By Emily Levesque, University of Washington
Which of these methods would you use to search for an extrasolar or exoplanet? Using a coronagraph to directly observe exoplanets, searching for planets blocking light in a star’s light curve, spotting the redshift effects of a planet’s gravity on its host’s motion, or relying on a gravitational lens. Which one do you think might have succeeded first? Astronomers today have used all of these methods to find exoplanets. However, the first confirmed detection of an exoplanet used something else entirely different—a pulsar.
It all started on the night of February 9, 1990, when astronomer Alex Wolszczan decided to look for a few pulsars. He was observing at the Arecibo Observatory in Puerto Rico, but because of a technical glitch, he couldn’t pursue his research.
Arecibo is an immense radio telescope with an unusual design: the 1,000-foot dish is built right into the ground, nestled in the natural depression of a sinkhole. The dish itself can’t be moved, but the telescope can still be pointed at different parts of the sky by moving a receiver suspended nearly 500 feet above the dish. Unfortunately, on Wolszczan’s night at the telescope that receiver was malfunctioning and couldn’t be moved. Unable to point to different parts of the sky, the telescope could only look straight up.
Still, unwilling to waste telescope time, Wolszczan decided to use Arecibo to search the sky overhead for pulsars.
Pulsars are rotating neutron stars, first discovered by Jocelyn Bell, emitting bright and perfectly regular pulses of radio light like a finely tuned ticking clock. However, one of the pulsars that Alex Wolszczan found at Arecibo that night, wasn’t quite right; its clock appeared to be stuttering, speeding up and then slowing down at regular intervals.
The variations were small—only a few milliseconds—but pulsars are supposed to be perfect. This pulsar was skewed, like something was nudging it back and forth in the sky.
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At the time, Alex Wolszczan was a published expert on the timing of millisecond-speed pulsars. He had grown up in Szczecin, Poland, with an early interest in astronomy encouraged by stories from his father that were based on the myths of the constellations. After getting his PhD at Nicolaus Copernicus University, Wolszczan eventually moved to the United States.
When he discovered this strange pulsar at Arecibo, he suspected right away that the effect could be coming from a planet around the pulsar, but there were a few problems.
Formation of Exoplanets around a Pulsar
To begin with, as we know, planets were born at roughly the same time as stars, coalescing out of the dense disks of gas and dust left around stars near the beginning of their lives. Pulsars, on the other hand, are formed in the supernova deaths of massive stars nearing the ends of their lives. However, stars massive enough to die as supernovae aren’t expected to successfully form planets. This is so because, mostly, early in their lives they get so hot that planet formation in these disks becomes very difficult. And even if exoplanets do manage to form, the violent supernova death of the star itself would likely obliterate them.
Thus, planet formation around pulsars isn’t impossible, but it is unlikely.
Today, we think that exoplanets like this could form if the pulsar used to have a companion star that was destroyed during the supernova, leaving behind leftover stellar material that could eventually form a disk around the pulsar. This disk could then accrete to form planets. They might be strange exoplanets, born long after their original sun had died, but they’re planets all the same, orbiting and tugging on the pulsar.
Calculating the Pulsar’s Position
The strange home for an exoplanet wasn’t the only problem; Alex Wolszczan also knew that there could be other explanations for the velocity shifts he was seeing with the pulsar. He wasn’t able to perfectly measure the position of the star with Arecibo. He was well aware that if the estimate was even slightly off, he might mistake the motion of the pulsar for the relative motion of the Earth.
Therefore, Wolszczan contacted a colleague, Dale Frail, requesting him to observe the very same pulsar with the Very Large Array in New Mexico. Thereon, the two began working together to carefully calculate the pulsar’s position and model possible explanations for its unusual motion. By 1991, they had a proposed answer; their wobbling pulsar was behaving strangely because it might be orbited by not one, but two exoplanets.
The First Extrasolar Planets
It took Wolszczan and Frail two more years to confirm that the exoplanets were there, and a couple of years later they discovered a third exoplanet around the same pulsar. In one fell swoop they had discovered the first extrasolar planets, the first multi-planet system, and the first evidence of planets forming around pulsars.
To say the least, this discovery was incredible, and officially launched the field of exoplanet research.
Common Questions about Discovering the First Exoplanet
Pulsars are rotating neutron stars, first discovered by Jocelyn Bell, emitting bright and perfectly regular pulses of radio light like a finely tuned ticking clock.
The other explanation for the velocity shifts Alex Wolszczan was seeing was, that, he wasn’t able to perfectly measure the position of the star with Arecibo. He was well aware that if the estimate was even slightly off, he might mistake the motion of the pulsar for the relative motion of the Earth.
Alex Wolszczan contacted a colleague, Dale Frail, requesting him to observe the very same pulsar with the Very Large Array in New Mexico. Thereon, the two began working together to carefully calculate the pulsar’s position and model possible explanations for its unusual motion.