Accretion (astrophysics)

In astrophysics, accretion is the process where a massive object grows by gravitationally attracting surrounding matter. This matter is typically in the form of gas or dust. As gravity pulls these particles inward, they often form a rotating structure called an accretion disk. This fundamental process is responsible for the formation of nearly all major astronomical objects, including galaxies, stars, and planets.

Scientists have spent decades developing models to explain how this accumulation happens. In 1944, Otto Schmidt proposed that Earth formed from meteoric material. Later, William McCrea introduced the protoplanet theory in 1960, and Michael Woolfson proposed a capture theory. In 1969, Viktor Safronov provided a quantitative look at how terrestrial planets form through specific stages. Modern science now uses intensive numerical simulations to study how these small pieces, called planetesimals, eventually build up into large worlds.

Star formation begins within giant molecular clouds made of cold hydrogen. These clouds can be massive, spanning roughly 10 to 100 light-years in diameter. Over millions of years, parts of these clouds fragment and collapse under their own gravity. These fragments create dense cores known as protostellar nebulae. As the cloud collapses, it loses potential energy and heats up, gaining kinetic energy. Because of the conservation of angular momentum, the collapsing cloud flattens into an accretion disk.

The birth of a star follows a specific sequence of stages. Initially, the central part of the nebula forms a hot, non-contracting core called a hydrostatic core. This core acts as the seed for the future star. As the surrounding envelope of gas falls inward, it accelerates, further shaping the disk. Eventually, the envelope becomes thin enough for a young stellar object (YSO) to be seen in infrared or visible light. At this point, the protostar begins to fuse deuterium.

Depending on the mass of the protostar, different outcomes occur. If the object has a mass above 0.08 solar masses, it will begin hydrogen fusion. If the mass is lower, it becomes a brown dwarf. During the Class I protostar stage, the central object has already gathered most of its mass. The remaining disk and envelope usually account for only 10% to 20% of the total mass. Later, the object becomes a classical T Tauri star, which features a prominent accretion disk and strong emission lines in its spectrum.

Planet formation is a complex journey from dust to worlds. It starts when microscopic dust grains collide and stick together through electromagnetic or van der Waals forces. These grains grow into larger aggregates, but they face the "meter size barrier." This is a problem where growing particles move too fast and collide destructively rather than sticking. To overcome this, particles may concentrate into pebbles that gravitationally collapse, or they may use a feedback mechanism called streaming instability to form massive filaments.

Once particles reach the size of mountains, they are called planetesimals. Over 0.1 to 1 million years, these planetesimals collide to form Moon-sized planetary embryos. Finally, these embryos collide over 10 to 100 million years to create full planets. There is also a process called pebble accretion, where gas drag slows down small pebbles, causing them to spiral into a growing planet. This can be 1,000 times faster than traditional planetesimal accretion.

Different environments create different types of planets. Terrestrial planets are composed of rock and metal that condensed in the warmer inner regions of a solar system. In contrast, Jovian or giant planets form further out, past the frost line. These planets begin as large, icy planetesimals that capture massive amounts of hydrogen and helium gas from the surrounding nebula. This distinction in composition is a direct result of where the accretion took place relative to the temperature of the disk.