# Can a planet have more than one suns? **URL:** \ **Category:** Factual Questions\ **Created:** [June 18, 2004, 1:08am UTC](https://boards.straightdope.com/t/can-a-planet-have-more-than-one-suns/250957 "2004-06-18T01:08:21Z")\ **Posts on this page:** 19\ **Page:** 1
**Author:** ![Dog80](https://avatars.discourse-cdn.com/v4/letter/d/6bbea6/32.png) [@Dog80](https://boards.straightdope.com/u/Dog80)\ **Post date:** [June 18, 2004, 1:08am UTC](https://boards.straightdope.com/t/can-a-planet-have-more-than-one-suns/250957/1 "2004-06-18T01:08:21Z")
There are many Sci-Fi stories that feature planets with two or more suns. But is it possible? Are there any planets like this? ---
**Author:** ![jawdirk](https://avatars.discourse-cdn.com/v4/letter/j/df705f/32.png) [@jawdirk](https://boards.straightdope.com/u/jawdirk)\ **Post date:** [June 18, 2004, 1:20am UTC](https://boards.straightdope.com/t/can-a-planet-have-more-than-one-suns/250957/2 "2004-06-18T01:20:15Z")
Yes, it is possible. Dual stars are very common. However, the planet would almost certainly have to be much closer to one of the suns (the one it orbits). If it was not, it would likely have an unstable orbit, and would soon crash into one of the suns. I don’t know if such a planet has ever been discovered, but it is unlikely. The number of planets discovered is very small (astronomically speaking) and I would guess we have only looked for planets in the best places to look, which would not be close to two stars (it would be too bright to easily see the shadow of the planet, and more complex to isolate the results). ---
**Author:** ![Lobsang](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lobsang/32/4067_2.png) [@Lobsang](https://boards.straightdope.com/u/Lobsang)\ **Post date:** [June 18, 2004, 1:23am UTC](https://boards.straightdope.com/t/can-a-planet-have-more-than-one-suns/250957/3 "2004-06-18T01:23:11Z")
I have seen simulations of binary star systems where a planet does a figure-of-eight between the two stars and the stars themselves do a kind of dance using eachother’s gravity. but weither this happens for real I have no idea. I would be interested to see real examples posted in this thread. ---
**Author:** ![Papermache\_Prince](https://avatars.discourse-cdn.com/v4/letter/p/f05b48/32.png) [@Papermache\_Prince](https://boards.straightdope.com/u/Papermache_Prince)\ **Post date:** [June 18, 2004, 1:26am UTC](https://boards.straightdope.com/t/can-a-planet-have-more-than-one-suns/250957/4 "2004-06-18T01:26:10Z")
As I understand it - and IANAAstronomer- binary star systems are not rare. [CNN 10/29/02](http://www.cnn.com/2002/TECH/space/10/09/binary.planet/) > [@](#): > > The first planet found in a close-knit double star system suggests that the galaxy possesses many more planets than previously estimated. > > Unlike our sun, a rare solitary specimen, the majority of Milky Way stars have companions. The discovery means that similar binary star systems could host planets as well, astronomers announced Wednesday. > > The planet is more massive than Jupiter and orbits the primary star in the double system, slightly farther than the average distance between Mars and the sun, the scientists said. It completes one orbit every 2.5 years. The question is - would these systems provide an environment suitable for life. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [June 18, 2004, 1:44am UTC](https://boards.straightdope.com/t/can-a-planet-have-more-than-one-suns/250957/5 "2004-06-18T01:44:07Z")
There are now [three known binary star planets](http://www.astro.uiuc.edu/~kaler/sow/gammacep.html). There are animations of the motion of the planet around Gamma Cephei [here](http://www32.brinkster.com/snefru/space/cephei/). Note that the planet and the companion star both orbit the main star. It’s believed that a planet couldn’t orbit both stars in binary system in a stable orbit, AFAIK. ---
**Author:** ![yabob](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/yabob/32/2821_2.png) [@yabob](https://boards.straightdope.com/u/yabob)\ **Post date:** [June 18, 2004, 1:47am UTC](https://boards.straightdope.com/t/can-a-planet-have-more-than-one-suns/250957/6 "2004-06-18T01:47:25Z")
I found that CNN article, too. Most sources seem to suggest that you are only going to get planets orbiting one of the partners - shared orbits like “figure eight’s” are unstable or at least highly unlikely. Given this, another question might be whether the secondary would be bright enough in the planet’s sky to be seen as a “second sun”, or would it just be a very bright star? ---
**Author:** ![jawdirk](https://avatars.discourse-cdn.com/v4/letter/j/df705f/32.png) [@jawdirk](https://boards.straightdope.com/u/jawdirk)\ **Post date:** [June 18, 2004, 1:48am UTC](https://boards.straightdope.com/t/can-a-planet-have-more-than-one-suns/250957/7 "2004-06-18T01:48:45Z")
Speculating again, Such systems could contain planets capable of sustaining human life. It mostly depends on the amount of light the planet receives. Since the light is inversely proportional to the square of the distance, there are plenty of circumstances where the total light from two stars would be about the same as it is on Earth. In most situations, the second star would hardly matter. About the figure-8 orbit: I suspect that, though it is possible to set up a stable 3-body orbit in a simulation, they are very rare in nature, and when they do occur, they likely degenerate relatively quickly due to outside influence. Not to say that it never happens – I would guess that just about every possible orbital configuration exists somewhere in the universe. ---
