The massive cloud of gas and dust that formed our solar system coalesced over hundreds of millions of years. The majority of matter was attracted towards the center thanks to gravity. This matter (mostly hydrogen and helium) gathered and eventually formed the sun. All the excess matter (less than 1%) formed the planets, asteroids, and comets. While they formed in the same manner, stars and planets have one central distinction.
The essential difference between stars and planets is size. Stars are essentially extremely massive gaseous planets. The sun, for example, contains over 99% of all the mass in our solar system! This means its immense mass creates extraordinary gravitational effects and pressure. The core’s tremendous pressure smashes hydrogen molecules into helium molecules. This process is called nuclear fusion and is responsible for the light and energy output of the sun. Planets do not contain enough mass to undergo nuclear fusion. Essentially, planets are puny stars. The extreme mass, pressure, and gravitational effects of stars means they have massive magnetic fields and create powerful bursts of radiation, known as solar wind.
An interesting mix between stars and planets are known as brown dwarfs. Brown dwarfs are extremely massive planets that are very near the mass required to undergo thermonuclear fusion. In other words, they are failed stars. Scientists have estimated that a brown dwarf is approximately 75-80% more massive than Jupiter. It’s also interesting to note their have been exoplanets observed orbiting around brown dwarfs, but because brown dwarfs do not fuse hydrogen into helium, they are still not classified as stars.
Stars and gaseous planets are composed mostly of hydrogen and helium. Just in our solar system all the Jovian planets fit this description (Jupiter, Saturn, Uranus, and Neptune). Think of the sun as the father of the solar system and the planets are its meek offspring.
Now, most stars and planets move in orbits (some may be lonely drifters between the interstellar medium after being flung into space) and all are round and spherical due to gravity. In a standard planetary system, planets revolve around stars, while stars revolve around their galactic centers. However, scientists have detected systems that contain multiple stars. In fact, many astronomers and cosmologists believe multiple star systems are the norm. This can have some pretty extreme effects on the orbits of the stars and planets in that system. Luckily, we live in a system where this is not the case and the Earth’s orbit is and will be consistent barring some random catastrophe or until the sun becomes a red giant.
The phases of stars is the other elementary distinction between stars and planets. Stars contain a finite amount of hydrogen. When a star is stable, it is on the main sequence of its lifetime. Our sun is a prime example. However, when its supply is depleted it will begin to fuse helium. This causes it to swell to an even greater size. This expansion of a star is called the red giant phase, ushering the end of the main sequence. After it runs dry on helium it will shed its outer layers creating a beautiful, but devastating planetary nebula. Depending on a stars mass it will end its life in different ways. If it not very massive (red dwarfs) a star will simply slowly (possibly trillions of years) fade into a white dwarf and its eventual extinction. A mid-sized star (like the sun) will shed its gas into a planetary nebula and form a white dwarf. Massive stars have two options. Either they will explode into a supernova and become a black hole or if they are not quite massive enough they will form neutron stars.
The main difference between stars and planets is size, pure and simple. When enough mass is attracted in a central region some pretty amazing chemical reactions can take place. I could not be writing about this and you could not read this if it were not for this amazing set of natural laws.
