A → circumstellar disk of gas and dust surrounding a
→ pre-main sequence star from which planetary systems form.
Protoplanetary disks are remnants of → accretion disks
which bring forth stars. Typically, their sizes are ~100-500 AU, masses ~10-2
solar masses, lifetimes ~106-107 years, and accretion rates
~10-7-10-8 solar masses per year.
According to the standard theory of planet
formation, called core accretion, planets come into being by the growth of
→ dust grains
which stick together and produce ever larger bodies, known as
→ planetesimals.
The agglomeration of these planetesimals of 100 to 1000 km in size into rocky
Earth-mass planets is the main outcome of this theory.
Beyond the → snow line
in the disk, if the masses of these cores of rock
and ice grow higher than 10 times that of Earth in less than a few
million years, gas can rapidly accrete and give rise to giant gaseous
planets similar to → Jupiter. If core building goes on
too slowly, the disk gas dissipates before the formation of
→ giant planets can start.
Finally the left-over planetesimals that could not
agglomerate into rocky planets or core of giant planets
remain as a → debris disk around the central
object that has become a → main sequence star.
An alternative to core accretion theory is formation of
planets in a massive protoplanetary disk by
→ gravitational instabilities.
The validity of these two theories is presently debated.
See also → protoplanet.
See also: → protoplanet; → disk.
