First steps of planet growth and destruction witnessed
SPACE TELESCOPE SCIENCE INSTITUTE RELEASE
Posted: April 27, 2001
A dramatic life-and-death game of planetary survival is taking place inside a gigantic cloud of gas and dust 1,500 light-years from Earth, and the outcome could have far-reaching implications for the number of planets in our Milky Way galaxy.
The bad news is that other observations suggest that any fledgling planets must try to quickly "beat the clock" by forming before they are evaporated away by a blistering flood of radiation from the nebula's brightest star. Called Theta 1 Orionis C, the star is part of the nebula's central Trapezium cluster and is visible through a small telescope.
In new research published today in Science Magazine, John Bally of the University of Colorado in Boulder and Henry Throop of the Southwest Research Institute, also in Boulder, used Hubble to assess if planets were beginning to grow in million-year-old dusty disks in Orion.
"This is the first time that large growing grains [which range in size from smoke particles to sand grains] have been seen in visible light in these protoplanetary disks," Throop says. "The dust we're seeing in the Hubble observations is large - completely unlike dust that we've seen in young star-forming regions like this before. We're seeing the very first stages of planetary formation happening before our eyes. We have two things happening in these systems: dust grains are beginning to stick together as a first step toward making planets, but then these bright stars are trying to tear everything apart. Which one wins is really a big question. It's like trying to build a skyscraper in the middle of a tornado."
A variety of Orion observations by Hubble and ground-based telescopes are helping astronomers converge on the idea that nurturing planets to maturity may be a dicey drama repeatedly playing out deep inside star-forming nebulas scattered across our Milky Way galaxy.
Because of Orion's hostile environment, which is typical of star-forming regions across the galaxy, "we're also seeing that planet formation is hazardous process," Bally says.
Depending on whether planets can form quickly or not, it could mean that planets may be more rare in the Milky Way than previously thought. The astronomers point out this is consistent with extrasolar planet discoveries so far. Those discoveries show that about 5 percent of the stars in our solar neighborhood have Jupiter-sized planets in small orbits.
Protoplanetary disks in Orion were first discovered in 1992 and dubbed "proplyds." At first glance, their existence seems to greatly improve the odds for planets being abundant in the galaxy, because they appeared to confirm a common model of planet formation.
But subsequent Hubble pictures revealed proplyds being blowtorched away by a relentless blast of radiation from the nebula's largest star. The doomed systems look like hapless comets, with wayward tails of gas boiling off the withering pancake-shaped disks.
Bally believes that the gaseous component of the disk will largely vaporize away but will leave behind a residual "gravel" disk of rocky pebbles that may successfully build terrestrial planets like Earth out of the grains he's seeing form.
If giant planets like Jupiter could collapse quickly out of the gas disk, they might survive, according to a theory proposed by Alan Boss of Carnegie Institutions of Washington. "Only time will tell. If we find lots of Jupiters around other stars, then it means they will have managed to grow rapidly in Orion-type environments," Boss says.
Throop agrees. "It looks like Jupiters must be formed either rarely or rapidly. It's a good bet that planetary systems in Orion will look nothing like our own solar system. Although they may have rocky planets like Earth and Mars, it looks hard to form either giant planets or comets."
The Hubble Space Telescope's majestic view of the Eskimo Nebula. This spectacular poster is available now from the Astronomy Now Store.