**Author:** ![Yeticus\_Rex](https://avatars.discourse-cdn.com/v4/letter/y/a698b9/32.png) [@Yeticus\_Rex](https://boards.straightdope.com/u/Yeticus_Rex)\ **Post date:** [June 18, 2004, 1:52am UTC](https://boards.straightdope.com/t/can-a-planet-have-more-than-one-suns/250957/8 "2004-06-18T01:52:11Z")
The question should be rephrased as “Do multiple star systems have planets?” [Space.com](http://www.space.com/scienceastronomy/astronomy/planet_binary_991104.html) [Ask a High Energy Astronomer…](http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980122c.html) [Multiple Star Systems](http://www.synapses.co.uk/astro/multistr.html) [Ask A Scientist…](http://www.newton.dep.anl.gov/askasci/ast99/ast99365.htm) [More Googled Results](http://www.google.com/search?hl=en&ie=UTF-8&q=planets+orbiting+multiple+stars) Enjoy. ---
**Author:** ![DougC](https://avatars.discourse-cdn.com/v4/letter/d/7feea3/32.png) [@DougC](https://boards.straightdope.com/u/DougC)\ **Post date:** [June 18, 2004, 1:52am UTC](https://boards.straightdope.com/t/can-a-planet-have-more-than-one-suns/250957/9 "2004-06-18T01:52:30Z")
- - - There are double, triple and at least one quadruple star system within view of small telescopes. You cannot _visually_ tell these systems are orbiting each other; it was deduced with redshifting, scientific methods and so on. They lie very close together (in astronomical terms) and one is moving slightly towards Earth, and the other is moving slightly away–therefore, they are orbiting. - Could you live on a planet orbiting one of these stars? Yes–but these star systems are spaced fairly far apart, and they don’t rotate once a day or once every few weeks; their orbital periods tend to run in the hundreds-of-thousands of Earth-years, or longer. So you could live on your planet, but you wouldn’t see “two suns” when you looked up in the sky, you’d see one normal-looking sun and one (or more) rather bright star(s). - It is not possible to have an Earth-like planet orbiting a pair of suns, with the suns orbiting each other (as in Star Wars/Tattooine). The ratio of distances required means that one of the three (sun1, sun2 and planet) would have to be _vastly_ farther away from the other two. ~ ---
**Author:** ![jawdirk](https://avatars.discourse-cdn.com/v4/letter/j/df705f/32.png) [@jawdirk](https://boards.straightdope.com/u/jawdirk)\ **Post date:** [June 18, 2004, 2:04am UTC](https://boards.straightdope.com/t/can-a-planet-have-more-than-one-suns/250957/10 "2004-06-18T02:04:06Z")
Another possible stable orbital configuration, is the two suns orbiting each other at an extremely close distance and the planet orbiting the center of gravity at a safe distance. This is probably very rare. I read that colliding stars are very rare even in colliding galaxies, and two suns orbiting each other at close distance is almost as rare as them colliding. It would probably be even more rare for such a system to have any planets. ---
**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 18, 2004, 7:30pm UTC](https://boards.straightdope.com/t/can-a-planet-have-more-than-one-suns/250957/11 "2004-06-18T19:30:25Z")
There are three possible stable configurations: You can have the two stars very far apart, with the planet orbiting around one of the stars, or you could have the stars very close together, with the planet orbitting around both stars as if they were a single object, as mentioned by others. You could also have a Trojan orbit, with the planet and the two stars forming a roughly equilateral triangle. Tatooine is presumably the second case: Both suns appear to be the same angular size, brightness, and color, which suggests that they’re at the same distance, and they’re much closer together in the sky than 60 degrees. On the other hand, the situation in Asimov’s “Nightfall”, with a half a dozen stars in irregular orbits, would not be stable for long times, probably not long enough for a planet to develop life. This may be the one case where Lucas did a better job on the science than Asimov did. ---
**Author:** ![Ethilrist](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ethilrist/32/4604_2.png) [@Ethilrist](https://boards.straightdope.com/u/Ethilrist)\ **Post date:** [June 18, 2004, 7:45pm UTC](https://boards.straightdope.com/t/can-a-planet-have-more-than-one-suns/250957/12 "2004-06-18T19:45:16Z")
> [@Yeticus Rex](#): > > The question should be rephrased as “Do multiple star systems have planets?” because otherwise, the answer is “Yes, actually, _all_ planets have more than one sun, but most of the suns are light-years away…” ---
