Planets orbiting stars beyond our Solar System are collectively called extrasolar planets, or exoplanets for short. While people throughout history have speculated about the existence of planets orbiting stars other than the Sun—and the possibility that life may be present on those worlds—the first planets to be discovered beyond our Solar System were only detected in 1992, by Aleksander Wolszczan and Dale Frail. That year, astronomers were confounded by the discovery of two planets, and then two years later by the discovery of a third, in the unlikeliest of places: the three planets were found in orbit at the heart of the supernova remnant PSR B1257+12.
Reading this object’s name from right to left, the “+12” gives its declination in degrees, the “1257” indicates that its right ascension is 12h 57m, the “B” indicates that these were its coordinates in 1950 (corrected for Earth’s precession), and the “PSR” tells us that the star is a special type of neutron star known as a pulsar.
In Module 5, you learned that a neutron star is created when a massive star implodes. The pressure from all the infalling matter drives electrons into atomic nuclei where they interact with protons, producing neutrons and high-energy neutrinos. The neutrinos create an enormous outward pressure that works against the infalling matter, eventually reversing its course and causing the star’s outer layers to explode outward as a supernova. What remains at the heart of the supernova is a neutron star, which spins rapidly due to conservation of angular momentum—the same physical mechanism you learned in Module 10 is responsible for the rotation of a protoplanetary disk.
A pulsar is such a neutron star, but one with an orientation that is particularly special relative to Earth, as the video clip above explains.
Since the PSR B1257+12 system was discovered, one other confirmed pulsar planet has been detected (PSR B1620-26 b), and at least one additional candidate has been proposed. However, such systems remain exceptionally rare.
As these planets orbit their central pulsar, the pulsar in turn must wobble due to its mutual gravitational interaction with the orbiting planets. This happens in much the same way as a pair of figure skaters spin about their common centre of mass during a “death spiral” (as demonstrated in the video below).
For the same reason, our Sun does not remain perfectly at rest at the centre of the Solar System, but wobbles about the Solar System’s centre of gravity, primarily due to its interaction with Jupiter. Because the Sun is about a thousand times more massive than Jupiter, their centre of gravity lies just outside the Sun’s surface, which it orbits at a speed of about 12 m/s over a period equal to Jupiter’s orbital period of 11.86 years. If an alien civilisation located in the ecliptic plane were to look closely at the Sun’s spectrum, it would observe a periodic Doppler shift as the Sun moved toward or away with a maximum radial velocity of ±12 m/s.
After Jupiter, the next most massive planet in our Solar System is Saturn, which adds a perturbation to the Sun’s wobble about 20% as strong as Jupiter’s. In contrast, despite Earth’s much smaller orbital distance, its mass is so much lower that its influence on the Sun’s wobble is only about 1% that of Jupiter’s.
If an alien civilisation were capable of measuring the Sun’s wobble with an accuracy of 1% of the primary wobble due to Jupiter, they could use a model of the orbital mechanics of many-body systems to determine the masses and orbital distances of all the planets in our Solar System except perhaps Mars and Mercury, which they’d need greater precision to detect.
In much the same way, when a pulsar is orbited by planets, their influence on the pulsar’s position is detectable through periodic increases and decreases in the frequency of its pulsations (you may wish to review the Doppler effect). In fact, in contrast to measuring Doppler shifts in the “noisy” spectrum of a star, detecting changes in the frequency of a pulsar’s signal is far simpler, because pulsars have the most regular, predictable signals of any astronomical object. This allows astronomers to detect minuscule variations in timing with astonishing accuracy.
To date, the innermost planet in the PSR B1257+12 system remains among the lowest-mass exoplanets ever discovered, at just 2% the mass of Earth (or about twice the mass of the Moon). Given the precision of pulsar-timing techniques, you might wonder why so few pulsar planets have been discovered. Scientists think the reason is that the conditions allowing planets to form—or survive—around a supernova remnant are extraordinarily rare.
The existence of pulsar planets was initially confounding to scientists, who expected that planets would be unable to survive the explosive end of a star’s life and remain in orbit around its dead core. Recent simulations suggest that the planets orbiting PSR B1257+12 formed after the supernova. The proposed scenario is that when the star exploded, it destroyed a companion star that it had previously orbited with (such binary systems are very common). The gas from the companion star is then thought to have settled into a protoplanetary disk, from which the three planets eventually formed.
This process appears so rare that nothing like it has since been observed. Of the two other planets known around pulsars, one is thought to have been captured, and the other is likely the stripped-core remnant of a former companion star.
In December 2015, after a public nomination and voting process, the IAU announced an official set of names for the four members of the PSR B1257+12 system. The pulsar itself was named Lich, after an undead creature in fantasy fiction that controls other undead beings. The innermost planet was named Draugr, after an undead spirit in Norse mythology; the middle planet, Poltergeist, after the “noisy ghost” of German folklore; and the outer planet, Phobetor, after the Greek god of nightmares who could take many forms in dreams.
These first discoveries demonstrated that planets can exist in environments far stranger and more hostile than anyone had imagined. The next discovery, only three years later, would challenge expectations again—this time around a star very much like our own Sun.
cosmiCave.org 