Spectral analysis of 12 white dwarfs has revealed the remains of disintegrated planetary bodies in their atmospheres. Most of them resemble rocks and meteorites in the solar system, and in two systems, possible signs of material originating from water-rich bodies were found.
An instrument built to map millions of galaxies and study dark energy has provided astronomers with a completely different kind of glimpse: a kind of "chemical autopsy" of planetary bodies that have disintegrated around dead stars.
Researchers analyzed observations of the spectroscopic instrument todark energy, Desi, at 12 White dwarfs Their atmospheres contain unusual amounts of heavy elements. The absorption lines in the spectra allowed us to identify between three and ten elements in each star, including oxygen, magnesium, silicon, calcium and iron – key components of rocky planets.
According to the study, published in the journal Monthly Notices of the Royal Astronomical SocietyThe composition of the material that fell into most white dwarfs is similar to the materials known fromSolar System The inner: ancient meteorites, reworked rocky material, and bodies in which an iron-rich core formed. Two of the six systems that allowed for particularly detailed analysis contained oxygen levels that might be consistent with bodies that were rich in water. (The scientific article)
The star's atmosphere serves as an evidence field
A white dwarf is the compact remnant left behind after a Sun-like star ends its active life and sheds its outer layers. Its mass may be similar to that of the Sun, but it is compressed into a body about the size of Earth.
The atmospheres of white dwarfs are usually composed mostly of hydrogen or helium. Due to their strong gravity, heavy elements should sink quickly below the visible layer. So when astronomers detect oxygen, magnesium, iron, or calcium in a white dwarf's atmosphere, the usual explanation is that the material arrived there relatively recently.
The source of the material may be asteroids, Planetsimals, moons or parts of planets left in the system after the star dies. The change in the star's mass and the gravitational perturbations between the remaining bodies can throw them off their orbits. A body that gets too close to the white dwarf is torn apart by tidal forces, and its remnants form a disk from which material falls to the star's surface.
The contaminated atmosphere thus becomes a kind of indirect sample of the interior of the disintegrated body. Unlike the usual study of planetary atmospheres, here astronomers obtain information about the solid material from which the body is built.
More than 1,750 white dwarfs swallow fragments of planetary bodies
More than 1,750 white dwarfs are currently known to be accreting fragments of planetary bodies. However, only a few dozen of them have spectral lines strong and clean enough to reliably calculate the relative abundances of many elements.
The new study, led by Paula Izquierdo fromUniversity of Warwick, focused on 12 metal-rich white dwarfs included in the first DESI data release. In astronomy, the term "metals" includes all elements heavier than hydrogen and helium, even if some of them are chemically non-metals.
All 12 white dwarfs contained some of the elements needed to build rocks. In six systems, the quality of the data allowed researchers to move from simply identifying elements to estimating the composition of the body from which they came.
Four of the source bodies were found to be dry bodies, composed primarily of rocky materials. In two other cases, an excess of oxygen was found relative to the amount needed to form the dry minerals measured. The researchers suggest that this excess may indicate oxides and material originating from water-rich planetesimals.
This does not mean that oceans have been found, and certainly not signs of life. The finding refers to the chemical composition of destroyed bodies, not directly observed liquid water. However, it does strengthen the possibility that bodies containing significant amounts of water are not unique to the solar system.
Is the solar system typical?
One of the central questions in the study of extrasolar planets is whether the rocky planets in the solar system were built from common materials, or whether their composition is unusual.
Most exoplanets are discovered by their effect on the star's light or motion. These methods allow us to calculate the planet's radius, mass, and sometimes density, but they usually do not provide a detailed list of elements. When a planetary body breaks up and falls onto a white dwarf, its chemical building blocks can be identified relatively directly.
The researchers found that the bodies in the sample generally resemble material known from the inner solar system. Some resemble primitive meteorites, while others may represent material that has been melted and separated into layers, including material originating from planetary cores.
However, the 12 systems are not a representative sample of all planetary systems in the galaxy. They were chosen because they were particularly rich in heavy elements and therefore relatively easy to analyze. In addition, the researchers need to correct the measurements for the different rates of deposition of the elements in the atmosphere and the stage of material absorption in which the star is located.
The findings therefore provide a detailed picture of some systems, but do not yet prove that planetary systems similar in composition to the Solar System are the majority.
The side mission of an instrument designed to study dark energy
DESI is mounted on the four-meter-diameter Mayell Telescope at Kitt Peak Observatory in Arizona. It splits the light from celestial objects into spectra using 5,000 robotic fiber optics, capable of observing thousands of targets in the same exposure.
Its main mission is to measure the redshift of tens of millions of galaxies and quasars and build a three-dimensional map of the universe. The map is used to study the expansion rate of the universe and to estimate the influence of dark energy.
However, when observing conditions are not suitable for the distant and faint targets of the main survey, part of the observing time can be directed to closer and brighter objects. This is how the white dwarfs included in the study were observed.
The ability to produce measurements useful for a completely different field illustrates the importance of large-scale spectroscopic surveys. DESI was not specifically designed to study planetary remnants, but the combination of a large field of view, thousands of fibers, and an extensive data archive could make it an effective tool in finding additional white dwarfs where the composition of planetary material can be measured.
As more observations are accumulated, it will be possible to compare hundreds of bodies that have broken up: to examine how common dry bodies are, how frequently water-rich materials appear, and how many of the systems contain material similar to the crust, mantle, or core of the planets in the solar system.
More on the subject on the science website
- Planet-devouring star observed for the first time
- Chaos and Destruction: A Dead Star's Cannibalism of Its Planetary System
- Early evidence for extrasolar planets in century-old photographic plates
- A 1917 photographic plate reveals the first evidence of an extrasolar planetary system
- Evidence of extraterrestrial life may be found in systems orbiting white dwarfs
Questions and Answers
What is a "metal-contaminated" white dwarf? It is a white dwarf whose atmosphere has been found to contain elements heavier than hydrogen and helium. Since these elements sink rapidly, their presence usually indicates ongoing or recent infall of planetary fragments.
How can the composition of a body that has already decomposed be restored? Each element absorbs light at characteristic wavelengths. Measuring the absorption lines in the spectrum of the white dwarf allows us to identify the elements and estimate the quantitative relationships between them.
Has the study discovered water on other planets? Not directly. In both systems, the oxygen-rich composition is found that might be consistent with planetesimals containing water. This is a chemical hint of water-rich material, not an observation of an ocean or evidence of life.
Why is DESI suitable for such research if it was built to study galaxies? DESI measures thousands of spectra simultaneously and with high sensitivity. The same capabilities that allow it to measure distant galaxies can provide very detailed spectra of nearby white dwarfs.
For the scientific article: Opening the scientific article
