In the early 1990s, planetary history was made. In 1992, two astronomers, Alexander Wolszczan and Dale Frail, published an article in Nature announcing the discovery of the very first planets outside the solar system.
These two extrasolar planets, or exoplanets, immediately intrigued. They were rocky worlds 4.3 and 3.9 times the mass of Earth, swirling in orbit around a type of dead star known as a millisecond pulsar, named PSR B1257+12, or Lich in abbreviated (Lich is a powerful undead creature in folklore). A third exoplanet 0.2 times the mass of Earth was confirmed orbiting the pulsar in 1994.
Now, an analysis of hundreds of pulsars has revealed that such exoplanets are incredibly rare – almost endangered.
Pulsars are quite rare; only about 3,320 are known in the Milky Way at the time of writing. According to astronomers, of these, less than 0.5% are likely to have rocky, Earth-like worlds in orbit. It’s just 16 pulsars.
Millisecond pulsars are even rarer, with about 550 known in the Milky Way. This makes humanity’s very first exoplanet discoveries quite incredible.
All dead stars are fascinating, but pulsars add a little spice to the interesting factor.
It is a kind of neutron star; it is the core 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 exceeded only by black holes. Neutron stars can weigh up to about 2.3 times the mass of the Sun, packed into a sphere just 20 kilometers (12 miles) in diameter.
A pulsar is a rotating neutron star whose poles emit beams of radiation. Its orientation is such that when the pulsar spins, its beams sweep across the Earth, making the star appear to be pulsating. Think of a very dense cosmic lighthouse.
And because some pulsars spin extremely fast – on the millisecond scale – these light pulses also happen on the millisecond scale. For a better idea of what this means, you can listen to the pulses of the pulsar translated into sound here.
It’s a pretty extreme environment. It is possible for them to have exoplanets; since the discovery of Lich and its worlds, a handful of other pulsars have been discovered with exoplanets. However, most of these planets are giants, and those that aren’t can get a little weird, like an ultra-dense world seen as the remnants of a pulsar-cannibalized white dwarf star.
A team of astronomers led by Iuliana Nițu from the University of Manchester in the UK set out to find out how common pulsar planets are. They conducted a survey of 800 pulsars monitored by the Jodrell Bank Observatory in the UK, looking for anomalies 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 under such extreme conditions. Finding out how common they are and what they look like is a crucial step towards that.”
Their search parameters were set to find worlds from 1% the mass of the Moon up 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 Lich’s two 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 extremely unlikely to host exoplanets much heavier than Earth, and less than 0.5% are likely to host 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 assess.
Draugr, with its small mass and 25-day orbit, would not be detectable in 95% of the team’s sample because it would get lost in the noise. It is not known how many pulsars would be likely to host such tiny worlds; or even whether it is possible that these worlds exist outside of a multiplanetary system.
Of the 800 pulsars, 15 exhibited periodic signals that could be attributed to exoplanets. However, the team thinks most of them can be traced to the pulsar’s magnetosphere. One pulsar in particular, PSR J2007+3120, looked like a promising candidate for follow-up investigations of exoplanets.
This means that only 0.5% of pulsars are likely to have Earth-like worlds, the team concluded, meaning that the likelihood of us stumbling upon a distant planet with a rare millisecond pulsar for a star is quite weak.
The team also discovered that the pulsar systems are not oriented to any exoplanet size or mass range. However, such 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 however 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.