[UC Riverside] A nearby star system with two suns could potentially support an Earth-sized planet in a region where liquid water could exist, according to a new paper examining the dynamics of the system
The paper, published in the Astrophysical Journal, focuses on a binary star system called 70 Ophiuchi that is trillions of miles from Earth, but in cosmic terms is considered relatively nearby at about 5 parsecs distance. For reference, the nearest star to our sun, Proxima Centauri, is 1.3 parsecs away.
70 Ophiuchi’s two stars are slightly smaller and cooler than the sun, at about 88% and 73% of its mass. Though astronomers have observed the system for decades, there are no confirmed planets orbiting either star.
The binary star system 70 Ophiuchi AB, or 70 Oph AB, can be seen near the center of this image taken by the U.S. Naval Observatory in 1937.
Using observations performed at the University of Michigan to better define how the two stars move around each another, researchers modeled whether an Earth-sized planet could maintain a stable orbit in the habitable zone around the larger star. The habitable zone, or HZ, is the area around a star where a planet is just warm enough that it could maintain liquid water on its surface.
Skylar D’Angiolillo, a UC Riverside doctoral student in Earth and planetary sciences, conducted the dynamical modeling for the paper, using computer simulations to see how objects in space move and affect one another over time . Here, D’Angiolillo and her advisor, UCR astrophysicist Stephen Kane, talk about what the results mean for the search for worlds capable of hosting life.
Q: Why is 70 Ophiuchi an interesting place to look for planets?
D’Angiolillo: It’s a binary system, so there are two stars. We have decades of observational data on the system, including measurements that tell us about the motion of the stars. Using those data, we were able to better constrain the architecture of the system and model what might happen to a planet there.
We calculated the boundaries of the habitable zone around the primary star. Then I tested whether an Earth-mass planet could maintain a stable orbit at different distances within that zone.
What we found is that stable orbits are possible in the habitable zone of the primary star. You might expect the second star to disrupt those orbits, but that isn’t necessarily what happens.
Kane: This is also one of the nearest star systems to us. It’s been known since antiquity, but what hasn’t been known nearly as well is precisely how the two stars orbit each other. Now that we have a much better picture of those motions, we can build a model on a computer and see what happens under different scenarios.
Q: How do you test whether a planet could survive there?
D’Angiolillo: We use simulations that model the gravitational interactions among multiple bodies, from objects as large as stars to objects as small as planets.
You start with the existing architecture, in this case, the two stars, and then inject a test planet into the system. We placed an Earth-mass planet at different distances from the primary star and followed what happened to it.
Does it remain in a stable orbit? Does it fall into the star? Does it eventually get ejected from the system?
You can think of Earth’s orbit as being fairly circular. 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.
Q: Why does finding stable orbits in a two-star system matter?
Kane: Our solar system, with only one star, is actually somewhat unusual. Many stars have stellar companions. That means when we search for planets around other stars, we have to contend with the gravitational effects of an additional star.
You might imagine that second star coming through like a wrecking ball and making it impossible for an Earth-sized planet to survive in the habitable zone. But these simulations show that isn’t necessarily the case.
When people think about finding another Earth, they often picture something that looks like our own solar system. But if we’ve learned anything from the past few decades of exoplanet discoveries, it’s that other planetary systems can look nothing like ours. Having a second sun may sound exotic, but we need to consider worlds that could be habitable without necessarily looking like Earth’s neighborhood.
Q: What happens next?
D’Angiolillo: One reason this kind of work is important is that there are a lot of steps we can take before trying to directly observe a planet. We can ask first: Is this system even capable of hosting a potentially habitable planet?
That kind of dynamical vetting could be useful for the Habitable Worlds Observatory, a NASA mission planned for the 2040s that is intended to help search for potentially habitable worlds. There are many other binary systems that could benefit from this type of analysis as scientists begin considering possible targets.
Kane: We also have more work to do on this particular system. There are no confirmed planets there yet, but now we know an Earth-sized planet could potentially maintain a stable orbit in the habitable zone.
Who knows? 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.
Author: Jules Bernstein
A Century of Radial-velocity and Astrometric Monitoring of 70 Oph AB: New PFS Data and Constraints on Possible Planetary Companions, The Astrophysical Journal (open access)
Astrobiology, exoplanet,
