Stars shine by the process of nuclear fusion in which lighter atoms, such as hydrogen, fuse to create heavier ones.
All stars, including the Sun, have a finite lifespan.
Stars shine by the process of nuclear fusion in which lighter atoms, such as hydrogen, fuse to create heavier ones. This process releases vast amounts of energy that counteracts the ever-present inward pull of the star’s gravity. Ultimately, fusion helps stars resist gravitational collapse.
This balance of forces is called “hydrostatic balance”. However, there will come a time when a star’s core fuel supply will begin to run out and it will eventually die.
Stars more than eight times the mass of the Sun will typically burn through their fuel in less than 100 million years. Once fusion stops, the star collapses – creating a massive instantaneous final burst of nuclear fusion that causes the star to explode as a supernova.
Supernovae release enough energy to outgrow the entire galaxy in which they occur. What’s left afterwards are damaged, dead stellar cores called neutron stars or, if the progenitor star was big enough, a black hole.
All planets orbiting a star when it goes supernova would disappear. Mysteriously, though, a handful of “zombie planets” have been spotted orbiting neutron stars. And they are some of the strangest worlds in the world.
Neutron stars are extremely dense, containing as much mass as the Sun crushed into a sphere only a few miles wide. Some neutron stars emit beams of radio waves into space – and planets have been found around these ‘pulsar’ stars.
As the pulsar spins, the radio beams pierce space creating regular radio flashes. Pulsars were discovered in 1967 – you can listen to radio broadcast sounds from some of them here.
The regularity of these radio pulses makes pulsars ideal for hunting nearby planets. If a pulsar has a planet, both will orbit a common center of gravity. This means that the radio emission will periodically stretch and compress in a predictable way – allowing us to detect the planet.
Phobetor, Draugr and Poltergeist
About 2,300 light-years from Earth is the pulsar PSR B1257 12. It flashes 161 times per second and is nicknamed the “Lich” after a dead creature in Western folklore. It orbits three rocky, terrestrial planets called Phobetor, Draugr, and Poltergeist.
These planets hold a special place in the history of astronomy, as they were the first beyond our Solar System (exoplanets) to be discovered in 1991. A few years ago, Nasa released this “zombie worlds” poster:
Their discovery challenged ideas about planet formation, which usually takes place as a new star forms. Instead, these planets must have formed after the dying star went supernova.
It is not yet known for sure how this happened. Material in a debris disk orbiting the pulsar may have coalesced into planets after the supernova.
Draugr, named after an undead creature in Norse mythology, is the innermost of the three. It has about twice the mass of the Moon and is the least massive planet known today, orbiting the 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 which can allow electrical currents to arc through the space between the pulsar and an orbiting planet. So if any of these planets have atmospheres, they may be constantly bathed in the eerie light of a powerful aurora (similar to our northern lights).
If you stood on the surface of one of these zombie worlds, you would see, through the strong hue of the aurora, the red-hot Flame in the sky projecting two powerful and tightly confined beams of light outward in opposite directions into the blackness of space.
Neutron stars can be extremely hot, carrying the residual heat left over from the supernova. The Lich is nearly 30,000 degrees Celsius, and the innermost of these worlds, the Draugr, is likely only a few degrees below zero on its surface.
The world of diamonds
The planet PSR J1719−1438b orbits a pulsar about 4,000 light-years away, orbiting its host in just over two hours.
It is the densest planet yet discovered – so dense, in fact, that it is thought to be largely diamond.
This “diamond world” is the core left over from a dead star called a white dwarf. They are known to be high in carbon (diamond is made of carbon) – but this particular white dwarf has lost 99.9 percent of its original mass, consumed by the strong gravity of the nearby pulsar.
This diamond ball is about half the size of Jupiter and orbits PSR J1719-1438 at a distance of 600,000 km (just 1.5 times as far as our Moon from Earth). At such close proximity to its host pulsar, it is possible that this world has a very hot surface.
Orbiting the Milky Way (and many galaxies) are globular star clusters – globular groups of up to a million stars each. These are some of the oldest stars in the universe.
The globular star cluster Messier M4 is located about 5,600 light-years away and contains about 100,000 stars. Among them is a planet nicknamed Methuselah, after the son of Enoch in the Book of Genesis, who supposedly lived for 969 years.
At the center of the star cluster M4 is a pulsar and a white dwarf that orbit their common center of gravity every 161 days. Given the short-lived nature of massive 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 pace 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 universe’s formation. It was then captured in orbit around the host pulsar, in which it has been orbiting ever since.
The high density of stars in globular clusters makes the chances of two stars having a close encounter quite high – and likewise the exchange of planets. Methuselah is the oldest known planet in the world, having formed about 12.7 billion years ago along with all the M4 stars.
Pulsar planets are worlds of extremes, yet even they may not be the strangest. A small number of theoretical studies have suggested the existence of planets orbiting black holes. So far, however, none have been found.
By Gareth Dorrian, Postdoctoral Researcher in Space Science, University of Birmingham
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