I like when people give nicknames to things, especially when they’re appropriate ones. Even though seemingly a lot of people frown upon naming astronomical objects, a couple of exoplanets managed to pick up nicknames back when there were only a handful known. One such object was PSR B1620-26 b, a gas giant in orbit around two dead stars, which acquired the rather apt nickname of Methuselah. This particular planet seems quite deserving of having a name, though. It’s fascinating in a lot of ways.
For a start, Methuselah is the oldest planet known. Indeed, it’s one of the oldest known objects of any kind in our galaxy. Though the age, admittedly, is an estimate. The binary pair it orbits, a pulsar and a white dwarf, lie in the midst of a globular cluster, Messier 4. Globular clusters are pretty unusual things in themselves; every star and planet in a globular is thought to form all at the same time. Messier 4 is 12.7 billion years old, and so Methuselah too, must be 12.7 billion years old.
12.7 billion years is a long time. That makes Methuselah almost as old as the Milky Way, and not a lot younger than the Universe itself. It’s interesting to wonder what a planet so ancient might have seen in its lifetime. And it must have seen some interesting events to be in such an unusual orbit. Orbiting those two stellar corpses, a neutron star and a white dwarf, is unusual in itself. The problem is that in life, that neutron star can’t have been one of the planet’s original host stars. Neutron stars form in supernovae, and a supernova at such close proximity would have simply vapourised Methuselah. A clue though, comes from the unusual type of orbit. Methuselah sits in a circumbinary orbit around the central pair. The white dwarf and neutron star form a close binary, orbiting each other with a separation of just 1 AU. Methuselah then orbits the pair 23 AU away, as if they were a single star. Only a handful of other known planets possess this type of orbit, and it’s led astronomers to hypothesise a rather turbulent history for Methuselah.
Once upon a time, it would’ve been a fairly normal gas giant around a fairly normal sun-like star, somewhere in the outskirts of Messier 4. 2.5 times the mass of Jupiter, but otherwise not unlike it. Over time, Methuselah’s host star migrated, spiralling slowly in towards the centre of the globular cluster. Globular clusters are bustling stellar cities, densely packed with stars of all kinds. In such a heaving throng, a close encounter was virtually inevitable. As it happened, Methuselah’s host star encountered a binary pair, consisting of a neutron star and some other unknown companion. Ejecting its original companion into space, the neutron star captured both star and planet into its gravitational clutches. Gradually as the star started to age, it started to bloat into a red giant. A red giant with a greedy neutron star in close proximity, ready to devour any stellar material within its grasp. Any material spilling towards the neutron star would’ve been hungrily accreted to its surface, causing explosions visible as novae because of the the runaway fusion at the neutron star’s surface. This process would also cause the neutron star to spin up, rotating faster and faster until it became a millisecond pulsar rotating at hundreds of times per second. Eventually, the red giant would be completely consumed by the pulsar, leaving the original star as a helium white dwarf. Dead and used up. All the while, the planet would bear witness to its host star’s demise, relatively undisturbed, safely orbiting the two of them 23 AU away. There’s a possibility that the planet may even have been able to keep ahold of its moons during all of the chaos.
Such stellar drama is rare, but there are few other ways to explain Methuselah’s unlikely situation. This is one planet which has definitely led an interesting life. It’s quite likely too, that so close to the core of Messier 4, further drama is yet to come. An interesting thought is how many other planetary systems exist in the old star clusters in our galaxy. If it can happen once, it can certainly happen again. Perhaps somewhere, another similar cosmic soap opera is going on even as you’re reading this…