
A state of matter, often hydrogen and helium, that fills protoplanetary disks.Gas is one of the four fundamental states of matter, characterized by particles that are widely separated, move freely, and fill any container they occupy. In the context of planetary science and astrophysics, gas—primarily hydrogen and helium—is the raw material from which stars and planets form. Protoplanetary disks, the swirling clouds of gas and dust around young stars, are the birthplaces of planets. The behavior of gas in these disks, including its turbulence, pressure, and temperature, determines how dust grains clump together to form planetesimals and eventually planets. Gas also plays a critical role in the atmospheres of planets, influencing their climate and habitability.
The concept of gas as a distinct state of matter was formalized in the 17th century by scientists like Robert Boyle and later developed by Daniel Bernoulli, who proposed the kinetic theory of gases in 1738. The role of gas in planet formation became a major focus of astrophysics in the late 20th century with the discovery of exoplanets.
In a gas, particles are in constant random motion, colliding with each other and with the walls of any container. The pressure of a gas is the result of these collisions, and its temperature is a measure of the average kinetic energy of the particles. In protoplanetary disks, gas orbits the central star, but pressure gradients and turbulence cause it to move slightly slower or faster than the Keplerian orbital speed. This drags dust particles, causing them to spiral inward and collide, eventually growing into larger bodies. The gas also provides the material for giant planets like Jupiter and Saturn, which accrete massive envelopes of hydrogen and helium.
Understanding gas behavior is essential for modeling planetary atmospheres, predicting weather, and designing industrial processes like gas turbines and chemical reactors.
In the early universe, the only elements were hydrogen and helium gas; all heavier elements were forged later in stars and supernovae.




















