Artistic impression of a habitable planet near a pulsar (right). Credit: Amanda Smith, IoA, Cambridge

Astronomers have calculated that it is theoretically possible that habitable planets could exist around pulsars. Such planets would, however, require an enormous atmosphere to convert the deadly X-rays and high energy particles emitted by the pulsar into heat. This is the conclusion of a  scientific paper by astronomers Alessandro Patruno and Mihkel Kama, working in the Netherlands and the United Kingdom, and published in the journal Astronomy & Astrophysics.

Pulsars are known for their extreme conditions. Each is a  fast-spinning neutron star - the collapsed core of a massive star that has gone supernova at the end of its life. Only 10 to 30 km across, a pulsar posseses enormous magnetic fields, accretes matter, and regularly give out large bursts of X-rays and highly energetic particles.

Surprisingly, despite this hostile environment, neutron stars are known to host exoplanets. The first exoplanets ever discovered were around the pulsar PSR B1257+12 - but whether these planets were originally in orbit around the precursor massive star and survived the supernova explosion, or formed in the system later remains an open question. Such planets would receive little visible light, but would be continually blasted by the energetic radiation and stellar wind from the host. Could such planets ever host life?

For the first time, astronomers have tried to calculate the 'habitable' zones near neutron stars - the range of orbits around a star where a planetary surface could possibly support water in a liquid form. Their  calculations show that the habitable zone around a neutron star can be as large as the distance from our Earth to our Sun. An important premise is that the planet must be a super-Earth, with a mass between one and ten times our Earth. A smaller planet will lose its atmosphere within a few thousand years under the onslaught of the pulsar winds. To survive this barrage, a planet's atmosphere must be a million times thicker than ours - the conditions on a pulsar planet surface might resemble those of the deep ocean floor on Earth.

The astronomers studied the pulsar PSR B1257+12 about 2300 light-years away as a test case, using the X-ray Chandra space telescope. Of the three planets in orbit around the pulsar, two are super-Earths with a mass of four to five times our Earth, and orbit close enough to the pulsar to warm up. According to Patruno, "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."

In the future, the astronomers would love to observe the pulsar in more detail and compare it with other pulsars. The ALMA telescope of the European Southern Observatory would be able to show dust discs around neutron stars. Such discs are good predictors of planets. Our Milky Way contains about 1 billion neutron stars, of which about 200,000 are pulsars. So far, 3000 pulsars have been studied and only 5 pulsar planets have been found.

Based on a NOVA press release

Reference:
Neutron Star Planets: Atmospheric processes and habitability. By: A. Patruno & M. Kama. Accepted for publication in Astronomy & Astrophysics [https://www.aanda.org, preprint: https://arxiv.org/abs/1705.07688]

Local IoA contact:  Dr Mihkel Kama (mkama AT ast.cam.ac.uk)