"We calculated the boundaries of the habitable zone around the primary star," said co-author Skylar D'Angiolillo of UC Riverside. "Then I tested whether an Earth-mass planet could maintain a stable orbit at different distances within that zone."

The result surprised even the researchers. "What we found is that stable orbits are possible in the habitable zone of the primary star," D'Angiolillo said. "You might expect the second star to disrupt those orbits, but that isn't necessarily what happens."

The reason is orbital shape. "You can think of Earth's orbit as being fairly circular," D'Angiolillo explained. "An unstable planet's orbit can become increasingly elongated until eventually the planet is essentially flung away from the system. By repeating the simulation at different locations, we can identify regions where an Earth-sized planet could remain stable."

Why our sun is the oddball

The finding matters because single stars like our sun are the exception, not the rule. Roughly 85% of all sun-like stars in the Milky Way have one or more stellar companions. If habitable planets can form in binary systems, then potentially life-friendly worlds could exist around the most common types of stars in the galaxy.

"Having a 2nd sun may sound exotic, but we need to consider worlds that could be habitable without necessarily looking like Earth's neighborhood," said co-author Stephen Kane of UC Riverside.

The work also has a practical purpose. Before spending scarce telescope time on direct observations, astronomers can first ask a cheaper question: is a given system even capable of hosting a habitable planet?

"There are a lot of steps we can take before trying to directly observe a planet," D'Angiolillo said. "We can ask first: Is this system even capable of hosting a potentially habitable planet?"

She noted that this kind of dynamical vetting could prove useful for NASA's Habitable Worlds Observatory, a mission planned for the 2040s that will search for potentially habitable worlds. Many other binary systems could benefit from the same screening approach as scientists weigh possible targets.

Caveats and next steps

The results come from simulations, not detections. A stable orbit is a necessary condition for habitability, not a guarantee of it — the planet's atmosphere, composition, and history would all matter too. And no planet has yet been confirmed around either star in 70 Ophiuchi despite decades of monitoring.

Still, Kane remains optimistic that a real discovery could arrive sooner rather than later. "Who knows?" he said. "We might actually discover an Earth-size planet in the habitable zone that Skylar has predicted could be there even before the Habitable Worlds Observatory launches."

The team plans to keep investigating 70 Ophiuchi in the meantime. If a planet does turn up, it would give scientists their nearest confirmed example of a rocky world living under two suns.