First steps of planet growth and destruction witnessed
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.

Four snapshots show dust disks around embryonic stars in the Orion Nebula being "blowtorched" by a blistering flood of ultraviolet radiation from the region's brightest star. Within these disks are the seeds of planets. The doomed systems look like hapless comets, with wayward tails of gas boiling off the withering, pancake-shaped disks. The Frisbee-shaped disks, called protoplanetary disks, are wider than our solar system and reside in the centers of the cocoons of gas. These cocoons were formed from material evaporating off the surface of the disks. Evidence from Hubble suggests that dust grains in the disk are already forming larger particles. But these particles may not have time to grow into full-fledged planets because of the relentless "hurricane" of radiation from the nebula's hottest star, called Theta 1 Orionis C. In the picture at top left, the disk is the green-colored oval near the center. Radiation from the hot star is heating up the disk, causing matter to dissipate, like steam evaporating from the surface of boiling water. A strong "stellar wind," a stream of particles moving at 4,500 to 8,900 miles per hour (7,200 to 14,400 kilometers per hour), is propelling the material away from the disk. The material is glowing because it is being energized by radiation from the hot star. Credits: NASA, J. Bally (University of Colorado, Boulder, CO), H. Throop (Southwest Research Institute, Boulder, CO), C.R. O'Dell (Vanderbilt University, Nashville, TN)
The good news is that NASA's Hubble Space Telescope is giving astronomers the first direct visual evidence for the growth of planet "building blocks" inside dust disks around dozens of stars in the giant Orion Nebula - the nearest, large "star-factory" to Earth.

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."

Orion Nebula mosaic from Hubble with outline of detailed picture below. Credit: C.R. O'Dell (Vanderbilt University)
The astronomers deduced the dust size from the way the disks allow light to pass through them. The fine dust normally seen in space scatters blue light but allows red light to pass through. The Sun appears red at sunset because atmospheric dust influences light in the same way as space dust. The dust disks in Orion, however, appear gray because they allow all colors of light to pass through. This is unusual in space and can only be explained if the dust is much larger than interstellar dust. Radio observations also provide tantalizing hints that much of the material in the disk may range in size from snowflakes to gravel.

Orion Nebula detail showing several proplyds. Credit: J. Bally (University of Colorado)
The Hubble observations show, for the first time, that it may be easy to start building planets. According to conventional theory, the grains will continue to snowball up through clumping, and then pull together under gravity, until they become the size of planets. This discovery helps confirm the long-proposed scenario for how Earth and the rest of the solar system formed around our Sun 4.5 billion years ago.

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.

Anatomy of a protoplanetary disk. Credit: Ann Feild, NASA/STScI
The researchers predict that within 100,000 years, 90 percent of the youngest disks - which started out being billions of miles across - will be largely destroyed. But planet formation will be "business as usual" in the 10 percent of the proplyds that are shielded from the ultraviolet radiation. These stars will probably become hosts of a variety of planets.

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."

Hubble poster
The Hubble Space Telescope's majestic view of the Eskimo Nebula. This spectacular poster is available now from the Astronomy Now Store.