Our solar system may have had another planet, that was swallowed up by the Sun. But before it was destroyed, this planet caused a lot of problems.

Space scientists John Chambers and Jack Lissauer of NASA think that along with Mercury, Venus, Earth, and Mars there was another planet, just outside of the orbit of Mars and inside the asteroid belt. They base their theory on computer modeling.

It is commonly believed that during the formative years of our solar system, between 3.8 billion and 4 billion years ago, the Moon and Earth were frequently hit by space debris. However, scientists aren?t sure if the impacts slowed down 3.8 billion years ago or if there was a sudden increase in the impact rate around 3.9 billion years ago, with quiet periods before and afterwards.

This period of time is known as the ?lunar cataclysm,? and was also the period when the first evidence of life is believed to have appeared on Earth. Astronauts brought back lunar rocks from the moon and virtually all of them were nearly the same age, 3.9 billion years, and none were older, indicating that this was a period of high impact. But some scientists claim that these samples were biased because they came from a small area of the Moon, where there happened to be many impacts.

Why there would have been so many impacts 3.9 billion years ago may be explained by the ?Planet V? hypothesis. ?The extra planet formed on a low-eccentricity orbit that was long-lived, but unstable,? says Chambers. About 3.9 billion years ago, Planet V was destabilized by gravitational interactions with the other inner planets. It?s orbit became highly eccentric orbit and crossed the inner asteroid belt, which was much larger then than it is today. Planet V?s close encounters with the inner belt of asteroids stirred up a large number of them, sending them hurtling towards the other planets, including Earth and the Moon. Planet V would have eventually self-destructed by bumping into the Sun.

The temporary existence of more than 4 planet-sized bodies in the inner Solar System is the currently favored model for the formation of the Moon. Chambers and Lissauer believe our Moon is a leftover from a massive collision between Earth and a Mars-sized body 50 million to 100 million years after the formation of the Solar System.

Wendell Mendell, a planetary scientist at NASA, says, ?This idea and others within the last few years show that the Solar System is filled with all sorts of gravitational resonances…that a lot of potential orbits in the Solar System are chaotic and unstable. My sense is that this is a new idea. It?s another thing to throw into the pot that?s not totally crazy. By thinking that the Solar System was really quite different in a major way with an extra inner planet, we might be able to develop some sort of self-consistent scenario that explains a lot of things.?

Apollo 17 astronaut Harrison ?Jack? Schmitt believes that in order to answer these questions, we need to return to the Moon. ?You?re going to have to be very, very specific on what sites you go to collect new samples,? he says. ??Numbers of targeted missions could get a lot of great information on some of these fundamental questions that we still haven?t been able to answer.?

Schmitt wants to explore the side of the Moon those of us on Earth never get a chance to see and says, ?Then we can do the kind of thing that Apollo did for the near side of the Moon.?

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Vesta is one of the strangest asteroids ever studied. It has many features that resemble our own Moon, including ancient lava flows that indicate the rock once had a molten interior, leading scientists to believe it used to be larger. Maybe it was even a planet, back when the solar system was young and chaotic and everything could change in one catastrophic collision.

Vesta is about 334 miles in diameter and has probably remained largely unchanged for about 4 billion years. At least one space impact may have torn large parts of Vesta away, and exposed subsurface that scientists can see.

And at least one meteorite found on Earth is a chip off Vesta, helping researchers to understand the asteroid and its chemical makeup. Vesta shares many characteristics of the rocky, so-called terrestrial planets like Earth. Soon scientists will find out more about it, since in December 2001, NASA approved the Dawn mission to study Vesta and the nearby asteroid Ceres. The mission will make a nine-year journey to the rocks and will orbit above their surfaces.

Both asteroids are in the main asteroid belt, between Mars and Jupiter, but they are very different from each other. Ceres has a surface that contains water-bearing minerals. It may have a very weak atmosphere and frost. Vesta is thought to be dry, having been resurfaced by lava flows.

Much of what?s known about Vesta so far comes from a 1995 Hubble Space Telescope study. ?The Hubble observations show that Vesta is far more interesting than simply a chunk of rock in space as most asteroids are,? says Ben Zellner, a Georgia Southern University researcher who worked on the study. ?This qualifies Vesta as the ?sixth? terrestrial planet.?

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