eso8701 — Communiqué de presse scientifique
Possible Planetary System Photographed Around Nearby Star
5 janvier 1987
Based on observations obtained at the European Southern Observatory (ESO), astronomers at the Space Telescope Science Institute (STScI) have uncovered the strongest evidence yet for the presence of a giant planetary or protoplanetary system accompanying a nearby star .
Using special observational and image analysis techniques, Francesco Paresce and Christopher Burrows, of STScI and the European Space Agency (ESA), have made the first visible light images of a large disc of material closely bound to the star Beta Pictoris. The disc is at least 80.000 million kilometres across, or more than three times the diameter of our solar system.
The observations were made at the ESO La Silla observatory in the Atacama desert in Chile. The astronomers will present their findings at the 169th meeting of the American Astronomical Society in Pasadena, California on January 5th.
An unusual excess of infrared radiation, indicative of circumstellar matter, was initially detected around Beta Pictoris by the Infrared Astronomy Satellite (IRAS) in 1983. Subsequent ground-based observations revealed the presence of a disc-like feature at near-infrared wavelengths.
When Paresce and Burrows made detailed observations of the disc at several regions of the visible light spectrum, they found that the reflectivity of the disc material was neutral, or wavelength independent. This means that the colour and spectral characteristics of light reflected from the disc almost exactly matched the spectrum of light emitted from the star itself.
This observation offers the strongest indications yet that the disc is made up of relatively large solid particles. If it were extremely fine dust, which is commonly found in interstellar space, it would scatter only the bluer wavelengths of starlight. The observational data alone cannot establish the true size of the reflecting particles but does set a lower limit of about 0.001 millimetre (1 micron). At this diametre or greater, the particles found around Beta Pictoris are at least ten times larger than material normally observed in interstellar space.
“The observations show unequivocally that an agglomeration process is in an advanced state, where fine interstellar grains stuck together to form larger clumps", reports Dr. Paresce. It is believed that as such a 'snowballing' process continues, the disc material may eventually accrete into planet-sized objects, if they have not done so already. Our solar system may have condensed or accreted out of thick dust grains which formed a circumstellar nebula that accompanied the birth of our sun, approximately 4600 million years ago.
The presently available observational data cannot determine the composition of the particles, though they likely contain silicates, carbonaceous materials, and water ice - common elements abundant within our own solar system.
The evidence for planetary formation is also supported by the fact that the large dust particles are arranged in a flattened disc. The disc likely formed out of an immense, protostellar nebula that contracted and collapsed into the feature seen today. Most of the nebula's gas and dust concentrated at the centre of the disc to form the star Beta Pictoris. The remaining material now continues to orbit the star.
At present it is not known if planets already formed within the disc or if it is still in a protoplanetary stage. “All that can be said for sure is that the disc has progressed from a 'fine sand' stage into at least a 'pebble' stage", says Dr. Paresce.
Beta Pictoris is a relatively young star estimated to be no older than 1000 million years, or about one fifth the age of our sun. Approximately 50 light years away, it is a socalled 'main-sequence dwarf', like our sun.
Paresce and Burrows made their observations of Beta Pictoris, which is visible as a fourth magnitude star in the southern hemisphere, with the ESO 2.2 metre telescope. Attaching a coronograph of their own design and fabrication, the researchers blocked out the brilliant image of the star, so that the faint circumstellar features could be photographed with a CCD (Charge Coupled Device) detector. To allow analysis of the disc at various wavelengths of light, a series of exposures were then taken through bandpass filters across the visible spectrum. These difficult observations were facilitated by the excellent atmospheric conditions at the ESO La Silla observatory.
As a control, an identical observing sequence was performed on the stars Delta Hydrus and Alpha Pictoris which are not expected to have prominent circumstellar disc features visible from Earth.
Through special data analysis techniques developed by Paresce and Burrows, the two stellar images were corrected for known instrumental effects, precisely registered, and differences between the two images were evaluated. This was an especially challenging task since the researchers were probing the near vicinity of Beta Pictoris and had to contend with intense scattered light from the star itself. They also had to be sure that they were seeing reflected light from a true disc feature and not contamination produced by the instrument optics.
Their resulting data yields the first true, photometrically accurate image of the Beta Pictoris disc, down to about four arcseconds from the star. Never before has such a relatively faint feature been photographed within such close proximity to such a bright star.
The resulting images reveal a highly flattened disc which extends symmetrically outward from Beta Pictoris, into a northeast and southwest direction on the sky. The disc's apparent angular width may indicate that it is slightly tilted to our line of sight. The disc dramatically increases in brightness toward its center, though its structure closer to Beta Pictoris is not visible due to the occulting finger which blocks out most of the light from the star.
Astronomers are eager to find evidence of extrasolar planetary systems to learn whether our own solar system was created out of very unique conditions, or whether it is the result of common and fundamental processes that accompany stellar formation. These questions can not be satisfactorily answered until astronomers have carefully studied examples of planetary formation other than our own solar system.
Paresce and Burrows have images of planetary or protoplanetary around other stars to analyze. They also plan to make detailed observations of Beta Pictoris with the NASA/ESA Hubble Space Telescope, which is now scheduled for launch in late 1988. With its significant increase in resolution over present ground-based instruments, the Space Telescope will have the capability to provide a far more detailed view of the disc's structure, closer to the star. It will also have the potential for detecting the extremely faint glow of planets which may accompany the star. It is also expected that this fascinating area of astronomical research will greatly benefit from future, giant telescopes on the ground, such as the ESO 16 metre Very Large Telescope (VLT), now in the final planning stage.
The Hubble Space Telescope is a project of international collaboration between NASA and ESA. The Space Telescope Science Institute is operated for NASA by the Association of Universities for Research in Astronomy (AURA). It is located on the Johns Hopkins University Campus in Baltimore, Maryland, U.S.A.
The European Southern Observatory is an international organisation, supported by eight countries (Belgium, Denmark, France, the Federal Republic of Germany, Italy, the Netherlands, Sweden and Switzerland). Its headquarters are located in Garching near Munich, F.R.Germany, and the observatory is at La Silla, 600 kilometres north of Santiago de Chile, South America.
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