NASA-Led Studies Show LISA Can Find Planets Around White Dwarf Pairs
Two NASA-led studies in The Astrophysical Journal show how the LISA gravitational wave observatory could find planets orbiting double white dwarfs.
A planet circling a pair of dead stars would leave a fingerprint in gravitational waves, and the future LISA observatory should be able to read it. Two studies led by NASA astrophysicists, published Tuesday in The Astrophysical Journal, lay out how, according to NASA. Together they describe a possible new class of exoplanets, meaning worlds outside our solar system, found throughout the Milky Way.
No planet has yet been found around a binary white dwarf. Planets have turned up around a few single white dwarfs, so detections by LISA would add a population that astronomers have never seen.
How a planet shows up in gravitational waves
Gravitational waves are ripples in space-time produced when massive objects accelerate, for example two stars orbiting each other. Albert Einstein’s 1916 general theory of relativity predicted them, and ground-based observatories first detected them in 2015. They move at the speed of light and pass through whatever lies in their path.
A double white dwarf is a tight pair of Earth-sized stellar remnants left behind by stars like the Sun. On its own, such a pair sends out gravitational waves at a nearly constant frequency. A sufficiently massive planet in the system tugs the pair, which makes the binary wobble. That motion adds a Doppler modulation to the signal, said Scott Noble, a theoretical astrophysicist at NASA’s Goddard Space Flight Center and a coauthor of the second paper.
In LISA’s data, the modulation would appear as sidebands, which are weaker signals just above and just below the main frequency. Casey McGrath, a postdoctoral researcher at the University of Maryland, Baltimore County and Goddard, compares this to radio, where a carrier frequency spills some energy into neighboring frequencies that carry the message. Here, the sidebands carry the evidence of a planet.
The idea resembles the radial velocity method, which uses starlight to find planets around nearby stars. There, a planet’s pull makes the star move toward and away from us, and its spectral lines shift between bluer and redder wavelengths. Japanese astronomer Naoki Seto first proposed the gravitational wave version in 2008.
Checking the candidates with Webb
McGrath wrote the first paper alone. It examines the detection method and works out the planet masses and orbital periods for which it works best. The second paper, by McGrath and coauthors, proposes confirming LISA’s candidates with NASA’s James Webb Space Telescope.
A planet in a double white dwarf system adds infrared light beyond what the two stars produce. Webb’s instruments can readily detect this excess out to at least 500 light-years, the team reports. For a test system at that distance, the researchers found that Webb could detect a planet about five times Jupiter’s mass. Under ideal conditions, LISA could detect a planet below Jupiter’s mass.
Mary Anne Limbach, an assistant research scientist at the University of Michigan and a coauthor of the second paper, said joint detections would let scientists establish that the signals come from real planets and not false positives.
What LISA will survey
Astronomers estimate the Milky Way holds 10 million double white dwarf systems with orbital periods under an hour, which emit gravitational waves in LISA’s range. They expect the observatory to resolve about 10,000 of them, plus around 100 mixed binaries that pair white dwarfs, neutron stars or black holes with similar periods.
LISA, short for Laser Interferometer Space Antenna, is led by the European Space Agency and slated to launch in the mid-2030s. It will use three spacecraft trailing Earth around the Sun, arranged in a triangle 1.6 million miles (2.5 million kilometers) on each side. Infrared laser beams between the craft will track changes in their separation smaller than the width of a helium atom. NASA supplies the telescopes, the laser system, devices that manage electric charge buildup on the proof masses, data analysis for gravitational wave sources, and engineering and science support.
Why it matters
The planets in these studies are predictions of what LISA should be able to see, and the observatory has not yet flown. If it works as modeled, it would reach binary white dwarfs, a type of system where no planet has ever been confirmed. The Webb method gives astronomers a way to check which LISA signals come from real planets.
Source: NASA
