Astronomers have discovered an ancient stellar cluster where they believe a planet was literally torn to pieces by the dying remains of a star.
Using telescopes including Nasa’s Chandra X-ray Observatory and the European Space Agency‘s International Gamma-Ray Astrophysics Laboratory, researchers based in Italy and London discovered a source of X-rays close to the centre of a globular cluster known as NGC 6388.
Previous observations had convinced astronomers the source of the unusual X-rays was a central black hole at least 200 to 300 times the mass of the Sun. But after studying the new data, they found the X-rays were not located in the right place for that to be true — and that the radiation gradually dropped at the same rate predicted by models of a planet being pulled apart by a white dwarf.
A white dwarf is born at the end of an average-sized star’s life cycle, once most of its hydrogen fuel is used up. They are both extremely small — about the size of Earth — and dense, with the same mass as our Sun. Unable to generate energy using fusion, the star is left to glow dimly and slowly cool. If a planet were to pass too close it might wreck this relatively calm scene, and be sucked in by the star’s intense gravity and torn apart. The planet would be destroyed and emit X-rays, which astronomers believe they have now detected within NGC 6388.
The official name for this brutal cosmic act of nature is “tidal disruption”, which arguably underplays the situation a little.
In their paper from the Monthly Notices of the Royal Astronomical Society, Melania Del Santo of the National Institute for Astrophysics and colleagues from Italian institutions and University College London explained the planet would have been about a third the size of Earth, and the star would have been slightly greater in mass than our own Sun.
The team say that it is still possible the X-rays are caused by something else, but after watching NGC 6388 for 200 days with the X-ray telescope on Nasa’s Swift Gamma Ray Burst mission, they explain that it does not appear to be a binary system with a neutron star, and is too faint to be a binary system with a stellar-mass black hole.
The team also published an image of NGC 6388 with the X-rays detected in pink, and visible light in red, green and blue, that shows the source of the X-rays was not the centre of the cluster as once thought, but a location to one side — the putative dead planet. A full list of the authors and the study can be seen at the Chandra project’s website and the Royal Astronomical Society.
The bad news for Earth is that eventually our own Sun will likely end up as a white dwarf star of exactly this type. Worse still, our planet will have been incinerated long before this, when our currently friendly star balloons to several times its existing size. However — there is good news too. The nearest existing White Dwarf is Sirius B, located a pleasingly distant 8.6 light years away.