Science1 publisher3 min readPublished
Simulations find stable Earth-mass orbits in the habitable zone of 70 Ophiuchi's larger star
UC Riverside simulations find an Earth-mass planet could orbit stably in the habitable zone of 70 Ophiuchi's larger star, about 5 parsecs away. A stable orbit is the first test a habitable world must pass, so the result tells planet hunters where in this pair to look.
The Scientist · Science desk
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What happened
- The modeling relied on University of Michigan observations that better define how the two stars move around each other.
- UC Riverside doctoral student Skylar D'Angiolillo ran the dynamical modeling, with astrophysicist Stephen Kane as her adviser.
- Each star is a little smaller and a little cooler than the sun; their masses come to roughly 88% and 73% of the sun's.
- Astronomers have watched the system for decades without confirming a planet around either star.
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Why it matters
- decision Planet searches that pass over binaries because the companion's gravity might clear out a habitable zone now have a modeled case for keeping this system's primary star on their lists.
- constraint Only the larger star and an Earth-mass planet were tested, so searchers cannot yet count the smaller star's habitable zone, or heavier planets, as dynamically safe.
- precedent Because many stars have stellar companions, this cheap inject-and-track screen is likely to become a routine first step for other binaries before anyone spends direct-imaging time on them.
Each simulation behind the paper in The Astrophysical Journal [1] starts with the two stars on their measured orbit. It then adds an Earth-mass planet at a chosen distance from the primary star and follows the gravitational interactions among all three bodies [8]. The test planet sits inside the habitable zone, the band where a planet would be just warm enough to keep liquid water on its surface [15]. "Then I tested whether an Earth-mass planet could maintain a stable orbit at different distances within that zone," D'Angiolillo said [17].
From there the planet has three possible fates: it stays in orbit, falls into the star, or is ejected [8]. Failure usually comes gradually. An unstable orbit grows more and more elongated until the planet is flung from the system, and repeating the run at different distances maps where an Earth-sized planet could last [9].
The companion's gravity is the hazard under test, so the answer can only be as good as the stellar orbit fed into the model. Kane said that orbit had been the weak point. "It's been known since antiquity, but what hasn't been known nearly as well is precisely how the two stars orbit each other," he said [11].
With the refined orbit as input, at least some test planets in the primary's habitable zone held on. "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." [10] Kane's reason for caring is how common binaries are. "Our solar system, with only one star, is actually somewhat unusual. Many stars have stellar companions," he said [12].
The thing this doesn't tell you is whether anything is there. A stable slot in a simulation is a precondition for a habitable world, and an empty system passes the same test. The published Q&A does not report what share of the habitable zone stayed stable, how long the runs lasted, or how elongated the stars' own orbit is. Those figures decide whether "possible" means most of the zone or a thin band. I think the result justifies keeping the primary star on habitable-zone target lists. How high it ranks depends on that width.
Distance is the other argument for attention. Proxima Centauri, the nearest star to the sun, is 1.3 parsecs away, so 70 Ophiuchi is roughly 3.8 times as far [3][1]. Kane still called it "one of the nearest star systems to us" [16]. The researchers described the modeling as groundwork: "there are a lot of steps we can take before trying to directly observe a planet" [14].
What to watch
- Stability maps from the full Astrophysical Journal paper: if only a narrow band of the primary's habitable zone keeps an Earth-mass planet over long runs, the case for ranking the system high gets weaker.
- Any planet search of 70 Ophiuchi's primary star that reports a detection, or firm limits on Earth-mass planets in its habitable zone.
- Equivalent modeling for the smaller star, at about 73% of the sun's mass, whose habitable zone this study did not test.