Star & Planet Formation

Artist’s concept of a planetary system forming around the star Fomalhaut, the brightest star in the constellation Pisces Austrinus. (Credit: David A. Hardy)

Throughout our Milky Way Galaxy and many other galaxies, new generations of stars are continually being born and old generations are dying. Studying the earliest dust-enshrouded phases of star and planet formation are the exclusive domain of radio astronomy. The Atacama Large Millimeter/submillimeter Array (ALMA) and Expanded Very Large Array (EVLA) are poised to make breakthroughs in our understanding regarding how stars and planets form, and ALMA will directly image planets around other stars.

Among the countless stars visible in the night sky, how many are surrounded by solar systems like our own? Scientists believe the number of other worlds may depend on the mechanism by which stars and planets are born.

A consensus theory that emerged over the last half of the 20th century is known as the standard model of planet formation. It suggests that planets may be common throughout our Galaxy, the Milky Way.

According to this theory, a planetary system begins as a disk of gas and dust rotating in one direction around a star. The protoplanetary disk is itself a by-product of the formation of the parent star from a dense interstellar cloud.

Birth of an Unusual Planetary System

Gravity causes the cloud to contract, and slight variations in the motion of different parts of the cloud build up into a net rotation. Since the cloud is spinning, the physical principle known as "conservation of angular momentum" causes it to flatten into a disk. Material in the core of the disk starts forming a new star.

The remaining disk is full of tiny dust grains of graphite and silicates, with outer coatings of water ice or frozen carbon dioxide (dry ice). These grains rain toward the plane of the disk, pulled inward by its gravitational force.

This process causes the grains to become concentrated and they begin to stick together, gathering mass and growing in size over several thousands of years. Eventually, the grains form pebble and rock-size objects orbiting in the same direction and plane.

These objects interact gravitationally, resulting in an uneven, clumpy distribution. Most of them accumulate into particular orbits, with nearly empty gaps between. At this point, there are trillions of orbits and gaps, most of them circular and some enlongated.

Within each crowded orbit, the matter collides to produce planetesimals, objects up to a mile or so in diameter. These planetesimals in turn collide and accrete new material, eventually building up into planet-sized objects. The type of planet that forms depends on temperature conditions and the type and quantity of matter available.

Birth of a Planetary Gap

In the case of our solar system, there was just enough material left over at the right temperature to form rocky planets like the Earth, Mars and Venus within 155 million miles of the Sun. Farther out, where temperatures were cooler and more material was available, gas giants formed.

Since 1995, scientists have discovered more than 150 new planets and planetary systems, though most do not resemble our solar system. Instead, scientists have been surprised to find gas giants orbiting very close to their host stars. Theorists speculate that some of these large planets may have migrated inward because of gravitational interactions with the protoplanetary disk.