Asteroid proves to be dwarf planet

Confirmed: Vesta, the second largest of the asteroids, is a runt planet. Observations by NASA’s Dawn spacecraft show that Vesta, formed within the first few million years of the solar system, has a surface which is in places as bright as snow and in others as dark as coal, and that it contains an iron core that may have even briefly generated a magnetic field – all very planetlike features. Both Vesta and Ceres, the largest asteroid and the Dawn’s next destination, are also heavy enough to show that gravity has made them round.

In the current parlance of astronomers, however, Vesta and Ceres, like Pluto, are dwarf planets, not planets. The definition of planet, set by the International Astronomical Union in 2006, not only takes into account size and appearance, but also requires that a planet be the gravitational bully in its orbit. While Vesta and Ceres are round, they have not vacuumed up or pushed away thousands of other rocks in similar orbits.

But that is in large part what makes Vesta intriguing. The asteroid belt between Mars and Jupiter seems to be the remnants of a planet that never came together, and a closer look at Vesta could reveal much about the planet-building process. Dust coalesced into larger chunks like Vesta, which is 330 miles wide, and these then combined to form the rocky planets: Mercury, Venus, Earth and Mars.

The scientists reported their Vesta findings in six articles in the journal Science.
One of the surprises is that two giant craters near the south pole of Vesta formed relatively recently: one of them a billion years ago, the other at least two billion years ago. On other solar system bodies like the moon, such giant impacts occurred very early in the history of the solar system, about four billion years ago. The mineralogical measurements also confirm that a class of meteorites that have been found on earth came from Vesta, very likely ejected by the impact that created the giant craters. Vesta’s pockmarked surface could also help tell why the pieces of the asteroid belt never became a planet.

After Vesta formed, the gravity of newly formed Jupiter stopped further growth. The traditional view is that Jupiter’s pull accelerated the Vesta-size asteroids so that they smashed themselves apart instead of gently coming together. Jupiter would have scattered away many of the original asteroids, but slowly.

An alternate idea, however, is that Jupiter, nudged by leftover gas and dust in the early solar system, migrated inward to where Mars orbits today, scattering away the asteroids, and later was pulled outward again by the gravity of Saturn, and in its wake pulled back some of the asteroids. Kelvin Walsh, a research scientist at the Southwest Research Institute in Boulder, Colorado, said computer simulations indicated that this could explain the current asteroid belt and why Mars is small compared with earth and Venus. He is not a member of the Dawn team.

The 2,000 craters on Vesta catalogued by the Dawn scientists could indicate that most of the other asteroids were scattered away early, and if Walsh is right, they might show that there were fewer impacts than would be predicted by the traditional picture. The Dawn spacecraft has been orbiting Vesta since last July. This August, it will propel itself away from Vesta and head toward Ceres, which is slightly farther out from the sun, but of a much different makeup.