30 years after its discovery, astronomers realize the first confirmed exoplanet is truly rare
In 1992, astronomers discovered the first exoplanet in an unexpected part of the universe: around a pulsar, a rapidly rotating stellar corpse.
Not many other pulsar planets have been discovered since then, and with potentially good reason: In new research detailed July 12 at the National Astronomical Meeting in the UK, astronomers are now discovering that such pulsar worlds may prove extremely rare.
Here’s the background- Astronomers discovered the first known exoplanet around the pulsar PSR B1257+12 in 1992, located about 2,300 light-years from Earth in the constellation Virgo. A pulsar is a type of neutron star, the corpse of a star that dies in a cataclysmic explosion known as a supernova, whose gravity is strong enough to crush protons together with electrons to form neutrons — but not massive enough to become a black hole.
The violent nature of supernovae often makes the remnants of their ancestral stars spin. A rotating neutron star can spin up to 700 times per second, emitting a narrow beam of radio waves from its magnetic poles that flash like a beacon of lighthouse, hence the name “pulsar”, which is short for “pulsing star”.
If a pulsar is orbited by a planet, it will wobble slightly due to the gravitational pull of this world. “We were able to detect pulses from pulsars that arrived earlier or later than expected because of this tiny motion,” said study senior author Michael Keith, an astrophysicist at the University of Manchester in the UK. Backwards. “This pulsar timing technique is very sensitive, because we can track each pulsar’s rotation over many years.”
PSR B1257+12 is now known to host at least three planets similar in mass to the rocky planets of our Solar System. Although scientists have detected nearly 5,000 exoplanets in the 30 years since then, only five other pulsars are known to host anything resembling a planet. It likely didn’t form around this pulsar like the regular world. The companions of four of the five pulsars are cold dead stars known as white dwarfs that have cooled and lost enough material to become “diamond planets”. The companion of the fifth pulsar, the super-Jupiter planet, was likely captured from the pulsar’s partner star.
Overall, PSR B1257+12 is currently the only known example of a neutron star with an Earth-mass planet. Much is still unknown about how such a world could have formed and survived around pulsars.
WHAT DO SCIENTISTS DO? — Astronomers are conducting the largest search for a planet orbiting a pulsar to date. Discovering how common pulsar planets may or may not be a key first step in figuring out how they could have formed in the first place, said study lead author Iuliana Nițu at the University of Manchester.
“Since the first pulsar planets were discovered, it has generally been considered very rare,” said Keith. “However, this is the first time we have conducted a very thorough search for another planet.”
The researchers analyzed data from about 800 pulsars observed by the Jodrell Bank Observatory in Cheshire, England, over the past 50 years. They searched for worlds ranging from one-hundredth the mass of the moon to 100 times the mass of the Earth. The hunt involved orbits that would take anywhere from 20 days to 17 years to complete, planets with circular oval shapes and more “eccentric”. A supernova may almost completely disrupt its ancestral stellar planetary system, “and planetary remnants may find themselves in highly eccentric orbits, barely attached to the pulsar,” Keith said.
WHAT DO THEY FIND? — Scientists found the first known exoplanets may be extremely rare – less than 0.5 percent of all known pulsars can host Earth-mass planets. “It’s surprising that one pulsar is known to have three planets, and we don’t see it anywhere else,” Keith said.
Two-thirds of the roughly 800 pulsars also appear highly unlikely to host companions more than two to eight times the mass of Earth. One pulsar, PSR J2007+3120, has the potential to host at least two exoplanets, with masses several times that of Earth and orbits taking between 1.9 and 3.6 years to complete, respectively. Long-term observations using highly sensitive instruments such as the upcoming Square Kilometer Array could help confirm whether it may or may not have a world, the researchers said.
“An Earth-mass planet around a pulsar would not be like Earth – possibly barren rock blown up by high-energy radiation from the pulsar,” Keith said.
One scenario in which a pulsar hosts the most planets suggests that material exploding from a star after a supernova could collapse to form worlds.
“The material that stars blow up is not very different from the material that originally formed the star, so it’s not impossible that a similar disk of gas and dust could form around pulsars similar to those of our own solar system,” said Keith. “This scenario seems very plausible, but it is a mystery why this appears to only occur in one pulsar.”
WHAT’S NEXT? — This new data on pulsars will help scientists learn more about these dead stars, “which are exotic objects at the extremes of physics,” notes Keith. Astronomers will also make their planet-hunting algorithms publicly available so other scientists can analyze their own databases for pulsar worlds, he added.
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