Planet-like bodies with liquid water formed very early in the history of the Solar System, or so scientists used to think. That scenario may now be due for revision after a finding with ESA's Infrared Space Observatory, ISO. The theory was based on the presence of certain minerals called carbonates in primitive Solar System objects. Carbonates are thought to form in liquid water, which can only exist in large, planet-like bodies. Using ISO, an international team has discovered large amounts of carbonates around two dying stars, where large bodies do not exist. This suggests that carbonates are not necessarily linked to liquid water. This is the first detection of carbonates outside the Solar System.

"Our finding suggests that not all carbonates found in the Solar System were formed in association with liquid water, and this of course sheds new light on the formation history of the Solar System," says Ciska Kemper, of the University of Amsterdam.

The Solar System formed out of residual material left-over from the formation of the Sun itself. About 5 000 million years ago 'clumps' in this material -which was swirling around the newborn Sun - began to grow, and some ended up forming the planets. Nowadays astronomers can study the earliest stages of the process by analysing Solar System objects that are known to be very primitive, such as certain meteorites. For instance, in several of these objects they have found very old carbonates. This is considered to be evidence that large, planet-like bodies with liquid water had 'formed' already 20 million years after the formation of the first clumps of material around the Sun.

Liquid water is thought to be a key ingredient for the formation of carbonates. On Earth, a large fraction of sedimentary rocks are carbonates - for instance, limestone and marble. They are the sediments that precipitate when a watery solution of carbon dioxide and another mineral, such as calcium, becomes saturated - the 'scale' in the tea kettle forms in the same way. Carbonates exist also in grains of dust between the planets, in asteroids and in meteorites coming from Mars, for example, in the famous meteorite ALH 84001, which some say contains fossilised bacteria.

However, ISO's discovery, for the first time, of carbonates in dying stars breaks the automatic association between these minerals and liquid water. Kemper and her colleagues have found large amounts of the carbonates calcite and dolomite in the nebulae NGC 6302 and NGC 6537, dubbed respectively the 'Bug Nebula' and the 'Red Spider Nebula'. These are old stars that have spent the last ten thousand years expelling material through dense stellar winds and are about to 'die' as white dwarfs - small, very dense and opaque 'corpses' of stars. It is in the expelled material, which now forms a shell around the central star, where ISO has identified the unmistakable chemical signature of the carbonates, their spectra. And these carbonates cannot have been formed through the interaction with liquid water: neither has the material from the stars condensed to form new planets, nor are the carbonates residual from a pre-existing planetary system destroyed by the dying star.

"The amount of carbonates we find is equivalent to at least 30 Earth masses, far too large to be the relic of a hypothetical planetary system present before the star became a planetary nebula. On the other hand, the age of the dust-shell in the nebula is about ten thousand years, which is too short for a new planetary system to form," explains Kemper.

Therefore, the carbonates around the Bug Nebula and the Red Spider Nebula must have formed through an alternative mechanism that does not involve liquid water. Kemper and colleagues suggest several possibilities but say none of them has been tested in the laboratory yet.

Was this alternative mechanism also at work in the early Solar System? The authors cannot say. But the mere possibility implies that the assumption that carbonates in primitive Solar System objects indicates the quick formation of planets with liquid water in the Solar System needs to be reviewed.