Orbiting a Binary Star
It’s an iconic scene from Star Wars. Luke Skywalker’s family just acquired two droids with knowledge of the rebellion. While cleaning them, he discovers a distress message that only fuels his desire to leave Tatooine to join the academy, like many of his friends already have. Work pressures and a seemingly uncaring uncle stand in the way and perpetually delay any hope of progress. As Luke feels the weight of his teenage world, he stares wistfully at the serene yet eerie setting of the binary suns that light Tatooine. Dramatic orchestral music highlights the crushing recognition that his dreams are slowly slipping away.
I can relate to Luke. Outside influences have squashed dreams, work has brought delays and constraints, and family members have impeded progress. Yet one burning question remains: How realistic is the double star sunset seen by Luke?
Fortunately, exoplanet discoveries made in the past decade help to illuminate the answer.
One of Tatooine’s stars exhibits a red glow, meaning it likely belongs to a group of stars called M dwarfs. M dwarf stars have less than half the mass of the sun and represent more than 75 percent of the stars in the galaxy. If we include stars that have up to 80 percent of the mass of the sun, the fraction climbs to almost 90 percent of all stars. Most of these stars have no companions, but the fraction of binary systems increases as the mass of the star increases. Calculations based on extensive observations indicate that around half of all stars (and 30 percent of M dwarfs) exist in binary systems. So, the binary star system visible from Luke’s home planet makes sense.
Many people consider binary star systems as unlikely places to look for habitable planets. Planetary formation in such systems may experience difficulties not seen in single star systems, and gravitational disruptions could eject any planets that do form. Yet in 2011, astronomers detected the first exoplanet that orbits around both stars in a binary system. Since then, astronomers have found a handful of these exoplanets, the latest named Kepler-1647b. This Jupiter-mass planet takes over three years to complete a circuit around two stars with masses similar to the sun—and it’s been doing so for the last 4.4 billion years. Studies show that as many as half of all binary systems could host planets. Further, many detected exoplanets orbit in binary systems, although they orbit only one star in a binary system with a large separation between the stars.
Clearly, binary star systems host exoplanets! Most research to date focuses on Jupiter-sized exoplanets because they are the easiest to detect, but it seems reasonable to conclude that researchers will also find Earth-sized planets orbiting in binary star systems. At this time, it even appears that binary stars host planets as frequently as single stars, although this may change as astronomers accumulate more data and make more precise measurements.
The existence of Earth-sized planets orbiting around binary stars is only one factor in the equation. The big question remaining is whether a truly habitable planet can form in such a scenario. One huge factor in Earth’s ongoing habitability is the incredible coordination between the changes in Earth’s atmosphere and surface that counter the increasing output from the sun. Adding another star to the mix seems to require an even greater degree of fine-tuning!