A Pulsar Habitable Zone?

Life and pulsars don’t seem to mix. But science fiction hasn’t shied away from making the connection, as witness Robert Forward’s Dragon’s Egg (Ballantine, 1980). In the novel, a species called the cheela live on the surface of a neutron star, coping with a surface gravity 67 billion times stronger than that of Earth. An interesting consequence: The cheela live at an accelerated rate, going from the development of agriculture to high-tech in little more than a month, as perceived by the human crew observing the course of their rapid development.

Now we have news that two astronomers are considering habitable planets in orbits around pulsars, a venue that to my knowledge Forward never considered, but perhaps more recent science fiction writers have (let me know if you have any references). Alessandro Patruno (Leiden University), working with Mihkel Kama (Leiden and Cambridge University) see reasons for thinking that life might emerge in such an environment, though the kind of atmosphere that would sustain it would be like nothing we’ve yet encountered.

The paper defines three categories of neutron star planets while explaining the conditions they would be subjected to:

Neutron star planets can be first-, second- or third-generation. First generation planets would be formed in the usual manner, as a by-product of the star formation process, and would likely be ablated or unbound during stellar death. Second generation objects would form in the supernova fallback disk around a freshly-formed neutron star. Third generation planets would form from a disk consisting of a disrupted binary companion (possibly previously overflowing its Roche lobe), thought to be essential for producing millisecond pulsars such as B1257+12. The supernova explosion, the accretion from a companion for millions up to billion years that MSPs [millisecond radio pulsars] undergo, and the emission of high energy X-ray/γ-ray radiation and MeV–TeV particles (the pulsar wind) are all disruptive processes that might destroy planets or disrupt their orbits.

In any case, neutron stars deal out bursts of X-rays and other particles, accreting matter around them and boasting huge magnetic fields. This is a very dicey environment, one would think, to be talking in terms of habitable zones. But in their paper in Astronomy & Astrophysics, Patruno and Kama find room for a habitable zone as large as 1 AU in breadth. To make this work, the planet must be a super-Earth with a mass between one and ten times that of Earth. Also required: An atmosphere a million times as thick as Earth’s.

Daunting conditions indeed. The work draws on studies of the pulsar PSR B1257+12, famous for its three known planets, which were the first exoplanets ever discovered, in 1992 (the third was found in 1994, still a year before the discovery of 51 Pegasi b). Aleksander Wolszczan and Dale Frail will forever be associated with the discovery. Patruno and Kama used the Chandra space telescope to study PSR B1257+12, which is 2300 light years out in Virgo.

Image: 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 called 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. Credit: NASA/JPL-Caltech/R. Hurt (SSC).

What we have around this pulsar are two super-Earths with masses between 4 and 5 times that of Earth, orbiting the pulsar at 0.36 and 0.46 AU respectively; the third, innermost planet is about twice as massive as the Moon. The pulsar itself shows a mass of 1.4 times the Sun’s, with a radius estimated to be in the range of 10 kilometers. All three planets are close enough to be heated by the pulsar, a daunting thought given the X-ray radiation and relativistic ‘pulsar wind,’ which could have devastating effects on a planetary atmosphere.

Nonetheless, the paper continues:

… the two Super-Earths may have retained their atmosphere for at least a hundred million years provided they contain a large atmospheric fraction of the total planet mass, with the atmosphere possibly still being present to these days. We also find that if a moderately strong planetary magnetosphere is present, the atmospheres can survive the strong pulsar winds and reach survival timescales of several billion years. The same argument applies to possible pulsar planets around more powerful objects than PSR B1257+12.

We are talking about a planet that would have an atmosphere accounting for about 30 percent of the planet’s mass. In this news release, the authors liken conditions on the surface of such a world to the deep sea floor here on Earth. Says Patruno: “According to our calculations, the temperature of the planets might be suitable for the presence of liquid water on their surface. Though, we don’t know yet if the two super-Earths have the right, extremely dense atmosphere.”

That pulsar wind remains tricky on several levels. It is not an indefinite process, but one that turns off once the pulsar reaches a slow enough rate of spin. The paper points out that young pulsars turn off the pulsar wind within about a million years, while millisecond radio pulsars do the same in about a billion years. Losing the pulsar wind turns off the planet’s energy source and would cause a dramatic drop in temperature, unless tidal heating, radiogenic effects or X-ray radiation can step in in a process called Bondi-Hoyle accretion, analyzed in the paper:

Isolated neutron stars are directly exposed to the interstellar medium and it is expected that all of them would accrete some of this material. Such accretion process generates extra power due to the conversion of the accreted gas rest mass into energy, with a typical efficiency of the order of 10–20%. This so-called Bondi-Hoyle accretion process should be continuous and might be the main source of power for these type of systems.

I’m thinking science fiction writers among our audience (of which there are more than a few) might want to look at this paper to see what kind of scenarios they can tease out of it. Bear in mind that to this date, we’ve found but five pulsar planets, out of some 3000 pulsars studied. But exotica are what science fiction thrives on, and the kind of habitable zone depicted here is made to order for the hard science fiction writer willing to dig into this paper’s equations.

Addendum: Didn’t Alastair Reynolds deal with a neutron star planet in the first book of the Revelation Space sequence? I need to revisit the series. Wonderful stuff.

The paper is Patruno & Kama, “Neutron Star Planets: Atmospheric Processes and Irradiation,” Astronomy & Astrophysics Vol. 608, A147, published online 19 December 2017 (full text).