A significant portion of one of my stories is planned to take place on Luyten B - https://en.m.wikipedia.org/wiki/Luyten_b - a potentially habitable exoplanet not far from the Sun. This is a rocky planet, around 3 times the mass of Earth, that orbits within the habitable zone of its red dwarf star and is possibly not tidally locked because of its high eccentricity (I’m going to assume it isn’t tidally locked). What would life be like on this planet? What sort of life would be likely to evolve, what would the climate be like, what would the sky look like?

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    $\begingroup$ Life would be quite good. The sky would look great. As Darwin puts it, "life, with its several powers, having been originally breathed into a few forms or into one, [...] endless forms most beautiful and most wonderful have been, and are being, evolved". The climate would be most interesting. Any further specification would be baseless speculation. $\endgroup$ – AlexP Jun 20 '19 at 11:26
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    $\begingroup$ Climate is probably both too broad and under-specified. (From Earth's orbital parameters alone, it'd be hard to be more specific than "it probably has liquid water".) Life is similar but even worse. $\endgroup$ – Cadence Jun 20 '19 at 11:26
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    $\begingroup$ Very close to winning the prize for most "too broad" a question. You were just beaten out by the question someone asked about what a universe with a different number of dimensions would look like. $\endgroup$ – StephenG Jun 20 '19 at 12:49
  • $\begingroup$ Hello Locaq. Our help center explains that questions must be specific and answerable, must include context, must include restrictions/requirements, and should include research. I'm afraid this question isn't specific or answerable. We are just discovering what might be habitable planets, but have yet to prove any are, much less what form that life takes. Because we have only one datapoint (Earth), what forms life would take in the conditions you specify is a guess, making the question broad and primarily opinion-based. $\endgroup$ – JBH Jun 20 '19 at 13:59

Luyten b orbits Luyten's Star at roughly one-eleventh the distance of the Earth from the Sun. Even though Luyten's Star's radius is only one-third that of the Sun, it would still take up far more space in the sky - roughly 15 times the apparent area, in fact. In turn, it would not be nearly as blinding, as the light from it is distributed on a larger apparent area and is heavier in red and infrared compared to our sunlight.

If the planet isn't tidally locked, it is likely to be in some sort of spin-orbit resonance, meaning that it may rotate twice per orbit, or three times per two orbits (like Mercury), or another similar resonance. Or it might have a retrogade 1:1 resonance like Venus.

The days will hence likely be on the same order as the orbital period (year) of 18.65 days. With 2:1 resonance, the day will be exactly as long as the year; with 3:2 resonance, it will be shorter; with 3:1 resonance, it will be longer, etc.

With no inclination, there will be no seasons on Luyten b. In turn, solar tides will be greater than lunar tides on the Earth due to the close proximity to the star.

The surface gravity is likely to be only roughly half again as great as on the Earth (ca. 3 times the mass, ca. 1.4 times the radius).

Prominent in the sky will be Luyten's innermost planet, which whizzes around the star in just 4.7 days at a bit more than one-third distance of Luyten b. It will exhibit phases like Venus. Two other detected planets are further out, at roughly 8 and 10 times Luyten b's distance; but they are both around 10 Earth masses and hence should be quite visible in the night sky.

Red dwarfs tend to have stronger solar winds and greater flare activity than G-type stars like our Sun, so there is likely to be very prominent northern and southern lights, if the planet has a magnetic field similar to Earth's (and it will need to have at least that to protect inhabitants from high-energy particles.

It is difficult to say anything about climate without knowing about the planet's atmosphere and composition, especially how much water there is on the surface. The thickness of the atmosphere can make the surface warmer or colder than on Earth.

Because of the (likely) longer days, winds will tend to be less powerful and will likely mainly be from the day side to the night side at the surface (with the opposite pattern at higher altitudes). The long days will also mean higher variations in temperature between nighty and day, though a thick atmosphere can even this out.

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  • $\begingroup$ Thank you very much! Luyten is pretty inactive, so I don’t think the flares would be a big issue. Why does the day length affect the wind and temperature variations in that way? $\endgroup$ – Locaq Jun 20 '19 at 13:18
  • $\begingroup$ Wind is mainly caused by two factors: (1) The rotation of the planet: Air moving away from the equator will move to a band where the rotational speed is smaller due to a smaller diameter, and vice versa, creating vortexes. (2) Sunlight heats and expands air on the day side, particularly over land, causing wind to flow towards the night side and towards the sea, with the reverse pattern happening at night, as the air cools. $\endgroup$ – Klaus Æ. Mogensen Jun 21 '19 at 9:16
  • $\begingroup$ With longer days and nights, the sun has more time to heat the day side, while the night side has more time to cool off. This is seen on the Moon, which has a day cycle of 28 days: The average temperature on the Moon (at the equator and mid latitudes) varies from -298 degrees Fahrenheit (-183 degrees Celsius), at night, to 224 degrees Fahrenheit (106 degrees Celsius) during the day. $\endgroup$ – Klaus Æ. Mogensen Jun 21 '19 at 9:17
  • $\begingroup$ A thick atmosphere can however distribute heat better. Venus, with its very thick atmosphere, has no day and night variation even though the day cycle is equal to the year length. In return, the lack of an atmosphere on the Moon makes the temperature variation listed above more extreme than if there had been an atmosphere. $\endgroup$ – Klaus Æ. Mogensen Jun 21 '19 at 9:20

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