Long before worlds such as Earth and Mars got to be proper planets, they were just grains of dust rotating around a young protostar. Dust grains collided to form pebbles, and pebbles smashed together to form boulders. Larger and larger chunks emerged until there were planetesimals, planetary embryos, and finally, rocky planets. At least, that's how we think rocky planets form around stars.
But according to Alan Boss of Carnegie Institution for Science, there's a problem with this story: It can't explain why many of those small chunks don't fall inward toward the protostar (and get destroyed) before they even get the chance to become planets. He's developed a new model that could fill this gap in our understanding. It's described in the new issue of The Astrophysical Journal.
A lot of dust and gases hang out in a disk around forming stars. Falling into a star is especially a problem for pieces measuring one to ten meters in radius: They're the most susceptible to the “gas drag” that sends them spiraling toward their stars. However, previous research has shown that stars like our sun experience occasional “explosive bursts” that last for 100 years. When these weird, luminous bursts happen, the stars and their disks become gravitationally unstable. This helps the one-to-ten-meter pieces to move away from the stars – and not toward them.
There's another piece in this puzzle, which is where Boss' new research really comes into play. Astronomers have recently found that young stars are often surrounded by spiral arms that look a little like those of galaxies such as the Milky Way. When Boss modeled these arms, he found that their force could scatter the small chunks to far outer regions where they can safely merge into big chunks. Eventually, they reach the point where they no longer have to worry about being dragged into the star's cauldron at the center of the developing system.
The only problem for these would-be planetary bits is simply getting big enough for the spiral arms to affect them. Boss found that smaller particles, measuring between one and ten centimeters in radius, are far more likely to fall back into the stars, regardless of the spiral arms.
“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 says in a statement. In other words, these factors – the disruptions and the spiral arms – could be key for astronomers hoping to find out where to look for possible rocky, Earth-like planets.