Dead Planets Around White Dwarfs Could Emit Radio Waves We Can Detect, Sending Out Signals for Billions of Years

When a star reaches the end of its life cycle, it will blow off its outer layers in a fiery explosion known as a supernova. Where less massive stars are concerned, a white dwarf is what will be left behind. Similarly, any planets that once orbited the star will also have their outer layers blown off by the violent burst, leaving behind the cores behind.

For decades, scientists have been able to detect these planetary remnants by looking for the radio waves that are generated through their interactions with the white dwarf’s magnetic field. According to new research by a pair of researchers, these “radio-loud” planetary cores will continue to broadcast radio signals for up to a billion years after their stars have died, making them detectable from Earth.

The research was conducted by Dr. Dimitri Veras of the Center for Exoplanets and Habitability at the University of Warwick and Prof. Alexander Wolszczan, the famed exoplanet hunter from the Center for Exoplanets and Habitable Worlds at Pennsylvania State University. The study that details their findings was recently published in the Monthly Notices of the Royal Astronomical Society.

Artist’s impression of a white dwarf star surrounded by planetary remains. Credit and ©: Mark A. Garlick/ of Warwick

This method of detecting exoplanets is actually quite time-honored. In fact, it was used by Dr. Wolszcan himself in 1990 to detect the very first confirmed exoplanet around a pulsar. This is possible because of the way a white dwarf’s powerful magnetic field will interact with the metallic constitutions of an orbiting planetary core.

This causes the core to act as a conductor, which can lead to the formation of a unipolar inductor circuit. Radiation from this circuit is emitted as radio waves which can then be detected by radio telescopes on Earth. However, Veras and Wolszcan sought to find how long these cores can survive after being stripped of their outer layers (and hence, how long they can still be detected).

Put simply, planetary cores orbiting a white dwarf star will inevitably be dragged inward due to the influence of the white dwarf’s electrical and magnetic fields (a phenomenon known as Lorenz drift). Once they get close enough, the planetary remnants will be ripped apart by the powerful gravity of the white dwarf and consumed – at which point, they will no longer be detectable.

In previous models, astronomers calculated the survivability of planetary cores based on how long it would take for the cores to drift inwards. However, Veras and Wolszcan also incorporated the influence of gravitational tides into their model, which may represent an equal or dominant force.

Arecibo Observatory, the world’s biggest single dish radio telescope. Credit: NAIC/Arecibo Observatory/NSF

They then conducted simulations using the entire range of observable white dwarf magnetic field strengths and their potential atmospheric electrical conductivities. In the end, their modelling revealed that in many cases, planetary cores could survive for over 100 million years and as long as a billion years. As Dr. Veras explained:

“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.”

“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 pair hope to use their results to inform future searches for planetary cores around white dwarfs. “We will use the results of this work as guidelines for designs of radio searches for planetary cores around white dwarfs,” said Prof. Wolszczan. “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.”

The Green Bank Telescope is the world’s largest, fully-steerable telescope. Credit: NRAO/AUI/NSF

They hope to conduct these observations using radio telescopes like the Arecibo Observatory in Puerto Rico and the Green Bank Telescope in West Virginia. These advanced instruments will allow them to observe white dwarfs in the same parts of the electromagnetic spectrum that allowed for the breakthrough discovery made by Prof. Wolszczan and colleagues in 1990.

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

Billions of years from now, after our Sun goes supernova and the planets in the inner Solar System are scorched balls of metal, it is somewhat encouraging to know that extra-terrestrial civilizations (or possibly our descendants) will still be able to study what remains of Earth.

Further Reading: Warwick, MNRAS