Astronomers decide what we call a star, and what we call a planet. For awhile now, the official definition has had to do specifically with hydrogen, if a ball of gas fuses hydrogen, we call it a star. Burning hydrogen gives stars their brightness and long life spans, lasting up to millions of years. But there's another class of object known as brown dwarfs. Instead of fusing hydrogen, they fuse something called deuterium, which is basically a hydrogen atom with an extra neutron. Because of this, brown dwarfs have few of the characteristics of what we normally call stars. They don't shine, and they can be small, sometimes only a bit larger than gas giants like Jupiter. Astronomers have referred to brown dwarfs as failed stars.
But is that an accurate description? What separates a brown dwarf from a gas giant? MinutePhsyics investigates in the video below.
There are a few things that make brown dwarfs look more like stars than like gas giants, though some of them are currently under debate. For one, brown dwarfs seem to form from compressed gas and dust, not from the extra gas and dust floating around stars, as planets do. Gas giants like Jupiter have more metal in them than stars, and more than brown dwarfs. Brown dwarfs also move more like stars, in clusters moving in generally the same direction, rather than like planets, which orbit stars.
Some scientists believe that there isn't really a way to differentiate between brown dwarfs and gas giants, blurring the line between planet and star. In any case, the reason to differentiate them isn't burning deuterium.
Source: minutephysics
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