April 21, 1998
The team of British and American astronomers report their findings, based on their observations of three well-known stars in our Milky Way Galaxy -- Vega, Fomalhaut and Beta Pictoris -- in the April 23 issue of the journal Nature.
"One of the most striking features we see is a central hole in the disk around Fomalhaut," said Wayne Holland, who led the astronomy team at the JAC. "The lack of bright emission close to the star suggests that dust is largely cleared out, and a probable explanation is that it has formed into rocky planets like the Earth, even though we cannot detect these directly."
"It is generally believed that our own Solar System formed out of such a disk," said Benjamin Zuckerman, UCLA professor of physics and astronomy, "but whether the newly discovered disks contain majestic planets like Jupiter and Saturn, or just comets and asteroids, remains to be seen."
For about 600 million years, our Solar System was bombarded by comets and asteroids, until, as Zuckerman said, "the gravitation of Jupiter and Saturn, the 'garbage men of the solar system,' cleaned out these massive objects that could have decimated life."
These star systems may teach us much about the history of our own solar system. "What we see is almost exactly what astronomers orbiting nearby stars would have seen if they had pointed a millimetre-wave telescope at our own sun a few billion years ago," said Jane Greaves, one of Holland's colleagues at the JAC.
"SCUBA uses detectors cooled to a tenth of a degree above absolute zero (-273 degrees Celsius) to measure the tiny amounts of heat emission from small dust particles at a wavelength close to one millimetre" Holland said.
The astronomers report on the following:
Fomalhaut, the brightest star in the constellation Piscis Austrinus (the Southern Fish), is believed to be about 200 million years old -- very young compared to our Sun. In the new image the brightest emission, and therefore the most dust, is surprisingly far from the star. The star appears to be surrounded by a huge disk of dust aligned roughly in a north-south direction with a hollow central cavity.
Why has the dust disappeared from the star's inner region? The most exciting explanation is that the dust has coalesced and formed into planets. Other possibilities include that the dust was absorbed into Fomalhaut or blown away from the star.
The image appears like a doughnut, with a hole in the centre comparable to the size of our own planetary system. The region with the brightest emission is located at a distance from Fomalhaut equivalent to the distance from the Sun of the Kuiper belt of comets out beyond Neptune and Pluto in our own system.
"We could be seeing a region around Fomalhaut that is rich with countless comets, although the images cannot reveal comets directly," Holland said.
The new SCUBA image of Vega shows it to be enshrouded in faintly glowing dust. As with Fomalhaut, the brightest peak is not centred on the star but at a distance from Vega of about seven thousand million miles -- about twice the distance from our own Sun to Pluto. However, this time the emission is concentrated in only one peak which puzzled the astronomers.
The astronomers made a deep search with the 10-metre Keck Telescope (also on Mauna Kea) -- the world's largest optical and infrared telescope -- for infrared light from possible planets or brown dwarfs. They detected no such objects, and have no explanation for this bright blob.
"This bright blob is a real mystery; we simply don't have an explanation yet," Zuckerman confessed. "If it is indeed associated with Vega, it's completely unknown. We can suggest explanations -- such as a dust cloud around a giant planet orbiting Vega -- but they are complete guesses at this point."
Beta Pictoris, in the southern constellation of Pictor (the Painter's Easel) is the only one of the three stars that had been previously imaged. It is also one of the youngest stars in the solar neighbourhood -- being only about 30 million years old. The emission from dust in this case seems to be mainly concentrated at the position of the star, but as with Vega, there is also an unidentified blob in line with the disk.
This source may be a disconnected fragment of the disk in orbit around an unseen companion planet, the astronomers said. It is far from Beta Pictoris -- some 10 to 20 times the distance from our Sun to Pluto. While the blobs in the Vega and Beta Pictoris images may be dust-enshrouded giant planets, planets are not supposed to be able to form at such great distances from stars, the researchers noted.
"In the Beta Pictoris picture, if this blob is indeed associated with the star," Zuckerman said, "presumably it's a planet-like object surrounded by dust -- which is completely unexpected, a total mystery. The blobs in the Vega and Beta Pictoris images seem to be telling us something very surprising, and we have more questions than answers at the moment."
Noting that none of the stars has retained a large enough mass of dust to form planets, the astronomers say that if these stars are orbited by planets, they have most likely already formed, or are well on their way to forming.
"If the blobs around Vega and Beta Pictoris surround planets, that could tell us that planets are a common phenomenon," Holland said.
"Even if planets are more common than was believed, these findings do not make intelligent life in the universe more likely," Zuckerman said.
