NASA researchers propose using gravitational waves to find a hidden class of exoplanets

Astrophysicists at NASA's Goddard Space Flight Center have outlined a method for detecting a type of exoplanet that has never been confirmed: worlds orbiting two white dwarfs locked in a tight binary. The approach does not rely on light at all. It uses gravitational waves, the ripples in spacetime produced when massive objects accelerate, as the signal that gives a hidden planet away.

The work, published this week in The Astrophysical Journal, was described in a NASA Science release on Tuesday. It centers on the Laser Interferometer Space Antenna, or LISA, a gravitational-wave observatory led by the European Space Agency with NASA as a partner, slated to launch in the mid-2030s. No data exists yet; the papers are a roadmap for what LISA should be able to see once it is operating.

How the method works

Casey McGrath, a postdoctoral researcher with the University of Maryland, Baltimore County working at NASA Goddard, is the lead author of the first paper. She compared the technique to an established method astronomers already use to find planets around ordinary stars.

"The most common source for LISA is expected to be double white dwarf systems," McGrath said in the NASA release, adding that a planet's gravity "becomes imprinted on the gravitational wave signal in a way we'll be able to uncover."

White dwarfs are the Earth-sized, burned-out remnants of stars like the sun. When two of them orbit each other closely, they generate a steady gravitational-wave signal. NASA Goddard astrophysicist Scott Noble, a co-author of the second paper, said a planet orbiting that pair would tug on the stars themselves. "Add a sufficiently massive planet to the system, and its presence causes the binary to experience reflex motion," Noble said, which shifts the frequency of the gravitational waves in a pattern similar to the Doppler shift astronomers use to find planets by watching a star wobble. That shift shows up as faint sidebands on either side of the main gravitational-wave signal.

What the numbers show

• The Milky Way may hold roughly 10 million double white dwarf systems with orbital periods under an hour, the kind of tight pairs LISA is built to detect.
• LISA is expected to individually resolve about 10,000 of those systems, plus roughly 100 more exotic pairings involving a neutron star or black hole.
• Under favorable conditions, LISA could detect a planet of roughly Jupiter's mass, or somewhat smaller, in these systems from thousands of light-years away.
• The second paper finds that NASA's James Webb Space Telescope could independently confirm such a planet, out to at least 500 light-years, by detecting the extra infrared glow the planet adds to the system.

No planet has yet been confirmed around a binary white dwarf of this kind. If LISA's data matches the predictions, researchers say it would represent a distinct population of exoplanets that current ground- and space-based telescopes are not equipped to find, because such systems are typically too faint and too close together to resolve optically at galactic distances.

Background and why it matters

The idea that gravitational waves might reveal planets is not new. Japanese astronomer Naoki Seto first proposed the concept in 2008, more than seven years before gravitational waves were first directly detected by ground-based instruments in 2015, confirming a prediction Einstein made in his 1916 general theory of relativity. The new NASA papers extend that idea by working out exactly which planet masses and orbital periods the method could realistically detect, and by proposing a specific way to double-check any detection with an independent instrument.

That second piece, confirmation with Webb, is where the practical stakes lie. Gravitational-wave data alone would likely narrow a planet's properties to a range of possibilities rather than a single definitive answer. Pairing a LISA signal with a Webb infrared observation would let scientists state with confidence that a specific signal corresponds to an actual planet, rather than noise or a misidentified stellar process.

Mary Anne Limbach, an assistant research scientist at the University of Michigan and a co-author of the confirmation study, said that cross-checking will be essential once the mission begins operating. "Once LISA captures its first exoplanet detections, it will be critical for us to validate some of them," Limbach said. "These joint detections will allow us to establish that the gravitational signals correspond to genuine planets."

What's next

LISA itself remains roughly a decade from launch. The mission calls for three spacecraft flying in a precise triangular formation, 1.6 million miles on a side, exchanging infrared laser beams to measure distance changes smaller than the width of a single helium atom. NASA's role is specific: the agency is contributing LISA's telescopes, its laser system, devices to manage the buildup of electric charge on the spacecrafts' free-floating test masses, and part of the data-analysis work needed to pick individual sources out of a background hum of overlapping gravitational-wave signals from across the galaxy. McGrath said the faint sidebands her method looks for carry messages about the presence of a planet, once scientists know how to read them. In the Webb-confirmation study's specific test case, the authors found the telescope could detect a companion planet roughly five times the mass of Jupiter, direct infrared evidence that would corroborate what a LISA gravitational-wave signal alone could only suggest. The mission's NASA page and the Webb-confirmation paper both note that the real test of the method will not come until LISA is collecting data, at which point Webb and other telescopes would be called in to attempt the follow-up observations the new research describes.