11 mins read 05 Aug 2022

Science Talk - What Are Pulsar Planets?

2022 marks the 30-year anniversary since the discovery of the first exoplanets, which happened to be orbiting a pulsar - one of the most unusual places we could find any planet. But what is a pulsar planet, and how did they get there?

Artist impression of the PSR B1257+12 system - a pulsar with three exoplanets orbiting around it. Credit: NASA/JPL-Caltech/R. Hurt.

It's been 30 years since the first exoplanet was discovered, and those first exoplanets ... well, they were found orbiting a pulsar. A planet orbiting a pulsar, you ask? YES! this is super weird and rare, so let's dive in.

Pulsars are born when a massive star (usually 8-25 times the mass of our Sun) explodes in a supernova event. These are extremely energetic events that blast most of the progenitor star apart. But just as most of the star gets flung out into space, the inner part of the star falls in on itself under the force of gravity. 

This creates some rather fascinating, and equally terrifying, outcomes. Firstly, about 1.4 times the mass of the Sun is crushed down into a diameter no bigger than a regional town (~20 kilometres across), so the density is off the charts. So dense, that a teaspoon full of this material would weigh as much as all of humanity combined into a big ball of mush. 

Though, the high mass and density are not the only remarkable things about these compact, remnant objects. They also exhibit extremely fast rotational speeds (spinning faster than your kitchen blender) and contain powerful magnetic fields (trillions of times as strong as your fridge magnet). These features are inherited from the progenitor star but intensify as they are squeezed down into a small volume. 

The complex nature of the magnetic fields of pulsars. As the pulsar rotates, beams of energy from the magnetic poles sweep past the Earth and we see it pulse. In most models, we assume the pulsar has a dipole (i.e., a north-south axis) but for some pulsars, like this example, we find it is asymmetrical and misaligned. Credit: NASA Goddard Space Flight Centre.

The fast rotation and strong magnetic field provide the perfect conditions to generate radio emissions from the magnetic poles of this rapidly rotating massive object, and as those beams sweep past our view, we see a pulse. Like a cosmic lighthouse flashing in radio wavelengths. 

We use radio telescopes to detect these ‘pulses’ (hence, pulsar), and what we find is that they are spinning very fast, but also, slowing down by a tiny fraction with each rotation (known as their period derivative). The measurements that we take are so accurate, that for some pulsars, we have their period derivative down to values as small as 10-21 or put another way, accurate values that stretch out to 21 decimal places before we hit a margin of error. 

This makes the regular ticking nature of pulsars some of the most accurate clocks we have in the Universe. That they are spread all over the Galaxy gives us an opportunity to conduct sensitive timing experiments with them, in conditions that we could never reproduce on Earth (can you imagine trying to reproduce that kind of magnetic field and that much gravity without destroying the planet in the process?).

After several decades of observations, we now know that a portion of pulsars live in binary systems, and just as we do with regular stars, we can measure the pulsating signal as it approaches us or moves away from us, which is known as the doppler shift.  And thanks to the accurate nature of those pesky ticks, we can do this with very high precision, which gives us insight into the intrinsic nature of the pulsar, and also any binary companion it might have.

Data collected on the PSR B1257+12 system, representing the variation in the observed pulsar period, over several years. The solid curve is the model predicted for a two-planet system, and the data points match the model, proving that a planetary system exists. Credit: D. Frail / A. Wolszczan.

On occasion we notice that the ticking from pulsars gets to us earlier or later than expected, creating a little bit of a wobble in the data we observe over time. This tells us that something must be pulling on the pulsar, and when we measure this wobbling over several cycles, we find it follows a regular pattern like the pulsar is going around a centre of mass, in an orbit 

This is similar to our Solar System too - Jupiter is big enough to cause the Sun to move around a central point known as the barycentre. So if you could measure data from the Sun from a distant point, you would see that it wobbled just a tiny amount over a cycle of about 12 years (which corresponds to the length of Jupiter’s orbit).

The thorough analysis of the data that these wobbles produce, allows us to learn about the period of the orbiting body, and its mass. And once again, thanks to the sensitivity that comes with measuring pulsar ticks, we can infer companion masses which can get lower than that of Earth’s Moon, even from all these light years away.  

That’s just what happened in 1992. Astronomers were observing a pulsar (PSR B1257+12) when they noticed a curious periodicity in the wobbling - like there was a mass pulling on the pulsar. What they soon came to realise was that they were looking at a planet orbiting a dead star. In fact, what they found was not one, but two planets orbiting the pulsar!

These became the first planets discovered outside our Solar System - or exoplanets. Since these two, there have been over 5,000 confirmed exoplanets, but from all of these, only been nine pulsar planets, including the first two that were discovered around PSR B1257+12. 

Life on a Pulsar Planet

The orbiting debris field around a pulsar, with materials that can slowly coalesce to form planets. Credit: NASA/JPL-Caltech/R. Hurt.

So, what would life be like on one of these pulsar planets?

Well ..... DEAD.

Pulsars emit enormous amounts of radiation (from radio waves all the way up to gamma rays) - so much so that life (as we know it) could not survive. You'd be living under a constant strobe effect of radiation as well ... some pulsars spin hundreds of times per second, so this wouldn't be pleasant. The magnetic fields of pulsars also create a 'wind' of relativistic particles - which sounds like the most extreme form of sandblasting in the history of the Universe. Under these conditions, no planet’s atmosphere could survive intact. And speaking of, if you wondered too close, both the magnetic field and their gravity would really inflict some damage.

