Updated: Jan 17, 2022
Jupiter is bigger than some of the stars in the universe. The smallest star yet discovered is EBLM J0555-57Ab. In our solar system, there are two objects larger than this star. One is obviously the Sun. The other is Jupiter, with a mean radius of 69,911 kilometers. Also the gas giant, is mainly composed of hydrogen and helium. Then why is Jupiter a planet and not a star?
First let us understand what makes a star a star:
A star is a huge celestial body made up of hydrogen and helium that emits light and heat from churning nuclear forges inside its core. In order for this fusion to occur, the star should be of a certain mass. Mass of a star is pivotal in dictating the course of its life.
Nuclear fusion in Sun
Jupiter does not have enough mass to cause nuclear fusion. Hence, it is not a star. EMBLM J005-57Ab has mass 85 times that of Jupiter. This is as light a star can be.
Even though Jupiter isn't a star it is still sizeable. Its mass is 2.5 times that of all the planets combined. As it is a gas giant, it has really low density: around 1.33 grams per cubic centimeter.
Jupiter and Sun are more alike than you think!
The Sun's density is 1.41 grams per cubic centimeter and Jupiter has a density of 1.33 grams per cubic centimeter. They are quite similar in their composition as well. By mass, Sun is about 71% hydrogen and 27% helium whereas Jupiter is about 73% hydrogen and 24% helium.
If Jupiter is similar to Sun then why is it called a failed star?
Jupiter may be similar to Sun in certain ways but it is only about 0.1% the mass of Sun. It has hydrogen and helium but it is not massive enough to have the internal pressure and temperature necessary to cause hydrogen to fuse to helium- the energy source that powers the sun and most of the stars. Hence Jupiter is called a failed star.