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New thinking on asteroid belt
Scientists have come up with an alternative explanation for planetary formation which may shed light on how the solar system's asteroid belt was formed.

Writing on the pre-press website arXiv.org Sergei Nayakshin from the University of Leicester claims his new hypothesis neatly shows why the asteroid belt exists between the rocky inner planets and the giant gas planets of the outer solar system.

Nayakshin's giant planet embryo theory involves all the planets forming through the collapse of gas from protoplanetary clouds in what is now the outer solar system.

He says planetary formation begins at distances in excess of 50 AU (Astronomical Units) from the host star.

An astronomical unit is the average distance between the Earth and the Sun which is about 150 million kilometres.

As these clouds grow more dense, they gain more mass and gravity, with any rocky material aggregating to the centre to form the planet's core.

Nayakshin believes these embryonic planets interact with the host star's gas cloud causing them to spiral towards the inner solar system.

As they migrate closer, the host star's irradiation and gravitational tidal forces destabilise the planet's gaseous atmosphere.
The critical radius

Once they get closer than a critical radius, they lose their gas envelopes together with any solids still mixed up in their outer atmospheres, leaving behind terrestrial rocky 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 conventional theory which suggests rock and dust clump together to form terrestrial planets which then attract the gases that form their atmospheres.

Under this standard theory of planetary formation the gas giants form when these rocky cores grow to at least ten times the size of Earth and attract huge gaseous envelopes.

But Nayakshin says conventional theory doesn't explain how metre-sized lumps of rock end up sticking together after smashing into each other at random.

And he says there's also a problem with planetary rotation, claiming planets forming from the random aggregation of rock and dust should have random rotation rather than the almost uniform directional rotation we see.

Nayakshin says his theory allows planetary cores to all rotate in the same direction as the original gas cloud because they from through the gravitational collapse of the cloud rather than by random collisions.

And he claims it explains how the asteroid belt formed and why it separates the outer solar system gas giants from the terrestrial planets of the inner solar system.
Standard model already explains Asteroid belt

Dr. Daniel Price from Monash University says it's an interesting hypothesis but that it 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 being the ingredients of a planet that failed to form because of the gravitational influence of Jupiter.

"There is a gap in our understanding of how planets grow from about metre size to kilometre size," he says.

Price says Nayakshin is playing on that question mark.

"His theory doesn't change our understanding of how the gas giants form, 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 says would have been effected by the star, so his theory really only applies to two of the solar system's planets."

Price says the test of any hypothesis is whether it can explain something better than the existing theory and it's not clear that he's done that.

"It's plausible but not terribly convincing."

Price says we now know of over 500 planets orbiting other stars and any theory of planetary formation must take them into account as well.

"It's an anomaly that he hasn't attempted to explain planets going around other stars".

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