Planetary history was made in the early 1990s. In 1992, two astronomers, Alexander Wolszczan and Dale Frail, published an article in Nature announcing the discovery of the first planets outside the Solar System.
These two extrasolar planets, or exoplanets, were immediately intriguing. They were rocky worlds 4.3 and 3.9 times the mass of Earth, orbiting a type of dead star known as a millisecond pulsar, called PSR B1257 + 12, or Lich for short. (Lich is a powerful dead-living creature in folklore). In 1994 it was confirmed that a third exoplanet 0.2 times the mass of the Earth orbited the pulsar.
Now, an analysis of hundreds of pulsars has revealed that these exoplanets are incredibly rare, almost disappearing.
Pulsars are quite rare; only about 3,320 are known in the Milky Way at the time of writing this article. Of these, astronomers now say, less than 0.5 percent are likely to have Earth-like rocky worlds in orbit. That’s just 16 pulsars.
Millisecond pulsars are even rarer, with about 550 known in the Milky Way. This makes the first discoveries of exoplanets of humanity surprising.
All the dead stars are fascinating, but the pulsars add a little kick to the interesting factor.
They are a kind of neutron star; this is the nucleus of a dead star that has reached the end of its atomic fusion life, ejected most of its outer material and collapsed into an object whose density is only exceeded by black holes. Neutron stars can have a mass up to 2.3 times that of the Sun, wrapped in a sphere only 20 kilometers (12 miles) in diameter.
A pulsar is a rotating neutron star that has beams of radiation coming out of its poles. Such is its orientation that, as the pulsar rotates, its rays pass through the Earth, making the star appear to pulsate. Think of a really dense cosmic beacon.
And because some pulsars have an extremely fast rotation, on millisecond scales, these pulses of light also occur on millisecond scales. To get a better idea of what this means, here you can hear the pulsar pulses translated into sound.
This is a pretty extreme environment. They may have exoplanets; since the discovery of Lich and his worlds, a handful of other pulsars with exoplanets have been discovered. However, most of these planets are giants, and those that are not can be a bit strange, like an ultradense world believed to be the remains of a white dwarf star cannibalized by the pulsar.
A team of astronomers led by Iuliana Nițu from the University of Manchester in the United Kingdom wanted to find out how common pulsar planets are. They conducted a survey of 800 pulsars monitored by the Jodrell Bank Observatory in the United Kingdom, looking for incidents in the time of the pulses that could indicate the presence of exoplanets in orbit.
“Bracelets are incredibly interesting and exotic objects,” Nițu said.
“Exactly 30 years ago, the first extrasolar planets were discovered around a pulsar, but we still need to understand how these planets can form and survive in such extreme conditions. Finding out how common they are and what they look like is a crucial step towards that “.
Its search parameters were set to find worlds ranging from 1 percent of the mass of the Moon to 100 times the mass of the Earth, with orbital periods between 20 days and 17 years. These search parameters would have detected the larger of the two worlds of Lich, Poltergeist and Phobetor, which have orbital periods of 66 and 98 days respectively.
The team found that two-thirds of the pulsars in their sample are extremely unlikely to house exoplanets much heavier than Earth, and that less than 0.5% are likely to house exoplanets in the mass range of Poltergeist and Phobetor. .
The presence of exoplanets similar to the smallest exoplanet in the Lich system, Draugr, is a little more difficult to measure.
Draugr, with its small mass and 25-day orbit, would not be detectable in 95 percent of the team’s sample, as it would be lost in noise. It is not clear how many pulsars could accommodate such small worlds; or even if it is possible for these worlds to exist outside a multiplanetary system.
Of the 800 pulsars, 15 showed periodic signals that could be attributed to exoplanets. However, the team believes that most of them can be attributed to the pulsar’s magnetosphere. One pulsar in particular, PSR J2007 + 3120, looked like a promising candidate for follow-up exoplanet surveys.
This means that only 0.5 percent of pulsars are likely to have Earth-like worlds, the team concluded, meaning the likelihood that we will stumble upon a distant planet with a rare thousand pulsars. lisegons for a star is quite small.
The team also found that the pulsar systems are not skewed toward any size range or mass of exoplanet. However, any exoplanet around a pulsar would have extremely elliptical orbits. This contrasts with the almost circular orbits observed in the Solar System and suggests that although they formed, the process was different from that produced by planets around baby stars just beginning their life.
The team’s research was presented last week at the National Astronomy Meeting in the UK and published in the Monthly Notices of the Royal Astronomical Society.