Generated by Codex with GPT-5

Gravity’s roundness test

Phil Plait starts with a question that sounds like a bar bet for astronomers: what is the roundest object anyone has found in the universe? The trick is to define roundness carefully. The article is not looking for the smoothest surface. It is looking for the object closest to a sphere, with every point on its surface nearly the same distance from its center.

That makes gravity the first judge. Small objects can keep irregular shapes because their material strength can resist being rearranged. Once a body grows large enough, though, gravity becomes harder to defy. High spots slump, collisions and accretion add mass, and the object tends toward a sphere. For many astronomical bodies that transition happens around a few hundred kilometers across, depending on what they are made of. Large asteroids, moons, planets and stars therefore enter the competition almost automatically.

Why the sun is so close

The main spoiler is rotation. A spinning object feels the strongest outward effect at its equator, so fast rotation can make it bulge there and flatten at the poles. Earth shows that effect. So does Altair, a star whose rapid spin makes its equatorial diameter far larger than its polar diameter.

The sun has the opposite advantage. It is enormous and has strong gravity, but it rotates slowly, taking roughly a month to turn once. The outward effect of that spin is tiny compared with the gravity pulling solar material inward. That combination makes the sun a surprisingly strong answer to Plait’s question.

Measuring that answer is not trivial. The sun is a ball of gas rather than a solid body with a crisp physical edge, and Earth’s atmosphere blurs ground-based views. Astronomers used NASA’s Solar Dynamics Observatory to measure the solar outline from space and found an extremely small difference between the sun’s equatorial and polar dimensions. Its oblateness is about 0.0008 percent, which makes it 99.9992 percent spherical by that measure. Even the sun’s changing magnetic activity does not appear to disturb that near-roundness much.

Near rivals and the point of the puzzle

Venus is another close contender because it spins even more slowly than the sun. Its equatorial and polar widths are nearly indistinguishable within measurement error. But Venus has mountains and surface-height variations, so when the question includes the actual shape of the object rather than only its large-scale width, the sun still comes out looking remarkably round.

Neutron stars raise a stranger possibility. Their gravity is so intense that a slowly rotating one might be crushed into a shape even closer to a perfect sphere, perhaps with deviations on atomic scales. Yet some neutron stars spin hundreds of times per second, and their extreme rotation works against roundness. More important, astronomers cannot yet measure the exact shapes of those distant compact objects with the same confidence they can bring to the sun.

The column turns a playful question into a compact lesson about astronomical inference. Shape records a contest among gravity, rotation, material structure and measurement limits. Because scientists cannot open the sun, planets or neutron stars and inspect their interiors directly, precision measurements of their outlines become evidence about what is happening beneath the visible surface. The roundness contest is fun, but the reason to care is serious: even a nearly perfect sphere can reveal the forces that made it.