Ian Robson, Director of the JCMT, said, "SCUBA really is a fantastic new instrument. This is just one of the exciting new discoveries that SCUBA is making that is revolutionizing submillimetre astronomy and our knowledge of the universe."
Holland's team also included astronomers Jane Greaves, Iain Coulson, Dolores Walther, William Dent, Walter Gear and Ian Robson, and Zuckerman's included UCLA graduate students Richard Webb and Chris McCarthy.
More information on the JCMT and SCUBA can be found on the JCMT Web Page.
NASA Headquarters, Washington, DC
Jet Propulsion Laboratory, Pasadena, CA
W. M. Keck Observatory, Mauna Kea, Hawaii
April 21, 1998
Scientists released an image of the probable site of planet formation around a star known as HR 4796, about 220 light-years from Earth in the constellation Centaurus. The image, taken with a sensitive infrared camera developed at NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA, shows a swirling disk of dust around the star. Within the disk is a telltale empty region that may have been swept clean when material was pulled into newly formed planetary bodies, the scientists said.
"This may be what our solar system looked like at the end of its main planetary formation phase," said Dr. Michael Werner of JPL, who co-discovered the region, along with Drs. David Koerner and Michael Ressler, also of JPL, and Dana Backman of Franklin and Marshall College, Lancaster, PA. "Comets may be forming right now in the disk's outer portion from remaining debris."
The discovery was made on March 16 from the giant 33-foot (10-meter) Keck II telescope atop Mauna Kea, Hawaii. Keck II and its twin, Keck I, are the world's largest optical and infrared telescopes. Attached to the Keck II for this observation was the mid-infrared camera, developed by Ressler at JPL and designed to measure heat radiation.
The four scientists reported their discovery in a submission to The Astrophysical Journal Letters. The disk was discovered independently and contemporaneously at the Cerro Tololo Observatory in Chile by another team of scientists, led by Ray Jayawardhana of the Harvard-Smithsonian Center for Astrophysics (CfA), Cambridge, MA, and Dr. Charles Telesco of the University of Florida, Gainesville.
Koerner of JPL said the finding represents a "missing link" in the study of how planetary systems are born and evolve. "In a sense, we've already peeked into the stellar family album and seen baby pictures and middle-aged photos," Koerner said. "With HR 4796, we're seeing a picture of a young adult star starting its own family of planets. This is the link between disks around very young stars and disks around mature stars, many with planets already orbiting them."
"This is the first infrared image where an entire inner planetary disk is clearly visible," Werner said. "The planet- forming disk around the star Beta Pictoris was discovered in 1983 by the Infrared Astronomical Satellite (IRAS), and also later imaged with the Hubble Space Telescope, but glaring light from the star partially obscured its disk."
The apparent diameter of the dust disk around HR 4796 is about 200 astronomical units (one astronomical unit is the distance from Earth to the Sun). The diameter of the cleared inner region is about 100 astronomical units, slightly larger than our own solar system.
HR 4796 was originally identified as an interesting object for further study by Dr. Michael Jura, an astronomy professor at the University of California, Los Angeles. The star, HR 4796, is about 10 million years old and is difficult to see in the continental United States, but is visible to telescopes in Hawaii and the southern hemisphere.
The discovery of the HR 4796 disk was made in just one hour of observing time at Keck, but the JPL team plans to return to Hawaii in June for further studies. They hope to learn more about the structure, composition and size of this disk, and to determine how disks around stars in our galaxy produce planets. They plan to study several other stars as well, including Vega, which was featured prominently in the movie, "Contact."
The Harvard/Florida research team that also found the HR 4796 disk included Drs. Lee Hartmann and Giovanni Fazio of Harvard- Smithsonian Center for Astrophysics, and Scott Fisher and Dr. Robert Pina of the University of Florida.
JPL's use of the Keck telescope is supported by NASA's Origins program, a series of missions to study the formation of galaxies, stars, planets and life, and to search for Earth-like planets around other stars that might have the right conditions for life.
The W. M. Keck Observatory is owned and operated by the California Association for Research in Astronomy, a joint venture between the University of California, California Institute of Technology (Caltech), Pasadena, CA, and NASA. Use of the Keck Observatory for Origins research is managed by JPL for NASA's Office of Space Science, Washington, DC. JPL is a division of Caltech.
The research of both teams was supported in large part by the NASA Origins Program, with additional support to the CfA- Florida team from the National Science Foundation, the National Optical Astronomy Observatories, and the Smithsonian Institution; and with additional NASA support for the CalTech/JPL-Franklin & Marshall team, including use of the Keck Observatory.