Earth-mass planets could orbit stably in 70 Ophiuchi's habitable zone

Sixteen light-years from Earth, in a binary star system called 70 Ophiuchi, there may be room for worlds like ours. A research team has modeled what happens when you place an Earth-mass planet in orbit around the primary star of this two-star system, and the results suggest something counterintuitive: the presence of a second star does not necessarily make such an orbit impossible. In fact, stable orbits appear feasible within the habitable zone—the region where liquid water could exist on a planet's surface.

The 70 Oph AB system consists of two K-class stars, both smaller, cooler, and dimmer than our Sun. The primary star has roughly 88 percent of the Sun's mass; the secondary star about 73 percent. Together, they are thought to be around 3 billion years old. For decades, astronomers have wondered whether planets could survive in such a configuration. The gravitational pull of two stars creates a complex dance of forces, and most models suggested that any planet orbiting one star would eventually be ejected or sent spiraling into one of its suns.

To test this assumption, the research team conducted an extensive computational study. They modeled Earth-mass planets at 2,000 different distances from the primary star and tracked their orbital behavior over 10 million years of simulated time. What they found was encouraging: stable S-type orbits—orbits that circle one star while the companion star remains at a distance—are indeed possible within the habitable zone. The second star's presence complicates the gravitational environment, but it does not eliminate the possibility of stable planetary orbits.

The team's analysis rested on decades of observational data. They drew on measurements from eight different instruments collected over roughly a century, achieving a precision better than one meter per second in their radial velocity measurements of both stars. This long baseline of data allowed them to rule out the presence of any Jupiter-sized gas giants orbiting within about five astronomical units of the primary star. Such massive planets would have left unmistakable signatures in the star's motion.

The history of 70 Ophiuchi adds texture to this search. In 1855, British astronomer William Jacob proposed that a planet orbited one of the stars in this system. The claim did not survive scrutiny. More recently, another signal emerged that might have indicated a Jupiter-mass world, but further analysis suggested it was simply stellar activity—the star's own variations in brightness and motion—rather than evidence of a planetary companion.

Earth-mass planets, however, remain invisible to current detection methods. They are too small to produce the gravitational wobble that astronomers can measure with existing instruments. Yet the modeling work provides a roadmap. By identifying the orbital zones where Earth-sized worlds could maintain stable paths around the primary star, the research narrows the search space for future observations. As detection technology improves—whether through more sensitive radial velocity measurements or direct imaging techniques—astronomers will have a clearer sense of where to look and what orbital configurations are actually possible in binary star systems like this one.