Solar Systems in the Making The formation of planets around stars is believed to start with the gravitational accretion of a disk of material. Current theory for the formation of our own solar system holds that a large gas cloud collapsed to form the Sun and that the planets accreted in time from a disk of leftover material swirling around the new star. Astronomers have identified many young stars, such as the two shown here, that are likely candidates for forming planetary systems. Each is surrounded by a disk of orbiting dust and gas, the material remaining from the birth of the star itself. The disk of Herbig-Haro 30 (above left) spans 40 billion miles and emits powerful gaseous jets. HK Tauri (above right) is actually a binary star system. The dark disk surrounding one member of the pair is 20 billion miles in diameter. Although these systems may be too young for disk material to begin accreting into planetary bodies, scientists have found evidence for a number of other extrasolar planets in recent years (page 140). So far, none appears to resemble the small rocky planet on which we live, but depicted at right is one scenario for how rocky Earth -like planets might form from the debris around a newborn star.

Dust in a protoplanetary disk accretes into rocks that collide and merge into ever-larger bodies (top). After about 100,000 years, some of these embryo planets may have up to 70 percent the mass of Earth (middle). As larger bodies pull smaller ones into elliptical orbits, violent impacts and mergers occur (bottom), venting volatile gases that may eventually form the new world's atmosphere.

ball. The ball itself is akin to the atom's shell of electrons . Although not quite rigid, the electron shell carves out a volume of space that other atoms may not enter.

In ordinary solid matter, atoms bind together in gridlike structures called lattices . The space between each atom is large compared to the sizes of the individual atoms. It's as though a network of interconnecting toothpicks holds the Ping-Pong balls in fixed positions relative to one another. The toothpicks represent the electrostatic repulsion among the clouds of electrons around the atoms. They resist compression by any force we could muster on Earth , even if we ground the brick to dust.

However, stars can turn that trick readily. We know that during its lengthy adulthood, a star like our Sun balances the tremendous inward crush of its own gravity with outward pressure released by its fires of thermonuclear fusion. This equilibrium will exist in our Sun for 5 to 7 billion years more. Observers in our future solar system will see the Sun's energy output decline temporarily as helium ash builds up in the interior. Then the full weight of the star's outer layers starts to bear down upon the core. For a time the Sun staves off implosion by fusing helium into carbon . This transition pumps out energy at a thousand times the previous rate, resulting in a spectacular swelling of the Sun's outer layers (like our balloon in the hot sunlight). Our home star expands to hundreds of times its current diameter, becoming a "red giant " that engulfs Mercury , Venus , and possibly Earth and Mars within its scorching atmosphere.

Sadly for the Sun, those helium fires die down in about 100 million years. Then, gravity wins in a dramatic fashion. The core collapses under the pressure, as surely as a steamroller flattens an egg. In our analogy the Ping-Pong ball atoms pour into the center of the star. The force of gravity shatters the toothpicks between the atoms until they push against one another in a dense globe--far more densely packed than any state of matter on Earth .

At that point the basic rules of quantum mechanics prevent a further collapse. Electrons in the star's core obey the Pauli exclusion principle : As the gravitational bear hug crams the electron shells closer together, the electrons orbit their atoms ever more excitedly to avoid falling into the same physical state. This creates a new kind of outward pressure. The shells of the Ping-Pong balls grow vastly more rigid than occurs in normal matter. Finally, when the star shrinks to an object the size of Earth , (continued)