Pulsars are small spinning stars of the size of cities like Cologne that emit a beam of radio waves. As the star spins and the radio beam sweeps repeatedly over Earth, radio telescopes detect a regular pattern of radio pulses.
For the newly discovered pulsar, known as PSR J1719-1438, the astronomers noticed that the arrival times of the pulses were systematically modulated and concluded that this is due to the gravitational pull of a small orbiting companion, a planet. These modulations can tell astronomers several more things about the companion. First, it orbits the pulsar in just two hours and ten minutes, and the distance between the two objects is 600,000 km - a little bit less than the radius of our Sun. Second, the companion is so close to the pulsar that if its diameter was any larger than 60,000 km (less than half the diameter of Jupiter) it would be ripped apart by the gravity of the pulsar.
"The density of the planet is at least that of platinum and provides a clue to its origin", said the research team leader, Prof. Matthew Bailes of Swinburne University of Technology in Australia. Bailes leads the "Dynamic Universe" theme in a new wide-field astronomy initiative, the Centre of Excellence in All-sky Astrophysics (CAASTRO). He is presently on scientific leave at Max Planck Institute for Radio Astronomy.
The team thinks that the planet is the tiny core that remained of a once-massive star after narrowly missing destruction by its matter being siphoned off towards the pulsar. They found the pulsar among almost 200,000 Gigabytes of data using special codes on supercomputers at Swinburne University of Technology, at The University of Manchester and at the INAF-Osservatorio Astronomico di Cagliari.
The project is part of a systematic search for pulsars in the whole sky involving also the 100-m Effelsberg radio telescope of the Max-Planck-Institute for Radioastronomy (MPIfR) in the Northern hemisphere. "This is the largest and most sensitive survey of this type ever conducted. We expected to find exciting things, and it is great to see it happening. There is more to come!", promises Prof. Michael Kramer, Director at the MPIfR in Bonn, Germany.
How did the pulsar acquire its exotic companion? And how do we know it's made of diamond? Pulsar J1719-1438 is a very fast-spinning pulsar-what's called a millisecond pulsar. Amazingly, it rotates more than 10,000 times per minute, has a mass of about 1.4 times that of our Sun but is only 20 km in radius. About 70% of millisecond pulsars have companions of some kind: astronomers think it is the companion that, as a star, transforms an old, dead pulsar into a millisecond pulsar by transferring matter and spinning it up to a very high speed. The result is a fast-spinning millisecond pulsar with a shrunken companion-most often a white dwarf.
"We know of a few other systems, called ultra-compact low-mass X-ray binaries, that are likely to be evolving according to the scenario above and may likely represent the progenitors of a pulsar like J1719-1438" said Dr. Andrea Possenti, of INAF-Osservatorio Astronomico di Cagliari.
But pulsar J1719-1438 and its companion are so close together that the companion could only be a very stripped-down white dwarf, one that has lost its outer layers and over 99.9% of its original mass. This remnant is likely to be largely carbon and oxygen, stars of lighter elements like hydrogen and helium just won't fit. The density means that this material is certain to be crystalline: that is, a large part of the star may be similar to a diamond.
"The ultimate fate of the binary is determined by the mass and orbital period of the donor star at the time of mass transfer. The rarity of millisecond pulsars with planet-mass companions means that producing such 'exotic planets' is the exception rather than the rule, and requires special circumstances", said Dr. Benjamin Stappers from the University of Manchester.
"The new discovery came as a surprise for us. But there is certainly a lot more we'll find out about pulsars and fundamental physics in the following years", concludes Michael Kramer.
Credit: CSIRO Astronomy and Space Science (CASS).
Transformation of a Star into a Planet in a Millisecond Pulsar Binary , M. Bailes et al., 2011, Science
Parallel Press Releases:
'The Dish' finds a 'diamond planet' , CSIRO Media Release, 26 August 2011.
Image and Short Movie in different resolution (Swinburne Astronomy Productions, Swinburne University of Technology).
Max-Planck-Institut für Radioastronomie (MPIfR).
Swinburne Centre for Astrophysics & Supercomputing (CAS).
ARC Centre of Excellence for All-Sky Astrophysics (CAASTRO).
Swinburne Pulsar Group at CAS.
Fundamental Physics in Radio Astronomy (Research Group at MPIfR).
Prof. Dr. Michael Kramer,
Director and Head of Research Group "Fundamental Physics in Radio Astronomy",
Max-Planck-Institut für Radioastronomie, Bonn.
E-mail: mkramer (at) mpifr-bonn.mpg.de
Prof. Dr. Matthew Bailes,
Swinburne Centre for Astrophysics & Supercomputing
at present: Max-Planck-Institut für Radioastronomie.
E-mail: mbailes (at) swin.edu.au
Dr. Norbert Junkes,
Max-Planck-Institut für Radioastronomie.
Press and Public Outreach,
E-mail: njunkes (at) mpifr-bonn.mpg.de