Could Jupiter Become a Star?

Why Jupiter Isn't a Failed Star

Two reasons Jupiter can't become a star are that it has insufficient mass and it formed like a planet and not like a protostar.
Two reasons Jupiter can't become a star are that it has insufficient mass and it formed like a planet and not like a protostar. Antonio M. Rosario / Getty Images

Jupiter is the most massive planet in the solar system, yet it isn't a star. Does that mean it's a failed star? Could it ever become a star? Scientists have pondered these questions​ but didn't have enough information to draw definitive conclusions until NASA's Galileo spacecraft studied the planet, starting in 1995.

Why We Can't Ignite Jupiter

The Galileo spacecraft studied Jupiter for eight years and eventually began to wear out. Scientists were concerned contact with the craft would be lost, ultimately leading Galileo to orbit Jupiter until it either crashed into the planet or one of its moons. To avoid possible contamination of a potentially living moon from bacteria on Galileo, NASA intentionally crashed Galileo into Jupiter.

Some people worried the plutonium thermal reactor that powered the spacecraft could start a chain reaction, igniting Jupiter and turning it into a star. The reasoning was that since plutonium is used to detonate hydrogen bombs and the Jovian atmosphere is rich in the element, the two together could create an explosive mixture, ultimately starting the fusion reaction that occurs in stars.

The crash of Galileo didn't burn Jupiter's hydrogen, nor could any explosion. The reason is that Jupiter doesn't have oxygen or water (which consists of hydrogen and oxygen) to support combustion.

Why Jupiter Can't Become a Star

Yet, Jupiter is very massive! People who call Jupiter a failed star are usually referring to the fact that Jupiter is rich in hydrogen and helium, like stars, but not massive enough to produce the internal temperatures and pressures that start a fusion reaction.

In comparison to the Sun, Jupiter is a lightweight, containing only about 0.1% of the solar mass. Yet, there are stars much less massive than the Sun. It only takes about 7.5% of the solar mass to make a red dwarf. The smallest known red dwarf is about 80 times more massive than Jupiter. In other words, if you added 79 more Jupiter-sized planets to the existing world, you'd have enough mass to make star.

The smallest stars are brown dwarf stars, which are only 13 times the mass of Jupiter. Unlike Jupiter, a brown dwarf can truly be called a failed star. It has enough mass to fuse deuterium (an isotope of hydrogen), but not enough mass to sustain the true fusion reaction that defines a star. Jupiter is within an order of magnitude of having enough mass to become a brown dwarf.

Jupiter Was Destined to Be a Planet

Becoming a star isn't all about mass. Most scientists think that even if Jupiter had 13 times its mass, it wouldn't become a brown dwarf. The reason is its chemical composition and structure, which is a consequence of how Jupiter formed. Jupiter formed as planets form, rather than how stars are made.

Stars form from clouds of gas and dust that are attracted to each other by electrical charge and gravity. The clouds become more dense and eventually start rotating. The rotation flattens the matter into a disc. The dust clumps together to form "planetesimals" of ice and rock, which collide with each other to form even larger masses. Eventually, about the time the mass is about ten times that of the Earth, the gravity is enough to attract gas from the disc. In the early formation of the solar system, the central region (which became the Sun) took most of the available mass, including its gases. At the time, Jupiter probably had a mass about 318 times that of Earth. At the point the Sun became a star, the solar wind blew away most of the remaining gas.

It's Different for Other Solar Systems

While astronomers and astrophysicists are still trying to decipher the details of solar system formation, it's known that most solar systems have two, three, or more stars (usually 2). While it's unclear why our solar system only has one star, observations of the formation of other solar systems indicate their mass is distributed differently before the stars ignite. For example, in a binary system, the mass of the two stars tends to be roughly equivalent. Jupiter, on the other hand, never approached the mass of the Sun.

But, What If Jupiter Became a Star?

If we took one of the smallest known stars (OGLE-TR-122b, Gliese 623b, and AB Doradus C) and replaced Jupiter with it, there would be a star with about 100 times the mass of Jupiter. Yet, the star would be less than 1/300th as bright as the Sun. If Jupiter somehow gained that much mass, it would only be about 20% bigger than it is now, much more dense, and maybe 0.3% as bright as the Sun. Since Jupiter is 4 times further from us than the Sun, we'd only see an increased energy of about 0.02%, which is much less than the difference in energy we get from annual variations in the course of Earth's orbit around the Sun. In other words, Jupiter turning into a star would have little to no impact on Earth. Possibly the bright star in the sky might confuse some organisms that use moonlight, because Jupiter-the-star would be about 80 times brighter than the full moon. Also, the star would be red and bright enough to be visible during the day.

According to Robert Frost, an instructor and flight controller at NASA, if Jupiter gained the mass to become a star the orbits of the inner plants would be largely unaffected, while a body 80 times more massive than Jupiter would affect the orbits of Uranus, Neptune, and especially Saturn. The more massive Jupiter, whether it became a star or not, would only affect objects within approximately 50 million kilometers.


Ask a Mathematician Physicist, How Close Is Jupiter to Being a Star?, June 8, 2011 (retrieved April 5, 2017)

NASA, What Is Jupiter?, August 10, 2011 (retrieved April 5, 2017)