**Author:** ![Gorsnak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gorsnak/32/7587_2.png) [@Gorsnak](https://boards.straightdope.com/u/Gorsnak)\ **Post date:** [June 18, 2004, 7:54pm UTC](https://boards.straightdope.com/t/can-a-planet-have-more-than-one-suns/250957/13 "2004-06-18T19:54:01Z")
> [@Chronos](#): > > On the other hand, the situation in Asimov’s “Nightfall”, with a half a dozen stars in irregular orbits, would not be stable for long times, probably not long enough for a planet to develop life. This may be the one case where Lucas did a better job on the science than Asimov did. I lent my copy of ‘Nightfall’ to a friend long ago, and never got it back, but I’m pretty sure that it was _moons_, not suns, that the planet had in overabundance. If Jupiter were just massive enough to have ignited, would Earth’s orbit still be stable? How bright would it appear in various orbital configurations? Would it be enough to destroy life as we know it? ---
**Author:** ![DougC](https://avatars.discourse-cdn.com/v4/letter/d/7feea3/32.png) [@DougC](https://boards.straightdope.com/u/DougC)\ **Post date:** [June 18, 2004, 8:16pm UTC](https://boards.straightdope.com/t/can-a-planet-have-more-than-one-suns/250957/14 "2004-06-18T20:16:06Z")
> [@](#): > > …You could also have a Trojan orbit, with the planet and the two stars forming a roughly equilateral triangle. Tatooine is presumably the second case: Both suns appear to be the same angular size, brightness, and color, which suggests that they’re at the same distance, and they’re much closer together in the sky than 60 degrees… - - - Such a situation is not possible for an Earth-like climate: to orbit the combined mass of two suns, the planet would have to be orbiting them both at a much farther distance than if it had only one sun–and then the amount of heat it received would be far lower than normal. A larger planet can hold in more heat, but a planet can only get so big before it begins to have a liquid-covered surface (Earth’s surface is already _mostly_-liquid covered!) … -So then, if we had a very large-diameter planet that had a very light core–Berylium, or–maybe a planet with a _hollow_ core, orbiting a pair of suns… ~ ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [June 18, 2004, 8:48pm UTC](https://boards.straightdope.com/t/can-a-planet-have-more-than-one-suns/250957/15 "2004-06-18T20:48:01Z")
> [@jawdirk](#): > > I read that colliding stars are very rare even in colliding galaxies, and two suns orbiting each other at close distance is almost as rare as them colliding. It would probably be even more rare for such a system to have any planets. It’s rare for stars to collide, but not the other. There are stars that orbit so close we can’t separate them with our best telescopes. Some of them have orbital periods of just a few days. We know they are there two ways: 1) they eclipse each other or 2) doppler shifts as they periodically approach and receed from Earth. In fact, these are also the two ways we know there are planets out there, since no one has directly observed a non-Solar System planet. Most are found via doppler shifts, but for one or two we’ve seen the dip in light when the planet eclipses the star. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [June 18, 2004, 9:35pm UTC](https://boards.straightdope.com/t/can-a-planet-have-more-than-one-suns/250957/16 "2004-06-18T21:35:30Z")
> [@Exapno Mapcase](#): > > There are now [three known binary star planets](http://www.astro.uiuc.edu/~kaler/sow/gammacep.html). This is an aside, but Huh!? From the site, Gamma Cephei A has a mass “a mass of 1.5 times that of the Sun”, and Gamma Cephei B is a “low-mass star”, presumably with less mass than Gamma Cephei A. But the planet has “a mass of at least 1.8 times that of the Sun”, larger than either of the stars. Color me purple… 😕 ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [June 18, 2004, 9:54pm UTC](https://boards.straightdope.com/t/can-a-planet-have-more-than-one-suns/250957/17 "2004-06-18T21:54:04Z")
> [@Gorsnak](#): > > I lent my copy of ‘Nightfall’ to a friend long ago, and never got it back, but I’m pretty sure that it was _moons_, not suns, that the planet had in overabundance. [Nope, suns.](http://www.wordiq.com/definition/Nightfall_%28Asimov%29) What’s more, there were at least two stable planetary orbits in the system (the civilization’s home planet and the occulting planet.) ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [June 18, 2004, 9:55pm UTC](https://boards.straightdope.com/t/can-a-planet-have-more-than-one-suns/250957/18 "2004-06-18T21:55:28Z")
> [@ZenBeam](#): > > From the site, Gamma Cephei A has a mass “a mass of 1.5 times that of the Sun”, and Gamma Cephei B is a “low-mass star”, presumably with less mass than Gamma Cephei A. But the planet has “a mass of at least 1.8 times that of the Sun”, larger than either of the stars. Color me purple… 😕 I suspect they screwed up and really meant the planet has 1.8 times the mass of **Jupiter**. ---
**Author:** ![Gorsnak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gorsnak/32/7587_2.png) [@Gorsnak](https://boards.straightdope.com/u/Gorsnak)\ **Post date:** [June 18, 2004, 11:25pm UTC](https://boards.straightdope.com/t/can-a-planet-have-more-than-one-suns/250957/19 "2004-06-18T23:25:31Z")
> [@MikeS](#): > > [Nope, suns.](http://www.wordiq.com/definition/Nightfall_%28Asimov%29) What’s more, there were at least two stable planetary orbits in the system (the civilization’s home planet and the occulting planet.) Hmm. I could swear it was moons. Maybe that was one thing reworked in the novel version with Silverberg? I’ve never read the short story, just the novel.