Two studies predict that the planned LISA observatory could find planets around pairs of white dwarfs and that the James Webb Space Telescope could help confirm some candidates. LISA remains in development, with launch planned for the mid-2030s.
Sole source: Two NASA-led studies describe how a future space observatory could find planets beyond the solar system by reading their effects on gravitational waves, and how another telescope could check some of those candidates.
The idea builds on a proposal made by Japanese astronomer Naoki Seto in 2008. Planets have been found around a few isolated white dwarfs—Earth-sized remnants of stars like the Sun—but not around a pair of them. Confirmed planets around such pairs would add a distinct population.
Gravitational waves are tiny ripples in space-time produced by orbiting masses. Pairs of white dwarfs in tight orbits are expected to be the most common sources for LISA, the Laser Interferometer Space Antenna. Binaries with orbital periods shorter than an hour emit gravitational waves in LISA’s detection range. Astronomers expect it to distinguish about 10,000 individual double white dwarf systems.
A sufficiently massive planet would tug the pair into a small wobble. As the pair moves toward and away from Earth, that motion would shift the frequency of its otherwise nearly continuous gravitational-wave signal, an effect called Doppler modulation. The researchers predict LISA would see secondary signals, called sidebands, above and below the main frequency.
In one paper published in The Astrophysical Journal, sole author Casey McGrath examines which planet masses and orbital periods would work best for detection. McGrath is a postdoctoral researcher at the University of Maryland, Baltimore County and NASA’s Goddard Space Flight Center. A second paper, which McGrath coauthored, examines confirmation with Webb.
If LISA finds suitable candidates, the researchers propose looking with Webb for infrared light beyond what the white dwarfs alone would produce. Mary Anne Limbach, a University of Michigan assistant research scientist and coauthor of the confirmation paper, said joint detections would help establish whether the signals come from planets rather than false positives.
For a test system 500 light-years away, the researchers predict Webb could detect a planet about five times Jupiter’s mass, while LISA could detect one less massive than Jupiter under ideal circumstances. NASA says LISA could detect a Jupiter-mass planet around a double white dwarf thousands of light-years away.
The European Space Agency leads LISA, while NASA is contributing telescopes, other hardware and scientific and engineering support. The planned observatory’s three spacecraft would follow Earth around the Sun and exchange infrared laser signals to measure tiny changes in their separation as gravitational waves pass.
LISA’s launch is planned for the mid-2030s. The proposed Webb checks would come after LISA identifies suitable candidates.
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