A nearby young star recently gave birth to millions of comets, according to new observations using NASA’s Far Ultraviolet Spectroscopic Explorer (FUSE) spacecraft. The result provides clues to how our own solar system formed and evolved.
The star, Beta Pictoris, is estimated to be about 20 million years old, relatively young for a star whose life will span billions of years. It is about 60 light years from Earth, in the direction of the southern constellation Pictor (Painter’s Easel) (One light year is almost 6 trillion miles, or about 9.6 trillion kilometers). A 400 billion-kilometer diameter disk of dust and gas surrounds Beta Pictoris, and previous observations have given hints that planets may be forming (or recently formed) deep within.
The new research shows how the chemistry of the disk implies that comets have formed around Beta Pictoris as well, and gives additional evidence that these comets are evaporating by the millions. These observations are a rare glimpse at the chaotic birth of a solar system; young planetary systems are surprisingly active; we are witnessing the birth and evaporation of millions and millions of comets.
A team of scientists from the Institut d’Astrophysique de Paris-CNRS (IAP, France), the Johns Hopkins University (Baltimore, MD), and the Laboratoire d’Astrophysique de Marseille (France), has found no evidence of molecular hydrogen (H2), the most abundant molecule in the universe, in the Beta Pictoris disk. This was unexpected because the same team, using the Hubble Space Telescope, had previously found carbon monoxide (CO) in the Beta Pictoris disk. What is seen in the Beta Pictoris system is thus contrary to what is commonly seen in the gaseous clouds of our galaxy, where CO is only seen when H2 is present. When these clouds collapse, they form new stars and planetary systems; thus, they are the raw material for new solar systems like Beta Pictoris.
In interstellar gas clouds, carbon monoxide gets broken apart by starlight in about 1,000 years, a relatively short time compared to the estimated 20 million year age of Beta Pictoris. Since we still see carbon monoxide, we think the amount originally present in the cloud that formed Beta Pictoris is now locked up in some kind of reservoir where it is shielded from starlight as it is slowly released back into the Beta Pictoris disk.
Add this to the fact that the amount of molecular hydrogen is too low to be seen by FUSE. This is strange because normally there are about 100,000 molecular hydrogen molecules for every carbon monoxide molecule in interstellar gas clouds. So we think the molecular hydrogen has been locked away in some kind of reservoir also.
Comets, which consist largely of frozen gasses, are a likely reservoir. In our own solar system, there is a swarm of thousands, perhaps millions, of comets beyond the orbit of Pluto, called the Kuiper belt. If a similar comet swarm surrounds Beta Pictoris, the comets would still be warm enough to slowly release carbon monoxide, but far too cold to release molecular hydrogen, which would remain locked up as water ice (H2O).
Previous observations of infrared emission from Beta Pictoris with the Infrared Space Observatory (ISO) actually contradict the FUSE lack of detection of ultraviolet absorption by molecular hydrogen along a narrow line-of-sight to the star. They claim to have detected a large amount of molecular hydrogen in the disk. If both results are correct, the only way for this conflict to be resolved is if the molecular hydrogen is not evenly distributed throughout the disk. Molecular hydrogen absorbs certain kinds of ultraviolet light; when it does so, FUSE can detect its presence, but molecular hydrogen has to be more or less evenly distributed in a cloud to block ultraviolet light from stars behind the cloud, or it won’t reveal itself to FUSE. However, molecular hydrogen can also emit infrared light; celestial objects containing molecular hydrogen will glow in infrared if they are warm enough, and can be seen directly by ISO, depending on how close they are and how brightly they glow.
The molecular hydrogen clumps could be left over gas from the formation of the star, or perhaps from failed protoplanets. The initial carbon monoxide would no longer be in gaseous form but rather condensed into cometesimals or comets. All we can say for certain is that carbon monoxide must be continuously generated in the disk.
Notes for Editors
FUSE is a NASA Origins mission developed and operated by The Johns Hopkins University in collaboration with NASA’s Goddard Space Flight Center, the Centre National d’Etudes Spatiales (France), the Canadian Space Agency, the University of Colorado, and the University of California, Berkeley. FUSE was launched on June 24, 1999, from Cape Canaveral on a three-year mission to obtain high resolution spectra in the far ultraviolet wavelength region (905-1185 Angstroms) of faint objects within and beyond our galaxy.
Peer reviewed publication and references
“Deficiency of molecular hydrogen in the disk of Beta Pictoris” A. Lecavelier des Etangs, A. Vidal-Madjar, A. Roberge, P.D. Feldman, M. Deleuil, M. AndrÈ, W.P. Blair, J.-C. Bouret, J.-M. DÈsert, R. Ferlet, S. Friedman, G. HÈbrard, M. Lemoine, & H.W. Moos Nature, 2001, August 16 issue
www.iap.fr/LaboEtActivites/ThemesRecherche/BetaPic/NoH2/NoH2_index.html
Dr Roger Ferlet
Centre National de la Recherche Scientifique
33-1 44 32 80 74
