# Life on a planet orbiting a Binary Star **URL:** \ **Category:** Factual Questions\ **Created:** [June 29, 2009, 6:44am UTC](https://boards.straightdope.com/t/life-on-a-planet-orbiting-a-binary-star/501322 "2009-06-29T06:44:13Z")\ **Posts on this page:** 15\ **Page:** 2
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 30, 2009, 6:25pm UTC](https://boards.straightdope.com/t/life-on-a-planet-orbiting-a-binary-star/501322/21 "2009-06-30T18:25:30Z")
> [@](#): > > A planet that orbits binary stars like Tatooine might have some major weather problems due to stars eclipsing each other. For any arrangement that could lead to a habitable planet, you’d probably be looking at the two stars having a period of mere days or so around each other. The eclipses would be both short enough and frequent enough that the effects would just average out. > [@](#): > > 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. The distance between A and B ranges from about 11 AU to about 36, which means that from our hypothetical planet, B would be somewhere between about 1/100 and 1/1000 as bright as A, were they the same intrinsic brightness. Actually, A is intrinsically about 3 times brighter than B, so you’d be looking at somewhere between 1/300 and 1/3000. That’s still considerably brighter than the Moon, but not much compared to full daylight. ---
**Author:** ![runcible\_spoon](https://avatars.discourse-cdn.com/v4/letter/r/9fc29f/32.png) [@runcible\_spoon](https://boards.straightdope.com/u/runcible_spoon)\ **Post date:** [June 30, 2009, 7:17pm UTC](https://boards.straightdope.com/t/life-on-a-planet-orbiting-a-binary-star/501322/22 "2009-06-30T19:17:31Z")
> [@Chronos](#): > > The distance between A and B ranges from about 11 AU to about 36, which means that from our hypothetical planet, B would be somewhere between about 1/100 and 1/1000 as bright as A, were they the same intrinsic brightness. Actually, A is intrinsically about 3 times brighter than B, so you’d be looking at somewhere between 1/300 and 1/3000. That’s still considerably brighter than the Moon, but not much compared to full daylight. Out of curiosity, would it be possible for a planet to orbit the less-bright of a pair? I don’t know if there’s a strong correlation between size and brightness, but would it be possible to have a planet orbiting a dim star, with another, much brighter, star farther out such that you’d get anywhere close to equal light from both of them? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 30, 2009, 7:32pm UTC](https://boards.straightdope.com/t/life-on-a-planet-orbiting-a-binary-star/501322/23 "2009-06-30T19:32:45Z")
A planet, sure, but you’d be hard-pressed to get life. The more distant star, in order to be the same brightness, would have to be significantly hotter, which means the spectrum would be rich in dangerous ionizing radiation. There’s a relatively narrow range of stellar temperatures that can support life as we know it. Commence quibbling about the “as we know it” part. ---
**Author:** ![Anne\_Neville](https://avatars.discourse-cdn.com/v4/letter/a/b9e5f3/32.png) [@Anne\_Neville](https://boards.straightdope.com/u/Anne_Neville)\ **Post date:** [June 30, 2009, 8:17pm UTC](https://boards.straightdope.com/t/life-on-a-planet-orbiting-a-binary-star/501322/24 "2009-06-30T20:17:01Z")
> [@runcible\_spoon](#): > > Out of curiosity, would it be possible for a planet to orbit the less-bright of a pair? Not only is it possible, we’ve discovered one that does. The planet that we know about in the [16 Cygni system](http://en.wikipedia.org/wiki/16_Cygni) orbits the second most massive star in the triple system. > [@](#): > > I don’t know if there’s a strong correlation between size and brightness For [Main Sequence stars](http://en.wikipedia.org/wiki/Main_sequence), there is a very strong correlation between mass, radius, and brightness. If a star’s not a giant or white dwarf (both of which are stars near the end of their lives), it’s probably a Main Sequence star. I wouldn’t expect to find life as we know it on a planet orbiting anything other than a Main Sequence star, just because stars change so rapidly (on the timescale on which life evolves) once they go off the Main Sequence. There’s also a correlation between a Main Sequence star’s mass and its lifetime. The brighter the star, the shorter the time it will stay on the Main Sequence. A star going off the Main Sequence might mess up not only life on planets orbiting it, but on other planets in its system (depending on how close they are to it). If a star is extremely massive, it can go supernova, which can harm life in other systems up to 25-100 light-years away. A very bright star won’t live very long before it goes off the Main Sequence and cooks any life on nearby planets. ---
**Author:** ![Sapo](https://avatars.discourse-cdn.com/v4/letter/s/ec9cab/32.png) [@Sapo](https://boards.straightdope.com/u/Sapo)\ **Post date:** [June 30, 2009, 8:20pm UTC](https://boards.straightdope.com/t/life-on-a-planet-orbiting-a-binary-star/501322/25 "2009-06-30T20:20:06Z")
> [@Enola\_Straight](#): > > Could a planet in such a binary star system have a stable figure-8 orbit? Just for fun, read “Placet is a crazy place” by Fredric Brown ---
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [September 4, 2018, 4:59pm UTC](https://boards.straightdope.com/t/life-on-a-planet-orbiting-a-binary-star/501322/26 "2018-09-04T16:59:05Z")
