LISA Could Detect Planets in Systems of Two White Dwarfs

A Gravitational Signal That Could Give Away a Planet

Two studies led by NASA astrophysicists describe how the LISA space observatory could detect a population of exoplanets not yet observed: planets orbiting binary systems of white dwarfs. The proposed method looks for a planetary signature in the gravitational waves emitted by the two compact stars. These are not detections that have already been made: LISA is in development, and its launch is scheduled for the mid-2030s, according to NASA.

White dwarfs are Earth-sized remnants left behind by stars like the Sun. Although exoplanets have been found around some isolated white dwarfs, NASA says none have yet been discovered around a pair of white dwarfs. If the strategy works, these worlds would constitute a distinct population of exoplanets.

The proposal is based on the fact that the two stars in a close binary produce gravitational waves at an almost continuous frequency. If a sufficiently massive planet orbits the pair, its gravity causes the binary to undergo a reflex motion. That displacement modulates the signal: sidebands appear, secondary signals at frequencies slightly higher and lower than the main one. According to the team, these variations could reveal the planet’s presence.

From Stellar Motion to Gravitational Waves

The reasoning has a methodological parallel with the radial velocity method, which uses light to search for planets around nearby stars. In that method, the planet’s gravitational pull causes the star to wobble slightly; the motion periodically changes the spectrum of its light through the Doppler effect. For LISA, the signal being sought would not be a shift in spectral lines, but a Doppler modulation of the gravitational waves emitted by the pair of white dwarfs.

NASA compares the sidebands to those generated in radio communications around a carrier frequency. In the analogy, the secondary signals contain information about the planet, although interpreting the shape of the gravitational-wave signal will be necessary for the actual observation. The possibility of searching for exoplanets in binary white dwarf systems with LISA was first proposed in 2008, according to the same source.

The mission will consist of three spacecraft that will follow Earth in its orbit around the Sun. Together, they will form a triangular configuration with sides measuring 1.6 million miles (2.5 million kilometers). The spacecraft will measure minute changes in the distances between them using infrared laser signals; these changes will make it possible to detect passing gravitational waves. NASA is participating in the mission, led by the European Space Agency, and is contributing telescopes, other components, and engineering and scientific support.

What the Studies Would Investigate and What Remains to Be Demonstrated

The two papers described by NASA appeared on Tuesday in The Astrophysical Journal. The first, authored solely by Casey McGrath, analyzes the detection method and determines which planetary masses and orbital periods would be most favorable. The second, written by McGrath and coauthors, examines how to confirm candidate planets with the James Webb Space Telescope. The available material does not specify the calendar date corresponding to “Tuesday.”

The proposed confirmation approach takes advantage of Webb’s infrared sensitivity. According to the team, a planet in a binary white dwarf system would produce an excess of infrared light compared with what would be expected from the stars alone. Webb’s instruments could detect this excess out to a distance of at least 500 light-years. In a test system, the researchers estimate that Webb could detect an exoplanet with approximately five times Jupiter’s mass; under ideal circumstances, LISA could detect one with less than Jupiter’s mass. These are estimates for the case described, not results from observations of confirmed planets.

Confirmation matters because a candidate signal alone is not enough to establish that a planet exists. NASA describes joint observation as a way to validate some candidates and distinguish genuine planetary signals from false positives. The available information presents a strategy for future detection and confirmation; it does not report observational findings of exoplanets in binary white dwarf systems.

A Possible Population, Not a Confirmed Census

Astronomers estimate that the Milky Way contains 10 million systems of two white dwarfs with orbital periods of less than an hour, which generate waves in the range suitable for LISA. The estimate cited by NASA is that LISA could resolve about 10,000 individual systems, as well as around 100 mixed binaries—with white dwarfs, neutron stars, or black holes—with similar periods. These figures describe estimates of systems the mission could observe, not a tally of detected planets.

The scope of the research depends on whether LISA can measure the predicted modulation and whether candidate signals can be confirmed. Webb would provide a complementary infrared check, but the reported sensitivity figures apply to a test system and specific conditions. For now, the proposal expands potential avenues for studying exoplanets: it uses gravitational waves to search for worlds in a stellar configuration different from those explored so far, without this meaning that this population has already been discovered.

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