Thirty years ago, the first exoplanets were found around a pulsar, a star that rotates extremely quickly. Astronomers have now discovered that these planets might be extremely uncommon.

Iuliana Nițu, a PhD candidate at the University of Manchester, will present the new work at the National Astronomy Meeting (NAM 2022) tomorrow, Tuesday, July 12, at 8:00 a.m.

It is still unknown what processes lead planets to develop and persist around pulsars. The Jodrell Bank Observatory has been keeping an eye on 800 pulsars for the last 50 years. Their research has shown that this first discovered exoplanet system may be very rare: less than 0.5 percent of all known pulsars could have planets the size of Earth.

Pulsars are a type of neutron star. Neutron stars are the most dense stars in the universe. They are born when a normal star explodes violently at the end of its life.

They rotate quickly, have tremendously powerful magnetic fields, and are exceptionally stable. From their magnetic poles, pulsars release brilliant radio emission beams that seem to pulse as the star rotates.

“[Pulsars] produce signals which sweep the Earth every time they rotate, similarly to a cosmic lighthouse,” adds Nițu. “These signals can then be picked up by radio telescopes and turned into a lot of amazing science.”

The first exoplanets were found in 1992, orbiting a pulsar named PSR B1257+12. At least three planets with masses comparable to the rocky planets in our Solar System are known to exist in the planetary system.

Since then, a few pulsars that contain planets have been discovered. But because pulsars are born and die in very violent ways, it’s unlikely that planets form in a “normal” way, and many of the planets we’ve found are strange objects (like planets made mostly of diamond) that are different from those in our Solar System.

A group of astronomers from the University of Manchester did the most extensive search to date for planets that orbit pulsars.

The scientists specifically searched for signs of planets with companions that have masses up to 100 times that of the Earth and orbital periods of 20 days to 17 years.

The system PSR J2007+3120, which may include at least two planets with masses a few times that of the Earth and orbital periods of 1.9 and 3.6 years, is the most promising of the ten possible detections.

The study’s findings show no bias in favor of specific planet masses or orbital periods in pulsar systems.

But the results do tell us something about the shape of the orbits of these planets. Unlike the nearly circular orbits of planets in our Solar System, the orbits of these planets would be very elliptical.

This suggests that pulsar-planet systems form very differently from conventional star-planet systems.

“Pulsars are incredibly interesting and exotic objects. Exactly 30 years ago, the first extra-solar planets were discovered around a pulsar,” explains Nitu, “but we are yet to understand how these planets can form and survive in such extreme conditions. Finding out how common these are, and what they look like is a crucial step towards this.”

Image Credit: NASA/JPL-Caltech

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