Future gravitational-wave observatories built to detect colliding black holes and dead stars might also be able to do something nobody designed them for: find planets. That is the premise behind an illustrative study highlighted by NASA's Goddard Space Flight Center on Nov. 10, which argues that a planned space-based detector could pick out exoplanets orbiting pairs of burned-out stars called white dwarfs.
The method relies on a subtle wobble. When two white dwarfs orbit each other closely, they emit gravitational waves, ripples in spacetime, as they spiral together. If a planet also orbits that pair, its gravity tugs the binary back and forth, and that tug shows up as a faint, periodic shift in the gravitational-wave signal. NASA scientist Scott Noble likened the effect to sidebands in a radio signal: extra frequencies layered just above and below the main tone. In a test case run for a system 500 light-years away, the modeling found that a sub-Jupiter-sized planet could, under ideal conditions, be detectable this way, with Jupiter-sized planets potentially identifiable thousands of light-years off.
A decade-old idea, revisited
The underlying concept dates back further than this month's release. In a 2019 paper in Nature Astronomy, covered in depth by Scientific American, astrophysicists Nicola Tamanini, of the Max Planck Institute for Gravitational Physics, and Camilla Danielski, of France's CEA, first proposed using the planned Laser Interferometer Space Antenna, or LISA, to hunt for planets this way.
In practice, LISA should allow us to make detections across the entire Milky Way.
Nicola Tamanini, Max Planck Institute for Gravitational Physics
The pair estimated that if even 1% of the tens of thousands of tight white dwarf binaries expected in the galaxy host planets, LISA could turn up hundreds of them. Other researchers have urged caution. Kaze Wong, a Johns Hopkins astrophysicist not involved in either study, has said the gravitational pull of a single planet on its binary stars may simply be too faint for LISA to pick up at all, and Danielski herself has acknowledged nobody has firmly modeled whether planets can even survive, let alone form, in such close orbits around two dying stars. A null result, she noted, would still be scientifically useful, narrowing down where such planets cannot exist.
LISA, a joint ESA-led mission with NASA contributions, is not expected to launch until around 2035, so any detections remain projections rather than results. NASA says that if the method works, the James Webb Space Telescope could follow up on the closest candidates, within a few hundred light-years, to confirm a planet's presence directly.