So how does a pulsar planet form in such extreme conditions? 

Firstly, the progenitor system undergoes a supernova - which is one of the most violent events that can occur in our Universe. A massive star, literally blowing itself up. Pulsar planets can’t be former planets from this old system, because prior to the supernova, the massive star would have expanded into a red giant and consumed the inner worlds. Even the worlds located further away - when that star exploded, the sudden change of mass, would cause a big change in gravity across the system causing it to destabilise, and bringing lots of grief for anything that was left behind. 

So maybe the pulsar planets are forged in the ashes of the remaining debris after the supernova has happened - the shredded remains of any former planets, mixed with lots of star guts. This could be an option, but the debris disc needs to be orbiting with a constant or high enough velocity, to avoid it falling back down onto the pulsar (which still has a fairly strong localised gravitational field). 

Sometimes pulsars have companion stars that eventually merge with them. During this process, material from the companion can remain in orbit after, and after long periods (millions to billions of years), this debris can start to coalesce and become small planets as well. In this scenario, the debris field will need to be far enough away from the pulsar, not to get pulled in. 

This is likely what happened for PSR B1257+12’s planets (which now number three, an additional was found in 1994 once again using the ticks from the pulsar) - with astronomers believing that two white dwarf stars merged into each other, creating the pulsar, and the debris disc eventually coalescing to form the planets. 

Another option is that the pulsar might steal a planet from a binary or wondering companion. As the secondary star and its planets get close, the pulsar ejects the stellar object but captures the planetary body, adopting it as its own. Welcome to hell, planetary friend. 

And lastly, pulsar planets can be all that remains of a companion star that strayed too close to the pulsar. All that radiation, that relativistic wind and energy can slowly evaporate a companion in a close orbit, until only its small, planetary-like core remains. 

In all of these scenarios, life (as we know it) would really struggle to find a way to live, given the intense amount of ongoing radiation that bombards them from the pulsar.

Companions of pulsars that stray too close can become ablated by the enormous amounts of radiation poured out by the pulsar. Eventually, a small, planetary-mass object might be left behind, and in some cases, even this has been completely evaporated. Credit: NASA Goddard Flight Centre.

Currently, only a handful of pulsar planets exist. We think it is because these systems are extremely rare in forming, under all of the different formation models that are described above. Some of these are several times the mass of Jupiter, whilst the smallest is only twice as massive as our own Moon.

Pulsars are also tiny objects, roughly only about 20 kilometres across. This makes them impossible to see directly with visible light, though neutron stars (which pulsars are) have been observed in x-ray light because their surfaces are extremely hot and slowly cooling - that’s a product of the supernova and being the former core of a very hot star. 

Often the only way we know they are there (and that they are pulsars) is because we’re lucky enough to have their radio beams pointed in our direction as they spin, allowing us to measure their ticks. And with these ticks, we can measure wobbles. And with wobbles, we can find pulsar planets. 

But of the Galactic population of pulsars, there are many whose beams never shine in our direction - and so, we don’t even know they exist. Even if we were able to notice them in the x-ray bands because of their hot surfaces, we can’t see them ticking along, and so, we can’t conduct our sensitive timing experiments, such as measuring how much a tiny planet is making them wobble. 

So, maybe there are lots of pulsar planets out there, and we just can’t measure their influence on their parent pulsars? Well, a bunch of scientists recently looked at this and determined that even if we factor in this observational bias, pulsar planets are still fairly rare. 

The Real Survivors: Pulsar Planets

The long journey that PSR B1620-26b has been on. It now is a pulsar planet that orbits a central system that features a pulsar and a white dwarf. Credit: NASA/A. Feild.

Of the very small population of pulsar planets we know of, there are a few cases in which baffle how these objects have survived so long for us to observe them. 

One such case is that of a planet named PSR B1620-26b, which orbits both a pulsar and a white dwarf. In other words, the two massive objects (pulsar and white dwarf) orbit each other in a tight configuration at the centre of the system, whilst the pulsar planet orbits much further out, and around both of the inner stars. 

Theory has it that this pulsar planet has been on quite the journey. Originally it orbited a normal Sun-like star, that lived inside a globular cluster - these are very dense cities of stars that orbit the Milky Way and other galaxies. They have large populations of stars held together with their mutual gravity, in small ball-like configurations. 

As this star and planet wandered through a dense region of the globular cluster, they encountered a neutron star and its own companion. This intervention ejected the neutron star’s original companion, leaving only the neutron star and this new star, along with its planet. 

Eventually the new star, after billions of years, ended its hydrogen fusion production and expanded into a red giant, from which the opportunistic neutron star started to steal matter from. This caused the neutron star to spin up to a millisecond pulsar, and the original star to be left as nothing but a white dwarf. 

All along, the helpless planet, remained in an orbit in the outer edges of this system, slowly circling around and around, watching all the drama unfold in the centre of the system. And because of the age of the globular cluster stars, and the time it takes for a regular Sun-like star to live its whole life until it stops burning hydrogen in its core, astronomers have deduced that this system is old - very old. In fact, PSR B1620-26b is the oldest exoplanet known, aged at about 12.6 billion years, making it about three times the age of the Earth. 

The things this pulsar planet has seen and lived through …..