# If a planet had two suns...

by baobeiiiii
Tags: planet, suns
 P: 1 Hey, this is a research question for a novel. All advice, comments are much appreciated. Can a planet have two suns? If so, what are the implications on the length of day and night, casting of shadows, temperatures, etc. Does the planet revolve around just one sun or both at the same time? Sorry that this is such a general question, thanks!
 P: 1,958 I think there are two realistic cases: (1) Two stars orbiting each other, and a planet orbiting around both of them. (2) A planet orbiting a star, and a second star orbiting the first star, but much further out. The first case looks pretty much like life on Earth except there would be two close together suns where we have one. Day and night would be about the same as we have now, with some minor variations. Imagine a second sun in Mercury's orbit. Since Mercury is always close to the sun in the sky anyway, nothing much changes. I think for a novel, the second case would be the most interesting. Imagine putting a second sun in Saturn's orbit. During part of the year there would be a second (but probably dimmer) sun in the sky when it would otherwise be night. Temperatures, etc. would depend on the relative brightnesses and distances of the two stars.
 P: 284 Orbits of a planet around two suns as in phyzguy's first case are quite interesting. In the movie K-Pax the lead character claimed he came from a planet with two suns. After looking at the possible orbits for such a system I came to the conclusion that life on such a planet was not likely. Of course it all depends on relative sizes of the stars and how far apart they are etc. A stable circular or elliptic orbit with not too much eccentricity, possibly necessary for life, is a long way out making it a very cold place. Closer in orbits can be very complicated like figure eights etc, making the temperature variations very large. If you do a search you will find some interesting information about these orbits. Recently some people posted calculations showing that some more stable orbits suitable for life are possible but I haven't verified these claims.
P: 50

## If a planet had two suns...

the permutations are endless.
if the planet is close enough to the suns then the suns would most likely be orbiting each other.
we then have questions re size of planet which in turn will dictate how close to the suns the planet has to be to be in a stable orbit.
in a complex kelplerian system such as this that has lasted the the potential of being swallowed by the suns or being flung out you need to consider tidal heating and/or syncronization orbits...
a single sun is so much easier....
Mentor
P: 9,636
http://arxiv.org/abs/0705.3444
 Terrestrial planet growth within circumbinary disks was uninhibited around inner binary star systems with binary apastrons (maximum separation) less than ~0.2 AU. Results from our simulations can be scaled for different stellar and disk parameters. Approximately 50 - 60% of binary star systems - from contact binaries to separations of nearly a parsec - satisfy these constraints. Given that the galaxy contains more than 100 billion star systems, a large number of systems remain habitable based on the dynamic considerations of this research.
Kepler-16b is an example of such a planet, and it is within the habitable zone - if it would be smaller, it could have conditions similar to earth. It has one of the largest maximal angle (as seen from the planet) between the stars.

