A Recipe for Constructing Solar Systems
- Within galaxies stars form from vast clouds of interstellar gas and dust which
contract due to their own gravity.
- If a cloud has even the slightest rotation it will
tend to form a flat rotating disk as it contracts under the influence of gravity.
- Most of the material works its way to the
center of the disk and forms a star.
- A small fraction of the material remains in the thin flat disk around the newly forming star.
- Planets form from the small amount of material left behind
in the disk. Surprisingly, the initial growth occurs among the material that is the rarest in the cosmos - all of the elements heavier than hydrogen and helium in the periodic table, the "other" 1%.
- The disk is made of a typical mix of interstellar
material -- mostly hydrogen and helium and a tiny amount
of the other elements.
- The hydrogen and helium is in gaseous form.
- Ice and dust grains -- solid particles -- contain
most of the heavy elements in the disk.
- The solid particles (ices and rock)
can stick together and grow to larger size. The gas, at least initially, only goes along for the
- At first the solid particles are microscopic and grow
by sticking together when they happen to run into one another.
- As the clumps grow larger (meters to kilometers in size)
their own gravity becomes significant and they attract more and
- These planetesimals are the basic building blocksfor planets.
- The planetesimals have been
largely constructed from the solid particles in the disk and
thus are made mostly of the elements other than hydrogen and helium.
- Billions of these objects will continue to collide and
grow until all that is left is a handful of large planets (and some
leftover planetesimals -- asteroids and comets).
- This process of growth by accumulation, which builds the
planets, is called accretion.
- The accumulation of debris in the latest stages of planetary
growth leaves planetary surfaces heavily cratered.
- The last stages of accretion can be quite violent as large objects collide.
- The accretion of the last bits of material continues even until the present era.
- The Earth sweeps up many tons
of material every day (mostly in the form of tiny dust
- When you see a meteor streak across the sky you are
watching accretion happen!
- Close to the Sun only the rocky grains can survive
the heat. The ice grains are evaporated.
- How do the outer Gas Giant planets fit into the picture?
- They began like the other planets from the accumulation
of solid planetesimals dominated by ice.
- Since they grew in a region where ice was abundantly available...
- Their cores grew quickly and became sufficiently massive to attract and retain gas directly from the nebula via their strong gravity.
- The Jovian planets have slushy cores left over from this
In a nutshell...
- Gravitational collapse of a huge diffuse interstellar cloud will naturally
produce a thin flat disk of material if the cloud has even the slightest rotation.
- The cloud is composed of a typical Galactic mix of material -- 99% hydrogen and helium; 1% other elements.
- Planets accrete initially from solid microscopic particles in a flattened disk around a forming star.
- The solid particles are largely composed of the 1% of elements other than hydrogen and helium.
- The temperature at any point in the disk determines which solid particles form/survive
- Close to the star (hot) -- rock and metal (mostly rock)
- Far from the star (cold) -- ice, rock, and metal (mostly ice)
- Hydrogen and helium are in gaseous form regardless of the temperature, so although they are the most abundant materials, they don't play a major role
in the initial formation of the planets.
- The "giant" planets -- which today are made mostly of hydrogen and helium -- formed from icy/rocky "seeds" which eventually grew
large enough to attract the more abundant hydrogen and helium gravitationally.
Revised March 14, 2005