Protoplanetary Disks Around Newly Formed Stars
16 December 1992
The NASA/ESA Hubble Space Telescope has uncovered the strongest evidence yet that many stars form planetary systems.Dr. C. Robert O'Dell of Rice University, Houston, Texas and colleagues have used Hubble to discover extended disks of dust around 15 newly formed stars in the Orion Nebula, a starbirth region 1500 light-years away.Such disks are a prerequisite for the formation of solar Systems like our own. "These images provide the best evidence for planetary 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. Our solar system is considered a relic of just such a disk of dust that accompanied our Sun's birth 4.5 billion years ago.
Before the Hubble discovery, protoplanetary disks have 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. (He calls them Proplyds following the suggestion of his wife, Gail Sabanosh, who noted that for him protoplanetary disks was too much of a tongue twister.)
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 proplyds are bright enough to have been seen previously by ground-based optical and radio telescopes as stars. Their true nature was not recognized until the Hubble discovery.
Each proplyd appears as 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 the rate of about one half the mass of our 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 proplyd's initial mass would be at least 15 times that of the giant planet Jupiter.
Many of the youngest and hottest stars in our 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 of Orion the Hunter. The region of Orion studied intensely by O'Dell and collaborators 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 Astrophysical Journal next spring, title: "Discovery of New Objects in the Orion Nebula on HST Images: Shocks, Compact Sources, and Protoplanetary Disks."
