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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 mean...
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HDE 226868
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2020-06-02T19:01:55Z (almost 4 years ago)
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<h2>Yes, if the orbit isn't circular.</h2><p>Seasons can definitely occur on a tidally locked planet.</p><p>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.</p><p>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 <a href="https://en.wikipedia.org/wiki/Tidal_circularization" rel="noreferrer">circularize the orbit</a>, 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.</p><h2>An example</h2><p>Let's do some calculations with an exoplanet known to be tidally locked.</p><p>Astronomers believe that the planet <a href="https://en.wikipedia.org/wiki/Tau_Bo%C3%B6tis_b" rel="noreferrer">Tau Boötis b</a> is tidally locked to its parent star. However, its orbit isn't perfectly circular - in fact, it has an eccentricity of <span class="math-container">$e=0.023\pm0.015$</span> (almost twice that of Earth's!). It orbits at a distance of <span class="math-container">$a=0.0481$</span> AU. Therefore, its closest approach to the star is <span class="math-container">$0.0467$</span> AU, and its farthest point is <span class="math-container">$0.0492$</span> AU. The star has a luminosity of <span class="math-container">$L=3.06L_{\odot}$</span>.</p><p>Putting this together, we see that the planet should reach a <a href="https://en.wikipedia.org/wiki/Effective_temperature" rel="noreferrer">temperature</a> 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.</p><h2>Some interesting differences</h2><p>Now, seasons on this planet would be <a href="https://physics.stackexchange.com/a/29294/56299">a little bit different</a> 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:</p><ul><li>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.</li><li>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.</li></ul><h2>Other ways to get seasons</h2><p>Now, our planet can get seasons though other mechanisms. For instance, <a href="https://worldbuilding.stackexchange.com/a/73178/627">I've argued that if its parent star is a variable star</a>, 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.</p><p>Essentially, you have some room to play around. Even if you're not satisfied with the orbital eccentricity approach, there are other options.</p>
- <h2>Yes, if the orbit isn't circular.</h2>
- <p>Seasons can definitely occur on a tidally locked planet.</p>
- <p>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.</p>
- <p>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 <a href="https://en.wikipedia.org/wiki/Tidal_circularization" rel="noreferrer">circularize the orbit</a>, 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.</p>
- <h2>An example</h2>
- <p>Let's do some calculations with an exoplanet known to be tidally locked.</p>
- <p>Astronomers believe that the planet <a href="https://en.wikipedia.org/wiki/Tau_Bo%C3%B6tis_b" rel="noreferrer">Tau Boötis b</a> is tidally locked to its parent star. However, its orbit isn't perfectly circular - in fact, it has an eccentricity of <span class="math-container">$e=0.023\pm0.015$</span> (almost twice that of Earth's!). It orbits at a distance of <span class="math-container">$a=0.0481$</span> AU. Therefore, its closest approach to the star is <span class="math-container">$0.0467$</span> AU, and its farthest point is <span class="math-container">$0.0492$</span> AU. The star has a luminosity of <span class="math-container">$L=3.06L_{\odot}$</span>.</p>
- <p>Putting this together, we see that the planet should reach a <a href="https://en.wikipedia.org/wiki/Effective_temperature" rel="noreferrer">temperature</a> 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.</p>
- <h2>Some interesting differences</h2>
- <p>Now, seasons on this planet would be <a href="https://physics.stackexchange.com/a/29294/56299">a little bit different</a> 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:</p>
- <ul>
- <li>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.</li>
- <li>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.</li>
- </ul>
- <h2>Other ways to get seasons</h2>
- <p>Now, our planet can get seasons though other mechanisms. For instance, <a href="https://worldbuilding.stackexchange.com/a/73178/627">I've argued that if its parent star is a variable star</a>, 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.</p>
- <p>Essentially, you have some room to play around. Even if you're not satisfied with the orbital eccentricity approach, there are other options.</p>
#1: Attribution notice removed
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System
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2020-06-01T14:14:00Z (almost 4 years ago)
Source: https://worldbuilding.stackexchange.com/a/177235 License name: CC BY-SA 4.0 License URL: https://creativecommons.org/licenses/by-sa/4.0/