ESA Science News

The earliest stages of formation of planetary systems remain very poorly known because of the thick layers of opaque dust that hid them. 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 JosÈ 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.
For this work, ISO observations were complemented with the 30 metres radio telescope of the Institute de Radioastronomie MillimÈtrique (IRAM), in Granada (Spain).
Footnote about ISO
The European Space Agency’s infrared space observatory, ISO, operated from November 1995 to May 1998, almost a year longer than expected. An unprecedented observatory for infrared astronomy, able to examine cool and hidden places in the Universe, ISO made nearly 30,000 scientific observations.
JosÈ Cernicharo, Instituto de Estructura de la Materia (CSIC)
Tel: +34 91 5901611
Martin F. Kessler, ISO Project Scientist
Tel.: + 34 91 813 1253
* ISOCAM homepage
* ISO Science homepage
* SCIENCE journal
Mid-infrared spectra between 5 and 17 mm towards several Class 0 sources as observed with ISOCAM. All of them show strong ices and silicates absorption. The three spectral windows are indicated by arrows in the VLA1 panel. The data for L1448-C, L1448-N, Cep E and IRAS2 have not been analysed as carefully as those of VLA1+VLA2 and VLA4 in HH1-2, and represent a first look to the infrared emission detected by ISO. These spectra need additional refinements like a fit of the instrumental point source function to the data. The astrometry for all the sources is better than 2-3 arcsec and has been derived from other bright objects in the observed fields (except for L1448-C which is at the edge of the field of view of ISOCAM and could have a larger position uncertainty). The VLA1 fit (continuous line) is obtained from the empirical absorption derived for VLA4 (continuous line) by scaling the total visual absorption by a factor 2. Image credit: Science journal.