HIGHLIGHTS
Circumbinary means
orbiting two suns.
Everything on this planet
would have two shadows.
Both suns may be in the
sky together, or one at a
time, so that there’s
no nighttime.
Planet is likely dark and cold.
for image ideas
You look toward the horizon, taking in the spectacular view of the two setting suns. The larger sun has already dipped halfway behind the mountains, the smaller one is just starting to touch the highest peak. Your shadows are stretching across the desert floor like lopsided arrows.
Does this scenario sound familiar? It may be because it reminds you of the iconic scene in the first Star Wars movie, where Luke Skywalker watches the suns set on his home planet Tatooine. What was science fiction then has now become reality: Tatooine-type planets exist!
Planets that are orbiting two stars instead of one are called circumbinary planets. In this scenario, the two stars orbit each other and form a binary system. The planet typically circles this pair of suns, though if the distance between the two suns is large enough, a planet can also orbit just one of the two stars. Depending on the distance between the stars, you may see only one or both suns in the sky. They could rise and set as a pair, or tag team so that you never experience the darkness of night.
The double pull of the stars’ gravity could easily swallow a young planet, so circumbinary planets tend to orbit far away from their stars. That means that even though the planet receives light from two suns, they are so distant that the planet is likely to be dark and cold. For example, the surface temperature of Kepler 16b (the first circumbinary planet discovered) is a chilly -73°C. However, the planet experiences variations in the light it receives as the binary stars eclipse each other, so the planet’s temperature would fluctuate.
The push and pull of the star pair also creates unstable orbits. In the case of Kepler 16b, it never completes its orbit exactly where it started. That means Tatooine-type planets may lead short lives: they are either pulled into their stars, or they are bumped out of the system, turning into rogue planets that wander space in solitary darkness.
It’s tricky to spot a planet circling multiple stars, and to date, only large gas giant planets have been spotted. As instruments become more powerful, though, the likelihood of finding a rocky planet just like Tattoine becomes increasingly likely.
Kepler-16: A Planet Orbiting Two Stars
The existence of a world with a double sunset, as portrayed in the film Star Wars more than 30 years ago, is now a scientific fact. NASA’s Kepler mission has made the first unambiguous detection of a circumbinary planet – a planet orbiting two stars – 200 light-years from Earth.
Watch the video
High school student observes “Tatooine”
Waipahu senior Laura Daclison used Hawaii’s Keck Observatory to observe the circumbinary planet Kepler 35. Laura was intrigued by Kepler 35 system because its stars are similar to our Sun, and she wondered how the planet stays in their orbit.
Watch the video
Circumbinary System Explainer
MIT Professor Joshua Winn explains how circumbinary systems form, how we detect them, whether or not they are rare, and what these systems can tell us about star formation and the hunt for exoplanets.
Watch the video
TO LIFE
HABITABILITY
It’s challenging to predict whether a circumbinary planet may host life because its orbit is somewhat unpredictable. However, scientists speculate that it is theoretically possible for a Tatooine-style planet to be habitable. Life on such a world would have to be very adaptable to the variations in temperature, light, and radiation coming from its two suns. Depending on where the planet is in its orbit, it may be covered in ice, liquid water, or a thick atmosphere of snow crystals. It may be dark for a long time if both suns set at the same time, or there may be uninterrupted daylight if there is always one of the two stars in the sky. That may influence whether organisms can rely on photosynthesis to stay alive.
STORY IDEAS
In a changeable, dynamic environment like a circumbinary planet, how would you prepare for changes in temperature and nature around you? How would you adapt?
With two suns in the sky, how would you measure the time? Could you use a sundial? What would your shadows look like?