Imagine a planet in orbit around a dead star. The world would be bathed in a lethal cocktail of X-rays and charged particles, emitted by a star so faint in visible light that it will scarcely cast a shadow on this world's surface. This may all sound like science fiction, but bizarre worlds like this really do exist.
We're steadily discovering more and more exoplanets around distant stars and, excitingly, we're finding planets that are more and more Earth-like. That said, it's easy to forget that the first exoplanets discovered weren't actually very Earth-like at all. In fact, the first exoplanet to be discovered was in orbit around a pulsar - a star that is long dead.
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Pulsars are the tiny, corpse-like remnants of once mighty stars. A type of rapidly spinning neutron star, pulsars are tightly compacted balls of bizarre neutron-rich matter, formed when some of the largest stars in the universe explode as supernovae. These may not seem, at first, to be good places to look for planets. Supernovae are, frankly, quite apocalyptic events that would easily vaporize any ill-fated planets in orbit around the exploding star.
Weird Worlds Nonetheless, we know of a handful of planets orbiting these strange, undead suns. The first such discovery was made over two decades ago, around a pulsar known as PSR 1257+12. Pulsars emit two beams of radiation from their magnetic North and South poles. Because the star's magnetic poles don't line up with the way the it rotates, this means that we see flashes whenever a beam is pointing towards us - exactly the way we see flashes of light from a lighthouse on the horizon.
The pulses we see from here on Earth are so regular that you could set your watch by them, but this also means that any changes in the pulse timing is quite easy to spot. If a pulsar carries planets in tow, the tiny gravitational tugs they make as they orbit can offset that timing ever so slightly. The effect is miniscule, but it's there.
PSR 1257+12 in particular, is a millisecond pulsar - it spins so rapidly that these tiny changes can be noticed fairly easily. So easily in fact, that this particular pulsar is now known to have a system of three planets around it. Two of these are super-Earths, and one is barely more massive than Earth's moon - it was the smallest known exoplanet until quite recently.
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Meanwhile, around another pulsar is a planet known as PSR B1620-26 b. This one is actually a giant, two and a half times as massive as Jupiter, and it's no less unusual. PSR B1620-26 b is the oldest planet we know of. Its venerable age of 12.7 billion years makes it nearly as old as the Universe itself, earning it the nickname of "Methuselah" among some people*, and hinting that planets may have been forming in our Universe for a very long time.
Worlds such as these are most certainly "alien" planets, in that they're so different to anything we know of that it's difficult to even guess at what they may be like close up. If these worlds have an atmosphere, then they may have dazzling planet-wide aurorae. Bathed in charged particles from the pulsars they orbit, molecules in these planets' atmospheres would be constantly torn apart, causing them to emit huge bursts light. On the other hand, if a pulsar planet has no atmosphere, its surface will most likely be scoured by lethal x-rays.
As for Methuselah, it's difficult to say for certain what happens to a gas giant after 12 billion years. The giant planets in our own solar system are actually still cooling. Jupiter, in particular, is known to emit more energy in infrared light than it receives from the sun. This is because of a process called Kelvin-Helmholtz heating, and it means that Jupiter is actually shrinking by around 2 centimeters each year. Over the course of a human lifetime, this is barely noticeable. But Methuselah is over 8 billion years older than Jupiter.
Curiouser and Curiouser Yet other pulsar planet is, somehow, stranger still. PSR J1719-1438 b, discovered in 2011, is believed to be made up mostly of carbon, crystallised into diamond. It's technically an ultra low mass white dwarf star, which had most of its mass stolen by the the pulsar it orbits. The remnant has no more mass than Jupiter, making it more planet-like than star-like.
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Because of this unusual history, PSR J1719-1438 b is considered a planet. In fact, it's the densest planet ever discovered, with intense pressures found below its surface which would cause carbon to crystallize. This sounds beautiful, but unfortunately for future sightseers, the gravity at the surface of this strange world would be enough to crush any visitors instantly. Provided they could survive the radiation from the pulsar, that is.
An interesting question that may spring to mind is, with the recent conjectures about possible life sustaining white dwarf stars, could any pulsar planets be home to some kind of life? Frankly, it's extremely unlikely, as disappointing as that may seem.
I never like to use the word impossible, but conditions around a pulsar are so hostile that the kind of molecules that life as we know it are made from would be fragmented rapidly. To speculate briefly, even if any life could exist on planets such as these, it would have to live deep below the surface of its planet, and would probably be so different that we'd have trouble even recognizing it.
In the past few years, fewer pulsar planets have been discovered, and a few previous detections have been disproven. There's a good chance though, that this may be because not many people are looking anymore. Most exoplanet researchers are being kept quite busy by almost 900 confirmed exoplanets so far discovered, and over 18,000 further candidates. Thanks to missions like Kepler, there's a lot of data to sift through.
All the same, I do hope people don't stop looking for pulsar planets altogether. There's mounting evidence that old stars may go through a second bout of planet formation. One pulsar, 4U 0142+61, has even been observed to have a planet forming disk around it. Considering all of this, our galaxy may have plenty of planets far stranger than the ones in our own solar system.
*Bear in mind that this is only a nickname. In other words, the name "Methuselah" isn't officially recognized, no matter how apt it may be!
Image: Artist's impression of PSR 1257+12. Credit: NASA/JPL-Caltech/R. Hurt (SSC)