The LISA (Laser Interferometer Space Antenna) observatory, now in development, will detect gravitational waves, the tiny ripples in space-time caused by orbiting masses. Now two studies led by NASA astrophysicists show in new detail how LISA could reveal a new and unique population of exoplanets — worlds beyond our solar system — throughout our galaxy.
“The most common source for LISA is expected to be double white dwarf systems, which are binaries composed of white dwarfs in tight orbits,” said Casey McGrath, a postdoctoral researcher at the University of Maryland, Baltimore County and NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “If an exoplanet also orbits within one of these systems, its presence becomes imprinted on the gravitational wave signal in a way we’ll be able to uncover.”
Two papers published Tuesday in The Astrophysical Journal detail the findings. The first, authored solely by McGrath, examines the detection method and determines the planet masses and orbital periods where it would work best. The second paper, by McGrath and coauthors, investigates how such planets could be confirmed using NASA’s James Webb Space Telescope.
The possibility of detecting exoplanets around LISA double white dwarf systems was first proposed by Japanese astronomer Naoki Seto in 2008.
Led by ESA (European Space Agency), the LISA mission is slated for launch in the mid-2030s. As a collaborative partner, NASA is contributing the telescopes, other critical hardware, and engineering and scientific support as part of its mission to better understand how the universe works.
The LISA detection method is similar to one called radial velocity that uses light and is already employed to find exoplanets around nearby stars. As a planet orbits, its gravity tugs on its host star, imparting a small reflex motion, or wobble, onto it. This wobble causes the star’s spectrum to oscillate, with spectral lines shifting from redder wavelengths as the star moves away from us to bluer ones as the star moves toward us. The shift is due to a phenomenon called the Doppler effect.
“Something similar occurs with gravitational waves,” said Scott Noble, a theoretical astrophysicist at NASA Goddard and a coauthor on the confirmation paper. “By themselves, double white dwarfs will emit gravitational waves at a nearly continuous frequency. Add a sufficiently massive planet to the system, and its presence causes the binary to experience reflex motion, adding a Doppler modulation to the gravitational signal.”
For LISA, this modulation will appear as sidebands, secondary signals at both higher and lower frequencies than the primary one.
McGrath likens the situation to modern radio communication, where some of the energy of a main radio carrier frequency goes into forming sidebands at adjacent upper and lower frequencies that carry messages (like voices or music). For gravitational waves, the sidebands carry messages about the presence of a planet.
White dwarfs are the Earth-sized remnants of stars like the Sun. While exoplanets have been detected around a few isolated white dwarfs, none have yet been found around a binary white dwarf, which means the LISA detections would represent a distinct new exoplanet population.
Astronomers estimate that our Milky Way galaxy contains 10 million double white dwarf systems with orbital periods of less than an hour, which generate gravitational waves in the right range for LISA to find. They expect LISA will resolve about 10,000 individual systems, as well as around 100 mixed binaries — with white dwarfs, neutron stars, or black holes — orbiting in similar periods.
“Once LISA captures its first exoplanet detections, it will be critical for us to validate some of them,” said Mary Anne Limbach, an assistant research scientist at the University of Michigan and a co-author on the confirmation paper. “These joint detections will allow us to establish that the gravitational signals correspond to genuine planets rather than false positives.”
The team described a new method for confirming these planets by taking advantage of the Webb telescope’s infrared sensitivity. A planet in a double white dwarf system produces a larger portion of infrared light than what’s expected from the stars themselves. Webb’s instruments can easily detect this infrared excess out to a distance of at least 500 light-years.
Using this technique for a test case system at this distance, the researchers find that Webb could detect an exoplanet about five times Jupiter’s mass, while LISA could detect a sub-Jupiter exoplanet under ideal circumstances.
The LISA observatory will consist of a trio of spacecraft following Earth as it orbits the Sun. They will form a vast triangular array stretching 1.6 million miles (2.5 million kilometers) on each side. Each spacecraft will include two telescopes that will use infrared laser beams to simultaneously transmit and receive signals between adjacent spacecraft. Through these telescopes, the spacecraft will measure miniscule changes — smaller than the width of a helium atom — in their relative distances, the signals of passing gravitational waves.
Gravitational waves were predicted by Albert Einstein’s 1916 general theory of relativity and first detected by ground-based observatories in 2015. The waves form whenever massive objects accelerate, such as two stars in orbit around each other. They travel through space-time, moving at the speed of light, and are unaffected by objects they encounter along the way. These properties make them a valuable tool for probing the cosmos.
Additional NASA contributions to the mission include the laser system, devices to manage the buildup of electric charge on the proof masses, data analysis for identifying and characterizing individual gravitational wave sources, and additional scientific and engineering expertise.
To learn more about the LISA mission, visit: