Brown dwarfs

What do you call something too big to be a planet and too small to be a star? A brown dwarf. A star shines because its centre is hot and squeezed enough to burn hydrogen. That takes at least 80 times the mass of Jupiter. Anything lighter never lights properly. Brown dwarfs are the ones that fell short: balls of gas between about 13 and 80 times the mass of Jupiter, made the way stars are made, but without a star’s fire.

The odd thing is their size. You might expect something 80 times as heavy as Jupiter to be far bigger. It is not. Pile more gas on to Jupiter and it hardly grows at all, because the extra weight squeezes the inside tighter. Every brown dwarf is about as wide as Jupiter, give or take a tenth or so. So the heaviest are packed astonishingly tight: about ten times as dense as lead.

For thirty years they were only an idea. They were predicted in the 1960s and named in 1975, and nobody could find one, because something small and barely glowing is very hard to see. Then in 1995 two were confirmed within months. One, Gliese 229B, was caught as a faint dot beside a small star 19 light years away. Its light showed methane, a gas that cannot survive on anything as hot as a star.

A brown dwarf does glow, with the heat left over from its making. It has nothing to keep that heat topped up, so it spends its whole life cooling and fading. A young one can be as hot as a small star, around 2,500 degrees. An old one can be colder than a kitchen. The coldest known, found in 2014, is about 3 to 12 degrees Celsius.

And they have weather. The warmer ones are hot enough for clouds of molten iron droplets and of fine rock dust. In 2014 astronomers made a rough map of the nearest brown dwarf as it turned, and found light and dark patches: banks of cloud with gaps between. It spins once in about five hours, and on another brown dwarf winds have been clocked at over 2,000 km an hour.

How many are there? Nobody knows well, because they are so hard to see. One estimate puts 25 to 100 billion in our galaxy. Two of the Sun’s four nearest neighbours are brown dwarfs, and neither was known before 2013. Better infrared telescopes keep finding more of them, close by.