Barren world of stars

A multi-coloured disc of dust surrounds the young star

By BBC News Online science editor Dr David Whitehouse

Astronomers say that a game of planetary survival is taking place inside a gigantic cloud of gas and dust 1,500 light-years from Earth.

It could mean that planets may be rarer in the Galaxy than previously thought.

The dramatic events are taking place in the Orion Nebula - the nearest, large stellar nursery to Earth.

The observations suggest that any fledgling planets must try to form quickly, or they will be destroyed by a flood of radiation from the nebula's brightest star.

Planets around young stars may have to form quickly....

In research published in Science magazine, John Bally of the University of Colorado in Boulder, US, and Henry Throop of the Southwest Research Institute, also in Boulder, used Hubble to see if planets were beginning to grow in million-year-old dusty discs that surround young stars in Orion.

First steps

"This is the first time that large-growing dust grains, which range in size from smoke particles to sand grains, have been seen in visible light in these protoplanetary discs," Throop said.

"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."

..or else they could be destroyed.

"We have two things happening in these systems: dust grains are beginning to stick together as a first step towards making planets, but then 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."

The astronomers deduced the dust size from the way the discs 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 discs in Orion appear grey because they allow all colours of light to pass through. This is because the dust is much larger than interstellar dust.

Radio observations also provide tantalising hints that much of the material in the disc may range in size from snowflakes to gravel.

Hostile environment

The Hubble observations show that it may be easy to start building planets. According to conventional theory, the grains will clump under gravity, until they become the size of planets.

Astronomers suggest that nurturing planets to maturity may be a dicey drama repeatedly playing out deep inside star-forming clouds scattered across our Galaxy.

Because of Orion's hostile environment, typical of star-forming regions, "we're also seeing that planet formation is a hazardous process," Bally says.

The outcome could have far-reaching implications for the number of planets in our Galaxy.

Depending on whether planets can form quickly or not, it could mean that planets may be rarer in the Galaxy than previously thought.

Astronomers point out this is consistent with extra-solar planet discoveries so far. They show that about 5% of the stars in our solar neighbourhood have Jupiter-sized planets in small orbits.

If giant planets like Jupiter could collapse quickly out of the gas disc, they might survive, according to a theory proposed by Alan Boss of Carnegie Institutions, 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."