Astronomers have found thousands of planets beyond our solar system by watching starlight dim or wobble. Now NASA scientists say a future observatory could find a completely new population of worlds using something very different: ripples in space-time itself.

Two studies led by NASA astrophysicists, published Tuesday, November 10, in The Astrophysical Journal, show in new detail how the LISA mission could detect planets orbiting pairs of dead stars, and how the James Webb Space Telescope could confirm them, according to NASA.

What is LISA?

LISA, the Laser Interferometer Space Antenna, is a space observatory now in development and led by the European Space Agency, with launch planned for the mid-2030s. NASA is a partner, contributing the telescopes, the laser system and other critical hardware, plus engineering and science support.

LISA will consist of three spacecraft following Earth around the Sun in a vast triangle about 1.6 million miles (2.5 million kilometers) on each side. Each spacecraft will use telescopes and infrared lasers to measure tiny changes in the distances between them, smaller than the width of a helium atom. Those changes are the signature of passing gravitational waves.

Gravitational waves, briefly

Gravitational waves were predicted by Albert Einstein’s general theory of relativity in 1916 and first detected by ground-based observatories in 2015. They form when massive objects accelerate, such as two stars orbiting each other, and travel at the speed of light without being blocked by anything in their path.

Where the planets come in

LISA’s most common targets are expected to be double white dwarf systems, pairs of the Earth-sized remnants of Sun-like stars locked 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,” said Casey McGrath, a postdoctoral researcher at the University of Maryland, Baltimore County, and NASA’s Goddard Space Flight Center.

The idea works much like a familiar planet-hunting technique called radial velocity. As a planet orbits, its gravity tugs on its host, creating a small wobble. With light, that wobble shows up as a shift in a star’s spectrum. With gravitational waves, it adds a Doppler modulation to the signal.

“By themselves, double white dwarfs will emit gravitational waves at a nearly continuous frequency,” said Scott Noble, a theoretical astrophysicist at NASA Goddard. “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.”

In LISA’s data, that modulation would appear as sidebands, faint secondary signals just above and below the main frequency. McGrath compares it to radio, where sidebands next to a carrier frequency carry voices or music. Here, they would carry the message that a planet is present.

Why this would be a new kind of planet

Planets have been found around a few single white dwarfs, but none has yet been found orbiting a pair of them. Any LISA detections would therefore represent a distinct new exoplanet population, potentially spread across our galaxy and up to thousands of light-years away.

Astronomers estimate the Milky Way holds about 10 million double white dwarf systems with orbits shorter than an hour. LISA is expected to resolve about 10,000 of them individually.

Webb as the fact-checker

The second paper looks at confirmation. A planet in a double white dwarf system gives off more infrared light than the stars alone would produce, and Webb’s instruments can detect that infrared excess out to at least 500 light-years.

“Once LISA captures its first exoplanet detections, it will be critical for us to validate some of them,” said Mary Anne Limbach of the University of Michigan. In a test case at that distance, the team found Webb could detect a planet about five times Jupiter’s mass, while LISA could detect a planet smaller than Jupiter under ideal conditions.

The bottom line

LISA will not launch for roughly a decade, but this work shows that its science could stretch well beyond black holes and merging stars. If the method works, the same instrument built to hear the universe’s gravitational hum could also reveal planets that have survived the deaths of their stars, a population no telescope has seen before.

Image: J0806 (binary star – artist's conception) by Credit: Tod Strohmayer (GSFC), CXC, NASA – Illustration: Dan via Wikimedia Commons, Public Domain Mark 1.0