> [@Chronos](#): > > 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. > [@Anne\_Neville](#): > > 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. In the 9 years since this thread was made, there have been several “Tatooine like planet discovered” stories in the media. Has any of them actually been of the type 1 that **Chronos** mentions or is that still theoretical? ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [September 4, 2018, 9:54pm UTC](https://boards.straightdope.com/t/life-on-a-planet-orbiting-a-binary-star/501322/27 "2018-09-04T21:54:58Z")
Also, Liu Cixin’s “Thee Body Problem” novel (and sequels) have been translated into English, winning the Hugo and Nebula awards. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [September 4, 2018, 11:50pm UTC](https://boards.straightdope.com/t/life-on-a-planet-orbiting-a-binary-star/501322/28 "2018-09-04T23:50:44Z")
> [@AK84](#): > > In the 9 years since this thread was made, there have been several “Tatooine like planet discovered” stories in the media. Has any of them actually been of the type 1 that **Chronos** mentions or is that still theoretical? At least [one planet has been found in a Type P orbit around a binary star](https://www.nasa.gov/mission_pages/kepler/news/kepler-16b.html); however, it is not a terrestrial planet and is not within the “habitable zone” around its stars. Although I suspect that we will at some point discover a rocky world around a binary system within a zone that would be at a steady enough range of irradience to support liquid water on a world with an atmosphere, a [“Tatooine-like” world](https://goo.gl/images/Zr92dL) is problematic for two reasons. One, the stars of Tatooine appear to be compariable in size and output, presumably F or G class stars, but are separated by enough distance (at least five degrees in that picture) that the orbit around them would not be stable inside the habitable zone. For instance, if Tatooine had to G2 class stars, they would have a separation of ~13 million km or ~ 0.1 AU in that picture. That is almost half of the perihelion of Mercury, and would produce enough gravitational perturbation to destabilize any planet inside the orbit of Jupiter pretty quickly. There could exist a stable orbit within the habitable zone of a much hotter pair of stars like a B or an A sitting much further out, but the stars would appear far more distant, almost pin-pricks, and such stars have much shorter lifespans so the formation of complex native life and a habitable ecosphere is unlikely. Alternatively, you could have much smaller K or M stars that are closely bound to one another that a planet could orbit close enough to appear of significant size, but the habitable zone around such stars is so small that it would be unlikely to support life on the surface; such stars also tend to have active magnetospheres and eject charged particle radiation and flares regularly, which would be lethal to any life as we know it. Binary stars probably have interacting magnetic fields especially if they are in close orbit which would increase this kind of activity. Type S orbits, where the planet orbits a star and the system is orbited by a second, more distant star, is plausible, but of course the second star is just going to look like a really bright, very distant planet, which is far less impressive. However, there is still kind of a possibility for this; many astrobiologists have hypothesized that while stable conditions to support life on terrestrial worlds is likely very rare, conditions on smaller worlds with an icy crust but kept active by internal seismic activity, hydrocarbon-rich moons, and watery moons around jovian and superjovian planets driven by tidal energy like Enceledus or Europa are actually much more promising candidates to support life, because they are protected against direct impingment of solar radiation (so they can survive in tight orbits around flare-prone M-class dwarfs), are more thermodyanically stable over evolutionary periods, and can serve as incubators for nascent life via accumlation of nutrients and materials around seismically driven vents or other geoenergetic features just as hydrothermal vents may have been the locations for the abiogenesis of early life on Earth. In such a case, you could have “worlds” in close enough orbit to see two separated stars, or a moon flying with a view of the rings of a supergiant (although not orbiting underneath the rings and in close or crossing orbit, obviously) and other visual tropes of science fantasy, even if not on a desert world with a breathable atmosphere. Stranger ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [September 5, 2018, 12:52pm UTC](https://boards.straightdope.com/t/life-on-a-planet-orbiting-a-binary-star/501322/29 "2018-09-05T12:52:41Z")
