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Would an Earth analog exoplanet which rotates around a solar analog, but without axial tilt, have no seasons?

Would it be similar to how Los Angeles does not have very noticeable seasons compared to other parts of the Earth?

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There are several factors that cause seasons. In approximate order from least to most speculative:

  • Axial Tilt: This is by far the significant factor in determining what the season is on Earth. As you may know, axial tilt affects the seasons because the light is "spread out" when it falls on an angled surface. Note that tilt causes different seasons in different hemispheres. An untilted planet could still have seasons for any of these other reasons.
  • Eccentricity: The Earth's orbit is very close to circular, so eccentricity has next to no effect on causing its seasons (and in fact, IIRC, currently Earth's eccentricity's tiny effect is actually opposing its seasons). An exoplanet with a high eccentricity could easily have planet-wide seasons due to varying distance from its sun.
  • Star Systems: As a commenter mentioned, another factor might be how many stars a system has. If such a solar system is stable (which, surprisingly, it might be, although I can't find the article at the moment), you could get seasons depending on where the stars are in their mutual orbits. A binary star system, for example, would have periods of winter when the stars align with the planets (so one blocks the other) and periods of summer when the stars are side-by-side.
  • Star Variation: (From a comment): believe it or not, the sun actually has "seasons" (here) that are on the order of a decade or so. These don't have much effect on Earth's climate since on the sun they mainly cause magnetic disturbances, but on an exoplanet the star's magnetic disturbances could be relatively larger, which would lead to significant temperature/brightness variations that would translate into seasons.
  • Internal Energy: Planets could have varying/cyclic sources of energy within them, just like solar masses. In our own solar system, Jupiter's seasons are partly due to this. On a terrestrial planet, variations might come from, for example, volcanic activity. Negative feedback loops of this sort will exhibit oscillations. These sorts of changes might not count as "seasons" for you, though, as they occur over larger timescales.
  • Stored Energy: Earth's seasons are complicated by the oceans, which greatly affect the intensity of the seasons, especially in the Americas. In particular, there are various cycles that work like subseasons underlying the main ones. The Pacific Decadal Oscillation, for example, affects seasons years apart. I don't see why an exoplanet couldn't have similar cycles that, unlike in the case of Earth, are the main factor in determining seasons. This could be stored gravitational energy too; for example in a tidally-locked exoplanet/moon system.

If you mean a true Earth-like analogue (in the sense of being exactly-like-Earth-in-every-way-except-the-axial-tilt), then no--it would not have noticeable seasons, since none of the above factors really apply.

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  • $\begingroup$ By the way, what about variations in solar activity? $\endgroup$ – Anixx Oct 10 '14 at 11:26
  • $\begingroup$ "Any negative feedback loop will exhibit oscillations" - Not all negative feedback - only feedbacks where the rate of change (or higher derivative) of a variable is related inversely to the variable itself (e.g. spring acceleration is inversely proportional to distance stretched). $\endgroup$ – naught101 Oct 10 '14 at 11:49
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    $\begingroup$ Earths eccentricity only opposes northern hemisphere seasons. It amplifies those in the southern hemisphere by the same small amount. IIRC Mars has an orbit eccentric enough that it currently dominates over axial tilt in generating seasons. $\endgroup$ – Dan Neely Oct 10 '14 at 13:44
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    $\begingroup$ @naught101: Close; the dynamical system needs to be C0, have a nonvanishing second derivative on a nondense subset of its input, and be either undamped or else underdamped and forced by a different process. I made this simplification because it is immensely improbable that negative feedback cycles we'd could observe wouldn't be. $\endgroup$ – imallett Oct 10 '14 at 18:43
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Exactly. Seasons and its difference between northern and southern hemisphere are caused due to the tilt of the Earth's axis. But this is not the only thing involved. An other important fact is the orbit's eccentricity: a very eccentric orbit would cause a whole planet "summer" when around the perihelion and a whole planet "winter" near the aphelion.

There would be also extreme permanent hot zones near the equator and extreme permanent cold zones at poles.

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  • $\begingroup$ damn you beat me by a minute $\endgroup$ – usethedeathstar Oct 10 '14 at 6:32
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    $\begingroup$ A third factor would be when the planet is part of a multiple star system. $\endgroup$ – Dieudonné Oct 10 '14 at 7:37
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    $\begingroup$ Then there's the Mercury situation: tidal lock leading to permanent seasons on a longitudinal basis! $\endgroup$ – Carl Witthoft Oct 10 '14 at 15:06
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If it would not have any axial tilt, and it rotates in an orbit thats not really eccentric, it would have no seasons. If it has (really) high eccentricity, it would have seasons, but they would be hot all over the planet a part of the year, and than cold all over the planet the other part of the year. On earth, due to the axial tilt, we have winter in the northern hemisphere, at the moment that there is summer in the southern hemisphere.

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I considered this as an explanation for George R. R. Martin's books, where several years could go by during each season. If the planet had no axial tilt and very little eccentricity in its orbit, there would be no annual seasons. But if solar energy output would vary sufficiently, you could have seasons, with inconsistent cycle times that measured in decades.

So I thought, "Ha, it is possible."

Then some character in the book noticed "the days were getting shorter" and I had to throw it out. In such a world, day length would always be the same, and it would always match night length.

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