The other option ("2" in phyzguy's post) would be similar to an extremely bright planet - might be bright enough to give visible shadows for a human eye.
 P: 76 What do you mean by "might be bright enough to give visible shadows for a human eye". You mean multiple shadows as in stadiums lit by flood lights?
 Mentor P: 9,636 At night, without the main sun and during the right season to see the other star at night. To get a planetary disk around one star, the second star needs some reasonable separation - I might be visible during daytime (but it does not have to, it depends on the distance), but I doubt that you could see multiple shadows with human-like eyes.
P: 284
 Quote by mfb At night, without the main sun and during the right season to see the other star at night.
If you can see the other star it wouldn't be night would it :-)
PF Patron
P: 10,394
 Quote by cosmik debris If you can see the other star it wouldn't be night would it :-)
It might. If the 2nd star is far enough away or dim enough it would be similar to having a full moon or three. It's still night, it's just not pitch black.
 P: 1,958 Suppose you took a star one tenth as bright as the sun and put it in Saturn's orbit. I think this is small enough that the Earth's orbit would still be stable. It would have an apparent magnitude of about -19, which makes it about 250 times as bright as the full moon. Not like a second sun, but much brighter than anything in our sky.
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P: 10,394
 Quote by phyzguy Suppose you took a star one tenth as bright as the sun and put it in Saturn's orbit. I think this is small enough that the Earth's orbit would still be stable. It would have an apparent magnitude of about -19, which makes it about 250 times as bright as the full moon. Not like a second sun, but much brighter than anything in our sky.
A red dwarf such as Proxima Centauri has only about 1.7% the luminosity of the Sun and would probably be a very common star in binary systems. I'm not sure what the apparent magnitude would be if it were at Saturn's orbit.
P: 1,958
 Quote by Drakkith A red dwarf such as Proxima Centauri has only about 1.7% the luminosity of the Sun and would probably be a very common star in binary systems. I'm not sure what the apparent magnitude would be if it were at Saturn's orbit.
It would be about 5 times fainter than what I calculated, so about magnitude -17. Still about 50 times brighter than the full moon.
 P: 535 Why not choose Sirius as your binary star.
P: 571
 Quote by baobeiiiii Hey, this is a research question for a novel. All advice, comments are much appreciated. Can a planet have two suns? If so, what are the implications on the length of day and night, casting of shadows, temperatures, etc. Does the planet revolve around just one sun or both at the same time? Sorry that this is such a general question, thanks!
The coolest case would be where it orbits the suns in a figure-8 pattern. There would be great extremes of temperature. The orbit would be chaotic. You could have the inhabitants advanced enough to have some sort of control over the orbit, since a chaotic orbit can be influenced by very small changes. Then the orbit of the planet would be a big political issue. Some beings would have evolved to favor one star or the other, so there would be a huge debate.
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P: 10,394
 Quote by phyzguy It would be about 5 times fainter than what I calculated, so about magnitude -17. Still about 50 times brighter than the full moon.
So night time would be similar to twilight here on Earth then?
Mentor
P: 9,636
 Quote by phyzguy Suppose you took a star one tenth as bright as the sun and put it in Saturn's orbit. I think this is small enough that the Earth's orbit would still be stable. It would have an apparent magnitude of about -19, which makes it about 250 times as bright as the full moon. Not like a second sun, but much brighter than anything in our sky.
I don't think you get a real planetary disk if the companion star is so close, unless there is some mechanism which captured the planet (or the star) after the initial formation of the system.

Known exoplanets in binary systems
Table A.4 and A.5 list 59 known exoplanets orbiting a single star with a single known companion. The closest companions are at a separation of 20-21 AU, where the planets have an orbital radius of 0.02 to 2.4 AU.
The other values are lower limits on the separation (projected distance as seen by us) - two additional systems have a minimal separation below 100 AU, all other 40 systems (with 52 planets) have a separation of more than 100 AU.
Table A.6 lists 11 exoplanets in systems with 3 or more stars. Two planets might be in systems with less than 100 AU separation between the stars.

A closer look on those close (20-21 AU) systems:
γ Cephei A with 1.4 solar masses has an exoplanet with an orbital radius of 2 AU, Companion star γ Cephei B is a red dwarf with 0.4 solar masses
Gl 86 A has a planet with an orbital radius of just 0.11 AU.
HD 41004 has an orange and a red dwarf, both have one known exoplanet each. HD 41004 Bb has an orbital radius of <0.02 AU, HD 41004 Ab has some weird orbit with an eccentricity of 0.74.
HD 196885 A has a red dwarf as companion, the exoplanet has an eccentricity of ~0.5.

A binary separation of ~10 times the orbital radius (this corresponds to earth<->saturn) is possible, but it is very rare and leads to eccentric orbits.
 P: 284 The list of binary star planets is obsolete. Consider Toliman B planets. Bb is generally believed to exist. Since planets are lettered in order of discovery, not location, it shall stay Bb. Since the bolometric luminosity of Toliman B is estimated at 0,500 solar, the habitable zone lies at 0,707 AU. Letīs call a planet there as Bc. The smallest distance between B and A is 11,2 AU at periastron. Then the distance between Bc and A goes down to 10,5 AU. The luminosity of Toliman A is 1,52 solar. Thus, at the nearest approach, the light cast by Toliman A on BC is almost 1,4 % of sunlight. Regarding comparisons: full sunlight is about 129 000 lux. Clear blue sky with Sun at zenith but in shade is about 20 000 lux, in Earth atmosphere at sea level. The illumination of a horizontal surface right at dawn or dusk by clear sky illuminated by sun right below horizon is 400 lux. With Toliman A at closest approach and at zenith - not only would it cast shadows (so does Moon) but the sky would be blue, and the direct light would be enough not only for general illumination but for demanding tasks. The blue sky would be only slightly dimmer than at setting or rising Toliman B.

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