Welcome to Friday Fiction Facts: sciency things that fiction writers need to know.

Alien Sunset (NASA/JPL-Caltech)

Alien Sunset (NASA/JPL-Caltech)

So you’ve got this great idea for a story and you decide, just to make life interesting, you’ll place it on a planet that has two suns. Why not? It worked for Star Wars, it can work for you.

Yes it can, but there are few things you need to know before you just throw that extra sun up in the sky and start imagining your main characters wrapped in the rosy glow of dual sunsets. Even Star Wars didn’t always get it right.

Figuring out how any fictional planet will look and act is complicated. It would take a whole big pile of math to be technically accurate – a lot more than I can give you here. But just focusing on the two suns, we can consider a few basics here.

Binary stars

Binary stars (artist rendering Casey Reed, NASA)

Binary stars (artist rendering Casey Reed, NASA)

What, exactly, does it mean to have two suns? It means that your planet is orbiting a binary star. Binary stars are two stars that are so close together that they orbit around a common center of mass.

Take a look at this NASA simulator and play with the numbers. You’ll see how changing the size and mass of the stars changes how the suns would behave in relation to each other.

Now let’s add the planet to the mix.

“There are actually three ways a planet can orbit in a binary star system. It can orbit one star close in, both stars farther out, or both stars in a figure 8 orbit.” – Phil Plait:

We’re going to assume, for this discussion, that your planet orbits both suns as a unit, so does not pass between them.

Watch this simulation and notice how there would be times during the day when you’d see both suns and times when you’d see only one because one is eclipsing the other:

Illustrated in the video is the planet Kepler-16b, discovered in 2011 by astronomers using the NASA Kepler space telescope. It is “the first confirmed, unambiguous example of a circumbinary planet – a planet orbiting not one, but two stars” (Quote: Josh Carter) (Video: NASA)

How the suns look to viewers from your planet can vary. One might appear brighter or larger than the other. This would depend on both size and distance.

Sunrise / Sunset

While your planet is circling the suns, it is also rotating, providing day and night. Let’s assume, for simplicity, that your planet is earth-sized and rotating at a similar rate.

To know how sunrise and sunset would work, you’d have to know the exact relationship between the two suns – the mass and size of each, how far apart they are, and how distant they are from your planet. But rather than figure all that out, let’s take a look at a real planet with two suns.

Kepler-16b orbits a large orange star and a small red one. Laurance Doyle, from the California-based SETI Institute explained how sunsets would occur on Kepler-16b:

“Sometimes the red star would set first, sometimes the orange star, sometimes they’d set (as if) touching each other and sometimes they would set together … Two sunsets are never the same.”

Kepler-16 orbits a slowly rotating K-dwarf that is, nevertheless, very active with numerous star spots. Its other parent star is a small red dwarf. (NASA/JPL-Caltech)

Kepler-16 orbits a slowly rotating K-dwarf that is, nevertheless, very active with numerous star spots. Its other parent star is a small red dwarf. (NASA/JPL-Caltech)

As well, your day lengths would also vary depending on how far apart the two suns were (from your vantage point) on any given day.

Shadows & Daylight

Of course, shadows will be different if you have two suns. Again quoting Doyle —

“You would have two shadows because the orange star would cast a shadow, but the red star would fill it in because it’s at a different angular point in the sky. If you would want to tell the time by sundial, you would need calculus.”

Different suns might be different temperatures, so would give off different colored light. This would affect, not only how things look to your characters but would also affect things like plants and photosynthesis.

Temperature & Seasons

Temperature on your planet would depend on the size of your suns and your planet’s distance from them, just like here on earth. Seasons are dependent on the angle of your planet’s axis.

You could just decide that your suns and planet are a lot like ours, but at the very least, you’d need to take into account the large difference between times and seasons when both suns are in the sky for long periods and times when they are both absent.

Length of a year

Our earthly year is 365 days long because that’s how long it takes us to orbit the sun. Kepler 16-b orbits its suns every 229 days. Conceivably, you could choose whatever you want for your year, but it may even turn out that you have cycles of long years and short years:

Now we have to consider two “years”: the time taken for the planet to orbit Sun #1, and the time taken for the two stars to orbit one another, which will necessarily be longer – perhaps up to 100,000 times longer. I’ll call this second year the “long year” and the other one the “short year”. We can imagine that a short year is something like an Earth year.” (Stack Exchange Physics).

By the Light of the Moon

artist's animation shows the view from a hypothetical moon in orbit around the first known planet to reside in a tight-knit triple-star system, HD 188553 Ab  (NASA/JPL-Caltech)

Click image to view an artist’s animation showing the view from a hypothetical moon in orbit around the first known planet (HD 188553 Ab) to reside in a tight-knit triple-star system (NASA/JPL-Caltech)

Do you want your planet to have a moon? If so, keep in mind that the phases of the moon are just variations in how the sun reflects off the moon as viewed from earth. With two suns, you can’t just throw a full moon willy-nilly into your sky.

Assuming that “The moon orbits the planet, the planet orbits Sun #1, and Sun #1 forms a binary star system with Sun #2. … the Moon would reflect light from both Suns. It would typically look like two moon phases super-imposed on top of one another. If two stars were different temperatures (thus giving out different coloured light), I imagine this would look really cool.” (Stack Exchange Physics).

It’s life Jim, but not as we know it …

Finally, we haven’t even considered how two suns would affect living things on a fictional planet. In reality, everything would be different, even on a remarkably earth-like planet. Just reeling a list off the top of my head — the presence of two different suns of different temperatures, the variation in day-length and seasons, the effects of “colored” sunlight on the evolution of visual systems, the presence of an atmosphere – all of these would completely change the life on your planet.

Not to mention the cultural and psychological effects on your planet’s inhabitants. For a haunting extreme example from science fiction, see “Nightfall” by Isaac Asimov, a short story about a planet with six suns, where people have never experienced darkness – except this once.

But those can be a topic for another day.


Live Science: What Would Earth Be Like With Two Suns?

Stack Exchange Physics Forum: Two suns, one moon, and one planet?

The Guardian: A planet with two suns? Maybe sci-fi author Isaac Asimov can enlighten us.

UCSB Science Online: “If our solar system had two suns, would it be summer year round and would the sun ever set (i.e. would we ever experience nighttime)?”

Phil Plait on Bad Astronomy: Review Star Wars the Phantom Menace : “Tatooine orbits a binary star system. Yet in all the scenes we only see single shadows.”

Wikipedia: Binary Stars in Fiction

Finally, if you really want to get it right, you might try out Universe Sandbox to simulate your imagined world.