Tuning-in to broadcasts from dead planet cores

‘Tuning-in’ to radio waves emitted around white dwarfs could help astronomers in the search for the cores of dead exoplanets.

How long can the cores of planets survive after their outer layers are violently stripped away?

That’s the question that astronomers aim to answer by ‘tuning-in’ to radio waves emitted by the cores of dead exoplanets.

To aid them in this hunt, researchers from the University of Warwick have selected white dwarf stars that are the best candidates to host surviving planetary cores. Their new research — published in the Monthly Notices of the Royal Astronomical Society — also looked for strong radio signals that earthbound astronomers can better tune-in to.

Dr Dimitri Veras from the Department of Physics led in the study in which he and his team assessed the survivability of planets that orbit stars which have burnt all of their fuel and shed their outer layers — in the process, destroying nearby objects and removing the outer layers of planets.

The team determined that the cores which result from this destruction may be detectable and could survive for long enough to be found from Earth.

The team will use their results to obtain observation time at telescopes such as Arecibo in Puerto Rico and the Green Bank Telescope in West Virginia to facilitate their search.

Veras explains the team’s selection process: “ 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. 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.”

Professor Alexander Wolszczan from Pennsylvania State University is positive the team’s method will yield positive results: “We will use the results of this work as guidelines for designs of radio searches for planetary cores around white dwarfs.

“Given the existing evidence for a presence of planetary debris around many of them, we think that our chances for exciting discoveries are quite good.”

Dead air radio

The technique developed by the team closely resembles the method used by the paper’s co-author professor Alexander Wolszczan from Pennsylvania State University to discover the first confirmed exoplanets in the 1990s.

The two exoplanets were discovered by Wolszczan and Dale Frail orbiting the pulsar PSR 1257+12 by detecting radio waves emitted by the star. The new research will observe white dwarfs in a similar band of the electromagnetic spectrum, in the hopes of a similar breakthrough.

PSR 1257+12, the pulsar around which the first exoplanets were discovered (NASA)

Veras continues: “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.

“Therefore, a discovery here would represent ‘firsts’ in three different senses for planetary systems.”

The radio waves that astronomers hope to exploit are generated when the magnetic field between a white dwarf and an orbiting planetary core forms and inductor circuit. The core is able to act as a conductor due to its remaining metallic constituents.

Radiation from this crude circuit is emitted as radio waves and is, therefore, detectable from Earth by radio telescopes. The effect is similar to the circuit created by Jupiter and its moon Io.

Jupiter and its moon Io form a similar circuit to the one formed by white dwarf stars and orbiting exoplanet cores (NASA)

Current models developed by scientists suggest that planetary cores could survive in excess of 100 million years, with some estimates suggesting they could hang on for as long as a billion years.

Dr Veras concludes: A discovery would also help reveal the history of these star systems because for a core to have reached that stage it would have been violently stripped of its atmosphere and mantle at some point and then thrown towards the white dwarf.

“Such a core might also provide a glimpse into our own distant future, and how the solar system will eventually evolve.”