Planets of failed stars

Infrared Image of 2M1207 and 2M1207b. Credit: ESO

Infrared Image of 2M1207 and 2M1207b. Credit: ESO

A brown dwarf is a failed star. It’s about the size of Jupiter, but anywhere from 13 to 80 times as massive. It is too small to shine by burning hydrogen, like a star, but it is massive enough to fuse the heavy hydrogen isotope deuterium. There’s not much deuterium in a star, but there’s enough for the brown dwarf to shine bright red under its own power for a few hundred million years (for comparison, a small star can shine for trillions of years). After that, they cool down slowly, over billions of years. Most brown dwarfs are about the same temperature as the hottest planets–one or two thousand degrees.

Brown dwarfs are thought to form from the same gas clouds as stars. Since they form in the same way, can they have planets? Yes, but with some caveats. First, as with nomad planets, not all astronomers agree that these objects should be called planets, since they don’t orbit true stars. “Planemo” seems to be the most popular alternative.

The first planet to be discovered orbiting a brown dwarf was 2M1207b, pictured above with its parent brown dwarf. That is an actual photograph (albeit in false color), one of very few we have of planets outside our Solar System. It was taken by the Very Large Telescope in Chile with an infrared camera. That’s why the brown dwarf appears blue in the image. The blue denotes hotter objects, as seen in the infrared, and the red denotes cooler objects. To the naked eye, both brown dwarf and planet would look either deep red or magenta.

2M1207b is about four times as massive as Jupiter and is separated from its parent brown dwarf more distantly than Neptune from the Sun. That means that it almost certainly did not form from a disk like the planets in our Solar System, but collapsed from the same primordial cloud as the brown dwarf, which brings us to the second caveat about planets around brown dwarfs.

Some astronomers say that only objects that formed like the planets in our Solar System should be called planets, and that objects like 2M1207b should be called something like “sub-brown dwarfs”. This definition is not very popular, since it would mean that if we found that Jupiter was not formed like the other planets (which we could tell if it did not have a solid core), then it would no longer be considered a planet. However, the International Astronomical Union has not produced a formal definition for exoplanets, so there is no strict rule about naming.

We do know, though, that some brown dwarfs have disks that could form planets like Earth. If it orbited close enough, such a planet could stay warm from heat of its parent brown dwarf and from tidal forces for billions of years and could even produce life. This may be the weirdest kind of place where we would expect to find life–at least life as we know it.

About Alex R. Howe

I'm a full-time astrophysicist and a part-time science fiction writer.
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