A team of astronomers led by UNSW Sydney have piloted a new method to find planets – and in the process, found 27 potential new worlds in double star systems.

There’s so little we know about circumbinary planets – planets that orbit two stars instead of one – that they can feel like the stuff of fantasy.

And for good reason: to date, we’ve only confirmed the existence of 18 circumbinary planets, compared to the more than 6000 planets we know about in single star systems.

Even the most widely-known circumbinary planet is, quite literally, fiction: the desert planet Tatooine from Star Wars, aka the birthplace of Anakin Skywalker.

But a study led by UNSW has now detected 27 potential circumbinary planets in one sweep, using a new planet-finding method that broadens the typical type of planets we can find.

The findings are published today in the Monthly Notices of the Royal Astronomical Society, just in time for May the 4th, Star Wars Day.

“Most of our current knowledge on planets is biased, based on how we’ve looked for them,” says Ms Margo Thornton, lead author of the study, astronomer and PhD candidate at UNSW. “We’ve mostly found the easiest ones to detect.

“This new method could help us uncover a large population of hidden planets, especially those that don’t line up perfectly from our line of sight. It could help reveal what the true population of planets in our universe might look like.”

The planet-finding method, called apsidal precession, has been used to characterise binary stars before, but not in a large-scale search for planets.

It involves monitoring how the binary stars’ orbit of one other – made visible by their stellar eclipses – change over long periods of time.

If there’s a variation in their (normally predictable) eclipse schedule that can’t be explained by general relativity or stellar interactions, it means a third body could be influencing the stars’ orbits – and that body could be a planet.

The findings were made using data from NASA’s Transiting Exoplanet Survey Satellite (TESS), a space telescope launched in 2018 with the mission to search for exoplanets.

“I’m excited about the potential for how many planets we could find with this method,” says Scientia A/Prof. Ben Montet, astronomer and senior author on the study. “I wasn’t expecting to find 27 already at this point from the pilot study.

“Now we get to start the really fun project of figuring out which ones are real planets.”

A new way to find planets

Almost all planets have been discovered by the ‘transit’ method, which is when a planet crosses in front of its star, creating a mini-eclipse.

This eclipse causes a dip in the starlight signal sent to Earth, suggesting there might be a planet orbiting there.

But the transit method restricts us to only discovering planets that cross between Earth and their star. If a planet orbits its star (or stars, in this instance) at an irregular orbit, or an orbit that isn’t in our direct line of sight, it can slip under our radar.

“We’re missing a huge part of the architecture for these systems,” says A/Prof. Montet.

The new method helps astronomers detect planets like these that we might have otherwise missed – helping to build our knowledge of what type of environments can support planet development.

“With this method so far, we have 27 strong planet candidates in environments completely unlike our own solar system,” says Ms Thornton, who made these findings just one year into her PhD.

“By learning more about different types of planets, we can better understand how planets form and evolve, especially in these complex environments with two stars.”

The planets are called ‘candidates’ for now as the team need to confirm, or deny, their planet status using an additional observation method.

Ms Thornton has started work on this process and hopes to have a follow-up paper ready within the next year.

Our circumbinary neighbours

The planet candidates range from objects that could be as small as the mass of Neptune to 10x as large as the mass of Jupiter.

The closest is about 650 light years away from Earth, and the furthest about 18,000 away. To put this in perspective, one light year is 9.4 trillion kilometres.

“The candidates are scattered across both our southern and northern skies,” says A/Prof. Montet.

“This means that any time of the year, no matter when you’re looking, at least one of these star systems is out there visible for you to look towards – as long as you have a telescope.”

Even though the candidates stretch across immense distances, they’re still relatively close to our ‘neighbourhood’ in the Milky Way – although our list of circumbinary planet neighbours may soon be growing.

“We found 27 planet candidates out of 1590 binary star systems, which is an almost 2% rate of binary systems that could potentially host planets,” says A/Prof. Montet.

“That implies there could potentially be thousands, or tens of thousands, of possible planets to be found with data from the Vera C. Rubin Observatory’s new 10-year sky survey, the Legacy Survey of Space and Time.

“So it’s a really exciting first step – and it also shows that there’s going to be a lot of work to do over the next few years.”

 

Learning about other worlds

Most of the planets we know about in the universe are in single star systems, like our solar system.

But cosmically speaking, systems like ours are in the minority: more than half of the stars in the universe are in binary or multiple star systems.

“We’ve painted half a picture, and the other half of the canvas is completely blank,” says A/Prof. Montet.

Astronomers still have a lot of questions about planet formation in these systems – and this new planet-hunting method could help fill some of those knowledge gaps.

“We can start asking questions like how common these planets are overall and if they could be habitable,” says A/Prof. Montet.

“If circumbinary planets do turn out to be habitable, that means life could be anywhere. Life could be everywhere. The sheer numbers are really exciting.”

Ms Thornton says the search for other planets can help us learn more about our own place in the universe.

“Understanding the diversity of other worlds out there is the first step in understanding if anyone else is out there. If we are alone or not,” she says.

“That’s what a lot of this comes back to. We just want to understand where we came from, what our place looks like in the universe, and what else exists out there.”

 

From star gazer to space explorer

Ms Thornton spent a big part of her childhood on family camping trips, gazing up at the night sky.

She looks back on these moments as integral to her future in astronomy.

“I was always out under the stars and just always had questions that my parents couldn’t answer,” says Ms Thornton. “So, I wanted to be able to answer them.”

Now, many years and answered questions later, astronomy became a passion that she could pursue as a career.

“My supervisor Ben often talks about this moment in astronomy where you’re the only person in the world who’s seen evidence of something exciting,” says Ms Thornton.

“When the first system I looked at had a clear signal that these stars were precessing, and we were able to rule out all the other causes of it, we were left with these plots and numbers that suggested we might have just found a planet.

“For a little while, we were the only people on Earth who knew about it. It was a very exciting feeling – and it’s a great part of working in astronomy.”

 

Answering new galactic questions

Over the next few months, Ms Thornton will be studying the spectra of these binary stars – that is, the light that makes up these stars – using the Anglo Australian telescope in Coonabarabran. The telescope has a remote observing room accessible from UNSW Sydney’s campus.

The team will also be collaborating with researchers in the US, UK and China later in the year to learn more about the candidates visible from the Northern Hemisphere.

Studying the spectra can help the team rule out whether the bodies they detected could be higher mass objects, like stars, brown dwarfs, white dwarfs or even black holes.

If nothing else can explain the objects, they could be confirmed as planets.

In the meantime, the team are also planning on applying the same planet-searching method to larger samples, and running simulations to better understand how the planet candidates formed and how they might evolve over time.

“I was surprised by how effective the method was and how small of a signal we could pick up on using the TESS data,” says Ms Thornton. “There’s good promise this method could potentially help us find objects as small as Earth.

“I’m excited for what’s to come next with this project. The universe is a lot more complex than we can directly see, and there could be a lot more of these real-life Tatooines out there.”

 

- ENDS -

Notes to journalists

Ms Margo Thornton and Scientia A/Prof. Ben Montet are available for interview on request.

An animation about the research is available on YouTube. To view this story, visit the UNSW Newsroom.

Contact details:

Sherry Landow, News & Content Coordinator, UNSW Sydney
E: [email protected]  P: +61 2 9065 4039

Isabelle Dubach, Senior News & Content Manager, UNSW Sydney
E: [email protected] P: 0432 307 244