INVESTIGATING THE HISTORY OF EARTH containing zirconium that they dated to 4 4 billion years ago Moreover meteorites which likely formed at about the same time as Earth have been dated to about 4 5 billion years ago These findings help pin down a likely age of about 4 55 billion years for Earth itself THE FoRMATION OF EARTH It is believed that Earth formed along with the rest of the solar system from a cloud of gas and dust called a nebula Nebulae are seen today in other parts of the Milky Way and also in other galaxies Astronomers have found evidence of stars and planetary sys tems forming in many of these nebulae as the mutual gravitational attraction of the nebula's particles pulls them together According to this theory called the nebu lar hypothesis the cloud consisted mainly of hydrogen and helium most of which ended up in the Sun The outer planets Jupiter Saturn Uranus and Neptune are quite different from Earth They were far enough from the Sun and therefore cold enough that these gases along with other compounds such as methane ammonia and water were able to gravitationally collect around small rocky or metallic cores I2
EARTH's ORIGINS Disks of gas and dust around young stars in the Orion nebula about 1 500 light years away may be new solar systems in the making Four images taken by the Hubble Space Telescope show disks surrounding four different stars that are only about a million years old The planets of our solar system are thought to have arisen from such a protoplan etary disk about 4 6 billion years ago The central glow in each image is the star The disks range in size from about two to eight times the diameter of our solar system NASA 3
INVESTIGATING THE HISTORY OF EARTH The inner planets Mercury Venus Earth and Mars were warmer and also exposed to a stronger solar wind These effects drove most of the light gases out into space leaving these planets as solid bodies Venus and Earth and to a lesser extent Mars retained atmospheres but only as fairly thin envelopes surrounding the rock and metal The planets formed by accretion a col lecting together of smaller objects mainly because of gravity The late stages of accre tion took the form of impacts of meteoroids and comets Some impacts must have been tremendously energetic likely melting much or all of Earth s early crust The most widely accepted theory of the Moon's formation suggests that a protoplanet at least half of Earth s diameter struck blasting mate rial from both it and Earth into space Some of this material then collected together to form the Moon This event probably hap pened sometime during Earth's first 100 million years FIRST BILLION YEARS The earliest period of Earth's history has been called the Hadean after Hades the underworld abode of the dead in ancient 14
EARTH's ORIGINS FORMATION OF THIE SoLAR SYSTEM Astronomers believe that the solar system formed as a by product of the formation of the Sun itself some 4 6 billion years ago According to the prevailing theory the Sun and its many satellites condensed out of the solar nebula a huge interstellar cloud of gas and dust The solar system began forming when the gravity of this interstellar cloud caused the cloud to start con tracting and slowly spinning This could have been caused by random fluctuations in the den sity of the cloud or by an external disturbance such as the shock wave from an exploding star As the interstellar cloud squeezed inward more and more matter became packed into the center which became the protosun the mate rial that later developed into the Sun The contraction caused the cloud to spin faster and faster and to flatten into a disk Eventually the center of the cloud collapsed so much that it became dense enough and hot enough for nuclear reactions to begin The Sun was born Meanwhile away from the center the gas and dust in the spinning disk cooled Solid grains of silicates and other minerals the basis of rocks condensed out of the gaseous mate rial in the disk Farther from the center where temperatures were lower ices of water meth ane ammonia and other gases began to form 15
