Jupiter and Saturn are mainly balls of gas but the more distant Uranus and Neptune contain relatively large cores of ice. This difference is confirmed by French and German astronomers who used ISO to measure heavy hydrogen in the planet's atmospheres. Although the result fits well with current ideas about planetary origins, it casts doubt on the part played by comets.

Image Above: A Jupiter-like planet evolves in a disk of gas and dust around a young star, in computer model of the process devised by Pawel Artymowicz of Stockholm University.

Water ice and other types of ices, including methane, were a raw material during the birth of the giant planets. Little of it survived close to the Sun, where the Earth and other rocky planets formed.

In cooler regions the ices supposedly made quite large planets, with gravity strong enough to bind gases into the thick atmospheres that make them giants.

The accompanying picture from Stockholm University illustrates a theoretical study of the building of a giant planet in the young Solar System.

Although ISO could not see the icy cores, now hidden deep inside the giant planets, it found a fingerprint of ice -- an excess of heavy atoms in the hydrogen gas of their atmospheres.

Due to chemical reactions at low temperatures ices possess an excess of heavy hydrogen, or deuterium. Some of this deuterium has found its way into the atmospheres of the planets, where ISO's short wavelength spectrometer (SWS) detected it.

"We have the first accurate values of deuterium's abundance in Uranus and Neptune," says Helmut Feuchtgruber of Garching, Germany. "And we can compare them with those in Jupiter and Saturn, obtained with the same instrument. This enables us to draw conclusions about the deep interior of the giant planets, which are completely hidden from view".

Uranus and Neptune have three times more deuterium in their hydrogen gas than Jupiter or Saturn. This is consistent with ices making up more than half of their mass, whereas in Jupiter and Saturn ices represent a much smaller part of the total -- too small to make any noticeable enrichment of the deuterium in the atmospheres.

The four giant planets may have started with similar icy cores, each roughly ten times the mass of the Earth. The differences then arose from the availability of gases to fill their atmospheres, with Jupiter gathering about forty times more gas than Uranus or Neptune did.

The ISO results confirm in broad outline the present theories of planetary formation, but they contradict a popular belief that the ice came from comets like those still seen today.

Comets are icy survivors from the origin of the Solar System, so it might be easy to imagine congregations of comets building the icy cores of the giant planets.

But the proportion of deuterium in comets is five times greater than in Uranus or Neptune, and the ISO scientists calculate that the cores were made from ice containing much less deuterium.

So in the swirling cloud of gas and dust around the Sun, from which the planets were born, theorists have to imagine a different ice-making machine.

An extensive study of the outer planets, using ISO observations, is led by Th�rese Encrenaz and Emmanuel Lellouch of the Observatoire de Paris-Meudon and Helmut Feuchtgruber of the Max-Planck-Institut f�r Extraterrestrische Physik.

A full account of results so far has been included in a new book from ESA, "The universe as Seen by ISO" (ESA SP-427), available from ESA Pubications Division.

The European Space Agency's infrared space telescope, ISO, operated from November 1995 till May 1998, almost a year longer than expected. As an unprecedented observatory for infrared astronomy, able to examine cool and hidden places in the universe, ISO successfully made nearly 30,000 scientific observations.