An Earth-Sized Planet Could Survive With 2 Suns Just 16 Light-Years Away

Planets with two suns have intrigued our cosmic imagination ever since they appeared in a popular work of fiction set in a galaxy far, far away.

That's right, the globally famous King in Yellow, published in 1895. Oh, and Star Wars, of course.

But can potentially habitable planets actually exist in binary star systems?

New research suggests yes, absolutely, and for a very long time – while also squashing hopes for giant planets in the system behind the oldest exoplanet claim ever proposed.

Our Solar System is unusual because it's the only one we know of with robots zipping between its planets.

But also for astrophysical reasons:

"Our solar system, with only one star, is actually somewhat unusual. Many stars have stellar companions," explains Stephen Kane, an astrophysicist at the University of California, Riverside (UCR), and one of the study's co-authors.

"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."

An immense astronomy team from more than a dozen institutions focused on the binary system named 70 Ophiuchi (70 Oph AB), whose two stars are K dwarfs that are approximately 88 percent and 73 percent the mass of the Sun and possibly around 3 billion years old, according to the team's calculations.

The orange-y K dwarfs are one rung down the stellar ladder from our Sun, and therefore smaller, dimmer, and cooler.

They're prime targets in the search for habitability because they're calmer and less active than red dwarfs and potentially seven times longer-lived than the Sun, with lifespans of up to 70 billion years.

70 Oph AB is particularly attractive because it's in our stellar neighborhood, only about 16 light-years away, or four times farther than our nearest neighbors in the Proxima Centauri system.

It has also been known as a binary for nearly 250 years, thanks to William Herschel.

So the team incorporated recent and archival radial velocity (RV) measurements of both stars, including data from eight instruments for the primary star, across a century of observation.

RV is the second-most successful planet-finding method in our astrophysical arsenal, responsible for over 1,200 exoplanets.

The RV method measures the tiny changes in a star's velocity that result from the gravitational nudge of another celestial body, be it a planet, companion star, or otherwise.

As that star thus draws closer to Earth, its light becomes ever-so-slightly bluer, and then correspondingly redder as it's moving away from us.

Amazingly, the researchers achieved an RV precision of better than 1 meter per second.

For perspective, that's like taking a leisurely stroll and quickening your pace to overtake a fellow pedestrian, or quickly escape your local donut shop's tempting aromas.

That astronomers can measure this change in the velocity of an entire star, a giant nuclear furnace trillions of miles away, is a testament to science.

The researchers then injected fake planet signals into their treasure trove of data to see what they could detect. They also simulated Earth-mass planets at 2,000 different distances from the primary star, tracking each one over 10 million years.

"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," explains Skylar D'Angiolillo, an Earth and planetary sciences researcher at UCR and one of the study's co-authors.

"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."

The researchers were also able to rule out the presence of Jupiter-mass-or-larger giant exoplanets around the two stars, going out to about five astronomical units (AU) from the primary star, or about the distance from the Sun at which a Jupiter-mass gas giant exists in our own Solar System (hint: Jupiter).

This is intriguing because in 1855, British astronomer William Jacob claimed that 70 Oph AB had a planet, making it the first host of an exoplanet candidate, University of Michigan (U-M) researchers point out.

Jacob's claim didn't survive scrutiny, but more recent measurements showed a 'wobble' that could still have been explained by a Jupiter-sized planet.

Instead, this supposed planetary signal appears to have been due to plain old stellar activity, the bane of the RV method.

"It's a mix of disappointment and excitement," says Yiting Li, an astronomer at U-M and the study's first author.

"We're finding out that there isn't a Jupiter-sized planet, but we're also opening the latest chapter on this historic system."

However, terrestrial planets remain below the detection threshold.

So, importantly, this suggests that Earth-mass planets could still exist here, as the researchers modeled where they're most likely to be found and to be stable – providing a framework that can be used for other binaries as well.

And, according to NASA, "More than half of all stars in the sky have one or more partners," so this can significantly expand our future exploration efforts.

Especially when next-(next)-generation observations come online, such as NASA's promising Habitable Worlds Observatory (HWO), designed to specifically explore and directly image alien worlds.

"Overall, our results demonstrate that it is possible for the 70 Oph system to host planets on S-type orbits around the primary star, including terrestrial planets on stable orbits within the [habitable zone] region," the researchers conclude.

"If stellar binaries can host Earth-like planets, 70 Oph is one of the best places to look."

The research has been published in The Astrophysical Journal.