By analyzing Hubble Space Telescope images of a suspected young planetary system recently discovered around the star HR 4796A, Scott Kenyon and Kenny Wood of the Harvard-Smithsonian Center for Astrophysics and Barbara Whitney and Michael Wolff of the Space Science Institute have produced a computer model that suggests the ring around that object probably is a common feature of all planetary systems.

Indeed, the well-known Kuiper Belt of asteroids in our own Solar System may even be the residual remains of such a ring.

Most theories of planetary formation predict that planets like Earth, Mars, and Jupiter, grow from the coalescence of much smaller bodies, so-called planetesimals embedded in a very thin disk-like nebula of dust and gas rotating around a hot young star. The planetesimals, ranging in size from one meter to one kilometer in diameter, are in constant collision, careening off each other like pinballs in an arcade game.

Caption: Credit for center and lower images: S Kenyon and K Wood (CfA) and B Whitney and M Wolff (Space Science Institute). Credit for upper image: G Schneider (Steward Observatory, University of Arizona) and collaborators.

Eventually, however, enough of the colliding bodies stick together to create either rocky planets like Earth and Mars, or icy ones like Pluto. The long, slow growth process has been compared to the proverbial snowball that grows larger and larger as it rolls down a snowy slope.

As the larger objects grow even larger, they stir up and accelerate smaller bodies in the nebula. The grinding, shattering effect of constant high-speed collisions produces untold millions of micrometer- sized particles that reflect light from the central star, which is seen through telescopes--especially those sensitive to infrared radiation--as a dusty disk.

When Kenyon and his colleagues simulated this process on the computer, the shining dust first appeared once planets like Pluto began to form. Moreover, the dust formed in a distinct and compacted ring, rather than in a diffuse and flattened disk. In fact, the presence of a ring may be the signature of an emerging planetary system, with the ring itself serving as a clear demarcation line between inner and outer regions. Inside a ring, new planets serenely circle the central star; outside, the cosmic construction project continues.

The scientists feel our own Solar System probably had just such a dust ring during the first 10 to 100 million years of its life. Today, a reminder of that evolutionary feature can be seen in the Kuiper Belt, which contains a host of massive bodies that never coalesced into larger planets, analogous to the scraps of building materials left over at a construction site. Indeed, they argue, the Kuiper Belt may contain several primordial "Plutos" still waiting to be discovered.

The paper "Forming the Dusty Ring in HR 4796A"by Kenyon, Wood, Whitney, and Wolff will appear in the 20 October 1999 issue of Astrophysical Journal Letters.

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