The LISA space observatory (Laser Interferometer Space Antenna), currently under development, will detect gravitational waves - tiny ripples in space-time from moving massive bodies. Two new studies led by NASA astrophysicists show how LISA could open up a new population of exoplanets - worlds beyond the Solar System.
"The most common source for LISA is binary white dwarfs, that is, pairs of dense stellar remnants in tight orbits," explained Kasey McGrath of the University of Maryland and NASA's Goddard Space Flight Center. "If such a system also has an exoplanet, its presence is imprinted in the gravitational-wave signal in a way that we can detect."
Both papers are published in The Astrophysical Journal. In the first, McGrath described the detection method and determined the planet masses and orbital periods at which the approach works best. The second, written with co-authors, is devoted to confirming such planets with NASA's James Webb telescope. The idea of searching for exoplanets around binary white dwarfs was proposed back in 2008 by Japanese astronomer Naoki Seto.
The LISA method is similar to the Doppler search for exoplanets using light. A planet's gravity slightly "tugs" its star, making it wobble. As a result, lines in the star's spectrum shift alternately toward the red and the blue. "Something similar happens with gravitational waves," says Scott Noble of NASA Goddard. "Binary white dwarfs emit waves at an almost constant frequency. Add a sufficiently massive planet, and the system begins to wobble, and modulation appears in the signal."
For LISA, such modulation will show up as sidebands - additional signals at frequencies above and below the main one. McGrath compares this to radio communication: part of the carrier frequency's energy goes into sidebands that carry the message. Here the "message" is the presence of a planet.
White dwarfs are remnants of stars like the Sun, about the size of Earth. Exoplanets have already been found around individual white dwarfs, but none has yet been detected around a binary. LISA discoveries would therefore mean a separate new population of exoplanets. Astronomers expect LISA to distinguish about 10,000 individual binary systems, as well as roughly 100 mixed pairs with a neutron star or black hole.
"When LISA makes its first detections, it will be important for us to verify some of them," says Mary Anne Limbach of the University of Michigan. "Joint observations will make it possible to confirm that the signals correspond to real planets rather than false positives." The team proposed confirming planets via an infrared excess: a planet gives off more infrared light than the stars themselves. Webb's instruments are capable of noticing this excess at a distance of at least 500 light-years.
LISA will consist of three spacecraft trailing Earth around the Sun. They will form a triangle with sides of 2.5 million kilometers. Each spacecraft will have two telescopes that, using infrared lasers, simultaneously transmit and receive signals between neighboring spacecraft. This measures changes in the distances between them smaller than the width of a helium atom - these are the signals of passing gravitational waves.
Gravitational waves were predicted by Einstein's general theory of relativity in 1916 and were first registered by ground-based observatories in 2015. They arise when massive bodies accelerate, travel at the speed of light and do not change when they encounter obstacles. So far this concerns theoretical calculations and modeling, not actual observations: LISA has not yet been launched.