WELLESLEY, Mass., Jan.

WELLESLEY, Mass., Jan.

In a new model of the early solar system, Jupiter is found to have been a youthful traveler, roaming as close to the Sun as where Mars is now and back out again before settling in its position as the fifth planet from our Sun.

Planetary scientists have long wondered why Mars is only about half the size and one-tenth the mass of Earth.

Asteroids with moons, which scientists call binary asteroids, are common in the solar system. A longstanding question has been how the majority of such moons are formed.