How Gravitational Waves Can Reveal New Worlds

The LISA (Laser Interferometer Space Antenna) observatory, now in development, will detect gravitational waves, the tiny ripples in space-time caused by orbiting masses. New studies explore 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. 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. 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. 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. Scientists liken 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. Using this technique for a test case system at a distance of 500 light-years, researchers have found that LISA could detect a sub-Jupiter exoplanet under ideal circumstances. Music credit: “A Light Left Burning” Lukas Knoebl [SESAC], Universal Production Music Credit: NASA’s Goddard Space Flight Center Producer: Scott Wiessinger (eMITS) Animator: Adriana Manrique Gutierrez (eMITS) Science Writer: Francis Reddy (University of Maryland College Park) Editor: Scott Wiessinger (eMITS) This video can be freely shared and downloaded at https://svs.gsfc.nasa.gov/15117. While the video in its entirety can be shared without