Astronomers have discovered a handful of planets orbiting the collapsed stars known as pulsars. Almost all of these confirmed or purported pulsar planets are small, only a few times the mass of Earth, and are thought to have formed out of the gas and dust left over from the supernova explosion that created the pulsar. However, PSR B1620-26 b, discovered in 1993, did not fit that pattern.
PSR B1620-26 b was several times as massive as Jupiter, and that was not the weirdest thing about it. It was the first planet discovered orbiting a binary star: a pulsar and a white dwarf, it has a very wide orbit that lasts 100 years, and, strangest of all, it resides in the globular cluster M4, yet another place where planets were not expected.
A globular cluster can have over a million stars in a space less than 100 light-years across. With that many stars buzzing around, one would expect that their gravity would knock any planets out of orbit. In fact, we think that is what happened here.
Astronomers believe that PSR B1620-26 b first formed around a solitary normal star at the same time that the cluster formed–12 billion years ago–which also makes it the oldest known planet. Millions of years later, its original sun passed close to a binary star: the pulsar we now see and another normal star now lost to history. Through a complex gravitational interaction, the pulsar’s original partner was kicked out, and the planet’s original sun replaced it, bringing the planet with it. Eventually, that sun aged and died, becoming the white dwarf we now observe.
Meanwhile, the planet lives on. Not having been present for the supernova that created the pulsar, it still retains its thick atmosphere, and, if it avoids the dense center of the star cluster, it may continue circling its old sun and its newer companion for billions of years to come.