Our planet has four seasons, thanks to its 23.5-degree tilt and the shape of its orbit around the sun. But do other planets have seasonal changes, too? (Image credit: ANDRZEJ WOJCICKI/SCIENCE PHOTO LIBRARY via Getty Images)
Every year on Earth we observe the usual seasonal changes: when summer begins in the Northern Hemisphere, winter begins in the Southern Hemisphere, and vice versa.
But are there seasonal cycles on other planets?
In fact, other planets, dwarf planets, and moons in our solar system also have their own seasonal cycles, and they can be significantly different from those observed on Earth, experts told Live Science.
To understand how seasons happen on other planets, we need to consider what causes seasonal changes on our own planet. “Earth has four seasons because of its axial tilt,” Gongjie Li, an astrophysicist at Georgia Tech, told Live Science. That means our planet rotates at a slight angle of about 23.5 degrees.
So when Earth is on one side of the sun, the Northern Hemisphere faces toward it and the Southern Hemisphere faces away, explained Shane Byrne, a professor of planetary science at the University of Arizona. When our planet passes the other side of the sun six months later, the Northern Hemisphere faces away.
Mars has an axial tilt of about 25 degrees. Because this value is close to Earth's tilt of 23.5 degrees, seasonal changes on Mars are similar to those we see on our planet.
“Like Earth, the polar regions have constant darkness and constant light, depending on whether it's winter or summer; then you can switch between the two every six months,” Byrne told Live Science. Interestingly, though, winter on Mars is dominated by carbon dioxide (or dry ice), rather than water ice, which can form cracks in the planet's surface.
In contrast, some planets in our solar system have significantly different tilts, resulting in more pronounced changes throughout the year. Mercury, for example, has no tilt, so there is “virtually no seasonal variation,” Byrne said. At the opposite end of the spectrum, Uranus has a 90-degree tilt, which causes its poles to either face the sun entirely or not be illuminated at all, meaning its seasons are very intense: Summers feature long periods of uninterrupted sunlight, while winters are plunged into perpetual, freezing darkness.
However, tilt is not the only factor that influences seasonal changes. The shape of a planet’s orbit can also influence climate change. This is because planetary orbits are typically elliptical rather than perfect circles. As a result, some planets are far from the sun at times and very close to it at other times. Mercury, for example, has an “eccentric” orbit, which contributes to its seasonal changes, Lee noted. Pluto also has a highly elliptical orbit, which leads to extreme variations in its climate, Byrne added.
These two factors — the planet’s tilt and the shape of its orbit — can also change over time. Byrne, who studies climate data on Mars, explained that the Red Planet hasn’t always been tilted at 25 degrees. In fact, research published in the journal Earth and Planetary Science Letters in 2018 found that Mars’ tilt has varied from 10 to more than 40 degrees over billions of years. This has led to significant variations in the planet’s annual cycle.
“So it's almost a coincidence that it looks like modern Earth,” Byrne said.
“We’re very lucky on Earth that the axis of rotation is fairly stable,” Lee said. That has given us relatively stable seasonal cycles that have persisted for millennia, although the broader climate sometimes changes as Earth’s entire orbit shifts further or closer to the sun.
Such stability likely helped life as we know it evolve here, Li added. Scientists like her are now studying planetary conditions
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