A U.S. researcher says he may have partially solved a long-standing mystery of how small, rocky worlds formed within in our Solar System, which could hold true for other systems, too.

It is commonly accepted that planets are formed when grains of dust in the disk of matter surrounding a developing protostar begin to attract each other and accrete in bodies that eventually become large enough to have their own gravity, which allows them to start attracting even more more material and accelerate their development into planets.

So, what's the problem? It's this: how do those dust grains and their very first small accumulations avoid getting sucked into the star before they can get big enough to fight the star's gravity with their own?

In the spinning disks of dust and gas that surround a star in its early stages of formation, dust grains will collide to form small pebbles, pebbles collide to create boulders and the process goes on up to the size of planetesimals — planet embryos, so to speak — and eventually, bona fide terrestrial, rocky planets.

So far, so good, but there are issues with that nice, clean description of planetary formation, says Alan Boss of the Carnegie Institution in Washington, D.C.

One of the biggest is the question of the pressure gradient in the gas that also makes up the star's surrounding disk, pressure that could create a "headwind" slowing the orbiting pebbles and boulders, causing them to spiral inward to be consumed by the young star.

Boulders between about three to 10 feet in diameter would have the most acute difficulties with this, since their size would make them the most susceptible to such gas-induced drag, Boss notes.

Also, if enough accretions of boulder size or smaller were swept into the star, it wouldn't leave enough material to form planetesimals and, finally, planets, he says.

A possible solution, Boss suggests, is tied to observations of young stars still in possession of their surrounding gas disks that have been seen experiencing periodic explosive episodes, perhaps lasting 100 years or so, episodes in which the star's brightness increases and it exhibits gravitational instability that affects that disk.

Boss says his study suggests such a phase can push the bodies of a size most at risk outward away from the young star, rather than inward toward it.

Spiral arms have been observed around young stars that are believed to be similar to ones believed to be involved in the short-term disruptions of the disk, and those arms could have gravitational forces sufficient to keep boulder-sized bodies away from the star long enough for them to form planetesimals large enough to resist the drag from gas in the disk.

"This work shows that boulder-sized particles could, indeed, be scattered around the disk by the formation of spiral arms and then avoid getting dragged into the protostar at the center of the developing system," Boss says. "Once these bodies are in the disk's outer regions, they are safe and able to grow into planetesimals."

However, the spiral arms and their gravity wouldn't be able to save particles smaller than around four inches across, which would probably be dragged back into the young star and lost, he says.

"While not every developing protostar may experience this kind of short-term gravitational disruption phase, it is looking increasingly likely that they may be much more important for the early phases of terrestrial planet formation than we thought," Boss suggests.