Pulsar planet is a gem of a discovery

A former white dwarf star has been transformed into a planet made of diamonds while circling a rapidly spinning pulsar. This dazzling discovery was made using the 64 metre Parkes radio telescope in Australia where irregularities in the signal from the pulsar alerted astronomers that something was orbiting it.

Radio pulses emanate from the pulsar as the diamond planet orbits it. The entire binary would fit inside our own Sun.
Radio pulses emanate from the pulsar as the diamond planet orbits it. The entire binary would fit inside our own Sun. (Image: Swinburne Astronomy Productions)

After follow up observations were made with the Lovell radio telescope in the UK and one of the Keck telescopes in Hawaii, the international team realised that there is something very unusual indeed about this mysterious object.

The variations in the pulses received on Earth allowed the astronomers to infer that the object orbits the pulsar, named J1719-1438, every two hours and ten minutes. The distance between the two is only 600,000 kilometres, which is slightly less than the radius of the Sun. This tight orbit means that the object orbiting the pulsar must be small in size, as a larger objet orbiting at this distance would lie too close to the pulsar and would be torn apart by its immense gravity.

The size is estimated to be half that of Jupiter, while its mass is slightly greater than Jupiter. “Our ‘thing’ is below the mass for fusion, orbits a stellar remnant and has the minimum mass/size,” Matthew Bailes from Swinburne University of Technology in Australia and leader of the research team tells Skymania News. It can thus be classed as a planet.

The small size and high mass of the planet mean that like the pulsar it orbits, it is also very dense. If the planet were composed of light elements such as hydrogen and helium, then it would be too large for the orbit and would be ripped to pieces by the pulsar. However, a planet made of carbon and oxygen, which are heavier elements, would enable the planet to survive around the vicious stellar remnant.

Because of the large density, it is likely that the carbon is in crystalline form which means that the planet could be comprised of diamond. According to Bailes, carbon is the most likely main component of the planet. It could have “neon, maybe magnesium, but it is more likely to have layers including carbon,” he explains. “If we were very unlucky the orbit may be very face on, and then you might just squeeze a helium white dwarf in, but how to explain its origin would be more difficult.”

The pulsar planet is not the first “diamond planet” discovered. WASP-12b beat it to the punch with a carbon-rich atmosphere suggesting a possible diamond or graphite interior. However unlike hot Jupiter WASP-12b, the planet orbiting pulsar J1719-1438 has a more unusual history; it used to be a star.

A pulsar is an extremely dense stellar remnant which emits regular beams of radio waves which sweep past the Earth like rays from a lighthouse. Despite having a mass nearly one and a half times that of our Sun, J1719-1438 is only 20 kilometres in diameter. This pulsar is actually known as a millisecond pulsar, due to the fact that it rotates 10,000 times every minute.

A millisecond pulsar is expected to have such a high spin rate if another nearby stellar companion has caused it to spin up. This happens when a companion star transfers matter to the pulsar. While planets have previously been found around pulsars, they would not have enough mass to spin up a pulsar, which means this diamond planet most likely used to be a white dwarf. Over 99.9 per cent of the white dwarf’s mass was stolen by the pulsar in its need for speed, leaving an unusual object now classed as a planet.

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By Amanda Doyle

I am an astrophysics postdoctoral research assistant at the University of Warwick. I obtained my PhD from Keele University in 2014 and my thesis title was "Spectral analyses of solar-like stars". My research involves refining stellar parameters with the aim of improving our understanding of both stars and planets. I completed my masters in astronomy at Swinburne University of Technology via the Swinburne Astronomy Online programme in 2010, and I obtained my degree in physics with astronomy from Dublin City University in 2008. When I'm not doing research, I like to write about all aspects of astronomy. I am a freelance science writer and I contribute to Astronomy Now, NASA's Astrobiology Magazine, BBC Sky at Night magazine, Skymania News, and Sen. I am also the editor of Popular Astronomy magazine.

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