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I'm building a medieval-high-fantasy world and I want it to have two suns. I'd like it to make it somewhat realistic at least as far as seasons and light/biomes go, while the actual feasibility of this kind of binary system can kinda be handwaved away. I also would like the two stars to be different, in the following example I opted for a yellow-reddish sun and a white one.

I'll make a few assumptions:

  • the Earth has an 8-shaped orbit around two stars, A and B, as detailed in this answer on Physics.SE
  • the planet's orbit is on the same plane as the two stars
  • the Earth's axial tilt relative to the orbit's plane is the same as our current one
  • A is a white star similar to Altair (~1.75M⊙, ~10L⊙)
  • B is a yellow dwarf (~1.25M⊙, ~1.2L⊙)
  • the two stars are roughly distant 3 AU from each other (I'm not really fixated on this, you can change this value as you wish if it helps in any way)

My first question is: would this configuration be tolerable and able to support life on the planet, or would there be problems (e.g. the white star being far too luminuous)? If not, what are some things I could change to make it viable and still have two suns that look different?

If it is viable, here's my main questions:

  • what would seasons and average temperatures be like on the planet?

My best guess is that each hemisphere would have two summers and one winter per year. E.g. the northern hemisphere would be in summer when the planet and the stars are aligned as A-B-Pl, would be in winter when they're aligned as Pl-A-B, and would be in strong summer when they're aligned as A-Pl-B (during which both hemispheres are in summer). I also suppose that the planet's revolution speed would be at its slowest when the planet is between the two stars. Would this setup make any areas on Earth less habitable for humans? more so?

EDIT: I'm only interested in how it would affect human life (climate + how they experience seasons) and in keeping suspension of disbelief, I don't want a 100% realistical solution completely rooted in physics, I just want help in making it so that I can explain the seasons and the look of the stars to a group of TTRPG players without them raising an eyebrow :)

As far as the biomes and temperatures part goes, everything I ask for is something along the lines of "the planet will have a mega-summer in the northern hemisphere when it's located XXX, reaching average temperatures of roughly YY°C (+Z°C than what we see on Earth)". Even a simple percentage increase/decrease in temperature is totally fine.

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    $\begingroup$ Too many questions in one post. Please split them in separate posts. $\endgroup$
    – L.Dutch ♦
    Commented Nov 18, 2020 at 15:21
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    $\begingroup$ The first line of the linked Physics SE answer is "...the orbits wouldn't be stable" (so no life). Therefore seems like you must be using magic to maintain a stable system. Once you open the door to magic, you can do anything you want with climate and seasons regardless of the physics. $\endgroup$
    – user535733
    Commented Nov 18, 2020 at 15:45
  • $\begingroup$ @user535733 I'm interested in making everything believable from a bystander looking at the sky with the naked eye while sitting on the planet's surface. So, as I stated in the question, the only part where I want to get more "technical" is the seasons and climate supposing such a system exists stably. If this isn't clear by reading my question, could you suggest a way of making it more so? Thanks $\endgroup$
    – Hankrecords
    Commented Nov 18, 2020 at 16:07
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    $\begingroup$ Good for you for letting us know you're looking for a suspension-of-disbelief answer. Sometimes it's hard to separate "in real life, that won't work" from "it's your world, it'll work great!" However, asking for biomes is too much and a reason to close (you're basically asking us to invent what life would be like on said planet...). I'd ask for the seasons and simply declare the life you want to be. $\endgroup$
    – JBH
    Commented Nov 18, 2020 at 16:43
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    $\begingroup$ @JBH I edited the question, hopefully that makes it clearer and narrows the scope down $\endgroup$
    – Hankrecords
    Commented Nov 18, 2020 at 23:50

2 Answers 2

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There's a lot of questions here - so I'm just going to focus on your main one about "what are the seasons and days like?" (the mods will probably make you edit your question down to this shortly anyway.)

That said I believe your planet will be habitable but on the hot side, I haven't done the calculations, because there are mitigation strategies available to you as an author - big mountains casting shadows, layers of ash from a permanently erupting volcano, white fluffy cloud layers reflecting sunlight, etc. If you run the physics and it's too hot, cool it down.

Here's the orbit and axial tilt as I understand it.

enter image description here

The relationship between the axial tilt and the plane is important here. I have guessed a relationship here - north pole pointing to the right, but you can tweak it to change things.

You have 8 seasons over your "Year" in the Northern hemisphere:

  • (A) Spring. ~18 hour days.
    • One sun rises, then another ~6 hours later.
  • (B) Mega summer. 24 hour days.
    • One sun rises when the other sets.
  • (C) Fall-lite. ~18 hour days.
    • One sun rises, then another ~6 hours later.
  • (D) Warm winter. ~10 hour days.
    • The suns rise at about the same time.
  • (E) Warm-to-hot spring. ~18 hour days.
    • One sun rises, then another ~6 hours later.
  • (F) Mega summer again.
    • One sun rises when the other sets.
  • (G) Fall. ~18 hour days.
    • One sun rises, then another ~6 hours later.
  • (H) Mild summer. ~13 hour days.
    • The suns rise at about the same time.

