More than 4,000 exoplanets are now known to orbit other stars. Indeed, astronomers suspect that such worlds are ubiquitous, estimating that, on average, every star in the Milky Way must have at least one planetary companion. But therein lies the rub: Although exoplanets seem to pop up everywhere, “everywhere” is far from the truth in describing where astronomers have actually looked. The vast majority of exoplanet surveys have stuck to either stars closely neighboring the sun or those farther off, in the direction of the Milky Way’s central galactic bulge. Truth be told, no one yet knows the true abundance of planets throughout the Milky Way or, for that matter, the prevalence of planets in galaxies other than our own.
According to a study published on July 8 in Nature Astronomy, a major step toward completing this exoplanet census could begin in 2034, with the launch of the European Space Agency’s LISA mission. LISA stands for Laser Interferometer Space Antenna, a name that hints at the mission’s primary purpose: to detect ripples in spacetime—gravitational waves—by looking for minuscule changes in the distances between three satellites arranged in a triangular constellation with sides 2.5 million kilometers long. LISA will be custom-built to tune in to gravitational waves from merging supermassive black holes, but it could apparently listen to gravitational waves from some exoplanet systems, too.
Nicola Tamanini, a co-author of the recent study and an astrophysicist at the Max Planck Institute for Gravitational Physics in Potsdam, Germany, explains that LISA could uniquely probe for a wide range of planets in certain types of binary star systems. “Moreover,” he says, “in practice, LISA should allow us to make detections across the entire Milky Way, into the Magellanic Clouds and even in the Andromeda galaxy, if conditions are optimal.”
The detection works like this: When two stars form a binary system, they do not technically orbit each other; instead they orbit a common center of mass. If a planetary body resides in such a system, the planet’s gravitational influence will perturb this center of mass, causing it to cyclically wobble back and forth in sync with the planet’s to-and-fro tugging. The wobble’s strength would provide an estimate of the planet’s mass, and its recurrence over time would reveal the planet’s orbital period. For light-based telescopes, this wobble would almost always be far too small to be seen. But the wobble could be discerned as a subtle periodic modulation of the gravitational waves emanating from the binary system. For one class of binaries—systems composed of two white dwarfs, burned-out remnants of sunlike stars—such modulations should be detectable by LISA.
Astronomers already know that white dwarf binary systems are out there in abundance and expect to see tens of thousands of them in our galaxy alone using LISA, Tamanini says. The gravitational waves such systems generate will, in fact, be used to calibrate the mission’s observations, guaranteeing that LISA will be tuning in. Even if just 1 percent of the Milky Way’s white dwarf binaries harbor planets, he says, LISA should find such worlds by the hundreds.
Kaze Wong, a Ph.D. student in astrophysics at Johns Hopkins University and an affiliate member of the LISA Consortium who was not involved in the recent study, welcomes its findings. “Previous research has looked at the potential for using gravitational waves to detect exoplanets that orbit a single star,” he says, “but work we undertook suggests that the gravitational waves produced by such a combination will be too weak to be detectable.” This method shows more promise, Wong says, because the gravitational waves produced by white dwarf binaries are so much stronger than those made by a planet twirling around a lone star. “Once the gravitational waves are detected,” he says, “we can point our electromagnetic instruments at them and determine further details of the exoplanetary source.”
As promising as this new method would be for detecting planets all across our galaxy and even in other adjacent galaxies, astronomers would still struggle to learn much more about those newfound worlds. Beyond the orbital period and mass estimate provided with the initial gravitational-wave-based detection, follow-up observations with traditional telescopes could perhaps pin down a planet’s precise mass and size and additional orbital parameters. According to study co-author Camilla Danielski, a researcher at the French Alternative Energies and Atomic Energy Commission, even accounting for future technological advances, such telescopic observations of any planets found by LISA would be limited to relatively nearby regions of the Milky Way, within roughly 10,000 to 20,000 light-years of our solar system.
Furthermore, the technique would be unlikely to yield any habitable, Earth-like planets. White dwarfs are essentially stellar corpses, the slowly cooling cores left behind after sunlike stars exhaust their nuclear fuel. But before a star becomes a white dwarf, it first goes through a red giant phase in which it balloons in size, swelling to scorch or even swallow any close-in planets. Consequently, Wong says, any surviving worlds will probably be uninhabitable—and those producing the strongest gravitational-wave signals will be uncomfortably close to their white dwarf hosts. More fundamentally, there is no guarantee that LISA will find any exoplanets at all—because so little is known about whether and how worlds can arise and persist in such extreme binary systems.
But Danielski points out that even if planets are not found around white dwarf binaries, that null result would still provide useful insights about planetary evolution.
“So far nobody has thought about modelling exoplanets around binary systems like these,” she says, “so we have very little knowledge about their existence.” Finding those planets would show that such worlds can somehow survive—or be resurrected from—the deaths of their stars. Finding none at all would provide a new constraint, all across the galaxy, on where planets simply cannot be. Knowing where not to look would be a useful thing in a universe supposedly so chock-full of worlds to explore.
“Ultimately, using LISA in this way,” Danielski says, “we will establish something certain about planetary formation throughout the Milky Way.”