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I'm using Kapteyn-b for a story, which orbits its red dwarf star every 49 days. It's almost 5 times larger than Earth, though I've made a couple changes to the observed data to make it more plausible. I've added a CO2 rich atmosphere for a mild greenhouse effect and prevent it from being tidally locked, as well as a Mercury-sized moon to help me fudge the gravity down to 1.4g.

The one thing I don't know is the axial tilt. I'm leaning towards something extreme to give my colonists a real challenge, say between 35 and 50 degrees. With that short a year, what kind of seasonal variation could they expect?

Edit: Kapteyn b makes a complete orbit around its parent star with about 1.2% of the Sun's luminosity about every 48.62 days at a distance of 0.17 AU (compared to Mercury, which orbits at a distance of around 0.39 AU. It has an eccentricity of 0.21, meaning its orbit is mildly elliptical.

2nd Edit: I'm going to go with an 18 hour day length to simplify things.

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  • $\begingroup$ To give you any ideas about climate, you need to know the distance from the star, and the star's luminosity. You have to make sure both of those match the planet's orbital period (and mass) since those things are all tied together by Kepler's Laws. As of right now, there is not enough information to answer this question. But as a side note, a greenhouse effect can have a very wide range of effects, so it is very reasonable to just make the climate however you like. $\endgroup$ – kingledion Aug 20 '18 at 22:29
  • $\begingroup$ @Hewholooksskyward aww, you're no fun. Making it tidally locked would REALLY make things interesting for your colonists. $\endgroup$ – Morris The Cat Aug 20 '18 at 23:00
  • $\begingroup$ Hey, this planet is borderline enough as it is! :) $\endgroup$ – Hewholooksskyward Aug 20 '18 at 23:03
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Having such a short year would actually have a stabilizing effect on the climate because of the issue of time lag. Note that on earth the hottest and coldest days of the year are usually not on the longest and shortest days (though on those days the temperature is probably changing the fastest). The hottest and coldest days are usually month or so later.

It would certainly matter more how close to the poles you are if the axial tilt is high. Also, you haven't indicated how long a day is on your planet. That could make a really big difference if there are huge temperature swings from day to night because each day is 100 hours long.

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  • $\begingroup$ If it were tidally locked, you could have a day and a year be the same length. THAT'D mess with those feisty colonists. $\endgroup$ – Morris The Cat Aug 20 '18 at 22:52
  • $\begingroup$ Agreed. February is cold because the earth sheds heat faster than it absorbs it all through the short days of fall and early winter. August is hot because the earth absorbs heat during the long days of march, april, may, june, july. On this planet that wouldn't happen, so the climate would fluctuate much less. $\endgroup$ – workerjoe Aug 21 '18 at 1:27
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Well, according to the article you linked, the surface temperature would start at -90F, without dramatic greenhouse activity, so in the equatorial zone of your world, your climate would be pretty similar to what you get in the Arctic and Antarctic regions of Earth. I'd imagine it'd be a lot like the weather in near-arctic places like Siberia and Scandinavia, except that everything would be cycling faster.

This would mean any plant life you had would have VERY quick growing cycles. You might look at cacti for a good example of plants that need to do their growing VERY quickly when resources are available, and be very protective when they aren't.

The primary impact of your extreme axial tilt would be more turbulent weather, as different areas of the planet are warming and cooling much more rapidly than they do here. You'd probably get lots of storms on a planet like that, and lots of wind.

The final thing I'd point out is that the orbital eccentricity could have a noticeable effect, creating a scenario where one of your hemispheres is going to have a much wider seasonal swing than the other, because in one case your 'summer' is reinforced by closest approach to the star, and the winter with its furthest, while in the opposite hemisphere you're further away in the summer, and closer in the winter, making for a less dramatic shift in temperature between seasons.

This is only true if the major axis of the orbit and the axial tilt of the planet are closely aligned. I'm not sure if there's any reason they would have to be, but it would make things more interesting for your colonists if it were.

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