Life on a planet orbiting a Binary Star

How would it be (presuming for instance it exists). I presume that while it would vary depending on local factors (size of the suns, distance, orbital times) it would be very different from what we know. Would night even exists? Will we have different types of “days” depending on a the position of the suns; a day with one sun at noon, the other at sunrise or sunset, or any other possible permutation?

Yes, depending on orbits of the relevant bodies and stars, you’d have wildly different skies than on earth. If it was complicated enough, no two days would be very much the same, and calendars based on them, would most likely have very long and confusing periods.

Well, if the planet is orbiting one of the stars, instead of orbiting their common center, it’s probably going to have one big sun in the sky, with the other getting bigger and smaller as it approaches the prime sun. You could probably get a calendar just based on the primary sun, but, similar to how there are solar and lunar calendars here, there might be a variety of different systems, with very different results. As far as I know, binary stars usually take a loooooong time to orbit each other, so it might not really be worth having a year based on the secondary star.

As for day/night/sunset/sunrise, yeah, you’d get weird permutations. If the secondary sun was sufficiently distant, it would be somewhat analogous to how we treat the moon, basically you don’t pay a whole lot of attention to it’s setting and rising times, and just go with the main sun’s day. On the other hand, there might be times when the second sun is much closer (although I haven’t the astronomical chops to say that definitely), in which case day and night might be somewhat less useful distinctions. Of course, if you’re sufficiently far north or south, that already happens on Earth.

Life would be so difficult that the primary occupation would be moisture farming. The environment would be so harsh as to only support a few species such as the Womp rat, the bantha, the Sarlacc, and the Krayt Dragon.

And because I stole this from Wikipedia, I don’t even remember the Sarlacc pit.

Star Wars aside (where apparently the twin susn can’t be stopped from setting at the same time anymore than a young slave boy can be made a Jedi), I don’t think it would necessarily be a desert world.

In Binary Star systems, is there usually a great divergance between the sizes of the suns? is there any thing such as a satellite star?

We have an example of a small star orbiting two larger stars very near us. Alpha Centauri A and B are each about the size of the Sun. They are orbited by Proxima Centauri, a red dwarf that is about 0.12 times the mass of the Sun. Note that the Alpha Centauri system contains an example of both two suns about the same size and a much smaller star orbiting a larger one (or pair of stars, in this case).

Research is ongoing into whether or not the Alpha Centauri system includes planets.

I’ve read that some orbits in certain binary systems are unstable-IIRC there’s some orbits in Alpha Centauri which are stable while orbiting either A or B, but many which are not.

WRT the Star Wars thing, if both stars are sufficiently close then a planet could orbit both stably.

Almost all orbits would be unstable. There are only three different sorts of situation where you could get a stable orbit:

1: Two stars very close together; planet much further out and orbiting the center of mass of the two stars. This appears to be the situation that Tatooine has, since it’s the only one that’ll put the suns close together in the sky.

2: Two stars very far apart, with a planet orbiting close to one of them and basically ignoring the other. The influence of the second star would range from “brightest thing in the night sky but it’s still night when it’s out” to “not even visible to the naked eye from the planet”, depending on the distances and stars involved (from the position of the main two stars of alpha Centauri, you couldn’t see Proxima, for instance).

3: Something resembling a Trojan orbit, with the two stars and the planet forming the points of a triangle that keeps its shape as it rotates (it’s not quite an equilateral triangle: That depends on the two primary objects having significantly different masses). In this case, the suns would always be 50-some degrees apart in the sky, and you’d have double-day for about a third of the day-night cycle, full night for about another third of the cycle, and single-day with only one of the two suns up for the times in between.

Brian Aldiss’s Helliconia trilogy imagines life in a binary system. There is a two thousand year climate cycle, during which civilization rises and largely falls.

A writer named Isaac Asimov wrote a story called “Nightfall”. It took place on a planet that was part of (IIRC) six start system and darkness came only once every thousand years at which time people became so crazed by the lack of light that they burned everything in sight, went totally bonkers and civilization had to restart from scratch. It is generally considered Asimov’s best writing. The idea was suggested to him by John Campbell (who was a total nut, but was a superb editor) based on a quote from, IIRC, Emerson to the effect that if the stars came out only every thousand years it would drive people nuts.

However, he never considered the question of orbital stability. From what he said it didn’t sound like there was only one star that it orbited primarily, but that each one gave enough light to blot out the fixed stars, but it wasn’t just orbiting a center of gravity of a tight bunch of stars.

The original quote was something to the effect that if the stars came out only once every thousand years, people would appreciate their beauty. Campbell turned that around to say that it’d probably drive them bonkers instead, and Asimov took that idea and shaped the story around it.

A planet that orbits binary stars like Tatooine might have some major weather problems due to stars eclipsing each other. According to Wikipedia, the difference in solar energy due to the Earth’s tilt can be 25% between summer and winter at 65 degree latitude. Yet, a star eclipsed by its binary twin would mean a reduction of 50% of the solar energy (assuming the two are equal in output). Plants an animals would have a very hard time coping - every once in a while, you’d have to deal with cold snaps in the middle of a spring or summer that could interrupt growing seasons. An eclipse during winter would make the winter that much worse - an otherwise Earth-like planet would be getting a quarter of the solar radiation it was used to during the summers.

Could a planet in such a binary star system have a stable figure-8 orbit?

I’m pretty sure it’s not possible, except maybe for relatively short timescales. Over the typical lifetime of a stellar system, other passing stars would perturb its orbit into something unstable.

Wiki says the apparent magnitude of Proxima from alpha AB is 4.5. Dim, but not invisible.

*** Ponder

Although not widely accepted, there is a theory that we’re living on a planet in a binary system right now – that there is a small star (dubbed Nemesis) orbiting so far out that it periodically disturbs the Oort Cloud, deflecting many of the comets there into the inner solar system. This is meant to explain the periodicy of mass extinctions on Earth some have observed in the anthropological record. Nemesis roils up the Oort bodies, they come shooting in towards us, and because of their great number, one or two smacks into the Earth.

Fingers crossed, huh?


The Bad Astronomer deals with possible orbits in a binary system here.

The scary thing if this is true is that she’s WAY late…the last mass extinction occurred about twice as long ago as the normal interval, IIRC. Which means that the “normal” interval was just basic coincidence or we’re really overdue for another dinosaur-killer.

This is the only one of the three possibilities that we’ve actually found in observations. The other two are theoretically possible as far as we know, but we haven’t found any examples.

I have nothing else to add, other than to provide the actual quote which inspired Asimov’s story.

Suppose, for purposes of sorting out the issues, you have a Terrestrial planet orbiting Alpha Centauri A (the G-type component) at about one A.U. From what I’ve gathered, Alpha Centauri B, a K-type, is at approximately Uranus distance from A, with Proxima (technically Alpha Centauri C) a measurable fraction of a light year out.

Okay, given the above:

  1. While A and B would technically orbit a common center of mass (along with C and any planets), what likely effect would B have on our hypothetical planet. Would it of necessiy be in a noticeably elliptical orbit thanks to B’s gravity, our would it take a low-exccentricity orbit like Sol’s planets, predominantly controlled by A’s gravity?

  2. Supposing the latter, what would be the visual effect of B (and of Proxima) in the sky? Measurable orb? Bright point source? How bright?

  3. Would life be able to form on this hypothetical planet? Would the temperature range be anything like Earth’s?

I’m basically asking what orbital mechanics has to say about such a hypothetical planet – the sole contrafactual assumption is that there is a Terrestrial planet with a semimajor axis of ~1 A.U. from ACA, which is the predominat gravitational influence on it.