The Orion Nebula. Image credit: NASA

By: Marcus Woo, Inside Science

(Inside Science) -- Interstellar space can be a dusty place, filled with tiny flecks no bigger than a bacterial cell.

But now astronomers have detected particles as big as pebbles, possibly a previously unknown type of dust that may kick-start the production of planets. The presence of these big particles may also suggest that star formation is more efficient than previously thought.

Astronomers know that large particles gather in the dense disks of gas and dust that surround newborn stars. Those particles, which themselves are the product of tinier specks that have stuck together, can clump up and accumulate to form asteroids and planets.

But the new discovery is the first time such large dust grains have been found outside planet-forming disks, said Bruce Draine of Princeton University, in New Jersey, who was not involved in the findings.

"That was not expected," he said. "The fact that they're seeing what appears to be an abundance of millimeter-sized grains does come as a surprise."

Dust grains absorb ambient starlight and heat up, causing them to glow in infrared light. To detect these big grains, a team of astronomers led by Scott Schnee of the National Radio Astronomy Observatory, in Charlottesville, Virginia, looked at clouds of gas and dust in the constellation Orion, measuring infrared light at wavelengths of 1.2 and 3.3 millimeters.

Dust grains can't efficiently emit light at wavelengths larger than their sizes, so bigger grains glow better at longer wavelengths. By comparing the brightness of the dust at the two wavelengths, the researchers determined that the grains were a few millimeters in size, with some possibly as large as a centimeter.

"We really expected them to be much smaller," Schnee said. "It's nice when that kind of thing happens, when you think you know what you're going to get and you get something totally different."

If these big grains are common throughout the galaxy, then that could carry implications for planet formation, he said. The particles in the dusty disks that form planets start out only about a micron in size, about as big as E. coli bacteria. It takes time for these specks to bunch together and build planets. But if pebble-sized grains are already floating around, then planets may be able to form faster.

The discovery, accepted for publication in the Monthly Notices of the Royal Astronomical Society, might also be important for star formation. Where there's dust, there's gas, Schnee explained. And the gas contains most of a cloud's mass. The researchers found that in the Orion clouds, most of the dust is in the form of big grains, which means less is in small grains. With fewer small grains, there are fewer grains overall, which in turn implies there's less gas and thus less mass.

In fact, the team calculated that there's two-thirds less mass in the Orion clouds than in previous estimates. Meanwhile, the clouds are still churning out the same number of stars, suggesting that clouds don't need to be as massive as once thought to efficiently form stars.

Although the Orion clouds are much denser than other parts of space, they're still not dense enough for small flecks of dust to easily cluster and grow a thousand-fold into the observed particles, Draine said. So how do such big grains form?

One possibility is that these are leftover grains from other planet-forming disks. Not all the big grains that form in a disk end up in planets, and so they might get dispersed into interstellar space. Another idea, Schnee said, is that small dust particles are simply better at clumping together than thought.

These big grains could be the answer to another cosmic mystery, Draine said.

When meteors careen through Earth's atmosphere, they strip the electrons off the atoms in their paths, which scientists can measure using radar. In doing so, they've discovered tiny meteors that bombard Earth at speeds so high that they must've come from outside the solar system.

"When I first heard about it, I thought it was crazy," Draine said.

These particles are much bigger than the micron-sized dust particles thought to populate interstellar space. Where could such big particles come from?

The Ulysses spacecraft, which studied the sun and completed its mission in 2009, also detected curiously large particles coming from interstellar space. It turns out, Draine said, that both these particles and the radar meteors are about the same size as the ones found in Orion. Whether they all are the same remains to be seen.

For now, Draine said the big grains in Orion must be confirmed with further measurements. In fact, Schnee and his colleagues are already analyzing additional data on the clouds to verify their observations. The next step, Schnee said, is to search for big grains elsewhere in the galaxy. 

Marcus Woo is a freelance science writer based in the San Francisco Bay Area who has written for National Geographic News, New Scientist, and other outlets. He tweets at @sucramoow.

Reprinted with permission from Inside Science, an editorially independent news product of the American Institute of Physics, a nonprofit organization dedicated to advancing, promoting and serving the physical sciences.