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Image left: An optical telescope image of the crab nebula. The central pulsar is shown inset, in a sequence of images demonstrating its rhythmic pulse. Image courtesy of the Kitt Peak 4-meter Mayall telescope.
Stranger than fiction:
In science fiction, there is always a way for a major character to come back from the dead. In astrophysics, it seems there are ways for a star system to come back from utter destruction- a process that mirrors the birth of a planetary system in a bizarre way.
It isn't often mentioned in these heady days of many hundreds of extrasolar planet discoveries, but the first planets found outside our own star system did not orbit a pleasant yellow dwarf like our sun, or a grumpy flare spitting red dwarf, nor even a bloated super bright red supergiant.
Worlds tied to a dead star:
They were found orbiting the corpse of a star which had been destroyed in a type 2 supernova explosion.
This discovery happened in 1990 five years before the current era of planet discoveries. The planets were found by the huge Arecibo radio telescope, during a hunt for pulsars.
Video above: The heart beat of an undead star. The radio 'noise' of a pulsar, as heard by a radio telescope like Aricebo. Visuals are an image generator, but pretty! Video courtesy of IDNmitigatioAPIs via YouTube.
A pulsar is the left over core of a giant star that has gone supernova. The core material of the star is mostly inert iron by the time it hits old age, and iron does not give out energy when it fuses- so there is no energy holding up it weight, and it collapses, and then bounces of itself in an unimaginable explosion. These momentary crucibles of colossal power are responsible for most of the heavy elements in the universe
This leaves the core mostly intact, by transmutes it: The pressure of the explosion, coupled with unmerciful gravity crushes that iron cinder until its atoms start to give in under the strain. If the core weighs more than about 1.4 solar masses, and less than about 3, then it collapses so far individual atoms are smooshed together, positive protons combining with negative electrons, and the dead core becomes a city sized ball of neutrons.
Video Above: The tormented Crab nebula around the Crab nebula pulsar seethes, huge waves of matter and energy visible even from 6500 light years away. In this video made by the Chandra x-ray space telescope and the Hubble space telescope the visible light image is on the right, and its x-ray counterpart on the left. the innermost ring is about a light year across, demonstrating how much punch one pulsar packs. Video courtesy of NASA
This 'neutron star' can be as heavy as several of our sun, but is so dense they would seldom be more than 20km across. This makes them great objects for probing natural laws as they show us the effects of incredible gravity, pressure, exotic forms of matter and temperature.
They do one more thing: They spin, some of them nearly a thousand times a second. As they have intense beams of radiation slicing out of the magnetic poles, and the poles of rotation and magnetism don't align, from a great distance these look like the flashing of a very very fast lighthouse, if you sit in the right spot. Close to..... the business end of 'Vaders death star on high beam is more accurate.
You would not expect something that is born in an explosion which outshines a galaxy, has the mass of a giant star crammed into a space the size of London city center, and gives out scorching twin beams of high energy radiation to have any planets going around it.
But you'd be wrong, and so many astronomers were (not all!). Arecibo showed that something (several somethings) with the right masses to be Earth-sized planets were pulling the spinning neutron star or 'pulsar' PSR 1257 +12 hither and thither, and throwing the timing of its lighthouse flashes off.
Two of these worlds were the mass of Earth, and one was roughly twice the mass of Mars. Later a fourth, dwarf, planet joined the odd little family.
Image above: The PSR 1257+12 system, including orientation to Earth. Image courtesy of extrasolar.net.
Bizarrely and despite having already suffered a horrible, searing, death, this star system had brought itself back!
How does that happen? After all a supernova in the face is about as lethal as lethal gets. Three possibilities jostle for consideration among astrophysicists, and all involve many of the same processes that formed our own solar system in miniature:
Possibility one: A rise from the ashes.
Image above: Supernova remnant Simeis 147. Could part of such a cloud of stellar debris form new planets around its supernovas ticking corpse? Image Courtesy of NASA Astronomy Picture Of the Day.
The supernova left behind more than just the remnant core, it left behind the guts and meat of a giant star, shattered and smeared over that part of space.
In the first scenario not all of that matter stayed smeared over space: some of it came back to its burned home, and collected in orbit about the neutron star, like smoke around a furiously deadly firefly. It formed a protoplanetary disk around the neutron star, and by the same processes that formed our terrestrial (rocky) worlds, (planetesimal accretion, protoplanetary formation, runaway growth Oligarchial growth etc) pulsar PSR 1257 +12 grew new planets.
The protoplanetary disk will have been tiny compared to that which gave rise to us: only ten or so Earth masses. As a result these planets are small, rocky worlds- there isn't enough mass in a whole disk to form one gas giant. And to boot the disk is made from supernova ash, which is going to be loaded with heavy elements, and radioactive isotopes.
