A new theory suggests that Mars and Mercury could have formed out of the “byproducts” of Earth and Venus, according to Brad Hansen, an astronomer at the University of California, Los Angeles, who presented his research earlier this month at the annual meeting of the American Astronomical Society.
Current scientific consensus says that the Earth and other planets were formed around 4.5 billion years ago, from a gigantic disk of gas and dust that surrounded the Sun. The force of gravity caused the minuscule particles of dust to come together, eventually forming pebble-sized objects that in turn grew larger and larger to eventually create the planets.
The theory is not without its problems, however. Scientists normally assume the initial dust disk was the same thickness throughout, but crunching the numbers in a computer simulation shows that this would result in planets of similar size with circular orbits. In fact Mars and Mercury are much smaller than Venus and Earth, and orbit the Sun in an ellipse.
This discrepancy is normally explained away by the presence of Jupiter, the largest planet in the solar system, which exerts a heavy gravitational force. Hansen believes he has a different answer.
In his model, the dust disk is not uniform; rather, it is clumped at different distances from the Sun. He suggests that Earth and Venus formed in a particularly large clump, known as an “annulus”, capturing much of the proto-planet debris as they immerged from the dust.
Not all of the material is captured and some particles can be ejected into a different orbit. It is these ejected particles that would go on to form Mars and Mercury – Hansen estimates that only 10% of the initial material would make up these two planets, with the majority gobbled up by Earth and Venus.