The globular star cluster Messier 4 has a couple of claims to fame. It’s the closest globular to Earth -- about 7,000 light-years away. And it’s home to the first circumbinary planet ever seen -- a planet that orbits both stars in a binary system. The planet was first reported 30 years ago this month.

Such planets are rare -- astronomers have discovered only about 15 of them.

It’s probably hard to make such a planet, and for it to hang around. The gravity of the stars in a binary should keep the planet-building materials close to them stirred up, preventing the birth of planets. And their gravity could easily kick a planet that does form out of the system.

The one in M4 is especially hard to explain. It orbits two dead stars -- a neutron star and a white dwarf. The neutron star is the corpse of a supernova -- a star that blasted itself to bits. The white dwarf died in a gentler process, but one that isn’t very kind to planets. So the planet either survived a double cataclysm, or it was born later, from the debris of the stellar deaths. Either way, it’s ancient: perhaps 12 billion years old -- almost as old as M4 itself.

Messier 4 stands due south as the sky gets dark now. It’s close to the right of Antares, the bright orange star at the heart of the scorpion. Through binoculars, it looks like a fuzzy star. But it’s really the combined glow of a hundred thousand stars -- and at least one planet.
 

Script by Damond Benningfield

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