This will have a profound effect on the little rocky worlds going around their diminutive and baleful lord. No gas giants, only terrestrial planets, and these are going to be hot, heavy and radioactive. Taking around a hundred thousand years to start to form they will be dense worlds, and even with masses on par with Earth this will make them smaller in radius, and their surface gravity higher. The planets will be infernally hot compared to their counterparts around normal stars, because the abundance of heavy elements will mean that they generate more internal heat, and for longer.
Possibility two: The legacy of stellar murder.
Image left: The site of supernova SN 1987A. Analysis of the debris shows the blast was asymmetric. Image courtesy of the Hubble space telescope.
It is also possible that the supernova blast was seriously lopsided, and threw the neutron star sideways into a companion star. This wouldn't have done the companion much good, but the neutron star would have left the carnage carrying a disk of matter, like cannibalistic interstellar loot. This disk would be similar in composition to the star from which the material was stolen. That allows a lot of scope for the detailed composition to vary, it is possible that these worlds may be chemically very different from the terrestrial worlds we know-  very carbon rich for example, depending on the type of star the matter forming them was stolen from.
But on the whole this process of planet formation would be much more like the formation of our own solar system in miniature: there still wouldn't be enough mass to form anything other than crusty hard terrestrial planets, but these would, in density, composition, and internal radioactivity, probably be more like the terrestrial planets in our solar system.
That doesn't mean that they would have to be dull orbs of irradiated rock: Our own Earth still has plenty of internal kick to power things like volcanoes, even the much lighter Mars kept surface activity and a dense atmosphere going for many many eons. These pulsar planets could be active worlds, with their own cycles of energy and volatiles, haunting the radiation soaked darkness. The magnetic and electrical environment alone would be more intense, frightening and fascinating than anything we have ever seen- these worlds would likely have they own magnetic fields, and these would be connected to one of the most power full dynamos in the universe. Geomagnetic and charged particle storms, aurora, even huge electrical circuits that are scaled up beyond imagining may fill the space above and around these worlds.
Possibility three: The legacy of a parasitic hunger.
The final possibility for how these worlds formed is less dramatic than the preceding two, but in its own way much more creepy. The pulsar may have fed off a still living companion, using its gravity like a siphon to strip away the living stuff of that luckless star, and accreting it onto its surface. Such feasts can end badly for white dwarfs, but in this case it seems that the pulsar may have slowly bled its companion dry, and left enough material still orbiting itself to coalesce into planets. This scenario is a close sibling to the second- much of the details of the planets formed Will depend upon the composition of the star that donated the material, but it would be a more familiar process than the heavy element rich processes of the first scenario, and its 'depleted uranium shell' worlds.
They will be dangerous, truly unique locals, changing violently on a daily basis. The radiation at the surface of each of these worlds will be intolerable to anything we'd think of as life: even if the death star beams from the pulsar don't actually connect with the planet the pulsar gives out intense x-ray, gamma ray, and particle radiation.
Yet in much the same manner as Io in our own solar system lacks nothing in beauty, majesty, complexity or awe inspiring power (click here for an amazing Io related blog), these little worlds orbiting their wizened little pulsar may well be as fascinating and complicated as anything we ave seen. The moons of Jupiter are proof that planets need not be orbiting a shining sun to have great character and active, complex behaviors.
Image left: Io, innermost moon of Jupiter. A complex, ever changing and fascinating world. Image courtesy of NASA.
The discovery of pulsar planets tipped off planet hunters that the theory of planet hunting by radial velocity measurements was sound, the very fact of their existence tells us how robust the planet forming process can be, and these rare, stunted, star systems give us a unique comparison to our own and its history.
How do we know all this?
Pulsar were found in 1967 by (then student) Jocelyn Bell. The mysterious high speed radio 'ticks' were initially suspected of being signals from an alien intelligence, such was their perfect timing. Over the following decades the study of these incredibly intense objects became a true child of 20th century technology-as most of the radiation they give out is invisible (and incidentally quite deadly at close range) to humans. Mostly their study is the province of space borne x-ray observatories like the Chandra telescope, and gamma ray telescopes such as the Fermi. However the lighthouse flashes do include visible light, and radio waves, allowing nearby ones to be imaged by radio telescopes, and the most powerful optical telescopes.
The pulsar planets are totally enigmatic- they are known only through studying the effect they have on their pulsars timing. They are likely to remain so, given their distance, size and dim central object. Most of what we know about them is , via theory and computer modelling, from their mass, their orbits, and what we understand of how pulsar form. However from the measurements made so far we can make some educated guesses- we know their rough mass and orbits, and we know what angle those orbits make, for two of the planets at least, to the radiation beams coming out of the central pulsar. So we can say that while the environment may be extreme, these worlds are probably spared the full fury of the pulsar, as they orbit well out of the beams path.
Depending on which model of how they form is closest to the truth we can make educated guesses about their composition, and perhaps make a rough guess at their radius,and whether they will have much internal heat. From the age of the pulsar we can guess the systems age. And from the positions and masses of these worlds a stab at the systems history can be made. For now, more details will have to wait future advances.....
Next: Young Sol comes of age....
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