What if Earth had two suns?

Every day, one Sun rises and one Sun sets.

What if there were two suns in our sky? What would days look like? Let's find out what scientists know.

Our solar system has just one star, the Sun. It feels normal to us, but NASA explains that systems like ours are actually in the minority: more than half of all the stars in the sky have one or more partner stars.

So what if Earth had two suns? What might shadows and sunsets look like? And are there real planets that circle two stars? This is a thought experiment. Choose a path and explore what astronomers have found, and what they think a two-sun sky might be like.

Multiple-star systems are common: NASA notes that more than half of all stars in the sky have one or more partners, in combinations ranging from mismatched pairs to systems of up to seven stars. Planets can orbit both stars of a close pair, a configuration called circumbinary.

This thought experiment asks what a second sun would change in a planet's sky, using two real circumbinary systems as evidence. Anything about Earth itself with two suns is an idea: Earth's orbit and climate are not modeled here, and the lab makes no prediction about them.

We have one Sun. So when you stand outside on a sunny day, you have one shadow.

One Sun rises in the morning. One Sun sets at night.

With one Sun, everything outside on a sunny day casts one sharp shadow, pointing away from the Sun. The Sun rises once and sets once each day.

In space, NASA says, stars often come in pairs that circle each other, held together by gravity. In such a pair, the two stars can be very different, such as a Sun-like yellow star partnered with a much smaller, cooler red dwarf.

Our single Sun gives one light source, one shadow per object, and one sunrise and sunset per day. That arrangement is not typical of stars in general.

In binary systems, NASA explains, partners can differ greatly in mass; for example, a mid-sized Sun-like star can orbit with a far smaller, cooler red dwarf. Seen from Earth, some binaries eclipse each other, and timing those eclipses lets astronomers measure the stars' masses, diameters and orbits.

Now imagine a second sun in the sky. It is smaller and a little red.

You might have two shadows! One dark, and one lighter.

Imagine adding a second, smaller, reddish sun near our own, like the pair of stars that the planet Kepler-16 b circles. NASA describes what someone standing on solid ground there, such as a moon of that planet, would see: two shadows, a deeper one from the bigger star and a fainter one from its smaller, reddish partner.

On Earth with two suns, you might see two sunsets on some evenings, one after the other. That is an idea: exactly how it would look would depend on how far apart the two suns were.

For the Kepler-16 system, NASA describes an observer on solid ground near the planet, such as a hypothetical moon, casting two shadows: a deeper one from the larger star and a fainter one from the smaller, reddish companion.

For an imagined two-sun Earth, the sky would depend on the pair's separation and the planet's orbit, which this lab does not model. Ideas such as paired sunsets, or a second sun sometimes rising before the first, follow from geometry, not from a calculation; they are offered as might-be scenes, not predictions.

A shadow is a dark spot where light can't reach.

Two suns shine from two places. So your body blocks light twice, and you get two shadows.

A shadow forms where something blocks light. With one Sun, light comes from one direction, so there is one shadow. With two suns in different places in the sky, each sun's light is blocked in a different direction, so you get two shadows.

The shadows wouldn't look the same. Where only the dimmer sun is blocked, the brighter sun still lights the ground, so that shadow is pale. Where the brighter sun is blocked, the shadow is darker.

Each light source casts its own shadow along the line from the source through the object. With two sources, there are two shadows, and each one is still lit by the other source. The shadow of the brighter star is filled in only by the dimmer star's light, so it looks deeper; the dimmer star's shadow is filled in by the brighter one and looks faint.

That fits NASA's description of the Kepler-16 shadows. How far apart the shadows point depends on the stars' separation in the sky, which changes as they orbit each other.

Scientists have found real planets that go around two stars!

One was found by a high school student. He spotted it about three days into his summer job at NASA.

In 2011, NASA announced Kepler-16 b, a planet about 245 light-years away that orbits two stars. It is a gas giant about the size of Saturn, so it has no ground to stand on, and its year lasts 229 days.

In 2019, Wolf Cukier, a high school student working as a summer intern at NASA, spotted another one in data from the TESS space telescope. The planet, TOI 1338 b, is about 6.9 times larger than Earth, between the sizes of Neptune and Saturn. Its two stars circle each other every 15 days, and it circles both of them.

Kepler-16 b, announced in 2011, lies about 245 light-years away; it is a Saturn-sized gas giant with a 229-day orbit, and NASA notes that only about a dozen of Kepler's confirmed discoveries are circumbinary planets.

TOI 1338 b, TESS's first circumbinary planet, was found by Wolf Cukier, a NASA Goddard summer intern, in 2019 and announced in January 2020. It is about 6.9 times larger than Earth, between Neptune and Saturn in size, and 1,300 light-years away; its stars, one about 10% more massive than the Sun and one about a third of its mass, orbit each other every 15 days. Its transits are irregular, every 93 to 95 days, because the stars move, which is why Cukier checked the data by eye.