The alien worlds around neutron stars

By Ben Pauley

This artist's concept depicts the pulsar planet system discovered by Aleksander Wolszczan in 1992. Wolszczan used the Arecibo radio telescope in Puerto Rico to find three planets - the first of any kind ever found outside our solar system - circling a pulsar named PSR B1257+12. Pulsars are rapidly rotating neutron stars, which are the collapsed cores of exploded massive stars. They spin and pulse with radiation, much like a lighthouse beacon. Here, the pulsar's twisted magnetic fields are highlighted by the blue glow. All three pulsar planets are shown in this picture; the farthest two from the pulsar (closest in this view) are about the size of Earth. Radiation from charged pulsar particles would probably rain down on the planets, causing their night skies to light up with auroras similar to our Northern Lights. One such aurora is illustrated on the planet at the bottom of the picture. Since this landmark discovery, more than 160 extrasolar planets have been observed around stars that are burning nuclear fuel. The planets spotted by Wolszczan are still the only ones around a dead star. They also might be part of a second generation of planets, the first having been destroyed when their star blew up. The Spitzer Space Telescope's discovery of a dusty disk around a pulsar might represent the beginnings of a similarly "reborn" planetary system. — *“Released to Public: Artist View of Pulsar Planet System by NASA/JPL (NASA)” by pingnews.com is marked with Public Domain Mark 1.0.*

Alien planets, such as the water-barren desert world of Arrakis from the Dune novels and films or the rebel base on the ice world of Hoth from Star Wars, are a staple of science fiction that push the boundaries of what we, as humans, consider habitable. But in our imaginings, these worlds often orbit around stars fairly similar to the one in our own system. Less explored is the possibility of strange, alien worlds around stars that are exceptionally different from our own Sun.

Planets don’t have to orbit stars that resemble the Sun, nor even other stars that lie on what’s known as the Main Sequence (a connection of most star types through their surface temperature and luminosity). Other kinds of stars include red and blue giants, white dwarfs, and neutron stars. A notable example in sci-fi of a world orbiting a different kind of star is in the film Interstellar, where the protagonists survey the habitability of planets surrounding a supermassive black hole. However, extreme situations like this are not limited to fiction, and, in fact, the first planets discovered outside our solar system orbit not a typical star but a type of stellar remnant called a pulsar.

The astronomer Jocelyn Bell Burnell first discovered pulsars in 1967 as a series of regular, precisely-timed radio bursts that appeared in the sky at distances too great to be from Earth-based interference. In the following decades, scientists learned that these pulsars are remnants from the explosive ends of some massive stars. After a massive star goes supernova, it can leave behind a ball of almost pure neutrons packed so tightly that around a Sun’s worth of material is condensed down to the size of a city and spinning around many times per second. These neutron stars can also emit a beam of light from their poles, which, due to their rapid rotation, appear as short, repeating pulses.

Some astronomers sweep the sky with telescopes while searching for evidence of previously unknown pulsars. They measure the rotation and energy outputs of known pulsars with ever greater precision, observing changes or unusual behavior. They do so because pulsars’ behaviors can give us insight into general relativity, gravitational waves, and the long-term fates of massive stars. In 1991, an inspection of the pulsar PSR 1257+12, discovered just 1 year prior, conducted with the Arecibo radio telescope, yielded strange results. Astronomers Aleksander Wolszczan and Dale Frail discovered that its pulses were consistently off from what they expected, either arriving too soon or too late, at predictable intervals of 67 and 98 days. With additional data obtained from the Very Large Array, they ruled out the possibility that these mistimings could be due to instabilities in the pulsar itself or the presence of a previously unknown binary companion.

What they were left with was the realization that at least 2 small, planet-sized objects orbited PSR1257+12, and that they pulled it closer or farther from the Earth over the course of their revolutions. Those intervals of mistimed pulses were actually the orbital periods of these companion planets, and their ability to measure these stretches of time and their relative impacts on the pulsar timing allowed the scientists to calculate several of their properties.

The closer of the 2 was at least 3.4 times Earth’s mass and orbited at a distance of approximately 50,000,000 kilometers. This is closer than Mercury’s orbit around the Sun. The farther planet, by contrast, was only 2.8 times Earth’s mass at minimum and orbited at a distance of approximately 70,000,000 kilometers, which is just under half the distance at which Earth orbits the Sun. Additionally, based on the energy output of the pulsar and the planets’ distances, the team could roughly estimate the surface temperatures of both planets, which came out to be around 500° Celsius.

The fact that these planets are around a pulsar led Wolszczan and Frail to ask how they got there. Under normal circumstances, planets form as part of the same process that their host stars undergo: clouds of gas and dust collapse into a rotating disk, with the center containing the bulk of the mass and becoming the star or stars, and the exterior coalesces into planets. The problem with that origin in this case is that the planets orbit close enough that, had they existed when the pulsar was a regular star, they would have been obliterated as it went supernova. This reasoning led the team to speculate that these planets are second-generation, formed after the supernova in a newly stable environment.

This discovery led to the emergence of the new field of neutron star planetary systems. While new examples have been found, they are scarce enough to suggest the phenomenon is rare. In 1993, astronomers detected a unique triple system comprising a pulsar and a white dwarf star orbiting each other, with a Jupiter-sized planet orbiting both at a distance between 2 and 8 times Jupiter’s orbit around the Sun. In 1994, scientists discovered a third, moon-sized object orbiting PSR 1257+12 with a 25-day period at a distance of 30,000,000 kilometers, about half the distance between Mercury and the Sun.

In 2011, an ambiguous case was found where a Jupiter-sized object was orbiting a pulsar. It was so dense that scientists couldn’t conclusively determine whether it was formed from leftover debris from a companion star or if it was itself the former companion star. Further research has revealed that pulsars can be surrounded by potentially planet-forming disks or even asteroid belts.

For the science fiction enthusiasts reading this, imagine an alien world where the inhabitants have never seen a star. In the distant past of their system, there was a star, somewhere between 8 and 20 times the size of our Sun and bluer. But it wouldn’t have mattered what color it was for long: it and any planets immediately surrounding it would have only existed 10s of millions of years at the longest before it blew up. Instead, their sky is filled with a perpetual lightshow following the magnetic field lines emanating from a source that to them would appear almost 70,000 times smaller than the Moon appears to us.

Perhaps these inhabitants rely on specially adapted high-energy photosynthesizers that make food from X-rays rather than sunlight. Or around a pulsar that mostly emits radio waves, perhaps the absence of starlight leads all these inhabitants to resemble creatures from the deep seas or caves on Earth: pale or translucent in color and reliant on smell, sonar, or electroreception to navigate the world. And if any of them ever became astronomers, rather than finding a universe filled with stars fundamentally like theirs, they would find themselves truly unique in the cosmos.

Work in this field is ongoing, focusing on potential origin scenarios for neutron star planets and their conditions after formation. Current research suggests these planets could be part of the 1st, 2nd, or even 3rd generation of planets in a given system. Then, depending on the exact conditions, these planets could potentially retain atmospheres and water for hundreds of millions to billions of years. So, to any prospective sci-fi writer who may think their alien setting is too outlandish, real planetary systems may be stranger than anything we can imagine.