Astronomers discovered the first exoplanets in 1992. They found a pair of them orbiting the pulsar PSR B1257+12 about 2300 light-years from the Sun. Two years later they discovered the third planet in the system.

Now a team of astronomers are trying to duplicate that feat by searching 800 known pulsars for exoplanets.

The team of astronomers is from the Jodrell Bank Centre for Astrophysics at the University of Manchester. Jodrell Bank has a group that works on Pulsars and time-domain astrophysics. Pulsars are objects of interest for a number of different reasons, and Jodrell Bank monitors 800 pulsars as part of their work.

The team is presenting their results in a paper titled “A search for planetary companions around 800 pulsars from the Jodrell Bank pulsar timing programme.” The paper’s first author is Iuliana Nitu and the paper will be published in the Monthly Notices of the Royal Astronomical Society.

Jocelyn Bell Burnell, an astrophysicist from Northern Ireland, discovered the first pulsar in 1967. It took a while for her and another astrophysicist to figure out what they were. There was the usual speculation about alien sources, but once other pulsars were discovered and studied, it became clear they were naturally-occurring objects.

Pulsars are rapidly rotating neutron stars that are highly magnetized and emit beams of electromagnetic radiation from their poles. When one of the poles is pointed at Earth, we can see it, kind of like a lighthouse. Pulsars are known to emit in radio, visible light, x-rays, and even gamma rays. As a pulsar rotates the beam is visible then invisible in intervals as small as several milliseconds. The intervals are very precise—more accurate than an atomic clock—and that makes pulsars useful tools for astronomers.

Their precise intervals make them ideal for searching for planets around them. Even a slight variation in their timing means the pulsar is moving back and forth. That means one or more planets could be tugging on it. Looking for exoplanets around pulsars is called the pulsar timing method.

The transit method is the more common method of looking for exoplanets. That involves watching the light from a star and looking for regular dips in its light. A dip in the starlight could signal the presence of a planet transiting in front of the star, and if the dip is repeated regularly, it’s evidence of an orbit. Scientists find most exoplanets with this method, although follow-up measurements with other methods are often used to help confirm a planet’s presence.

One problem with the transit method is its inherent selection bias. It’s much easier to detect large planets because they block more starlight. It’s also easier to find planets orbiting close to their stars because they orbit more quickly and cause dips in starlight more frequently.

This illustration shows a pulsar with its magnetic field lines shown in blue. The beams emitting from the poles are what washes over our detectors as the dead star spins. Image Credit: NASA
This illustration shows a pulsar with its magnetic field lines shown in blue. The beams emitting from the poles are what washes over our detectors as the dead star spins. Image Credit: NASA

But pulsar timing is different. Because pulsar timing is so precise, even small planets can tug on pulsars enough to signal their presence. The planets detected around PSR B1257+12 in the early 1990s were smaller than most exoplanets found with the transit method. The smallest of the three was only 0.002 Earth masses. As of 2019, the smallest exoplanet ever found with the transit method was 80% of Earth’s size.

This new effort to find exoplanets around 800 pulsars is different than other planet-hunting efforts. This effort isn’t a new survey or monitoring program. Instead, it’s based on searching the existing data on pulsars at the Jodrell Bank Centre. “The dataset used in this work is composed of observations of approximately 800 pulsars from the Jodrell Bank pulsar timing database,” the authors explain.

But what is the likelihood of finding more exoplanets around pulsars? Pulsars are extreme objects with long histories punctuated by episodic catastrophes. “The apparent rarity of systems like that of PSR B1257+12 may well be a consequence of the extreme conditions in which pulsars form,” the authors write
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