The European Space Agency's infrared space telescope, ISO, has measured the size of a proto-planetary system, surrounding a newly-born star, a Spanish team of astronomers report in tomorrow's issue of the journal Science.

ISO sees a very young 'baby-star' surrounded by a disk of the same diameter as Jupiter's orbit, in which planets are likely to form in the future.

Stars are born within thick 'cocoons' of dust very difficult to penetrate, and for this reason current models describing the process are very incomplete.

Astronomers know, in broad terms, that the future star begins to form within the dust cloud by accreting material which forms a disk, the same disk out of which planets, comets and all the components of a planetary system will probably form in the future -- the disk is actually called a 'protoplanetary disk'.

Once the star-to-be has gathered enough material, the high pressures and temperatures in its centre trigger the first nuclear reactions and the star 'lights up' -- it starts the 'ignition'. During this process the very young star or 'protostar' emits jets of material that can be detected with different techniques. Astronomers use these detectable signs to classify the evolutionary stages of the new-born stars.

The system observed by ISO was previously thought to be at the earliest evolutionary stage, in fact, so young that the protostar had not yet had time to ignite. However, ISO results contradict this belief.

"We are seeing the earliest stages of formation of a planetary system. There is already a central object hot enough to work as a star and to heat up its surrounding protoplanetary disk.

The star is already 'lit up'", says Spanish astronomer Jos=E9 Cernicharo, from the Instituto de Estructura de la Materia (CSIC), in Madrid, main author of the article being published in Science.

The system observed by ISO's infrared camera, ISOCAM, is 1200 light years away in a star-forming region in the Orion nebula. It's called VLA1/2. Cernicharo and his group estimate that the central star and its surrounding matter might be at an average temperature of at least 500 degrees Kelvin.

It is surrounded by a protoplanetary disk whose diameter is four times the distance from the Earth to the Sun, the same as Jupiter's orbit.

"This is the first time we can determine the size of the regions where where a low mass star and its planets are being formed", Cernicharo says.

ISO was also able to analyse the chemical composition of the large cocoon of material enshrouding both the star and its protoplanetary disk, a structure called by the researchers the 'placental' envelope.

It is much colder, and made up of grains of dust covered by ices of water, carbon dioxide, methane and probably methanol. This chemical information, another 'first' of the work, will contribute substantially to understanding the star-birth processes, say the researchers.

ISO results also indicate -- as highlighted by the team in Science -- that these systems will be observable with the new generation of large (8 metre class) ground-based infrared telescopes.

Current knowledge so far suggested that these very dusty objects could only be detected at far-infrared wavelengths not accessible from the ground, but ISO has shown that they can also be seen at certain very precise infrared wavelengths which do indeed cross the Earth's atmosphere -- the so-called 'infrared windows' at which ground-based infrared telescope work.