David Aguilar
(617) 495-7462

Christine Pulliam
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CfA Press Release

For Release: Wednesday, October 20, 1999

A Dusty Ring May Be the Tell-Tale Mark of anEmerging Planetary System

CAMBRIDGE, MA--The popular image of nascentplanetary systems as thin, spinning pancakes ofcosmic dust and debris may be changed by a newcomputer model that shows how that disk of debrisis transformed into a very distinct ring once Pluto-like bodies start to form.

By analyzing Hubble Space Telescope images of asuspected young planetary system recentlydiscovered around the star HR 4796A, Scott Kenyonand Kenny Wood of the Harvard-Smithsonian Centerfor Astrophysics and Barbara Whitney and MichaelWolff of the Space Science Institute have produceda computer model that suggests the ring around thatobject probably is a common feature of all planetarysystems. Indeed, the well-known Kuiper Belt ofasteroids in our own Solar System may even be theresidual remains of such a ring.

Most theories of planetary formation predict thatplanets like Earth, Mars, and Jupiter, grow from thecoalescence of much smaller bodies, so-calledplanetesimals embedded in a very thin disk-likenebula of dust and gas rotating around a hot youngstar. The planetesimals, ranging in size from onemeter to one kilometer in diameter, are in constantcollision, careening off each other like pinballs in anarcade game.

Eventually, however, enough of the colliding bodiesstick together to create either rocky planets likeEarth and Mars, or icy ones like Pluto. The long,slow growth process has been compared to theproverbial snowball that grows larger and larger as itrolls down a snowy slope.

As the larger objects grow even larger, they stir upand accelerate smaller bodies in the nebula. Thegrinding, shattering effect of constant high-speedcollisions produces untold millions of micrometer-sized particles that reflect light from the central star,which is seen through telescopes--especially thosesensitive to infrared radiation--as a dusty disk.

When Kenyon and his colleagues simulated thisprocess on the computer, the shining dust firstappeared once planets like Pluto began to form.Moreover, the dust formed in a distinct andcompacted ring, rather than in a diffuse and flatteneddisk. In fact, the presence of a ring may be thesignature of an emerging planetary system, with thering itself serving as a clear demarcation linebetween inner and outer regions. Inside a ring, newplanets serenely circle the central star; outside, thecosmic construction project continues.

The scientists feel our own Solar System probablyhad just such a dust ring during the first 10 to 100million years of its life. Today, a reminder of thatevolutionary feature can be seen in the Kuiper Belt,which contains a host of massive bodies that nevercoalesced into larger planets, analogous to thescraps of building materials left over at aconstruction site. Indeed, they argue, the KuiperBelt may contain several primordial "Plutos" stillwaiting to be discovered.

The paper "Forming the Dusty Ring in HR 4796A"by Kenyon, Wood, Whitney, and Wolff will appear inthe 20 October 1999 issue of "Astrophysical JournalLetters." An abstract is available at http://www.cfa.harvard.edu/~kenyon/hr4796/info.html. An image and caption can be found at http://www.cfa.harvard.edu/~kenyon/hr4796/pict.html.

Contact Information:

Harvard-Smithsonian Center for Astrophysics
Dr. Scott Kenyon, 617-495-7235, skenyon@cfa.harvard.edu
Dr. Kenny Wood, 617-495-7301, kwood@cfa.harvard.edu
60 Garden Street
Cambridge, MA 02138

Space Science Institute
Dr. Barbara Whitney, 608-221-3938, bwhitney@execpc.com
Dr. Michael Wolff, 303-492-3774, wolff@colorado.edu
1540 30th Street
Suite 23
Boulder, CO 80303

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