New thinking on the asteroid belt › News in Science (ABC Science)
Cosmic debris Scientists have offered an alternative explanation for planetary formation that could shed light on how the solar system’s asteroid belt formed.
Writing on the prepress site arXiv.org Sergei Nayakchin of University of Leicester claims that his new hypothesis perfectly shows why the asteroid belt exists between the rocky inner planets and the gas giant planets of the outer solar system.
Nayakshin’s giant planet embryo theory involves all planets formed by the collapse of protoplanetary cloud gas in what is now the outer solar system.
He says planetary formation begins at distances greater than 50 AU (astronomical units) from the host star.
An astronomical unit is the average distance between the Earth and the Sun which is approximately 150 million kilometers.
As these clouds become denser, they gain mass and gravity, with any rocky material clumping together in the center to form the planet’s core.
Nayakshin believes these embryonic planets interact with the host star’s gas cloud, causing them to spiral toward the inner solar system.
As they approach, radiation from the host star and gravitational tidal forces destabilize the planet’s gaseous atmosphere.
The critical radius
Once they approach a critical radius, they shed their gas envelopes along with any solids still mixed in their outer atmospheres, leaving behind rocky terrestrial planets like Mercury, Venus, Earth, and Mars.
Nayakshin says this critical radius corresponds to the asteroid belt. In other words, these asteroids are construction debris left over from the formation of the inner planets.
This hypothesis is the opposite of the conventional theory which suggests that rock and dust clump together to form terrestrial planets which then attract the gases that form their atmospheres.
According to this standard theory of planetary formation, gas giants form when these rocky cores grow at least ten times the size of Earth and attract huge gaseous envelopes.
But Nayakshin says the conventional theory doesn’t explain how meter-sized chunks of rock end up sticking together after crashing together at random.
And he says there’s also a problem with planetary rotation, saying that planets formed from the random aggregation of rocks and dust should have random rotation rather than the nearly uniform directional rotation we see.
Nayakshin says his theory allows the planetary cores to all spin in the same direction as the original gas cloud because they come from the cloud’s gravitational collapse rather than random collisions.
And he claims that this explains how the asteroid belt formed and why it separates the gas giants of the outer solar system from the ley planets of the inner solar system.
The Standard Model already explains the asteroid belt
Dr. Daniel Price from Monash University says it’s an interesting hypothesis but hasn’t been developed enough to throw the standard model of planetary formation out the window.
The Standard Model already explains the asteroid belt as the ingredients of a planet that could not form due to the gravitational influence of Jupiter.
“There’s a gap in our understanding of how planets grow from about a meter in size to a kilometer in size,” he says.
Price says Nayakshin is playing on that question mark.
“His theory doesn’t change our understanding of the formation of gas giants, so it really only applies to the four inner rocky planets.
“Then he makes a special case for two of them, Mercury and Venus which he thinks would have been affected by the star, so his theory only really applies to two of the planets in the solar system.”
Price says the test of any hypothesis is whether it can explain something better than the existing theory and it’s not clear he did.
“It’s plausible but not terribly convincing.”
Price says we now know of more than 500 planets orbiting other stars, and any theory of planetary formation must take this into account as well.
“It is an anomaly that he did not attempt to explain planets revolving around other stars.”