by Spring, summer, fall and winter – the seasons on Earth change every few months, around the same time every year. It is easy to take this cycle for granted here on Earth, but not every planet has a regular change in seasons. So why does Earth have regular seasons when other planets don’t?
I am an astrophysicist who studies the movement of the planets and the causes of the seasons. During my research, I discovered that the regular pattern of the Earth’s seasons is unique. The axis of rotation on which the Earth turns, along the North and South poles, is not perfectly aligned with the vertical axis perpendicular to the Earth’s orbit around the sun.
That slight tilt has big implications for everything from seasons to glacier cycles. The amount of that tilt can even determine whether a planet is habitable for life.
Related: A scorching alien planet brings the seasons to an end
Seasons in the world
When a planet has perfect alignment between its orbital axis and the rotational axis, the amount of sunlight it receives as it orbits the sun – assuming its orbital shape is a circle – is determined. Since seasons result from variations in how much sunlight reaches the planet’s surface, a perfectly aligned planet would have no seasons. But the Earth is not perfectly aligned on its axis.
This slight misalignment, known as obliquity, is about 23 degrees from the Earth’s vertical. Therefore, sunlight is more intense in the Northern Hemisphere during the summer, when the sun is more directly above the Northern Hemisphere.
Then, as the Earth continues to orbit the sun, the amount of sunlight the Northern Hemisphere receives gradually decreases as the Northern Hemisphere recedes from the sun. This is due to the winter.
The planets that spin on their axes and orbit the sun look like spinning tops – they spin and swirl due to the gravitational pull from the sun. As top spins, you could notice that it does not just stay perfectly upright and stationary. Instead, it may begin to tilt or wobble slightly. This tilt is what astrophysicists call spin precession.
Because of these wobbles, the Earth’s obliquity is not perfectly fixed. These small tilt variations can have a large effect on the Earth’s climate when combined with small changes in the shape of the Earth’s orbit.
Tilt wobbling and any natural changes in the shape of Earth’s orbit can change the amount and distribution of sunlight reaching Earth. These small changes contribute to the larger temperature changes of the planet over thousands to hundreds of thousands of years. This, in turn, can drive ice ages and periods of warming.
Obliquity to translate into seasons
So how do obliquity variations affect the seasons on a planet? Obliquity is low, meaning that the rotational spin axis is aligned with the orientation of the planet as it orbits the sun, resulting in stronger sunlight at the equator and low sunlight near the pole, as on Earth.
On the other hand, a high declination results in very hot or cold poles – meaning that the planet’s spin axis is pointing towards the sun or away from the sun. At the same time, the equator gets cold, because the sun does not shine above the equator throughout the year. This results in highly variable seasons at high latitudes and low temperatures at the equator.
When a planet has an obliquity greater than 54 degrees, the equator of that planet grows icy and the pole becomes warm. This is called reverse zoning, and it is the opposite on Earth.
Essentially, if obligates have large and unpredictable variations, the seasonal variations on the planet become wild and difficult to predict. A large dramatic change in obliquity can turn the entire planet into a snowball, where it is covered by ice.
Spinning orbital resonance
Most planets are not the only planets in their solar system. Their planetary siblings can interfere with each other’s orbits, which can cause variations in the shape of their orbits and orbits.
Therefore, the orbiting planets look like tops spinning on the roof of a car hurtling down the road, where the car represents the orbital plane. When the rate – or frequency, as scientists call it – at which the tops are bouncing, or spinning, matches the frequency with which the car is bumping up and down, something called orbital resonance occurs turning
These subtle variations can be created by the spin-orbit resonances, which is when a planet moves on its axis. Think of pushing a child on a swing. When you push at the right time – or at the resonant frequency – they will swing higher and higher.
Mars moves more on its axis than the Earth does, even though both are tilted about the same amount, and that has to do with the moon orbiting the Earth. Earth and Mars have similar spin frequencies, which match the orbital oscillation – the ingredients for spin-orbit resonance.
But the Earth has a huge moon, which pulls back the Earth’s spin and drives it to go forward faster. This slightly faster precursor prevents spin orbit resonance. Therefore, the moon stabilizes the obscurity of the Earth, and the Earth does not move on its axis as much as Mars does.
Exoplanet seasons
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Thousands of exoplanets, or planets outside our solar system, have been discovered in the past decades. My research group wanted to understand how habitable these planets are, and whether these exoplanets also have wild obliquities, or whether they have moons to stabilize them like Earth does.
To investigate this, my group is leading the first investigation into the axis-axis variations of exoplanets.
We investigated Kepler-186f, which is the first Earth-sized planet in the habitable zone. The habitable zone is the area around a star where liquid water can exist on the planet’s surface and life can develop and thrive.
Unlike Earth, Kepler-186f is located far from the other planets in its solar system. As a result, these other planets have only a weak effect on their orbit and movement. Thus, Kepler-186f generally has a fixed obliquity, like Earth. Even without a big moon, it doesn’t change wildly or have unpredictable seasons like Mars.
Looking ahead, further research on exoplanets will help scientists understand what seasons look like across the vast variety of planets in the universe.
