barrier

The term barrier (or growth barrier) has been used in astrophysics for issues that would seem to block some astrophysical process that is presumed to occur. If the process is the formation of observed objects, such a barrier is a point that requires explanation and if no adequate explanation exists, is a topic suitable for research. The term is currently commonly used regarding planet formation (planet formation barriers), and has occasionally been used in other formation processes such as star formation and galaxy formation.

Planet formation is presumed to arise from the nebula/circumstellar disk surrounding a newly-formed star, the amalgamation of some of the disk's dust and gas, growing to the mass of a planet. For this to happen, at each intervening mass from that of dust grains to that of an entire planet, the constituent material must come together, stick together, not be broken apart by collisions, not erode away, not drift into the star or escape, and sufficient material must be available for the growth, i.e., sufficiently close. The apparent barriers to this process go by different names, with some consistency of terminology, sometimes cited according to the general size of the object, and sometimes with terms reflecting the mechanism that would seem to prevent further growth. Terms regarding the size need to be taken as merely indications of the rough order-of-magnitude:

• meter size barrier
• kilometer size barrier
• centimeter size barrier

This terminology-by-size has some ambiguity: meter size barrier has been used problems growing much larger than a meter, but also for problems regarding growing toward meter-size. Some terms indicating the mechanism:

• electrostatic barrier aka charge barrier - a tendency protoplanetary-disk dust grains to be electrically negative, which should hold them apart.
• bouncing barrier - material meeting tends to bounce back apart rather than stick together; their relative velocities are a factor.
• fragmentation barrier - objects impacting tend to break each other apart rather than merge.
• drift barrier aka radial-drift barrier - material drifts inward or outward. The clear issue is an inward drift into the star: due to the force of gas pressure, the disk's rotation can be less than Keplerian, and above some size, the force of this pressure on an object is no longer significant and this lower speed allows the host star's gravity to draw it closer.

The term meter size barrier is often used to mean the fragmentation barrier and drift barrier. These four barriers generally apply to theories of rocky planet formation, and equally apply to rocky cores of gas planets for theories of gas planet formation in such manner, and to some extent, formation of ice cores, e.g., beyond the snow line. However, theories of formation of gas planets purely through the aggregation of gas have a different set of challenges.