Gravitational waves could reveal exoplanets
· BehindLines newsroom
NASA astrophysicists propose using the future LISA space observatory to spot planets around systems of two white dwarfs. According to NASA, two studies published in The Astrophysical Journal detail the method and a way to confirm these worlds with the James Webb Space Telescope.
The principle: a wobble in the signal
LISA (Laser Interferometer Space Antenna) is designed to detect gravitational waves, tiny ripples in space-time. According to NASA, its most common sources should be pairs of white dwarfs in tight orbits, which emit at a nearly constant frequency. If a planet circles such a pair, its pull makes the pair wobble slightly, and the signal then picks up a Doppler modulation. It shows up as "sidebands", secondary signals at slightly higher and slightly lower frequencies.
Casey McGrath, of the University of Maryland (Baltimore County campus) and NASA's Goddard center, who wrote the first study, likens this mechanism to the sidebands of radio. NASA notes that the method resembles radial velocity, which relies on light and is already used to find exoplanets around nearby stars. It adds that the idea of detecting planets this way was put forward as early as 2008 by Japanese astronomer Naoki Seto.
A population still unknown
NASA specifies that planets have been spotted around a few isolated white dwarfs, but not yet around a pair of white dwarfs: LISA would therefore reveal a new category of exoplanets. Astronomers estimate that our galaxy holds 10 million such pairs with an orbital period under an hour, and they expect LISA to isolate about 10,000 of them one by one.
Confirming with Webb
According to the second study, a planet in such a system adds to the starlight a surplus of infrared that Webb's instruments detect easily out to at least 500 light-years. For a test system located at that distance, the researchers calculate that Webb would spot a planet of about five times Jupiter's mass, whereas LISA might detect one less massive than Jupiter under ideal conditions.
A mission still in development
LISA is led by the European Space Agency (ESA), which describes it as the first space observatory devoted to gravitational waves. According to NASA, it will include three spacecraft arranged in a vast triangle 2.5 million kilometers on a side. NASA, a partner, supplies the telescopes among other things; launch is planned for around the mid-2030s.
Why it matters
LISA has not flown yet, and no planet has so far been found around a double white dwarf. According to NASA, if detections are confirmed, they would outline a population of exoplanets distinct from those already known, which will need validating, for example with Webb, to rule out false positives.