While numerous two- and three-star systems containing planets have been discovered (with planet-harboring binary-star systems possibly outnumbering planet-harboring single star systems), four-star systems with planets were at first thought to be if not impossible, highly unlikely. How can a planet maintain an orbit?
30 Ari a Double Double
However, we now know of two planet-harboring quadruple-star systems. First was Kepler-64b, found in 2012. The latest is a system called 30 Ari which lies 136 light-years away in the constellation Aries. The star system was first thought to be a triple-star system but a fourth star was discovered, making 30 Ari a double double-star system, with each binary orbiting a point in space between the two systems (the center of gravity for the entire quadruple-star system).
|A diagram of the newfound system show the two pairs of stars in orbit together, while a planet circles one of them. Credit: NASA/JPL-Caltech|
The one planet found to date is a giant planet 10 times the mass of Jupiter and orbits the main star in 30 Ari B every 335 days; it lies about 0.995 AU from the main star. The second star orbits at about 22 AUs away (for comparison, The Earth is 1 AU from the Sun, Uranus is around 20 AUs away, and Neptune is about 30AUs away).
The planet orbiting the main star in 30 Ari B might lie just within the habitable zone. While the planet itself is probably not habitable due to its mass (it is most likely a gas giant), this does not rule out a habitable moon.
A Complicated Sky
A sentient being on such a hypothetical moon would see sometime in the daytime sky (when the gas giant is not occluding or blocking the main star) one small sun and two bright stars. With a large telescope, it would see that one of the bright stars was a binary system (30 Ari A binary system).
Some days, depending upon the tilt of the gas giant and thus the orbital plane of the moon with regards to the orbital plane of the stars, as well as possibly depending upon latitude if on a large moon, the sentient being would see just one (the main) star in the sky, or the main star with one of the two bright stars. One of those stars would show retrograde motion on a "yearly" basis.
|Why celestial objects orbiting farther out display retrograde motion.|
Credit: Prof. Pogge, Ohio State University.
Night, or when the gas giant is blocking the main 30 Ari B star, the sentient being would sometimes see those two bright stars (they would be the brightest nighttime stars), or just one of them, and sometimes none. If the moon is on the sun-facing side of the gas giant, the nighttime side of the moon would face the gas giant which would probably block any stars of any kind from being seen.
In fact, with a gas giant 10 times the mass of Jupiter, it is possible the moon beings would not see the night sky for part of their year. When their moon-planet is between the main star and the gas giant, night is just the gas giant. When the orbit begins to take the moon to the other side of the gas giant, then a starry night begins to be seen, with the day being replaced by a starry night (and the nights always the gas giant filling the sky, or filling most of it). This does depend upon how far away the moon is orbiting from the gas giant. Further away and then they may be able to see the night sky framing the gas giant, though most of the sky would still be the gas giant.
What a complicated sky! What complicated early religions would arise from such complexity? A mighty light in the sky that has two smaller, wandering lights that has a complicated dance.
If the newfound star is on the same orbital plane as the main 30 Ari B and main 30 Ari A stars, then sometimes the lesser lights (but still brighter than any other star) would seem to merge together, only to separate again, on a repetitive basis. If not, then they would approach and pass each other, with the newfound star being the fastest moving.
The 30 Ari A system would be like a zodiac indicator, for when the beings are able to see the night sky, since the 30 Ari A system is always opposite of the 30 Ari B system (circling a point in space in between the two systems). But since the 30 Ari A system is moving (both systems dancing around that shared point), that pointer would slowly move over the centuries, where, for example, the first day of spring used to be when 30 Ari A was in constellation D, it has slowly moved to where it is now in constellation E.
I would think that sentient beings would be pattern seekers on some level--visual ones would try to make some sense of the dots in the night sky, whether they would have a close equivalent to our concept of a zodiac is another matter. The stars will align at certain times of the year, times that are important for a primitive people to survive (best time to hunt, when to prepare for lean season, etc), as well as any religious meaning that gets attributed to stars and the movement/dance of those stars.
What kind of Stonehenge would arise in such a complex system? Though the aliens would be able to handle it--we had to figure out when eclipses would occur and that is no easy matter.
Even More Complexity
Just as the two binaries orbit a shared center of gravity, the stars within each binary revolve around a shared center of gravity (or barycenter). The more alike in mass the two stars in a binary system are, the more the center of gravity will be outside of each star, in between. Two equally massive stars would orbit around a point essentially halfway between the two stars. A big imbalance in masses, however, could put the center of gravity inside the largest star. In our solar system the barycenter moves about as the planets have different masses, revolve at different rates, and are at different distances--sometimes the barycenter is inside the Sun, sometimes outside the Sun.
|Solar System Barycenter Orbit Around Sun, from Wikimedia Commons. Credit Carl Smith, Rubik-Wuerfel.|
All Hail the Sky Giant
Retrogrades are hard to explain if you believe your planet is non-spinning. Would seeing a large gas giant spinning above you help a race to think of their planet, the gas-giant's moon, as spinning as well? With ancient humans, the Moon pretty much always kept the same face toward the Earth (we can see a wee bit more as the Moon's orbit is not perfectly circular). A gas giant, even if the giant's moon was tidally locked, would appear to be spinning. Even if the gas giant was tidally locked, there would be at least some cloud bands that would circle the globe and spin. However, maybe those cloud bands would be looked at as just clouds and no proof of a spinning planet.
On the other hand, the sentient beings would probably quickly realize they are orbiting that gas giant. The moon would not be the center of the universe--the gas giant would be. They may try to at first explain the night sky as everything circling the gas giant.
A gas giant that is always looking down on them, a huge presence every night. The main star is not as constant, disappearing behind the gas giant for part of each year. The two bright wandering stars also disappear behind (or into) the gas giant at different times of the year. A sense of pattern helps to put a mind at ease--chaos is dangerous, especially to early societies that are always on the edge of death due to climate patterns (from too little rain to too much rain to devastating storms or floods or fires during times of droughts, etc). And the one constant--the only constant (even if it face changes a bit with the cloud bands)--the gas giant, the sky giant.
Would the gas giant then be the home of the gods? Heaven? Hell? Birth place of the universe? A god itself?
The Great Eye(s)
And if that gas giant had one or more giant storms, like Jupiter, a big eye or eyes peering down? The ever watching eye...if tidal lock and always see the eye. If not, then the eye moves--watching the heavens, scanning creation, turning its eye upon the inhabitants of the moon on a regular basis.
When the eye appears, is that the time to supplicate to the great sky god? When its eye is most directed to the moon?