"Tadpole Hunters" May Net Forming Planets

This will help astronomers hunt more effectively for these elusive objects, and better estimate how many planet-forming systems are out there.

"We were very surprised that we could see these blobs," says Dr Bärbel Koribalski of CSIRO's Australia Telescope National Facility. "It goes against the predictions."

Planet formation is a common process in the Universe, astronomers think. Planets and their parent stars are born from compact clouds of gas and dust. But very few of these have been found.

In 1994 the Hubble Space Telescope saw blobs of gas and dust in a star-forming region in the constellation of Orion. They were dubbed protoplanetary disks, or 'proplyds' for short. Each is thought to hide a forming star, with material around it that could later form planets.

Last year astronomers using the Hubble Space Telescope (HST) announced they had found three of the blobs on the outskirts of a region in our Galaxy where many stars are forming. Called NGC 3603, the region lies 20 000 light-years away in the Southern constellation of Carina.

A team led by Anita Mücke of the University of Montreal was independently using CSIRO's Australia Telescope to study massive stars in the same region. Tipped off by the optical studies, they scanned their data for the objects and found them there, unexpectedly bright.

Later, they found a fourth object that had not been picked up in the original HST study.

Apart from the objects in Orion and NGC 3603, only two other proplyds have been found.

There may be many reasons for this apparent scarcity.

Optical telescopes can see proplyds only when they are lying against a brightly lit background.

Many proplyds may have been mistaken for other kinds of objects - in particular, small blobs of glowing hydrogen gas called 'ultracompact HII regions'.

Astronomers detect a lot of those blobs - about four times as many as star-formation theories predict.

"It seems likely that many are really proplyds instead," says Dr Koribalski.

Just as radiation from the Sun blasts material from a comet into a sweeping tail, strong UV radiation from nearby hot young stars shapes the proplyds into sleek tadpole forms.

"We will be able to look through our catalogues of radio sources for tadpole-shaped objects," Dr Koribalski says. "Many of them may turn out to be proplyds. To confirm that, we'd get optical images of them to check that they have that tadpole shape."

The sizes of the proplyds may also make them hard to identify.

"The objects in NGC 3603 are much bigger than the proplyds in Orion. If you put the small Orion proplyds in NGC 3603, which is much further out, you wouldn't be able to see them," says Dr Koribalski.

On the other hand, if we saw the NGC 3603 proplyds closer, at the distance of the Orion nebula, "they wouldn't look like blobs at all - they might look like huge pillars of gas and dust," she adds.

But proplyds may also be genuinely scarce. The blasts of radiation that give them their tadpole shape may also destroy them before they can form planets.

A finding announced on 1 May at the Spring meeting of the American Physical Society notes that 90 percent of the proplyds in Orion will be destroyed in the next several hundred thousand years.

"NGC 3603 has about 100 times the ionizing power of the comparable region in Orion," says Dr Mücke. "The proplyds there may be blasted away by radiation long before they get a chance to form planetary systems."

The members of the observing team are Dr Anita Mücke (University of Montreal), Dr Bärbel Koribalski (CSIRO Australia Telescope National Facility), Dr Tony Moffat (University of Montreal), Dr Mike Corcoran (NASA Goddard Space Flight Center), and Dr Ian Stevens (University of Birmingham, UK).