Some dead planets aren't quite dead after all as astronomers discover that planetary cores can emit radio signals for a billion years more.

Astronomers plan to find out more about these "undead" cores of exoplanets by observing white dwarf stars that are most likely to host surviving planetary cores. They also considered the strength of the radio waves that can be detected.

Tuning In To The Dead

In a study published in the journal Monthly Notices of the Royal Astronomical Society, researchers analyzed the survivability of planets that orbit white dwarf stars.

White dwarfs are stars that have burnt out all their fuel and blown off their outer layers. These stars cause the destruction of nearby objects, including planets that get their outer layers stripped until only their cores remain. According to the authors, these planetary cores could survive long enough to be seen millions of light-years away in Earth.

The first such planet confirmed was discovered by co-author Alexander Wolszczan of Pennsylvania State University back in the 1990s. He achieved this with a method detecting radio waves coming from the star it orbits.

The study revealed that the magnetic field lying between a white dwarf and an orbiting planetary core can form a unipolar inductor circuit, with the metallic-rich core functioning as a conductor. Radiation from this circuit is emitted as radio signals that can be picked by Earth's radio telescopes. It's the same effect as a circuit between Jupiter and one of its moons Io.

Findings showed that planetary cores can survive without their outer layers from over 100 million years up to a billion years.

Lead author Dr. Dimitri Veras from the University of Warwick explained that there is a "sweet spot" for finding planetary cores, since a core too near a white dwarf would be destroyed and a core too far would be undetectable. He added that astronomers should only seek out planets around white dwarfs with weak magnetic fields and a distance of around 3 solar radii and the gap between the Mercury and the sun.

Future Research

The authors plan to find more planetary cores around white dwarfs by using telescopes like the Arecibo in Puerto Rico and the Green Bank Telescope in West Virginia.

"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 explained. "Therefore, a discovery here would represent 'firsts' in three different senses for planetary systems."

He continued that more discoveries could also help reveal the history of star systems, the evolution of the solar system, and the distant future of the sun and its planets.

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