What If the Earth Had Two Suns?

If Earth had two suns, you would get double sunsets, two faint shadows trailing behind you, and a calendar that barely made sense. It is not pure fantasy either: roughly half of all Sun-like stars exist in binary systems, and astronomers confirmed a real planet orbiting two stars, Kepler-16b, back in 2011. Actually living on such a world, though, would be brutally hard.

Two suns is normal in our galaxy

Our single Sun is actually something of an exception. About half of stars like it travel through space gravitationally bound to a partner, orbiting a shared center of mass.

So planets with two suns are not just a Star Wars invention. Kepler-16b, nicknamed a Tatooine world, became the first confirmed planet circling a pair of stars, and astronomers have found more such worlds in the years since.

That means somewhere out there are very real sunsets with two suns sinking together. Whether anyone is around to watch them is a different question.

The orbit problem

With one sun, Earth's path is a clean, stable ellipse it has held for billions of years. Add a second massive object and the gravitational tug-of-war gets messy fast.

A planet can settle into a wide circumbinary orbit around both stars, or it can hug just one of them. Either way, the second sun keeps yanking on it, and over long timescales those tugs can make the orbit wobble or even become unstable.

An unstable orbit is bad news. It can drift the planet too close and roast it, or fling it too far and freeze it, sometimes within a single lifetime's worth of years.

Astronomers have mapped out which arrangements stay stable. A planet generally has to orbit far outside a tight pair of stars, or stick very close to just one of a widely separated pair, to avoid getting its orbit wrecked by the second sun's pull.

What the sky and weather would do

The most fun part would be visible right from your backyard. Depending on where the two stars sat in the sky, you could watch two sunsets and cast two overlapping shadows from every object.

• Double sunsets and sunrises, sometimes minutes apart
• Two faint, overlapping shadows from everything outdoors
• Day-night cycles that drift instead of repeating cleanly
• Big temperature swings as the suns move closer together and farther apart
• Seasons that follow a messy, hard-to-predict rhythm

Could life survive it?

Life needs reasonably steady conditions to get going and stay going, and two suns make steadiness genuinely hard. Wild temperature swings and a shaky orbit are tough on chemistry that wants to stay liquid and warm long enough to evolve.

It is not flatly impossible, since stable habitable zones can exist around some binaries. Astronomers actively hunt for life-friendly two-star systems.

But Earth's calm, single-sun setup is a big part of why we are here to wonder about it at all. One predictable star turns out to be a serious advantage.

One sunset is romantic. Two is a scheduling nightmare.

What the two stars might look like

Binary systems come in flavors, and the type would completely change your sky. Two near-identical sun-like stars would give you two bright, similar discs.

But many binaries pair a normal star with a smaller, redder, dimmer companion. In that case one sun would blaze white-yellow while the other glowed a deep ember-red, painting very different colors across the day.

The distance between the two stars would matter just as much. Tightly packed stars might look like a single fat sun to the naked eye, while widely separated ones would clearly read as two distinct objects drifting apart and back together over the course of the year.

Either way, sunsets would become genuinely strange events. Imagine one sun dipping below the horizon while the other still hangs in the sky, casting long shadows in two directions and tinting the clouds two different colors at once.

Try It Yourself

Want to mess around with the ideas above? On whatifs.fun, Solar System, Size of Space and Survive in Space all let you do exactly that — free, in your browser, no download.

Keep reading: what happens if the sun disappeared and what if the sun were a different color. Both go deeper on the same rabbit hole.