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Dead planets give off ghostly radio waves which we should be able to detect

An artist’s impression of a white dwarf sitting in the center of the remnant of a planetary system. Mark A. Garlick / of Warwick

Dead planets orbiting dead stars can give off ghostly radio wave emissions that are detectable from Earth, allowing us to track planets even billions of years after their stars have run out of fuel.

Astronomers from the University of Warwick have proposed we could “tune into” radio waves given off by the cores of exoplanets that orbit around white dwarfs. A core is created when a planet orbits a star which has burned through its fuel and stripes away its outer layers, leaving a chunk of rock which can survive for a long time.

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These cores can be spotted by looking for the magnetic field between a core and the white dwarf it orbits. The core has metallic elements in it, so it acts as a conductor and together the core and white dwarf create a circuit. Radiation from the circuit is emitted into space as radio waves, we which should be able to detect on Earth.

“There is a sweet spot for detecting these planetary cores: A core too close to the white dwarf would be destroyed by tidal forces, and a core too far away would not be detectable,” lead author Dr. Dimitri Veras explained in a statement. “Also, if the magnetic field is too strong, it would push the core into the white dwarf, destroying it. Hence, we should only look for planets around those white dwarfs with weaker magnetic fields at a separation between about 3 solar radii and the Mercury-Sun distance.”

There is an open question about exactly how long a core can survive. Modelling suggests that cores can live as long as 100 million years, and maybe even up to a billion years.

The team plans to test the theory by using radio telescopes like the Arecibo in Puerto Rico and the Green Bank Telescope in West Virginia to searching for cores orbiting white dwarfs. “Nobody has ever found just the bare core of a major planet before, nor a major planet only through monitoring magnetic signatures, nor a major planet around a white dwarf,” Veras said. “Therefore, a discovery here would represent ‘firsts’ in three different senses for planetary systems.”

The findings are published in the Monthly Notices of the Royal Astronomical Society.

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