Imagine a planet orbiting a dead star. This world is bathed in a deadly cocktail of X-rays and charged particles emitted by a star in such a faint visible range that it hardly casts a shadow on the surface of this world. Sounds like science fiction, but such strange worlds can actually exist.
We are constantly discovering new and new exoplanets around distant stars. We are glad that many of them are similar to our Earth. However, it is easy to forget that the first exoplanets discovered were not at all similar to our home planet. The first exoplanets were discovered in the orbits of pulsars - stars that have long since died.
Pulsars are tiny corpses of once mighty stars. It is a type of fast-spinning neutron star, a dense ball of strange neutron-rich matter that forms at the site of a large supernova. At first glance, they seem to be not the best place to search for planets. In truth, we have supernovae on the list of the strangest objects in the Universe - these are events close to the apocalypse, easily evaporating planets in orbits that are not lucky enough to orbit around an exploding star.
Strange worlds
Oddly enough, we know the mass of the planets that revolve around these strange and lifeless suns. The first discovery was made several decades ago in the region of the PSR 1257 + 12 pulsar. Pulsars emit two streams of rays from the north and south poles. And since the magnetic poles do not always coincide with the axis of rotation of the neutron star, we see flares whenever the beam is directed towards us - like from a lighthouse on the horizon.
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The pulses seen from Earth are so regular that they can be used to check the clock. The other good thing is that any changes in pulse timing are easy to spot. If a pulsar carries the planet in tow, a tiny gravity tug that replaces its orbit, briefly, but effectively.
PSR 1257 + 12, by the way, is a millisecond pulsar. It spins so fast that tiny changes are easy enough to spot. Thanks to this, it became known that there are three planets around it. Two of them are super-Earths, one is slightly larger than the Earth's Moon. It was the smallest known exoplanet until recently.
Meanwhile, another pulsar has a planet known as PSR B1620-26 b. This is a real giant, two and a half times more massive than Jupiter, which, in principle, is not surprising. PSR B1620-26 b is the oldest planet known to us. It is about 12.7 billion years old and probably as old as the universe itself. She is called Methuselah, which is suggestive.
Worlds like these are unambiguously “alien” to us, since they differ significantly from everything that we know. It's hard to even guess what kind of close-up they will be. If they have an atmosphere, they can be full of dazzling auroras. Molecules in the atmospheres of such planets will constantly be torn apart, bathing in streams of charged particles from the pulsars around which they circle. On the other hand, if a planet has no atmosphere, its surface will be "licked" by X-rays and absolutely dead.
As for Methuselah, it's hard to say for sure what will happen to the gas giant 12 billion years later. The giant planets in our own solar system are still cooling down. Jupiter is known to emit more energy in the infrared spectrum than it receives from the Sun. This process is called the Kelvin-Helmholtz heating and means that Jupiter decreases by about two centimeters per year. Throughout your life, you are unlikely to pay attention to this. But Methuselah is 8 billion years older than Jupiter.
Curiouser and curiouser
Tellingly, there is another, even stranger planet near the pulsar. PSR J1719-1438 b was opened in 2011. It is believed to be composed almost entirely of carbon crystallized into diamond. Technically, it is an extremely small white dwarf star, mostly stolen from a nearby pulsar. The remainder of the mass does not exceed Jupiter's, thereby making the object more of a planet than a star.
Such an unusual story made a planet out of PSR J1719-1438 b. It is the densest planet ever discovered, and the pressure beneath its surface turns carbon into diamond. It sounds beautiful, but for future sightseers, the planet's gravity will be enough to instantly flatten any of them. If, of course, they survive after being exposed to a pulsar.
You have probably already asked yourself an interesting question several times: is life possible near a pulsar? To be honest, unlikely.
Nobody likes the word "impossible", but the conditions near the pulsar are so hostile that the set of molecules that we call "life" will instantly lose their meaning. Even if life existed on such planets, it would hide deep under the surface of its abode, and would probably be strikingly different from what we are used to seeing. Maybe, from our point of view, this is not life at all.
Not many planets around pulsars have been discovered in the past few years, and some past observations have been disputed. However, the chances of finding it are still quite high, since not many people are engaged in such searches. Most researchers are busy looking for exoplanets. Thanks to the recently deceased Kepler, we have accumulated enough data for analysis.
However, there is evidence that older stars may pass through the second pathway of planetary formation. One of the pulsars 4U 0142 + 61 was seen forming a planetary disk around it. Considering all of the above, it is worth considering that there may be many more strange planets in our galaxy than in our own solar system.