A planet orbiting a pair of dead stars could leave a fingerprint in the ripples of space-time, and NASA astrophysicists say a coming space observatory could read it.Two studies show how the Laser Interferometer Space Antenna, or LISA, could reveal a new population of exoplanets across the galaxy. The first, by Casey McGrath, a postdoctoral researcher at the University of Maryland, Baltimore County and NASA’s Goddard Space Flight Center, works out the detection method and the planet masses and orbital periods where it works best. The second, by McGrath and coauthors, shows how the James Webb Space Telescope could confirm such planets.Listening for a wobbleLISA is expected to hear mostly double white dwarfs, pairs of Earth-sized stellar remnants in tight orbits. By themselves, such a pair emits gravitational waves at a nearly constant frequency. A planet changes that. Its gravity makes the pair wobble, which adds a Doppler modulation to the signal, said Scott Noble, a Goddard theoretical astrophysicist and coauthor.In LISA’s data, the modulation would show up as sidebands, weaker signals at frequencies just above and below the main one. McGrath compares it to radio, where a carrier wave spawns adjacent frequencies that carry voices or music. Here, the sidebands carry the news that a planet is present. The idea resembles the radial velocity method, which uses the shifting colors of a star’s light to find planets around nearby stars.Why double white dwarfsAstronomers estimate the Milky Way holds 10 million double white dwarfs with orbital periods under an hour, the right range for LISA. They expect it to resolve about 10,000 of them, along with around 100 mixed pairs that include neutron stars or black holes.Planets have been found around a few single white dwarfs, but none around a binary, so any LISA detection would be a new kind of exoplanet. LISA could detect a Jupiter-mass planet in such systems up to thousands of light-years away, according to NASA.Webb as the second opinionMary Anne Limbach, an assistant research scientist at the University of Michigan and a coauthor, said some LISA detections will need checking to rule out false positives. The team proposes using Webb’s infrared sensitivity. A planet in a double white dwarf system emits more infrared light than the stars alone, and Webb can detect that excess out to at least 500 light-years.In a test case at that distance, Webb could detect a planet about five times Jupiter’s mass, while LISA could detect a sub-Jupiter planet under ideal circumstances.About the observatoryESA leads LISA, with launch slated for the mid-2030s and NASA as a partner. NASA is contributing the telescopes, lasers, charge-management devices, data analysis, and engineering and science support. The observatory will have three spacecraft flying in a triangle 1.6 million miles on each side. Telescopes on each will beam infrared lasers to their neighbors to measure distance changes smaller than the width of a helium atom.Gravitational waves were predicted by Albert Einstein’s 1916 general theory of relativity and first detected by ground-based observatories in 2015. The idea of finding planets this way goes back to Japanese astronomer Naoki Seto, who proposed it in 2008.The studies were published in The Astrophysical Journal.