Do seasons occur on a tidally-locked planet?
I have done some research and so much contradict one another or I simply fail to understand.
Do seasons occur on a tidally locked planet that isn't tilted on its axis?
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Depends on the orbit
I'm drawing on my answer to a different question here. Let's start with an overview of why there are seasons. I really like this description:
We have seasons because the earth is tilted (wonky) as it makes its yearly journey around the sun. The Earth's axis is tilted at an angle of 23.5 degrees. This means that the Earth is always "pointing" to one side as it goes around the Sun. So, sometimes the Sun is in the direction that the Earth is pointing, but not at other times. The varying amounts of sunlight around the Earth during the year, creates the seasons.
OK, so if you have a planet like the Earth with a (roughly) circular orbit and the same part of that planet is always facing its star and there's no tilt, then you would not have seasons. Being tidally locked doesn't matter. A planet in a circular orbit with no tilt will not have seasons.
What if the planet has a different type of orbit? Imagine your planet has an orbit like the one shown below. During the time that it is farther from its star, it will be winter on the entire planet. During the (shorter) time that it's near the star, it's summer.
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Yes, if the orbit isn't circular.
Seasons can definitely occur on a tidally locked planet.
Just like normal planets, tidally-locked planets don't need to have perfectly circular orbits. This means that over the course of a single orbit, this planet would receive different amounts of light from the star as it slowly moves away and then towards it. This will be the case for any orbit with a non-zero eccentricity.
The change in the energy received is likely to be small. Tidal locking requires long timescales, and over those same timescales, tidal forces from the star will work to circularize the orbit, reducing its eccentricity and therefore the magnitude of these seasonal differences. However, planets in closer to their stars tidally lock quicker, meaning that a planet close to its star could have a non-negligible seasonal variation while still being tidally locked.
An example
Let's do some calculations with an exoplanet known to be tidally locked.
Astronomers believe that the planet Tau Boötis b is tidally locked to its parent star. However, its orbit isn't perfectly circular - in fact, it has an eccentricity of $e=0.023\pm0.015$ (almost twice that of Earth's!). It orbits at a distance of $a=0.0481$ AU. Therefore, its closest approach to the star is $0.0467$ AU, and its farthest point is $0.0492$ AU. The star has a luminosity of $L=3.06L_{\odot}$.
Putting this together, we see that the planet should reach a temperature of 1706 Kelvin at its closest point, and a temperature of 1662 Kelvin at it farthest point. That's a difference of 46 Kelvin - certainly enough to cause some variation in climate.
Some interesting differences
Now, seasons on this planet would be a little bit different from seasons on Earth. Why? Well, the temperature variations are now entirely due to the orbit, rather than the tilt of the rotation axis. This has a couple of notable consequences:
- The changes due to the seasons will be more uniform, globally. Seasons due to axial tilt affect each hemisphere in opposite ways; in our case, the entire planet is moving closer and further from the star.
- The seasons will be different lengths. Winter comes because the planet is further from the star, but Kepler's second law tells us that planets further away move slower. Therefore, winter will be longer than summer.
Other ways to get seasons
Now, our planet can get seasons though other mechanisms. For instance, I've argued that if its parent star is a variable star, it can experience seasonal variations comparable to the ones we've discussed based only on orbital eccentricity. Indeed, these seasons will remain long after the orbit has circularized.
Essentially, you have some room to play around. Even if you're not satisfied with the orbital eccentricity approach, there are other options.
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Both the other answers (at this time) suggests making the orbit eccentric. The variable distance from the sun makes the planet hot or cold. This is correct.
But there is one important aspect of this situation neither answer mention.
If you have a tidally locked planet in an eccentric orbit, the tidal lock isn't perfect. There is an east-west "wobble". This is due to the planet moving at different speeds while rotating at a fixed speed.
For people living in the twilight zone will see the sun rise and set over the year and this will be much more important than the distance to the sun.
In one zone, the seasons will be
- Sunrise. Sun rises close and big and warm.
- Morning. Sun rises a few degrees off the horizon and also shrinks.
- Evening. Sun falls back to the horizon and shrinks some more.
- Sunset. Sun sets far and small and cold.
- Early Night. Dark and cold. Sun is getting closer, but also further below the horizon.
- Late Night. Dark, but warmer. Sun is getting closer and you can feel it.
On the other side of the planet, these season are reversed. (Cold mornings, warm evenings)
I can't think straight enough to say which of these zones is east and which is west.
If you follow the twilight band north or south the up-and-down wobble will be smaller and smaller but the near-far variation will be unchanged.
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