We have known since the 1990s that planets exist around pulsars, says Harvard’s chief astronomer, Avi Loeb, which are extraordinarily dense objects born out of the violent explosions of stars. It’s reasonable to assume that planets might also exist around black holes, which, perhaps surprisingly, have a much weaker impact on their environment than pulsars.
Inhabited planets says Loeb, might exist around the black holes that lie at the cores of most galaxies. It’s even possible that life may form on some of these planets, given that organisms on Earth have adapted to extreme conditions, including boiling heat, freezing cold, and acidic, highly salty and even radioactive environments.”
Our own galaxy, the Milky Way, harbors a black hole Sgr A* (Sgr stands for Sagittarius), with an innermost stable circular orbit that’s about the size of the orbit of Mercury. The downside to life near a black hole, writes Loeb, is the heat released by accreting supermassive black holes, posing an existential threat to civilizations residing near the centers of galaxies.
In a paper with John Forbes, of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, Loeb showed that a significant fraction of all planets in the universe are vulnerable to their atmospheres being stripped or their oceans being boiled off as a result of having been close to an active galactic nucleus sometime during their lives.
But what life on a planet orbiting a pulsar? Astronomers estimate that the Milky Way contains an estimated 1 billion neutron stars, of which about 200,000 are pulsars –neutron stars of only 10 to 30 kilometers in diameter with enormous magnetic fields, that accrete matter and regularly burst out large amounts of X-rays and other energetic particles. So far, 3000 pulsars have been studied and only 5 pulsar planets have been found. In 1992, the first exoplanets ever were discovered around pulsar PSR B1257+12.
In December of 2017, The Galaxy reported that it is theoretically possible that habitable planets exist around pulsars. Such planets must have an enormous atmosphere that convert the deadly X-rays and high energy particles of the pulsar into heat. This was the conclusion of the paper by astronomers Alessandro Patruno (Leiden University and ASTRON) and Mihkel Kama (Leiden University and Cambridge University) who suggested that there could nonetheless be life in the vicinity of these stars.
It is the first time that astronomers have tried to calculate so-called habitable zones near neutron stars. The calculations show that the habitable zone around a neutron star can be as large as the distance from Earth to the sun.
An important premise is that the planet must be a super-Earth with a mass between one and 10 times that of the Earth. A smaller planet will lose its atmosphere within a few thousand years. Furthermore, the atmosphere must be a million times as thick as that of the Earth. The conditions on the pulsar planet surface might resemble those of the deep sea.
The astronomers studied the pulsar PSR B1257+12 about 2300 light-years away in the constellation Virgo. They used the Chandra Space Telescope, which is specially made to observe X-rays. Three planets orbit the pulsar. Two of them are super-Earths with a mass of four to five times the Earth. The planets orbit close enough around the pulsar to warm up.
“According to our calculations, the temperature of the planets might be suitable for the presence of liquid water on their surface,” said Patruno. “Though we don’t know yet if the two super-Earths have the right, extremely dense atmosphere.”
Image at the top of the page is a NASA artist’s conception of the rotating, magnetized pulsar at the heart of the Crab Nebula.