In the early 1990s, planetary history was made. In 1992, two astronomers, Alexander Wolszczan and Dale Frail, published a paper 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 creature undead in folklore). In 1994 it was confirmed that a third exoplanet 0.2 times the mass of Earth was orbiting the pulsar.
Now, an analysis of hundreds of pulsars has revealed that such exoplanets are incredibly rare, almost disappearing.
Pulsars are pretty rare; only about 3,320 are known in the Milky Way at the time of this writing. Of these, astronomers now say, less than 0.5 percent are likely to have rocky, Earth-like worlds in orbit. It’s only 16 pulsars.
Millisecond pulsars are even rarer, with around 550 known in the Milky Way. That makes humanity’s first exoplanet discoveries pretty amazing.
All dead stars are fascinating, but pulsars add a bit of oomph to the cool factor.
They are a kind of neutron star; that’s the core of a dead star that has reached the end of its atomic fusion lifespan, ejected most of its outer material, and collapsed into an object whose density is second only to black holes. Neutron stars can be up to around 2.3 times the mass of the Sun, packed into a sphere just 20 kilometers (12 miles) wide.
A pulsar is a rotating neutron star whose poles emit beams of radiation. Such is its orientation that, as the pulsar rotates, its rays pass past Earth, making the star appear to pulsate. Think of a really dense cosmic lighthouse.
And because some pulsars rotate extremely rapidly, on scales of milliseconds, those pulses of light also occur on scales of milliseconds. To get a better idea of what that means, you can listen to the pulses of the pulsars translated into sound here.
This is a pretty extreme environment. They may have exoplanets; Since the discovery of Lich and its worlds, a handful of other exoplanet-bearing pulsars have been discovered. However, most of these planets are giants, and the ones that aren’t can get a little weird, like an ultra-dense world thought to be the remains of a pulsar-cannibalized white dwarf star.
A team of astronomers led by Iuliana Nițu of the University of Manchester in the UK wanted to find out how common planet pulsars are. They conducted a study of 800 pulsars monitored by the Jodrell Bank Observatory in the UK, looking for flashes in the timing of the pulses that could indicate the presence of orbiting exoplanets.
“Pulsars are incredibly interesting and exotic objects,” Nițu said.
“Exactly 30 years ago, the first extrasolar planets were discovered around a pulsar, but we have yet 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 this. “.
Its search parameters were set to find worlds from 1 percent the mass of the Moon to 100 times the mass of Earth, with orbital periods between 20 days and 17 years. These search parameters would have detected the larger of the two Lich worlds, 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 highly unlikely to host exoplanets much heavier than Earth, with less than 0.5 percent likely to host exoplanets in the Poltergeist and Phobetor mass range. .
The presence of exoplanets similar to the smallest exoplanet in the Lich system, Draugr, is a bit 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’s not clear how many pulsars could host such small worlds; or even if it is possible that those worlds exist outside of 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, seemed like a promising candidate for exoplanet-tracking studies.
That means only 0.5 percent of pulsars are likely to have Earth-like worlds, the team concluded, meaning the likelihood that we’ll stumble upon a distant planet with a rare millisecond pulsar for a star it is quite small.
The team also found that pulsar systems are not biased toward any size or mass range of exoplanets. However, any of these exoplanets around a pulsar would have extremely elliptical orbits. This contrasts sharply with the nearly circular orbits seen in the Solar System and suggests that, regardless of how they formed, the process was different from that which produces planets around baby stars just beginning their lives.
The team’s research was presented last week at the National Astronomy Meeting in the UK and published in the Royal Astronomical Society Monthly Notices.