Scienceline’s Guide to the Exoplanets: The Abyssal Watcher

PSR B1620 b: The Abyssal Watcher
Discovered by: Donald Backer et al.
Discovered: 1993
Distance from Earth: ~7200 light-years
Mass: ~2.5 Jupiter masses
Surface climate: Somewhere back in time
Habitability for humans: As much as a Floridian old age home

Last week, we talked about a pulsar planet. This week, we’ll talk about another one. I don’t have a reason — maybe because COVID-19? (Who knows? It’s as good a reason as any other.)

PSR B1620 b is a gas giant, rather larger than Jupiter, and orbiting in a peculiar old system: two binary partners, one a pulsar and another a white dwarf, both stars in their afterlives. They live together just outside the Milky Way, in the M4 globular cluster. Global clusters surround most galaxies; it’s not quite understood how they formed, but scientists know they are dense — and old. Imagine a suburb of high-rise tower blocks whose residents have all aged into retirement.

The planet, fittingly, is old. Some scientists estimate PSR B1620 b’s age at over 12 billion years — almost three times the age of the Sun. Other sources have nicknamed it “Methuselah,” after the biblical figure said to be the oldest ever human. It’s certainly one of the oldest exoplanets we’ve discovered yet.

Scientists think it unlikely that PSR B1620 b could have formed around a pulsar and survived the supernova. Instead, the planet is believed to have formed around a Sun-like star that was eventually captured by the pulsar and pulled into a tight dance. PSR B1620 b settled nicely into an orbit around both stars.

When that Sun-like star reached the end of its life and expanded into a red dwarf — as the Sun will do in a few billion years’ time — the pulsar siphoned off its outer layers, leaving behind a white dwarf.

PSR B1620 b now lives a fair distance from both its stars — around 23 AU, a bit further than Uranus is from the Sun — so it’s spared much of the direct radioactive blasting that last week’s PSR J1719-1438 b is constantly subjected to. From the “surface” of PSR B1620 b, you’d see two stark-white distant suns, one a pulsar whirling so dizzyingly fast you probably wouldn’t be able to tell it’s spinning at all.

Meanwhile, you’d be floating in the surface of gases that are so old they might have seen the formation of the Milky Way itself.