SETI [ASTRO] Observational Evidence That Terrestrial Planets Form Around Nearby Stars


Larry Klaes (lklaes@bbn.com)
Thu, 03 Jun 1999 11:11:44 -0400


>X-Authentication-Warning: brickbat12.mindspring.com: majordom set sender to owner-astro using -f >Date: Wed, 2 Jun 1999 23:41:23 GMT >From: Ron Baalke <BAALKE@kelvin.jpl.nasa.gov> >To: astro@lists.mindspring.com >Subject: [ASTRO] Observational Evidence That Terrestrial Planets Form Around Nearby Stars >Sender: owner-astro@brickbat12.mindspring.com >Reply-To: Ron Baalke <BAALKE@kelvin.jpl.nasa.gov> > >News Services >University of Arizona > >Contact(s): >Frank Low, 520-622-7074, flow@as.arizona.edu >Dean Hines, 520-621-3445, dhines@as.arizona.edu >Glenn Schneider, 520-621-5865, gschneid@as.arizona.edu > >Lori Stiles, News Services >Tel: 520-621-1877 FAX: 520-626-4121 >E-mail: lstiles@u.arizona.edu > >June 2, 1999 > >Astronomers report observational evidence that terrestrial planets form >around nearby stars > >Astronomers believe they have found the makings -- or the dust and >debris -- from a failed terrestrial planet circling a nearby system of two >young stars similar to our early sun. > >The belt of material around the binary star HD 98800B, which is only 150 >light-years away, is strikingly similar to the zodiacal dust bands in our >solar system's asteroid belt. These bands of dust and the asteroids that >produce them resulted from a failed planet between Mars and Jupiter, say >astronomers from The University of Arizona in Tucson, and they argue that >a similar explanation is needed to understand their new observations. > >Frank Low, Dean Hines and Glenn Schnieder of the UA Steward Observatory >made the discovery as part of the Hubble Space Telescope NICMOS >Instrument Definition Team's Environments of Nearby Stars program. They >are reporting the results this week at the 194th American Astronomical >Society meeting in Chicago. The team also is publishing a paper on this >work in the Astrophysical Journal Letters, "NICMOS Observations of the >Pre-Main Sequence Planetary Debris System HD 98800." > >"Because we find so many similarities between HD98800B and our own >solar system we conclude that the material responsible for the peculiar >infrared emission around these two very young stars must have been >produced in a way similar to the way asteroids formed around the sun," >Low said. "This is strong observational evidence that planets like Earth >form around nearby stars." > >The zodiacal dust bands in our solar system, discovered 16 years ago in >Infrared Astronomy Satellite (IRAS) observations by Low and colleagues, >formed and have been replenished for four billion years by collisions of >asteroids between Mars and Jupiter. Scientists say the belt of asteroids >would have been another planet in our solar system, except that Jupiter's >tremendous gravitational forces prevent planet formation so close by. > >During its flight in 1983 IRAS also found the huge infrared emission >from HD98800. However, only very recently, by making observations with >NICMOS on the Hubble Space Telescope and closely related ground based >observations, have astronomers fit the pieces of the puzzle into a model >that is remarkably similar in temperature, size, structure and mass to >the asteroid families and the zodiacal dust bands. Low emphasized that, >"It is only because HD98800 is so much younger than the Sun, about 5 >million years instead of 4 billion years old, that it has enough dust and >fine particles that we can see it clearly from space and from the ground." > >The astronomers used the high-resolution NICMOS camera in observations >spanning 306 days to study luminosity, or light intensity, for each >component of the HD 98800 system. They successfully measured how >much energy each of the two stars individually generates in the system, >which was no easy feat. > >The binary stars are "K dwarf" stars, not too different from our sun. When >K dwarf stars age, they typically end up slightly cooler and slightly less >massive than the sun. These companion stars are the same age, about 5 to >10 million years old, according to earlier research. > >But the astronomers didn't see the planetary debris disk in their NICMOS >images at all. And that, Schneider said, "is where the story got really >interesting." Like evidence in the Sherlock Holmes case from the dog that >didn't bark in the night, the evidence for the planetary debris disk that >NICMOS didn't get is very telling. > >The long light wavelengths radiating from the HD 98800 planetary debris >disk are in the far infrared, beyond the wavelength range detectable with >NICMOS, the Near Infrared Camera and Multi-Object Spectrometer. The >data from IRAS gave the energy output and temperature of the planetary >debris disk. Almost all of the material is at a single temperature -- 165 >degrees Kelvin -- just slightly cooler than our solar system zodiacal dust >bands at 200 degrees Kelvin. > >"That all this debris is at a single temperature is an amazing result," >Schneider said. "It behaves as a perfect 'blackbody' -- absorbing light >that falls on it from the stars and then re-radiating energy at particular >wavelengths characteristic of the nature of this material." > >That the debris particles are all at one temperature shows that they are >the same distance from the binary stars, their heat source. The team found >that the HD 98800 planetary debris disk is 4.5 AU from star B, the cooler >and slightly larger star in the binary system -- just as our solar system's >planetary debris at the asteroid belt is at the correct distance, about 4.5 >AU, for its temperature. At this temperature, the debris particles, which >previously have been shown to be silicates, the stuff Earth is made of, >must be bigger at least than 200 microns, which is about the diameter >of a human hair, Hines said. For all that's known at this point, Schneider >added, the particles could be millimeters in size, and many must be much >larger to remain in their orbits. > >The total mass of material in the belt is between one-half and one Earth >mass, the researchers say. The innermost and outermost particles in the >disk are probably no more than one AU apart. If you were to stand at the >center of the HD 98800B star, Hines said, you would see the band extend >12 degrees above and 12 degrees below the vertical. > >If the HD 98800 binary system is around 5 million years old and current >astronomical understanding is correct, Hines said, "Planets should have >formed in this system by now. Given evidence that this planetary debris >disk is so nearly identical to the zodiacal dust bands of our asteroid belt, >we can logically infer the next step: Our asteroid belt was formed by >a failed terrestrial planet. Now we see something that's essentially >analogous, or almost identical in properties, to our asteroid belt. A >terrestrial planet is trying to form here -- or it did form and was >disrupted, probably by perturbing forces of the binary system of stars." > >"In an evolutionary sense, what we are really seeing is the results of a >terrestrial planetary formation process or attempted formation process," >Schneider added. "There's been a kind of gap between finding planets by >radial velocity measurements and observing actual mechanisms of planet >formation." > >****** > >EDITORS NOTE: The American Astronomical Society meeting poster on >this story is viewable and downloadable via Netscape or other browers at: > http://nicmosis.as.arizona.edu:8000/AAS99_2/HD98800_POSTER.jpg > >The poster itself points to a journal preprint server from which the paper >on this research can be displayed or downloaded. Hines' artist rendering >of the planetary debris disk around HD 98800b can be viewed at > http://science.opi.arizona.edu/pics/hd98800b.jpg >(Contact Hines for a publishable print.) > >Low this week is in Tucson; Hines and Schneider return to Tucson Friday, >June 4. > >LINKS: >http://nicmosis.as.arizona.edu:8000/AAS99_2/HD98800_POSTER.jpg >http://science.opi.arizona.edu/pics/hd98800b.jpg > >



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