Earth’s seasons come from the planet’s tilt: as we orbit the sun, one hemisphere leans towards it, receiving more direct sunlight, while the other leans away.

Our star is a particularly calm one. Hot, massive “early-type” stars can spin almost 100 times faster than the sun, causing them to bulge in the middle. The gas around the star’s equator is then farther away from its centre, so it cools more than other parts of the star’s surface, in a phenomenon called gravity-darkening. The poles, meanwhile, remain hot and dense.

John Ahlers at the University of Idaho in Moscow wondered how gravity-darkening might change the seasons on a planet orbiting such a squished star.

If the planet orbits in the plane of the star’s equator, like the planets in our solar system do, then gravity-darkening could have no effect at all. But if the planet’s orbit is at an angle, it could receive much more light during some parts of its year than others. Ahlers found that the differences in exposure could mean changes of up to 15 per cent in the planet’s temperature over the course of each orbit.

Shifting radiation

Early-type stars emit mostly UV light, so this corresponds to yearly shifts of up to 80 per cent in the UV radiation hitting the planet. And because these planets would be directly over the star’s chilled equator twice in each orbit, it would go through two summers and two winters each year.

In extreme situations, the planet’s surface could rapidly oscillate between a boiling hellscape and a frozen tundra.

“Say you or I were standing on the surface of that planet,” says Ahlers. “Throughout the year, the shape and size and colour of the sun in the sky would change. And that would correspond to these very unusual changes in temperature.”

The scenario sounds plausible, says Jessie Christiansen at the NASA Exoplanet Science Institute at the California Institute of Technology in Pasadena. But there are many other factors that affect a planet’s seasons, like its atmosphere and composition. “You have to make a lot of assumptions about that planet to really start talking about whether it would experience ‘seasons’ as we know them,” she says.

Source: New Scientist