What If Earth Had Two Suns?

Planets with two suns are real. How orbits work around a pair of stars, what double sunsets and double shadows would be like, and whether such worlds can be habitable.

Two suns setting over the horizon sounds like science fiction, but it is real astronomy. A large share of stars — including many Sun-like ones — live in pairs or larger groups. And astronomers have found planets in these systems.

Two ways to orbit two stars

Around both stars. If the two stars orbit each other closely, a planet can circle the pair from far enough away. NASA's Kepler mission found the first confirmed example of a planet transiting two stars, Kepler-16b, announced in 2011. To be stable, the planet generally needs to orbit at several times the distance between the two stars.
Around one star. If the stars are far apart, a planet can orbit just one of them while the second sun shines from a distance. Our nearest stellar neighbour, Alpha Centauri, is a system like this: two Sun-like stars orbiting each other, with a small red dwarf, Proxima Centauri, far outside them.

Life under two suns

On a planet orbiting a close pair:

You would see two suns rising and setting close together, and objects would cast two shadows.
The suns would regularly pass in front of each other, causing brief dips in daylight.
The amount of light reaching the planet would vary in cycles, adding an extra rhythm to the climate.

On a planet orbiting one star of a wide pair, the second sun might appear as an extremely bright star — at times visible during the day — and for part of the year it could light up the night.

Could it be habitable?

There is no rule against it. The main challenges are:

Stable orbits. The gravity of two stars can disturb planetary orbits. There are stable zones, but they are narrower.
Building the planet. The second star stirs up the protoplanetary disk, which may make it harder for planetesimals to grow.
Climate. The habitable zone depends on the combined light of both stars and shifts as the stars move.

So far, a number of giant planets have been found around binary stars, and simulations suggest Earth-like planets are possible too.

How to build it

Pick a binary star system, then place your planet either far outside a tight pair or close to one star of a wide pair. Avoid the in-between zone, where orbits become chaotic.