Insights into the final phase of planetary demise: Astronomers have for the first time recorded directly how possible remnants of shattered planets tumble onto the surface of their burned-out parent star. The event was marked by a special flare: The researchers detected X-rays in a white dwarf that were released from the plasma when rock material hit the star’s surface.
Birth, development and death also characterize the history of the stars and their planets: At the beginning, matter accumulates more and more in a cloud of dust and gas, until finally nuclear fire ignites and a star floods space with radiation. Planets that orbit the center can then form from the remaining material in its vicinity. Such stars with planetary systems can then exist for billions of years. In the case of our solar system, an intelligent being was finally able to emerge on a small rocky planet that wonders how our cosmic homeland could develop further and one day end.
End-time processes in sight
As for stars of the category of our sun, astronomers assume that they will turn into white dwarfs. Over 300,000 specimens of these extinct stars have been discovered in our galaxy. These are the hot, but only faintly glowing remnants of stars in which nuclear fusion no longer takes place. This was preceded by processes that may have destroyed the celestial bodies of the star’s former planetary system or thrown them off course. The assumption is that white dwarfs can then ingest the debris from the planets, their moons and asteroids.
There was already astronomical evidence for this accretion process – but only indirect. They are based on spectroscopic studies of the light that reaches us from white dwarfs. It shows the signatures of certain substances. In this way it could be shown that the atmospheres of many white dwarfs are enriched with heavy elements, which presumably originate from material which they ingest. “Until now, accretion rates have been measured using spectroscopy and depended on white dwarf models,” says Tim Cunningham of the University of Warwick. However, astronomers have not yet been able to document the moment when remnants of the system’s former celestial bodies strike their extinct parent star.
X-ray pulses at the finale
That has now changed: Cunningham and his colleagues were able to capture direct signals of the impact in the form of X-rays. The astronomers focused on the white dwarf G29-38. Previous investigations have already shown that it has only recently reached its extinct state and is surrounded by a disk of fragments of celestial bodies. By analyzing data from the Chandra X-ray Observatory, the scientists were able to detect X-ray flares at G29-38, which they attribute to the impact of such fragments. As they explain, the impact of the material on the white dwarf creates extremely hot plasma, which then settles on the surface. As it cools, it emits the X-ray pulses that have now been detected.
According to the scientists, until now, detecting the faint X-rays reaching us from a white dwarf has been problematic because they can easily get lost between other X-ray sources in the sky. So they used Chandra, which is normally used for detecting X-rays from black holes and neutron stars. Thanks to the high angular resolution compared to other telescopes, they were able to isolate G29-38 from other X-ray sources and thus capture the changes in the white dwarf’s X-rays.
“Our discoveries provide the first direct evidence that white dwarfs are assimilating the remnants of their former planetary systems,” Cunningham concludes. “It is also the first time that we can derive an accretion rate that does not depend on detailed models of the white dwarf’s atmosphere. It is remarkable that the data agree very well with previous investigation results,” says the astronomer. “How we mapped accretion now provides a new way by which we can study these systems. This allows us to gain insight into the likely fate of thousands of known systems – including our own solar system,” says Cunningham.
Source: University of Warwick, professional article: Nature, doi: 10.1038/s41586-021-04300-w