Demystifying science | The mystery of round planets

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Why are the planets all round?
— Brigitte Meunier

It’s a matter of gravity.

“The moment a celestial body reaches a certain size, it becomes so massive that it compresses on itself,” explains Eve Lee, an astrophysicist at McGill University who specializes in planet formation. “There are forces in all directions that are equal, which leads to a round shape. »

This shape can be a little oblong in the presence of certain parameters. “If there are strong tides or rapid rotation, the shape will be less round,” says M.^me Lee. In most cases, planets are a little thicker in the center due to rotation. There is a centrifugal force at the equator. »

The Earth is thus 0.3% larger at the equator, which is equivalent to a greater radius of around twenty kilometers. This equatorial bulge, predicted by Newton at the beginning of the 18th century^e century, was confirmed from space by the American satellite Vanguard 1launched in 1958.

Exoplanets

In his work, M^me Lee is trying to understand how planets form around other stars, compared to what happens in our solar system. The first “exoplanets” were discovered almost 30 years ago and there are now 5,000 of them.

“There are two big differences,” she said. There are no planets between the Sun and Mercury, while in other solar systems there are many. And there is a type of exoplanet that does not exist in our solar system. They are called “super-Earths” or “mini-Neptunes”. »

And why are there no planets between the Sun and Mercury?

“There is no consensus,” said M.^me Lee. One hypothesis suggests that initially there was, but that the presence of the giant planets Jupiter and Saturn disrupted the center of the solar system. The planets closest to the Sun would have been expelled out of the solar system or would have collided with other planets. »

Another hypothesis is that our solar system had less matter than most other solar systems.

This particularity could finally be due to our current techniques for detecting exoplanets. “Currently our techniques are much more sensitive to large planets which are close to their star. We may be missing smaller planets located farther away from their stars. »

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