Are there any planets in the cosmos that aren’t round?

Every planet in our planetary system (solar system) is essentially spherical. But out in the cosmos, are there any planets that aren’t round?

Technically, planets are spherical, by definition; they ought to have sufficient mass to produce the gravity essential to pull themselves into a spherical shape. “Actually, one of the specifications for being a planet is, they have enough mass that makes them round,” says Susana Barros (a senior researcher at the Institute of Astrophysics and Space Sciences in Portugal). But that no way necessarily means planets are perfect spheres. “We call them round, but they’re not really perfectly round, including our own Earth,” says Amirhossein Bagheri (a planetary science and geophysics researcher at the California Institute of Technology).

Earth and planets akin to it often have a bulge around the equator by virtue of centrifugal force, the outward force experienced by objects that’re spinning. On Earth, the bulge is minor but significant: Due to variances in centrifugal force and the distance from Earth’s center, things weigh about 0.5% less at the equator than they weigh at the poles. But this effect can be drastic in the right circumstances. “If the planet is rotating very fast, the poles will flatten,” Barros says, resulting in a squished, football-like shape.

Centrifugal force isn’t the lone force that can alter the shape of a planet. “If the body is close enough to the host star, then these gravitational forces that are acting on the body become so large that the planet gets elongated,” Bagheri says. One such body is the exoplanet WASP-103 b, a gas giant double the size and 1.5 times the mass of Jupiter, that orbits a star nearly double as large as the sun.

WASP-103 b is also “really, really close to the star,” Barros says. That alters its shape. “There’s a balance between the force of the gas that’s called the hydrostatic equilibrium, that wants to expand the planet … and the strength of the gravitational attraction.” This pull from the host star results in a planet that’s “tear-shaped,” Barros said. This deformation can even alter the way the planet rotates. If a planet starts out with a noticeable bulge toward the home star and remains rotating normally, “then this bulge would not always be in the same place,” Barros opines. Moving that bulge around the planet as it rotates spends a lot of energy. “So they start like this, but then, very fast, they will align,” Barros says. The planet becomes tidally locked to its host star, with the same side of the planet facing the star always.

WASP-103 b is orbiting its star very quickly, leading to a flattening of its poles, Barros says. The resultant is a very squished planet. But even a squished sphere is still mostly spherical. Some researchers have posed the possibility of a toroidal — or doughnut-shaped — planet. This could theoretically happen if a planet were rotating fast enough for the outward centrifugal force to outweigh the gravitational force pulling the planet’s mass toward its center. But a toroidal planet has till now not been observed, and it’s not probable to be in the near future. “It’s more science fiction than science,” Bagheri says.