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Paula Cleggett-Haleim
Headquarters, Washington, D.C.               December 16, 1992
(Phone:  202/358-0883)

Jim Elliott
Goddard Space Flight Center, Greenbelt, Md.
(Phone:  301/286-6256)

Ray Villard
Space Telescope Science Institute, Baltimore
(Phone:  410/338-4757)


RELEASE:  92-226

HUBBLE DISCOVERS PROTOPLANETARY DISKS AROUND NEW STARS

     NASA's Hubble Space Telescope has uncovered the strongest 
evidence yet that many stars may form planetary systems.

     Dr. C. Robert O'Dell of Rice University, Houston, and 
colleagues have used Hubble to discover extended disksanetary 
systems," said O'Dell. "The disks are a missing link in our 
understanding of how planets like those in our solar system form.  
Their discovery establishes that the basic material of planets 
exists around a large fraction of stars.  It is likely that many 
of these stars will have planetary systems." 

     Hubble Space Telescope's detailed images confirm more than a 
century of speculation, conjecture and theory about the genesis of 
a solar system.

     According to current theories, the dust contained within the 
disks eventually agglomerates to make planets.  Earth's solar 
system is considered a relic of just such a disk of dust that 
accompanied the sun's birth 4.5 billion years ago.

     Before the Hubble discovery, protoplanetary disks had been 
confirmed around only four stars, Beta Pictoris, Alpha Lyrae, 
Alpha Piscis Austrini and Epsilon Eridani. 

     Unlike these previous observations, Hubble has observed newly 
formed stars less than a million years old which are still 
contracting out of primordial gas. 

     Hubble's images provide direct evidence that dust surrounding 
a newborn star has too much spin to be drawn into the collapsing 
star.  Instead, the material spreads out into a broad, flattened 
disk. 

     These young disks signify an entirely new class of object 
uncovered in the universe, according to O'Dell.

     Hubble can see the disks because they are illuminated by the 
hottest stars in the Orion Nebula, and some of them are seen in 
silhouette against the bright nebula.  However, some of these 
protoplanets are bright enough to have been seen previously as 
stars by ground-based optical and radio telescopes.  Their true 
nature was not recognized until the Hubble discovery.  

     Each protoplanet appears as a thick disk with a hole in the 
middle where the cool star is located.  Radiation from nearby hot 
stars "boils off" material from the disk's surface at a rate equal 
to about one-half the mass of Earth per year.  This material is 
then blown back into a comet-like tail by a stellar "wind" of 
radiation and subatomic particles streaming from nearby hot stars. 

     Based on this erosion rate, O'Dell estimates that a 
protoplanet's initial mass would be at least 15 times that of the 
giant planet Jupiter.

     Many of the youngest and hottest stars in the Milky Way 
Galaxy are found in the Orion Nebula.  The nebula is on the near 
edge of a giant molecular cloud which lies immediately behind the 
stars that trace the sword of the constellation Orion the Hunter. 

     The region of Orion is a bright part of the nebula where 
stars are being uncovered at the highest rate.  These results 
suggest that nearly half the 50 stars in this part of Orion have 
protoplanetary disks.

     O'Dell's co-investigators are graduate students Zheng Wen and 
Xi-Hai Hu of Rice University and Dr. Jeff Hester of Arizona State 
University.  Their results will be published in the Astrophysical 
Journal next spring under the title "Discovery of New Objects in 
the Orion Nebula on HST Images:  Shocks, Compact Sources and 
Protoplanetary Disks."

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     ___    _____     ___
    /_ /|  /____/ \  /_ /|     Ron Baalke         | baalke@kelvin.jpl.nasa.gov
    | | | |  __ \ /| | | |     Jet Propulsion Lab |
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