Here’s some basic data considering an Earth-like planet orbiting 1 AU around a Sol-like star with a Sol-like companion star not all that far away. Consider a distance for this “Earth” to the 2nd Sol equal to that of [50000 Quaoar](https://en.wikipedia.org/wiki/50000_Quaoar), a Kuiper belt object about 44 AU out. Per [this](https://en.wikipedia.org/wiki/Extraterrestrial_skies#50000_Quaoar_and_its_moon_Weywot), the 2nd Sol: “… apparent magnitude would be −18.56, in comparison the magnitude of the Moon at full phase is −12.74, so the Sun [Sol2] would appear on Quaoar as a point of light 212 times brighter than a full Moon on Earth.” And it is a point. Smaller than Venus appears to us at best. It would 1/2000th as bright at the Sun seen from Earth. Comparatively dim but enough light to give a weird dusky sky at night if it was on the opposite side of this “Earth” from the 1st Sol. Given the dimness and point-like source, you’d still see some stars but a lot would be washed out. 44 AU is close enough to cause orbital stability problems for this “Earth”, esp. since the two Sols are likely going to have non-circular orbits around their barycenter. Anything far enough out to give decent long term orbital stability is going to be just a brighter than usual star. Note an inherent problem. To get the 2nd Sol close enough to be a nice disk and not disrupt the orbit of the planet requires the planet to be _very_ close to the first Sol, which requires it to be a dim star so the planet is inhabitable. Which makes Sol1 smaller but still much bigger in the planet’s sky than our Sun. Etc. It really never balances out. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 6, 2018, 12:15am UTC](https://boards.straightdope.com/t/life-on-a-planet-orbiting-a-binary-star/501322/30 "2018-09-06T00:15:47Z")
To be fair to Tattooine, it’s not _very_ habitable, possibly only at all in a fairly small region of the surface, and there’s no evidence that any of the life found there evolved there. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [September 6, 2018, 12:29am UTC](https://boards.straightdope.com/t/life-on-a-planet-orbiting-a-binary-star/501322/31 "2018-09-06T00:29:58Z")
Well, there is the Sarlacc, which we can assume is a native creature, and the Sandpeople and Jawas, if not native, seem to inhabit the world without outside support. Not to be overly critical of _Star Wars_ as it is very deliberately a cinematic rendition of space opera with no real attempt to explain any of the science or technology behind it, but the planetology makes no sense, although it is not as evidently nonsensical as _Pitch Black_. Stranger ---
**Author:** ![TSBG](https://avatars.discourse-cdn.com/v4/letter/t/77aa72/32.png) [@TSBG](https://boards.straightdope.com/u/TSBG)\ **Post date:** [September 6, 2018, 8:36pm UTC](https://boards.straightdope.com/t/life-on-a-planet-orbiting-a-binary-star/501322/32 "2018-09-06T20:36:46Z")
> [@Stranger\_On\_A\_Train](#): > > Well, there is the Sarlacc, which we can assume is a native creature, and the Sandpeople and Jawas, if not native, seem to inhabit the world without outside support. Not to be overly critical of _Star Wars_ as it is very deliberately a cinematic rendition of space opera with no real attempt to explain any of the science or technology behind it, but the planetology makes no sense, although it is not as evidently nonsensical as _Pitch Black_ Can’t believe I’m going here, but… In “The Force Awakens,” Han and Chewie are trading in very dangerous beasts. It’s possible Jabba imported a Sarlacc for fun and murder. I don’t remember a ton about “Pitch Black” except that it was surprisingly enjoyable. In a nutshell, what’s wrong with the planetology? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 6, 2018, 8:58pm UTC](https://boards.straightdope.com/t/life-on-a-planet-orbiting-a-binary-star/501322/33 "2018-09-06T20:58:48Z")
Sarlaccs have been seen on other planets, in the Star Wars extended universe. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [September 6, 2018, 9:09pm UTC](https://boards.straightdope.com/t/life-on-a-planet-orbiting-a-binary-star/501322/34 "2018-09-06T21:09:25Z")
> [@TSBG](#): > > In “The Force Awakens,” Han and Chewie are trading in very dangerous beasts. It’s possible Jabba imported a Sarlacc for fun and murder. And somehow manages to feed it enough people for it to grow to vast proportions? Even if the sarlacc isn’t native, something has to be producing a breathable atmosphere. As I said, it’s best not to think too deeply about plaetology in the _Star Wars_ universe, or in space opera in general, because it just doesn’t make much sense. > [@TSBG](#): > > I don’t remember a ton about “Pitch Black” except that it was surprisingly enjoyable. In a nutshell, what’s wrong with the planetology? Every 22 years the planets align and somehow manage to produce an eclipse of both suns lasting for many hours, which allows some underground-dwelling bat creatures to come out and feed, although the surface is so barren it isn’t clear what the’d feed upon if not for the hapless crew whomcrashed there. This isn’t they way celestial mechanics, or ecology, or evolutionary biology works. It’s a pretty good thriller with some decent performances by about half the cast and essentially kicked off Vin Diesel’s career as an anti-hero, but from a scientific standpoint it is complete rubbish. Stranger ---
**Author:** ![Enola\_Straight](https://avatars.discourse-cdn.com/v4/letter/e/dec6dc/32.png) [@Enola\_Straight](https://boards.straightdope.com/u/Enola_Straight)\ **Post date:** [September 7, 2018, 2:19pm UTC](https://boards.straightdope.com/t/life-on-a-planet-orbiting-a-binary-star/501322/35 "2018-09-07T14:19:56Z")
> [@Chronos](#): > > Sarlaccs have been seen on other planets, in the Star Wars extended universe. Panspermia? [https://www.nature.com/scitable/blog/postcards-from-the-universe/life\_traveling\_in\_space\_a](https://www.nature.com/scitable/blog/postcards-from-the-universe/life_traveling_in_space_a) [Previous page](https://boards.straightdope.com/t/life-on-a-planet-orbiting-a-binary-star/501322.md?page=1)