The first detection of gravitational waves came via LIGO (the Laser Interferometer Gravitational-Wave Observatory)—an instrument that has to strain to overcome the constant background noise of vibrations and jolts that occur on Earth. Its success has helped push for the pursuit of a project that would rise above all that noise. LISA—the Laser Interferometer Space Antenna—would detect gravitational waves using the same technique as LIGO but place its hardware in space, free of any ground-based vibrations. Preliminary tests of prototype hardware have found that the idea should work.

LISA isn't expected to be put in place until the 2030s, but that hasn't stopped astronomers and physicists from contemplating the things that it might possibly detect. Two of these astronomers, Nicola Tamanini and Camilla Danielski, are now suggesting that LISA could be used to identify a very strange class of planets: heavy planets orbiting binary pairs of white dwarf stars. But because of its exquisite sensitivity, LISA could potentially spot them orbiting outside our own galaxy.

How would this work?

Gravitational waves are produced when any two objects with mass interact but are too tiny to be detected unless the objects in question are both massive and near to each other. The LIGO detector is sensitive enough to pick up things like neutron stars and black holes, all of which are both incredibly dense and have masses on the order of the Sun's and larger. But—due to its enhanced sensitivity and the frequencies of gravitational waves that it will be sensitive to—LISA will be able to pick up objects that are dense but not as massive.

A prime candidate here is a white dwarf star, which is the remains of a sun-like star after it has burned out most of its hydrogen and helium, producing a core that's primarily carbon and oxygen. Without the energy provided by fusion, gravity will crush these objects down to a dense ball of atoms, but they lack sufficient mass to crush the atoms themselves. If there are no other sources of mass, they simply stay as they are and gradually glow as they lose the heat they started with.

On its own, a white dwarf will not produce gravitational waves. But many Sun-like stars exist in binary systems, and some of them are what are termed "close binaries." These stars are close enough that, as they expand late in their lives, the two members of the binary will share a single envelope. The friction of orbiting through this can draw their cores even closer together. When this stage is over, the two resulting white dwarfs can be orbiting closely enough to produce gravitational waves.

LIGO is unable to detect these waves. LISA, on the other hand, would.

Stars

But detecting the binary system doesn't mean detecting an orbiting planet. While a massive planet orbiting nearby wouldn't be directly detectable via the gravitational waves it creates, it would alter the orbits of the two white dwarfs. And those changes would be detectable, as they would alter the frequency of the gravitational waves produced. The method is a bit like how we currently detect planets based on the Doppler shifts they create in a star's light as they drag it back and forth during their orbit.

The authors themselves specifically make that comparison. But it's not exact, as the technique will only work for planets far more massive than Earth and only if they're orbiting relatively near the binary white dwarfs. But in exchange, there are quite a few benefits. Tamanini and Danielski write that LISA "has the advantages that it can observe everywhere in the galaxy, is not affected by the activity of the stars, and does not need any observational pointing." In fact, they calculate that LISA may even be sensitive to binaries in the vicinity of the nearby Andromeda Galaxy, meaning it will certainly be able to pick up anything in the dwarf galaxies that orbit the Milky Way.

What can we learn?

Pairs of dead stars may sound extremely rare, and massive planets orbiting them is equally rare. But scientists estimate that 95 percent of the stars present in the Milky Way will end their lives as white dwarfs, which increases the odds of finding systems to observe considerably. Calculations indicate that LISA should be able to detect approximately 25,000 of these binary systems.

And finding anything would be informative. Right now, we are getting a clear picture of planet formation around lone stars, but binary stars are common. We have some indication that planets can form when the stars orbit at a distance, but we have a lot to learn about if or how they form around close binaries. It's possible that a single planet-forming disk forms around both stars, but we have little evidence to judge that. In addition, as Sun-like stars expand late in their lives, they eject huge quantities of dust and gas, which may trigger a late round of planet formation.

In addition to telling us about planet formation near binary stars, LISA's vast reach may make it possible to draw inferences about planet formation outside of the Milky Way. If we find similar frequencies of large planets orbiting white dwarfs in the Milky Way and the dwarf galaxies that orbit it, then that would support the idea that the mechanisms of planet formation are universal.

What LISA won't be able to tell us is much about the planet itself. Because we don't know what angle the planet orbits at relative to the line of sight with Earth, we can't tell whether it's a relatively light planet orbiting near the plane or if it's a massive planet orbiting at a large angle. That's because both should create similar alterations in the binary system. To really determine what's going on, we'd need to combine the gravitational wave data with visual observations using traditional telescopes, which requires that the stars be relatively nearby.

It's over a decade until LISA gets put into space, and Tamanini and Danielski note that the Transiting Exoplanet Survey Satellite, currently in operation, is expected to tell us about planets around close binary-star systems long before then. Still, the scientific case for LISA isn't based on exoplanet detection. But when it gets to orbit, this sort of preliminary work can ensure that we have the software in place to get this information out of its data—along with all the data that originally justified pursuing the mission.

Nature Astronomy, 2019. DOI: 10.1038/s41550-019-0807-y (About DOIs).