Dead Planets Around White Dwarfs Could Broadcast Radio Waves For Up To 1 Billion Years

Artist impression of a planet and a comet around a white dwarf. NASA, ESA, and Z. Levy (STScI)

Not every star ends up exploding in a supernova. Some are not massive enough, and once they have turned into a bloated red giant, they slowly lose their outer layers as they burn up through their helium reservoir. This process can also be destructive for planets, which get stripped of their outer layers as well, leaving just their cores behind.

This phenomenon is now being considered an advantage by a team of astronomers as they have come up with a way to observe these dead planets. According to a new study published in the Monthly Notices of the Royal Astronomical Society, it is possible – in some cases – that these dead planets are metal-rich and they can interact with their white dwarf’s magnetic field. This interaction produces radio waves, and the cores may survive for long enough to be detected by radio telescopes 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. Also, if the magnetic field is too strong, it would push the core into the white dwarf, destroying it," lead author Dr Dimitri Veras, from the University of Warwick, said in a statement.

"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.”

This radio emission has not been witnessed yet for white dwarfs, but astronomers have seen it happening between Jupiter and its volcanic moon Io. For this to happen around a white dwarf it will have to have the right conditions and timing.

The team modeled how long these planetary cores can survive after being stripped bare. They got a very hopeful answer that suggests they may be able to survive for between 100 million and 1 billion years. This could be long enough to spot some of these objects from Earth. Astronomers have previously found debris disks and even planets around 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. Therefore, a discovery here would represent ‘firsts’ in three different senses for planetary systems,” explained Veras.

The team is now planning an observation campaign proposal to radio telescopes such as Arecibo in Puerto Rico and the Green Bank Telescope in West Virginia.

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