Two studies led by NASA astrophysicists show how the planned LISA observatory could reveal a new population of exoplanets by reading gravitational waves from pairs of white dwarfs. The papers appeared Tuesday in The Astrophysical Journal.
LISA, the Laser Interferometer Space Antenna, is in development and is meant to detect minute ripples in space-time produced by masses in orbit. The work describes a way those signals could uncover worlds beyond the solar system across the Milky Way, according to a NASA article.
Planets have been found around a few isolated white dwarfs, but none around a binary white dwarf. White dwarfs are Earth-sized remnants of stars like the Sun. LISA detections would therefore mark a distinct population, the article said.
Double white dwarf systems in tight orbits are expected to be LISA’s most common sources. “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,” said Casey McGrath, a postdoctoral researcher at the University of Maryland, Baltimore County and NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
McGrath alone wrote the first paper, which examines the detection method and which planet masses and orbit lengths would suit it most. McGrath and coauthors wrote the second paper, which looks at confirmation with NASA’s James Webb Space Telescope.
The method resembles radial velocity, a light-based search already used around nearby stars. A planet’s gravity tugs its host and adds a small reflex motion. In gravitational waves, that motion would add a Doppler modulation, appearing as sidebands above and below the main frequency, according to Scott Noble, a theorist in astrophysics at NASA Goddard who coauthored the paper on confirmation.
Astronomers estimate the Milky Way holds 10 million pairs of white dwarfs whose orbits last less than an hour, the range LISA could search. They expect the observatory to resolve about 10,000 individual systems, plus around 100 mixed binaries that include white dwarfs, neutron stars, or black holes on similar periods.
The European Space Agency leads the mission, which is slated for launch in the mid-2030s. As a partner, NASA will supply the telescopes, other essential equipment, and support for engineering and science.
Three spacecraft will follow Earth as it orbits the Sun, forming a triangle 1.6 million miles, or 2.5 million kilometers, on a side. Infrared lasers will measure shifts in their separations tinier than a helium atom is wide.
Webb’s infrared sensitivity offers a check. A planet in one of these systems produces a larger share of infrared light than the stars alone would. The researchers find that Webb’s instruments can detect that excess out to a distance of 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 planet below Jupiter’s mass under ideal circumstances.
“These joint detections will allow us to establish that the gravitational signals correspond to genuine planets rather than false positives,” said Mary Anne Limbach, an assistant research scientist. She works at the University of Michigan and co-wrote the confirmation paper.
Japanese astronomer Naoki Seto first proposed in 2008 that planets around these LISA systems might be detectable. Gravitational waves, predicted in Albert Einstein’s 1916 general theory of relativity, were first detected by ground-based observatories in 2015.