The Birth of Stars and Planets

Stars are born when gravity pulls together a large, interstellar gas cloud.  These clouds are made almost entirely of hydrogen and helium--roughly three quarters hydrogen and one-quarter helium, by mass.  only about 2% of the cloud material consists of heavier elements (this fraction was smaller in the past, before stellar recycling made these heavier elements).  Because new stars are made from this interstellar material, they are born with the same chemical composition of roughly 98% hydrogen and helium and 2% other elements.

 

A typical star-forming cloud, such as the Orion Nebula (see picture0, gives birth to several thousand stars.  however, the stars are not all born at once; the individual stars may be born over a time span of many millions of years, depending on how rapidly gravity can make small pieces of the cloud collapse into stars. Stars for form not as isolated spheres but rather as the centeral objcets within broad, spinning disks of gas.  Planets can be born within these disks as part of the star formation process.  The disks form because of three critical processes that occur when gravity collapses a cloud of gas.

 As the cloud shrinks in size, it gradually spins faster and faster.  This spin-up occurs for the same reason that ice skaters spin faster when they pull in their arms ( a phenomenon known in physics as "conservation of angular momentum").  The Original cloud is so large and diffuse that its rotation may be imperceptible, but as it shrinks in size the rotation becomes noticeable, just as is the case with an ice skater.  As the rate of spin increases, the cloud fattens into a disk.  The flattening occurs because any gas particles that are not moving in the plane of rotation tend to collide with each other and with particles in the disk, which gradually forces all the particles into the same plane.  Near the cloud center, the temperature rises as the cloud density and pressure increases. (This occurs because as the cloud shrinks its gravitational potential energy is converted into thermal energy.)  When the temperature grows hot enough, nuclear reactions can begin and the central object ignites as a star.

Because all three processes must occur in any collapsing cloud, scientists believe that virtually all stars are surrounded by spinning disks as they are born.  Observations confirm that many young stars are surrounded by such disks.  There are many possible ways in which the disks can later be destroyed.  For example, in many star systems the central object rotates so fast that it splits into two stars (making a binary star system), and the competing gravitational tugs from the two stars may disrupt any disk.  Nevertheless, the existence of our own solar system (and other known planetary systems) proves that planets do form within these disks in at least some cases.

The very first generation of stars in the universe must have been made entirely of hydrogen and helium, because other chemical elements did not yet exist.  These first-generation stars must have lived and died before the birth of the old stars in the galactic halo, producing the relatively small amount of heavier elements that we find in halo stars.  As time passed, many generations of massive, short-lived stars added heavier elements to the galaxy.  By the time our solar system formed, about 4.6 billion years ago, interstellar gas in the Milky Way Galaxy already contained close to its present value of 2% heavier elements.  Although 2% may not sound like much, it is more than enough to trigger the process of planet formation and to make small, rocky planets like Earth.  Indeed, it's probably possible for planetary systems to form even around stars with much smaller abundances of heavy elements, such as around the stars in the halo, though we don't yet know for sure.

According to present theory, the process of planet formation begins as tiny solid particles condense from the gas in the spinning disk around a forming star.  (In cooler regions of the disk, some of the solid particles may actually be preexisting grains of dust that are commonly found in cool interstellar clouds.)  These particles condense for much the same reason that raindrops or snowflakes condense in clouds.  When the temperature is low enough, some atoms or molecules in the gas will bond together.  When the solid particles collide gently, they stick together, thus growing larger.  Eventually, they can become large enough so that their won gravity begins to attract more matter, enabling them to become larger still.  if nothing interrupts this process, gravity will eventually make full-fledged planets from what started as tiny solid "seeds." Because pure hydrogen and helium don not solidify, most of the material in the spinning disk always remains gaseous.  Only a small fraction of the matrial in the disk can condense to make the solid seeds.

The inner planets of our solar system (Mercury, Venus, Earth, and Mars) are so different in character from the giant outer planets (Jupiter, Saturn, Uranus, and Neptune).  In the inner regions of a spinning disk, near the central star, it is too hot for ices to condense.  But metal and rock can solidify at fairly high temperatures, so in these regions bits of solid metal and rock condense from the warm gas.  Because the heavy elements that make metals and rocks are so rare, there's not enough solid material in these inner regions to make planets much larger than Earth.  Moreover, these small planets lack the gravitational Earth.  Moreover, these small planets lack the gravitational strength needed to hold on to the abundant gas around them.  These inner planets therefore end up being made almost entirely of metal and rock. They are called terrestrial planets "Earth-like."

Bits of metal and rock also condense farther from the central star, but here the temperatures are cold enough for ices to condense as well.  Because the ices are made from more abundant elements than are rocks and metals, ices actually make up most of the solid material in these regions of the spinning disk.  Thus, the growing chunks of solid material in these regions of the spinning disk.  Thus, the growing chunks of solid material in an outer solar system are made mostly of ice, mixed with smaller amounts of metal and rock.  These iceballs can grow fairly large--perhaps 10 times the mass of the Earth or more.  At that point, their gravity begins to attract the surrounding hydrogen and helium gas, which makes them grow even bigger.  By the time the process is complete, these outer plantets have become giants made mostly of hydrogen and helium.  They are called Jovian planets "Jupiter-like."  The gas drawn into a jovian planet tends to form its own spinning disk, rather like a miniature version of the spinning disk around the star.  The same general processes then occur within these planetary disks, leading to the formation of moons.  This is one reason why the jovian planets tend to have many moons.