Scientists observe the final moments of planetary remnants

A novel study conducted by the University of Warwick (UW) reveals decades of indirect evidence for debris from disintegrating planets hurtling into white dwarfs across the galaxy.

What is a white dwarf?

A white dwarf is a star that has burnt up all its energy and shed its outer layers, potentially destroying, or unsettling any orbital bodies in the process. As material from those bodies is pulled into the star at a high enough rate it slams into the surface of the star, forming a shock-heated plasma. This plasma, with a temperature between 100,000 to a million degrees kelvin, then settles on the surface, and as it cools it emits X-rays that can be detected.

How were X-rays utilised in this study?

X-rays are similar to the light that our eyes can see, but they have much more energy. They are created by very fast-moving electrons (the outer shells of atoms, which make up all the matter around us). In astronomy, X-rays are the key fingerprint of material raining down on exotic objects such as black holes and neutron stars.

Detecting these X-rays is very challenging as the small amount that reaches Earth can be lost amongst other bright X-ray sources in the sky. So, the astronomers took advantage of the Chandra X-ray Observatory, normally used to detect X-rays from black holes and neutron stars that are accreting, to analyse the nearby white dwarf G29–38.

What does this evidence mean?

UW have discovered that under observation x-rays portray planetary debris heated to one million degrees as it falls into the dead core of its host star.

The collected results are not only the first direct measurements of the accumulation of rocky material onto a white dwarf, but also the confirmation of decades of indirect evidence of accretion in over 1,000 years; the observed event occurred billions of years after the formation of the planetary system.

The destiny of most stars, including our Sun, is to become a white dwarf. Over 300,000 white dwarf stars have been discovered in the Milky Way, and many are believed to be accreting the debris from planets and other objects that once orbited them.

In the past astronomers have employed spectroscopy at optical and ultraviolet wavelengths, both to measure the abundances of elements on the surface of the star, and to use it to work out the composition of the object it came from.

Astronomers have indirect evidence that these objects are actively accreting from spectroscopic interpretations, which show up to 50% of white dwarfs with heavy elements such as iron, calcium, magnesium polluting their atmospheres.

Astronomers have not been able to observe the material as it was pulled into the star, until now.

What does this new material mean regarding scientific advancements?

Dr Tim Cunningham of the University of Warwick Department of Physics said: “We have finally seen material actually entering the star’s atmosphere. It is the first time we have been able to derive an accretion rate that does not depend on detailed models of the white dwarf atmosphere. What’s quite remarkable is that it agrees extremely well with what’s been done before.

“Previously, measurements of accretion rates have used spectroscopy and have been dependent on white dwarf models. These are numerical models that calculate how quickly an element sinks out of the atmosphere into the star, and that tells you how much is falling into the atmosphere as an accretion rate. You can then work backwards and work out how much of an element was in the parent body, whether a planet, moon or asteroid.”

With Chandra’s improved angular resolution over other telescopes researchers could isolate the target star from other X-ray sources and viewed, for the first time, X-rays from an isolated white dwarf. Thus, this confirms decades of observations of material accreting into white dwarfs that have depended on evidence from spectroscopy.

Dr Cunningham concluded: “What is really exciting about this result is that we’re working at a different wavelength, X-rays, and that allows us to probe a completely different type of physics.

“This detection provides the first direct evidence that white dwarfs are currently accreting the remnants of old planetary systems. Probing accretion in this way provides a new technique by which we can study these systems, offering a glimpse into the likely fate of the thousands of known exoplanetary systems, including our own Solar system.”

This study has been published today in the journal Nature.

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