E28 # Life and Death of a Planetary System

The Basics

Life and Death of a Planetary System

How did we get here? How do stars and planets come into being? What happens during a star's life, and what fate will its planets meet when it dies? Come along on this interstellar journey through time and scientific detective work.

A Star Is Born

It all begins with an unimaginably cold cloud. This cloud contains the seeds of whole new worlds – stars and planets about to be born.

Molecules of hydrogen and helium gas, which normally zip around at high speeds, slow down and clump together because of gravity. Tiny grains of silicates, iron and carbon-rich material — together classified simply as "dust" — send some of the gas’s energy back out into space, making the cloud even colder. The dust grains spiral into the central knot of matter, like water running down a drain.

​From Cloud to Disk

The newborn star is a feisty baby, shooting out violent jets of magnetically accelerated material as it gets nourishment from the gas and dust whirling around it. Like a blob of pizza dough flattening out as a chef spins it, this material condenses into a flat disk. That "dough" has a preferred direction inherited from the collapse of the cloud. That same spin will remain with the system for its entire life, unless another star system gets close enough to interact with it.

Collisions, Collisions

A very young disk around a star contains mostly gas with dust -- no bigger than grains of sand -- swirling around in it. The baby star is still throwing out extremely hot winds, dominated by positively charged particles called protons and neutral helium atoms. A lot of the material from the disk is still falling on the star. But small groups of lucky dust particles are crashing into one another, clumping into larger objects. Planets will form from less than 1 percent of the mass of the disk.

​Moving Around

Based on just an image of baby planets in a protoplanetary disk, it is impossible to determine what the system will look like as it matures.

Settling Down

At approximately 100 million to 1 billion years old, planets tend to settle down in their orbits and stars don’t flare up as much. Our own solar system, about 4.5 billion years old, is the model for this idea of planetary "middle age." Mandell thinks of our planetary system as about 45 to 50 years old, when scaled down to a human lifetime.

Aging Into Gianthood

When our Sun approaches its red giant phase some 6 billion years from now, it will run out of fuel in its core. As hydrogen fusion slows, the core once again begins to contract. As the core gets smaller, it heats up until can kick off another round of nuclear reactions, fusing helium into heavier elements such as carbon, nitrogen and oxygen. The hotter core also makes hydrogen fuse in the “shell” of material surrounding the core. Meanwhile, extra heat produced deep within in the star causes its outer layer of gas to puff up.

Death and New Life

When the core of the former red giant has exhausted all of its fuel and shed all the gas it can, the remaining dense stellar cinder is called a white dwarf. The white dwarf is considered “dead” because atoms inside of it no longer fuse to give the star energy. But it still “shines” because it is so hot. Eventually, it will cool off and fade from view. Our Sun will reach this death about 8 billion years from now.

Credit: NASA