All stars, including the Sun, have a finite lifespan. Stars shine through the process of nuclear fusion in which lighter atoms, such as hydrogen, fuse together to create heavier ones. This process releases large amounts of energy that counteracts the star’s ever-present gravitational pull. Ultimately, fusion helps stars resist gravitational collapse.
This balance of forces is called “hydrostatic balance”. However, there will come a time when the fuel supply in a star’s core will begin to run out and eventually die. Stars with more than about eight times the mass of the Sun will generally burn up their fuel in less than 100 million years. Once the merger is complete, the star collapses – generating a massive, instantaneous final burst of nuclear fusion that blasts the star into a supernova.
Supernovae release enough energy to eclipse the entire galaxy in which they occur. What then remains are collapsed, dead stellar cores called neutron stars or, if the progenitor star was massive enough, a black hole. All planets orbiting a star when it goes supernova would be wiped out. Mysteriously though, a handful of “zombie planets” have been detected orbiting neutron stars. And they are some of the strangest worlds in the cosmos.
Neutron stars are extremely dense, containing as much mass as the crushed Sun in a sphere only a few kilometers in diameter. Some neutron stars emit beams of radio waves into space – and it is around these ‘pulsar’ stars that planets have been discovered. As the pulsar rotates, its radio beams sweep through space generating regular radio flashes. Pulsars were discovered in 1967 – you can listen to the radio emission sounds of some of them here.
The regularity of these radio pulses makes pulsars ideal for chasing nearby planets. If a pulsar has a planet, they will both orbit around a shared gravitational center. This means that the radio emission will be periodically stretched and compressed in predictable ways, allowing us to detect the planet.
Phobetor, Draugr and Poltergeist
Some 2,300 light-years from Earth is the PSR B1257+12 pulsar. It flashes 161 times per second and was nicknamed “Lich” after an undead creature in Western folklore. It orbits three rocky terrestrial planets named Phobetor, Draugr and Poltergeist.
These planets hold a special place in the history of astronomy, as they were the first planets beyond our solar system (exoplanets) to be discovered in 1991. A few years ago, Nasa released this poster ” zombie worlds” from them:
Their discovery challenged ideas about planetary formation, which normally occurs when a new star forms. On the other hand, these planets must have formed after the supernova of the dying star. It is not yet known for certain how this happened. Material from a debris disk orbiting the pulsar may have merged into planets after the supernova.
Draugr, named after an undead creature from Norse mythology, is the most intimate of the three. It has about twice the mass of the Moon and is the least massive planet currently known, orbiting Lich every 25 days. Its larger cousins, Poltergeist and Phobetor, orbit every 67 and 98 days respectively, and are each about four times the mass of Earth.
Pulsars have strong magnetic fields that can allow electric currents to travel through the space between the pulsar and an orbiting planet. So if one of these planets has an atmosphere, it could constantly be bathed in the unearthly light of powerful aurora borealis (similar to our aurora borealis).
If you were to stand on the surface of one of these zombie worlds, you would see, through the mighty hue of the aurora, the glowing lich in the sky casting two powerful, tightly confined beams of light outward. in opposite directions in the darkness of space. Neutron stars can be extremely hot, carrying waste heat from the supernova. Lich is nearly 30,000°C and the innermost of these worlds, Draugr, will likely only be a few degrees below freezing on its surface.
The diamond world
Planet PSR J1719−1438b orbits a pulsar about 4,000 light-years away, circling its host in just over two hours. It’s the densest planet ever discovered – so dense, in fact, it’s thought to be made up largely of diamond.
This “world of diamonds” is the residual core of a dead star called a white dwarf. These are known to be high in carbon (diamond is made of carbon) – but this particular white dwarf has lost 99.9% of its original mass, consumed by the powerful gravity of its nearby host pulsar.
This diamond sphere is about half the size of Jupiter and orbits PSR J1719-1438 at a distance of 600,000 km (only 1.5 times farther than our Moon is from Earth). At such a close distance from its host pulsar, it is likely that this world has a very hot surface.
Orbiting the Milky Way (and many galaxies) are globular star clusters – spherical groups of up to a million stars each. They are some of the oldest stars in the universe.
The globular star cluster Messier M4 lies about 5,600 light-years away and contains some 100,000 stars. Among these is a planet nicknamed Methuselah, after the son of Enoch in the book of Genesis who is said to have lived 969 years.
At the center of the M4 star cluster are a pulsar and a white dwarf that orbit their common gravitational center every 161 days. Given the short-lived nature of high-mass stars, the pulsar would have formed shortly after the formation of Messier 4 itself.
Methuselah also orbits this center, but at a much more leisurely rate of once every 100 years or so, at a distance similar to that at which Uranus orbits our own Sun. It is a gas giant planet about 2.5 times the mass of Jupiter. Methuselah is believed to have formed as a normal planet around a Sun-like star within the first billion years of the formation of the universe. It was then captured orbiting the host pulsar, on which it has been spinning ever since.
The high density of stars in globular clusters makes the chances of two stars colliding quite high – and so does the swapping of planets. Methuselah is the oldest known planet in the cosmos, having formed around 12.7 billion years ago along with all the stars in M4.
Pulsar planets are extreme worlds, but even they might not be the weirdest. A small number of theoretical studies have proposed the existence of planets orbiting black holes. So far, however, none have been found.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Gareth Dorrian does not work for, consult, own stock or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond his academic appointment.