Research led by astrophysicists Adam Fenton and Dimitris Stamatellos reveals an unexpected finding in the formation process of planets. New simulations suggest that large planets, including Earth-like ones, didn’t start their lives as spheres but rather as more of a saucer or flat disk shape.

Planets were ‘flat’ at first.

Simulation experiments indicate that large planets forming far from their host stars didn’t start their lives as spheres but rather as something called an “oblate spheroid.” As these protoplanets rotate, they gradually accrete material and eventually take on a round shape, much like Jupiter.

Stamatellos says, “We’ve been studying planet formation for a long time, but we’ve never previously thought about controlling the shape of the planets in simulations. We’ve always assumed they were spherical.” Despite the numerous planets discovered in the Milky Way, with over 5,500 confirmed so far, how they formed exactly remains unclear.

When a star is born, it forms from a cluster of gas and dust in a massive, dense cloud in space; this cluster collapses under gravity and begins to rotate. The surrounding material forms a disk, which spirals into the infant star and fuels its growth.

So how does the material in the disk come together?

For smaller terrestrial planets like Earth, Venus, Mars, and Mercury, scientists believe they were built up gradually by accretion and aggregation of rock fragments sticking together.

For larger gas giants, scientists believe something called disk instability could occur. This is where the rapidly cooling disk around the star breaks up into denser clumps under gravity, which then turn into planets.

Fenton, wanting to better understand the planet formation process, designed and ran complex simulations by varying aspects like gas density, temperature, and disk velocity.

Fenton says, “It was an extremely challenging computational project that required half a million CPU hours at the UK’s DiRAC [Distributed Research using Advanced Computing] High-Performance Computing Facility, but the results were incredible and well worth the effort!”

Simulation suggests it’s “flattened

These results revealed that protoplanets of gas giants form initially as a flattened shape as they rotate, which makes sense given the centrifugal force and the fact that the protoplanet is still a relatively loose collection of matter at that stage.

Even well-formed and much more compact planets in the Solar System have equatorial bulges due to centrifugal forces. The simulation also shows that material accumulates on the growing protoplanet more in the polar regions than around the equator.

The research suggests that the characteristics of a protoplanet embedded in a stellar disk can vary depending on the observer’s viewpoint. When viewed from the side, the pancake shape is more pronounced, but when viewed from the top, it’s easy to mistake its round shape for a sphere.

The researchers say that as we get better at detecting these developing planets, it’s important to understand what we’re looking at and how to interpret it.

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