the authors conclude that their tandem disk model not only reproduces the terrestrial planet distribution of the solar system but also offers a broader framework to explain exoplanet diversity. Future work, they note, will focus on chemical modeling of early Earth and better simulations of orbital evolution. If correct, this theory provides a natural way to form Earth-like planets without requiring dramatic migrations of giant planets.

Source: Nimura