# planetery system with two suns in K-PAX

In the movie K-PAX the character talks about his planet where there are two suns and he say that they only rise together once in many years. Wouldn't it be impossible to have one rising and the other setting? I mean was not expected to have the two always together?

• I would be curious about the orbits and gravitational effects of these stars. If a star were near enough to be considered a "sun," and not a member of a binary star system (in which both suns would nearly always rise together), how would its gravity affect the planet? – JYelton Feb 23 '11 at 16:37
• @Gilles, may i ask why not the movies tag? thanks – Jorge Florêncio Feb 24 '11 at 12:08
• The answer would apply equally to the movie and the novel, wouldn't it? – user56 Feb 24 '11 at 18:27
• Isn't this off topic? Shouldn't this be on Physics? – Reinstate Monica - Goodbye SE Dec 1 '11 at 18:25

Let's assume that the planet only orbits one of the stars. Say at an earth-like distance. The other star might be where Jupiter is. In this case, each year the two suns would rise together for a few months, then straddle the planet for another period of months.

It would be very hard for it to only happen ONCE in many years. Although I guess it would depend what you mean by "Rise Together." If you mean that they rise super-imposed on each other, then this would be highly likely.

It would be the same chance that Jupiter would be behind the Sun on a given day during sunrise. If the two suns weren't on the same (apparent) orbital plane, then only once in a great while would you get a sun-to-sun eclipse. A sun-to-sun eclipse at dawn could be what he means by "Rise Together."

• Surely this theory requires that one of the suns is massively lighter than the other. Remember Jupiter is 1000th the mass of our sun... – Timwi Feb 26 '11 at 23:52
• @Timwi: I'm primarily talking about basic distances and the orbital planes involved. However, it probably would require the suns be somewhat smaller than ours is, but not by that much. Although they could be the same size as each other. Remember, Jupiter is 4 times farther away from us than the sun is when it is closest. It would make for a kind of super summer/winter cycle too. And the double sunrise would take place when the Jupiter sun is about 6 times farther away from us than the sun. (during the super winter solstice) – DampeS8N Feb 27 '11 at 0:08

In practice binary star systems can have the stars very widely spaced. After all we can distinguish many binary stars optically from Earth, with even a small telescope. So the most likely situation is that the planet orbits one star, while that star (with the planet) orbits around the other.

Aldiss' Helliconia trilogy is set on such a star system. The planet in that case orbits a relatively cool red star, which in turn orbits a much hotter star over a period of several hundred years: as the distance from the hotter star varies so the climate of the planet of Helliconia varies enormously.

Clearly there will be periods of time in this case where both stars rise together (the further star is "behind" the nearer) and periods of time when only one star is seen at a time (the planet is between the two stars).

• Can we really see true binaries optically (and the definition of a binary system is one where the stars are within the gravitational influence of each other)? Somehow I doubt it - I'm sure I read it was impossible a few years ago, and I'm not aware that optical astronomical equipment has suddenly advanced that much – HorusKol Feb 23 '11 at 22:28
• Oh yes we certainly can: see en.wikipedia.org/wiki/Binary_star, this lists examples including Sirius (where the companion is too small to be seen easily but whose presence was deduced by observing the motion of Sirius A) and Albireo in the constellation of Cygnus. The stars in a binary system can be so far apart that they take 1000s of years to make a revolution and yet still be influencing each other gravitationally: they are just near each other relative to interstellar space, I guess. – AAT Feb 23 '11 at 22:39