Is Jupiter a Failed Star?

You often hear that Jupiter, the 5th planet from the Sun, is a "failed star".  Why is this?  For starters, stars, like our Sun, are composed of hydrogen and helium gas.  There are trace amounts of other gasses, but hydrogen and helium, especially hydrogen, dominate.  If it is hot enough in the core of the star, fusion of hydrogen to helium takes place and energy is released.  This is the definition of a star.  If fusion doesn't occur, the object never becomes a true star.

NASA/JPL/Space Science Institute [Public domain]

Since Jupiter is composed primarily of hydrogen and helium gas, yet doesn't have a high enough core temperature to fuse hydrogen, it is often called a "failed star".  This is a bit misleading.  A star needs to be larger than 8% the mass of the Sun for fusion to begin.  Jupiter has a mass that is 0.1% that of the Sun's.  In other words, Jupiter needs to be 80 times more massive to be a star.  That's a very large difference.  Jupiter is not even close to being a star.  Referring to it as a "failed star" implies that it has a mass that is somewhat close to the minimum mass needed to be a star.  I could understand calling Jupiter a "failed star" if it only needs 2 times its mass or 5 times its mass, but 80 times its mass?  That's a bit of a stretch.

To conclude, in my book, it's not correct to refer to Jupiter as a "failed star".  Its mass is no where near close enough to make it a star.  It's a planet orbiting our star, the Sun.  Nothing more.