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Given a planet in the habitable zone and able to produce life what would be the effect of a short year? For example NASA recently found 7 exoplanets with Trappist-1 d being in the habitable zone but having an orbital period of 18.2 days.

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You can give a look at places on our planet where the "habitable season" (in other word the part of the year where condition for sustaining life are met) is shorter than 1 year.

For example look at warm desert: normally there is lack of water and there is no vegetation, until some rain falls. In the few hours the water is moisting the soil, plants will grow up, bloom and produce seeds.

Coming back at your question, I think life is flexible enough to adapt to the local time cycle, provided a suitable energy source. Shorter year and thus shortes season? Faster development pace.

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If you are talking about human habitability then there may be a serious problem with growing crops humans could eat unless there is little or no seasonal variation. If you are talking about native life only then it should not be a problem because any life would evolve to fit the planet it springs from.

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This one is dependent on the seasonal variation. If seasons are negligible or irrelevant (which would mean that axial tilt is minimal, likely under 5 degrees), then the length of the year won't really have any importance. However, if you're thinking of an Earth-like planet with significant seasonal variation, you get a very different picture. Chief among the considerations would be the question of winter.

A longer year could render the polar regions effectively uninhabitable: even on Earth, with the benefits of modern technology, very few people live above the 60N latitude line relative to land area. If winter is cold enough to bring snow, it cuts down animal populations noticeably due to the lack of plant growth (and thus a reduced food supply) for that part of the year; a lot of animals die every winter in colder regions. As the snow sticks around for longer, population density plummets as each animal requires more and more land area to feed itself, with a corresponding lack of food to fuel any civilization that might arise; I think your limit is about three Earth months of sub-zero temperatures if you want a civilization in the area to have a strong population.

A longer year means that a winter taking the same proportion of the year will last longer: more of your planet's land area will exceed that three-month limit (which is more or less a fixed value, regardless of year length). In effect, a longer year will drive more of your population to the equatorial regions. You're also likely to see greater temperature swings over the course of a year, simply because a longer winter or summer would suggest that there is more time for the average temperature to go up or down in those seasons.

A shorter year, on the other hand, poses different complications. Winter is not so great a problem, but the seasons will necessarily be changing faster. I can't speak with absolute certainty here (you might simply get weaker seasons), but I think you would end up with nastier weather due to more and greater temperature fluctuations: more hurricanes, more tornadoes, generally more chaotic and unpredictable weather patterns, etc.

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Year length is contingent on distance from the star and so year length, distance, and average surface temp are all pretty much the same thing. It would have no effect other than that.

Until you add a tilt to the planet which causes seasons. The length of year then only matters only because it changes the length of a season and that change would only change the amount of time for a species in a season. If it's too fast I imagine that plants and animals would just adjust to an average rather than having drastic behavioral changes based on season as we do on Earth. Or if they do, it would be increase it's rate so that you would get life that would grow to normal Earth size in a much quicker period or you would reduce the max size that is need to reach maturity.

There wouldn't really be any other changes...

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