Pollution is a growing concern here on Earth, but in a nearby star cluster pollution is actually proving pretty useful. Astronomers analyzing Hubble data have found that a pair of white dwarfs — the tiny, final form most of the universe’s stars will assume — show signs of “pollution” from asteroid and planet-like debris falling upon them. The discovery not only underscores just how widespread rocky worlds are, but also opens the way to a new way of studying planet formation.
The two white dwarfs lie about 150 light-years away as part of the Hyades star cluster, some 625 million years old. The team found high concentrations of silicon in the two stars’ atmospheres, which likely came from a dusty disk of detritus surrounding and raining the debris upon each star. Making up each ring are asteroids less than 100 miles across that came too close to the star and were torn apart by its gravity. The high levels of silicon, combined with low levels of carbon, are similar to the concentrations found within the rocky worlds in our solar system. Most of the material swirling around and falling onto these stars would be recognizable to us as common rocks. In effect, the team saw evidence for the building blocks of rocky exoplanets around these cluster stars.
The fact that these stars are part of a cluster is significant because only four of the 800 or so known exoplanets orbit such stars. Since each member of a cluster has many similar characteristics (age, distance, makeup, surroundings) but can vary in size, clusters provide an ideal experimental setup to study the impact of a star’s size on planets surrounding it. These findings suggest that rocky planets might not be so rare around cluster stars, after all.
Studying white dwarfs has proven ideal for such research because their typical makeup includes only hydrogen and helium. Heavier elements like silicon (and carbon) would soon sink to hidden depths within the dead star, so a lasting detectable presence of these elements would indicate that something (such as a dusty ring of the debris) is continually polluting the star’s atmosphere.
The findings also offer a possible glimpse into our solar system more than 5 billion years into the future: After the sun has reached its own white dwarf phase, disruptions to the solar system could end up sending asteroids too close to the sun, producing our own dusty, polluting ring.
The team plans to continue studying white dwarfs to learn about the planets they might possess, as well as developing computer models to better extrapolate planetary conditions from this data.