Southern hemisphere is a bit cooler but very similar:

  • (A) Spring
  • (B) Mega summer. 24 hour days. Slightly cooler than Northern.
  • (C) Still hot a mild fall.
  • (D) Mild summer. Warmer than Northern's "H" however.
  • (E) A hot-to-mega-hot spring.
  • (F) Mega summer again.
  • (G) Fall
  • (H) Winter.

The southern hemisphere will on average be cooler, and have the coldest winter of anywhere. But the southern hemisphere also has 5 adjacent seasons of heat.

Tides will be weird. You'd have twice daily high tides at B/F, and once daily double-high tides at H and D. Tides would be once daily but uneven lengths (long high, short low) at A, E, G, and C.

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    $\begingroup$ What sort of gravitational effects would happen near point F in the orbit? Weird tides? Wouldn't sooner or later, without some other kind of natural force we don't yet know about, the differing gravities of the two stars tend to pull the planet more towards the stronger gravity star? $\endgroup$
    – elemtilas
    Commented Nov 19, 2020 at 6:54
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    $\begingroup$ I added a bit of info on tides to the question. Stability - real life this would break down pretty quickly, but for the purpose of this question it's handwaved to be stable. $\endgroup$
    – Ash
    Commented Nov 19, 2020 at 8:22
  • $\begingroup$ @elemtilas: You get twice-daily high tides on Earth. You're never going to get once-daily high tides. The strongest tides would be at point B/F, where the two suns are reinforcing each other (you get tides at the points on the planet closest to and furthest from the sun), and the weakest at the points near A, E, C, and G where the suns are at right angles in the sky. But you'll never get four-times-daily tides. $\endgroup$
    – Peter Shor
    Commented Nov 19, 2020 at 13:26
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    $\begingroup$ I won't downvote you this, but could you move the "B" a little bit to the left? It's not immediately visible. Might also move the "C" up a bit. Just some tidying. Otherwise, thanks for adding the bit about tides! $\endgroup$
    – elemtilas
    Commented Nov 19, 2020 at 14:13
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    $\begingroup$ Presumably, the "double-high tides" would be higher at D than at H, since the smaller sun is further away and exerting less pull on the planet. $\endgroup$
    – Darrel Hoffman
    Commented Nov 19, 2020 at 14:41
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I don't think your orbit is stable. What may work is to have a plant orbiting a small star in an eccentric orbit, while the small star orbits a larger brighter star in much larger eccentric orbit.

Now you have a wildly fluctuating climate that depend on:

  • Period around the small star.
  • Tilt of the planet compared to plane of orbit.
  • Rotational period.

So far no problem. This is much like earth, but with faster seasons due to a closer orbit.

  • period of of small star around large star.
  • tilt of planet orbit relative to plane of small star's orbit.

On Earth at temperate latitudes there is about a 2 month delay between the astronomical seasons and the climatological seasons. E.g. The least sun is on the winter equinox, 22 December in the Northern hemisphere, but the coldest month is usually February.

Similarly daily variation is going to be dependant on how long the day is. If earth had 48 hour days, the daily variation would be brutal. It won't be linear with day length, as I would expect cloud formation to increase with longer days.

Longer day length will also decrease coriolus effect which should make storms larger in extent, but also decrease the temperature difference between equator and poles.

A short fast orbit around the small star will mean that seasonal variation will be smaller. But an eccentric orbit means a long time in the cold end of the orbit and a very short time near the star. The more eccentric the orbit, the more extreme the difference.

The orbit around the bright star can act as a multiplier.

At this point my mental model of the spherical trigonometry fails me. I think you get a super roasting when the plane of the planet's orbit has the bright star in the same plane.

Anyone know of online models -- simplified climate models where you can change the plane of orbit etc?

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    $\begingroup$ This is a high-fantasy setting, not a science-fiction setting. "The gods have a sense of humor" is a perfectly acceptable reason for a planet to have a bizarre orbit. $\endgroup$
    – Mark
    Commented Nov 19, 2020 at 0:25
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    $\begingroup$ @mark NOPE. Op specifies "science-based" as requirement. $\endgroup$
    – user79911
    Commented Nov 19, 2020 at 8:35
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    $\begingroup$ @MarvinKitfox I think me specifying that (paraphrasing) "the only part I want to make sense physically is the seasons and temperatures, while the actual feasibility of the orbit can be taken for granted" is a valid use of the science-based tag $\endgroup$
    – Hankrecords
    Commented Nov 19, 2020 at 12:06
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    $\begingroup$ @Sherwood Botsford while your idea of putting the planet on a circular orbit around a small star while that star in turn orbits a bigger star is an interesting one, I did specify in my question that I wasn't interested in making the orbit itself science-based and reasonable, but rather to start from a system with that orbit in place and from that work out what the seasons on the planet would sensibly be like $\endgroup$
    – Hankrecords
    Commented Nov 19, 2020 at 12:10
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    $\begingroup$ The orbit you've described is the basis for Brian Aldiss' Helliconia series, where each of the larger seasons lasts for centuries based on the orbit of the larger star, and "normal" length sub-seasons on the smaller star's orbit. $\endgroup$
    – Darrel Hoffman
    Commented Nov 19, 2020 at 14:47

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