Watery, rocky planets may be common

An artist's impression of a massive asteroid belt in orbit around a star

An artist's impression of a massive asteroid belt in orbit around a star. New work shows that rubble around many white dwarf stars contaminates these stars with rocky material and water.

An international team of astronomers has discovered compelling evidence that rocky planets are commonplace in our Galaxy.

Leicester University scientist and lead researcher Dr Jay Farihi studied white dwarf stars—the compact remnants of stars that were once like our Sun—and found that many show signs of contamination by heavier elements and possibly even water, improving the prospects for extraterrestrial life.

White dwarf stars are the endpoint of stellar evolution for the vast majority (>90%) of all stars in the Milky Way, including our Sun. Because they should have essentially pure hydrogen or pure helium atmospheres, if heavier elements (in astronomy described as ‘metals’, examples including calcium, magnesium and iron) are found then these must be external pollutants.

For decades, it was believed that the interstellar medium, the tenuous gas between the stars, was the source of metals in these polluted white dwarfs.

Farihi and his team used data from the Sloan Digital Sky Survey (SDSS), a project that aims to survey the sky in infrared light, imaging more than 100 million objects and following up 1 million of these by obtaining their spectrum (splitting the light into its colours).

By examining the positions, motions and spectra of the white dwarfs identified in the SDSS, Farihi and his team show that this is no longer a viable theory. Instead, rocky planetary debris is almost certainly the culprit in most or all cases.

The new work indicates that at least 3% and perhaps as much as 20% of all white dwarfs are contaminated in this way, with the debris most likely in the form of rocky asteroid debris with a total mass equivalent to that of a single 140-km-diameter asteroid.

This implies that a similar proportion of stars like our Sun, as well as stars that are a little more massive like Vega and Fomalhaut, build terrestrial planetary systems. Astronomers are thus playing the role of celestial archaeologists by studying the ‘ruins’ of rocky planets and or their building blocks.

Adapted from information issued by RAS / NASA-JPL / Caltech / T. Pyle (SSC).

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