Pulsar Planets

Recently, I mentioned Kepler 10b.

One of 10b’s big selling points is its billing as the first definitely, definitely-solid exoplanet – i.e., one made of rock, not a gas giant. But actually, being strict, it isn’t the first definite rocky exoplanet – rather, it’s the first to be found around a main sequence star. As for the actual first, well, that’s a surprise.

In fact, the very first were discovered in 1990, orbiting PSR B1257+12. They orbit a neutron star!

If you’re surprised by this, so was the world of professional astronomy. You see, PSR B1257 is a pulsar – a rapidly-rotating stellar corpse, so dense that it’s composed mostly of neutrons with only a thin ‘normal-matter’ crust over them. It would be as little as only a dozen or so miles in diameter, despite weighing in at several solar masses. Its core density is that of nuclear matter – absurdly high. It has an extremely powerful magnetic field, which focus its rotation into two beams. As the neutron star spins, these beams sweep on and off across our line of sight, making it appear to ‘blink’ – to pulse. Hence the term pulsar.

How could such a bizarre object form? The answer is, violently.

It would originally have been a hot blue star, class O or B, and after only a few short million years that star would have ended its life in a supernova explosion. Supernovae are some of the most violent events that can occur in the modern universe – at least momentarily, the luminosity of a supernova can match the output of the galaxy it occupies. Inside them, temperatures and pressures reach levels not seen since shortly after the Big Bang. Due to their titanic luminosities, supernovae are visible over extreme distances – SN 1987A, which occured in one of Magellanic Clouds, was a naked-eye object despite being located 168,000 light-years away. With modern telescopes, we can see supernovae pretty much as far as we can see galaxies.

And if the Sun went supernova*, we would expect it to boil off the Earth. That’s right – no planet left!

In a nutshell, this is why no-one expected the pulsar planets. Pulsars are just not somewhere you’d ever expect to find a planet. In fact, there was some resistance to the idea within the community – and no wonder, given how basically crazy it is.

Anyway, no-one was expecting to find planets around pulsars. They were spotted by the anomalies they create in the beam – pulsars are some of the most precise clocks in nature. The planets’ gravity tug back and forth on the (ex-)star as they orbit it, causing slight variations in the timings of its rotation. Pulsars are so absurdly precise that even small bodies can be detected, hence the revelation of planets.

Further investigation has revealed a family of planetary bodies around PSR B1257, with one object about as heavy as our Moon, two more that are between 3.9 and 4.3 Earth masses and a fourth and final, miniscule object that may be the first detected dwarf exoplanet.

Also, it emerges that at least one more pulsar has a definite planet – PSR B1620-26. This too is a weird system – the pulsar is binary with a white dwarf, and the planet orbits both of them. The planet in this system is believed to be one the oldest known, perhaps barely a billion years younger than the universe itself.

The existence of pulsar planets leaves a lot of unanswered questions. The obvious one is, how did they get there? One possibility is that they’re what’s left of the cores of gas giants, Jupiter-size and upwards. When the star went BANG!, the outer atmosphere would have been stripped off, leaving behind a tiny nub from the core. However, there is controversy over this model. (An obvious problem with PSR B1257 is the tight spacings of its planets – could that many super-Jovians have stable orbits when so close together?) Another – and perhaps more likely – suggestion is that the system formed in situ, after the supernova had run its course. Perhaps the planets accreted from nova-debris, or from a subsequent interaction with another system.

What it does seem to tell us is that planet formation is a robust process. It can happen in some of the most extreme environments imaginable. This bodes well for the distribution of planets across the rest of the Galaxy.
*Which it can’t: the minimum conceivable mass for a supernova is the Chandraeskhar Limit of white dwarfs, which is 1.44 solar masses. And this is a hard limit, set by the basic physics – a supernova collapse can only occur when the inward pressure of a star’s weight is greater than that which electron degeneracy can support. In practise, a huge amount of material is flung out during a supernova detonation, so progenitor-stars weigh in with at least 8 solar masses.


4 Responses to “Pulsar Planets”

  1. I imagine a “sun-rise” on one of these planets would be quite spectacualar and highly irriatiated.
    As to where they came from, maybe they were formed from the remains of that star’s Kuiper belt/Oort cloud. I’d imagine the super nova that formed the pulsar would have had a pretty disruptive effect on these objects.

    • Dawn there would be bizarre. The ‘sun’, if it was visible at all, would be a tiny speck. A neutron star from several million miles wouldn’t look appreciably larger than a normal background star.

      Also, the planets’ orbits must periodically take them into the radiation beams. As well as light there’ll be electrons, protons and ions accelerated through the pulsar’s magnetic field – it’s really rather hard to imagine more inhospitable conditions. Not so much radiation burns as radiation barbecuing!

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