The Habitable Zone Around Neutron Stars

In the past 20 years there has been an explosion of discoveries in the field of exoplanets, i.e. planets located outside our solar system. Today we know more than 3,600 exoplanets around 2,700 different stars and the number grows day after day as new data comes in.

The number of stars in our Milky Way alone is estimated to be around 400 billions and based on the observations of the aforementioned planetary systems, astronomers believe that something like at least 100 billion planets might lurk out there.

Exoplanets are usually discovered around main sequence stars, which are stars that are converting hydrogen into helium in their cores, a process that generates energy. The Sun is a main sequence star and the light we see comes from this type of nuclear reactions.

However, there is a very small number of planets that are known to orbit an extraordinary object: a neutron star. In retrospect, the very first exoplanets ever discovered were actually observed more than 20 years ago to rotate around a neutron star, specifically a pulsar (named PSR B1257+12).
This planetary system contains a millisecond pulsar, that spins on its axis every 6 milliseconds, plus three planets. The closest planet has the size of the Moon, whereas the other two are so-called Super-Earths, 4 times more massive than the Earth. They orbit the pulsar at a distance which is a bit less than half the distance of the Earth from the Sun.  Since then, only very few more planets have been discovered around pulsars.

The environment around neutron stars is very harsh, since these are very extreme and energetic objects. Large flows of X-rays are constantly emitted with an intensity thousands to million times stronger than the Sun, which would be of course a deadly experience for any form of life developing on such planets.
Furthermore pulsars emit charged particles with speeds close to the speed of light which are called “pulsar wind”. This wind is capable of hitting the atoms in the outer layers of a planet and quickly evaporate its atmosphere. Furthermore, the collision generates heat that in turn produces gamma rays, the most deadly type of radiation.

In a recent work, however, we have considered in detail the atmospheric processed that both X-rays and pulsar winds induce on planets around pulsars and we have found something very surprising. It is true that the gamma and X-rays, together with the pulsar wind, evaporate the atmosphere of a planet. However, if such a planet is a Super-Earth, it can take several hundred millions to several billion years to remove completely its atmosphere. This is due to the fact that Super-Earths have a huge atmospheric mass, hundred thousands to million times thicker than the Earth, even if they are slightly more massive than our planet.

The main reason for this is that their gravity is stronger and thus they can retain a much larger gaseous mass. The atmosphere of the primordial Earth was indeed much ticker than it is today and we live in the thin and precious layer that is left over since those times.

But what is even more surprising is the fact that the two Super-Earths around the pulsar PSR B1257+12 might very well still possess an atmosphere despite the hundred million years spent bathing in the deadly radiation coming from the pulsar. Therefore these planetary atmospheres might still be able to shield the surface of the planets from the dangerous incoming high energy radiation.

Another surprising fact is that as the planet absorbs part of the X-ray radiation and pulsar wind, its atmospheric temperature can rise to levels which are compatible with life. We cannot say for sure whether the two Super-Earths have still an atmosphere and whether the amount of energy absorbed is sufficient (or whether it is even too much) to set the temperature to acceptable levels, but it seems that neutron stars can have an habitable zone (or a “Goldilocks zone“) and with a bit of luck the two Super-Earths might lie within this soft temperature spot.

Imagine what would be life on such planets: a huge pressure on the surface (due to the large atmospheric mass) able to crush anything we are familiar with. And completely dark. A very thick, black, warm fog. Indeed since gamma and X-rays cannot penetrate the whole atmosphere and reach the surface, neither will ultraviolet, optical or infrared light. It must vaguely look (and feel) like the deepest regions of the sea here on Earth with the difference that you have a whole planet at your disposal.

If we want to go way beyond with our imagination we can envision a pulsar planet where life has developed and evolved for billion years and become complex like on Earth. But I believe we cannot stretch it much beyond an ecosystem similar (but more extreme) than we have here in the Mariana trench. On Earth we have barophiles (a form of extremophiles), organisms able to live and thrive in such extreme conditions. We have huge amoebas like the xenophyophores, single celled organisms 10 cm in size.  Sea cucumbers flourish on the floor of the Challenger Deep  and a couple kilometers above them you can find “supergiants”, a species of gigantic shrimps and even snailfish.

Other extreme creatures here on Earth comprise the tardigrades, amazing creatures that seem immortal. They are able to survive both in space and at pressures of several thousands times the surface pressure of Earth. Could life on pulsar planets resemble such organisms? This is of course impossible to say at the moment, although one can imagine sci-fi scenarios where life evolves in such extreme conditions.

Of course someone will say what about intelligent life? Would it be possible? Would it even be conceivable? I don’t believe this is possible but imagine what would it be. What would it be for an intelligent organism to communicate in this immensely thick fog. And if they would manage to make it outside their enormous atmosphere what would they see? A neutron star spinning hundreds of times per second and emitting beacons of radiation. They would learn with little effort things which are incredibly complex for us. They would witness the effects of general relativity in front of their eyes. Neutron stars do indeed bend space and time in a way that is second only to black holes. They could learn about ultra-dense matter and the behavior of the strong force if they could measure the mass and radius of their neutron star. They would witness the effects of strong magnetic fields and complex electromagnetism by looking at the pulsar. And perhaps they would then look at the other stars, the “normal” stars, like the Sun, and wonder whether life would be possible around those large distant objects, whether such poor emitters of X-ray radiation could sustain life. Whether it would be even conceivable to have life around such pale, cold, weak stars.

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