LISA: Detecting Exoplanets Using Gravitational Waves

Humanity is experiencing a revolution in astronomy. Until recently, we've depended on the electromagnetic spectrum (i.e. light) to make discoveries from our solar system's backyard to the furthest-most reaches of the cosmos by using telescopes. Now, with the first historic detection of gravitational waves on Sept. 14, 2015, a whole new universe awaits us, one in which we can analyze the spacetime ripples washing over us from black hole collisions and, possibly, alien worlds as they orbit their distant stars.

In a study published July 8, 2019, in Nature Astronomy, a group of researchers have explored the latter possibility to reveal extrasolar planets, or exoplanets, that would otherwise remain invisible to traditional astronomical techniques.

"We propose a method which uses gravitational waves to find exoplanets that orbit binary white dwarf stars," Nicola Tamanini, of the Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI) in Potsdam, Germany, said in a statement.

So far, the gravitational waves generated by massive collisions in the deep cosmos have been detected by two observatories, the U.S.-based Laser Interferometer Gravitational-wave Observatory (LIGO) that uses two detectors in Washington and Louisiana, and the Virgo interferometer near Pisa, Italy. Both projects use L-shaped buildings that house advanced laser interferometers that can detect the minute fluctuations in distance as gravitational waves wash through our planet. LIGO was the first to detect the gravitational waves that were theorized by Einstein more than a century ago and now both LIGO and Virgo work in concert to make regular detections of black hole and neutron star collisions.

In 2017, another historic milestone was reached when both the gravitational waves and gamma-ray radiation were detected at the same time when two neutron stars collided in a galaxy 130 light-years away. This event launched a new era of "multimessenger astronomy" that enabled astronomers to pinpoint the location of the event, understand the physical mechanisms behind short gamma-ray bursts, confirm that colliding neutron stars are the culprit, and provide an intimate look at the nuclear processes that manufacture heavy elements (such as gold and platinum) in the cosmos.

Launching Detectors Into Space

With these incredible advancements facilitated by our new ability to detect gravitational waves, what does the future hold? Well, why not launch a gravitational wave observatory into space! As discussed in the Nature Astronomy study, the planned Laser Interferometer Space Antenna (LISA) will do just that and its extreme sensitivity will give us a brand new look at cosmic targets that are currently hiding in the dark. One of these targets will be binary white dwarf star systems that may be accompanied by orbiting exoplanets (with masses of 50 Earth-masses and greater) that cannot be seen using current exoplanet-detection techniques. Theoretically, LISA will be sensitive to gravitational waves coming from white dwarf binaries throughout our galaxy.

"LISA will measure gravitational waves from thousands of white dwarf binaries," said Tamanini. "When a planet is orbiting such a pair of white dwarfs, the observed gravitational-wave pattern will look different compared to the one of a binary without a planet. This characteristic change in the gravitational waveforms will enable us to discover exoplanets."

White dwarfs are the stellar corpses of sun-like stars that have run out of fuel and died long ago. Our sun will run out of fuel in 5 billion years or so, which will cause it to swell up into a bloated red giant. After the red giant phase, the star will shed its layers of hot plasma, creating a so-called planetary nebula, leaving a tiny spinning object approximately the size of Earth in its wake. This dense object will then be crushed under its own immense gravity, creating a blob of degenerate matter.

White dwarfs are well studied and represent the final, dead phase of our sun's life, but they could also be invaluable objects in our pursuit to find new worlds far beyond the solar system.

If, for example, two white dwarfs orbit one another as a binary system, the gravitational perturbations they create will act like a spinning child's toy in a swimming pool — ripples in spacetime will propagate in all directions, carrying energy away from the orbiting stars at the speed of light. Current gravitational wave detectors can only measure the most powerful cosmic clashes, but with LISA, these more subtle events that produce a weaker gravitational wave signal will be within reach.

Hidden Alien Worlds

Currently, astronomers use two primary methods to detect exoplanets orbiting other stars: the "radial velocity method," which uses very sensitive spectrometers attached to telescopes that can detect the Doppler shift caused by an orbiting exoplanet, and the "transit method," which NASA's Kepler space telescope (and others) use to detect the very slight dip in star brightness as a world orbits in front.

Although over 4,000 exoplanets have been discovered primarily by using these two methods, some exoplanets remain hidden and, in the case of binary white dwarfs, we know little about whether they can host exoplanets. But, if LISA can measure the space-time ripples emanating from these systems, it might also detect the slight tugging of exoplanets as they orbit, in a similar manner that the radial velocity method measures the Doppler shift of electromagnetic waves, only using gravitational waves instead.

LISA is a project led by the European Space Agency and is currently scheduled to launch in 2034. Consisting of three spacecraft flying in formation, they will beam ultra-precise lasers at one another to create a vast equilateral triangular laser interferometer with each spacecraft separated by 1.5 million miles (2.5 million kilometers). LISA will therefore be an interferometer a million times bigger than anything we currently have, or ever will have, on Earth.

"LISA is going to target an exoplanet population yet completely unprobed," added Tamanini. "From a theoretical perspective, nothing prevents the presence of exoplanets around compact binary white dwarfs."

If these binary white dwarf star systems are found to also host exoplanets, they will help us better understand how star systems like our own evolve and whether planets can survive after their binary star systems have run out of fuel and died. The researchers also point out that they could also reveal whether second-generation exoplanets (i.e. planets that form after the red giant phase) exist.

Beyond the gravitational wave detections of exoplanets, the possibilities are endless. If there's one thing that the current "new age" of gravitational wave astronomy has taught us, future space-based observatories like LISA could reveal phenomena that occur in the dark that we never thought we'd ever witness.