NASA Research Details How LISA Could Uncover Unique Population of Exoplanets

According to NASA News Releases, the Laser Interferometer Space Antenna (LISA) observatory, which is currently under development, is set to detect gravitational waves—tiny distortions in space-time caused by orbiting masses. Two studies conducted by NASA astrophysicists have provided further detail on how LISA might uncover a novel and unique group of exoplanets across the galaxy.

Casey McGrath, a postdoctoral researcher at the University of Maryland, Baltimore County and NASA’s Goddard Space Flight Center in Greenbelt, Maryland, stated that double white dwarf systems, which are binaries made up of white dwarfs in close orbits, are anticipated to be the most frequent source for LISA. McGrath noted that if an exoplanet orbits within one of these systems, its presence will leave an imprint on the gravitational wave signal that will be detectable.

Two papers published in The Astrophysical Journal on Tuesday detailed these findings. One paper, written solely by McGrath, focused on the detection method, determining the planet masses and orbital periods where detection would be most effective. A second paper, co-authored by McGrath, explored how these planets could be confirmed using NASA’s James Webb Space Telescope. The concept of finding exoplanets around LISA double white dwarf systems was initially suggested by Japanese astronomer Naoki Seto in 2008.

Led by the European Space Agency (ESA), the LISA mission is scheduled for launch in the mid-2030s. NASA is participating as a collaborative partner, providing telescopes, other essential hardware, and engineering and scientific assistance as part of its effort to better comprehend the universe. The method LISA uses for detection is comparable to the radial velocity technique, which utilizes light and is already employed to locate exoplanets around stars near our solar system. When a planet orbits, its gravity causes a slight wobble in its host star, which results in the star's spectrum oscillating; spectral lines shift toward bluer or redder wavelengths depending on whether the star is moving toward or away from Earth due to the Doppler effect.

Scott Noble, a theoretical astrophysicist at NASA Goddard and a coauthor on the confirmation paper, explained that a similar phenomenon occurs with gravitational waves. He stated that double white dwarfs will emit gravitational waves at a nearly constant frequency on their own. However, adding a sufficiently massive planet to the system causes the binary to undergo reflex motion, which introduces a Doppler modulation to the gravitational signal. For LISA, this modulation will manifest as sidebands, which are secondary signals appearing at frequencies both higher and lower than the primary signal.

McGrath compared this situation to modern radio communication, where some energy from a main carrier frequency is used to create sidebands at adjacent upper and lower frequencies that carry messages. In the case of gravitational waves, these sidebands convey information about the existence of a planet.

White dwarfs are remnants of stars similar to the Sun, and while exoplanets have been found around a few isolated white dwarfs, none have yet been discovered around a binary white dwarf. Therefore, any detections by LISA would represent a completely new category of exoplanet population.

Astronomers estimate that the Milky Way galaxy contains 10 million double white dwarf systems with orbital periods under an hour, which produce gravitational waves within the range LISA can detect. They anticipate that LISA will resolve approximately 10,000 individual systems, in addition to around 100 mixed binaries—those involving white dwarfs, neutron stars, or black holes—orbiting with similar periods.

Mary Anne Limbach, an assistant research scientist at the University of Michigan and a co-author on the confirmation paper, emphasized that once LISA makes its initial exoplanet discoveries, validating some of them will be crucial. She added that these joint detections will enable researchers to confirm that the gravitational signals correspond to actual planets and not false positives.

The team described a novel approach to confirming these planets by leveraging the infrared sensitivity of the Webb telescope. A planet within a double white dwarf system generates a greater amount of infrared light than what is expected from the stars alone. The instruments on Webb can easily detect this excess infrared radiation out to a distance of at least 500 light-years. Using this technique on a test case system at that distance, the researchers found that Webb could detect an exoplanet roughly five times the mass of Jupiter, whereas LISA could potentially detect a sub-Jupiter exoplanet under ideal conditions.

The LISA observatory will be composed of three spacecraft following Earth's orbit around the Sun. These spacecraft will form a large triangular arrangement with each side measuring 1.6 million miles (2.5 million kilometers). Each spacecraft will house two telescopes that will use infrared laser beams to simultaneously send and receive signals between neighboring spacecraft. Through these telescopes, the spacecraft will measure extremely small changes—smaller than the width of a helium atom—in their relative distances, which are the signals from passing gravitational waves.

Gravitational waves were predicted by Albert Einstein’s general theory of relativity in 1916 and were first observed by ground-based observatories in 2015. These waves are generated whenever massive objects accelerate, such as two stars orbiting each other. They travel through space-time at the speed of light and are unaffected by objects they pass. These characteristics make them a valuable tool for studying the cosmos.

Additional contributions from NASA to the mission include the laser system, devices designed to manage the buildup of electric charge on the proof masses, data analysis for identifying and characterizing individual gravitational wave sources, and supplementary scientific and engineering expertise.