Rarely even acknowledged as the first planets discovered beyond our solar system, pulsar planets were first found in the radio spectrum circling rapidly rotating, hyper dense neutron stars, known as pulsars. But three decades later, they remain one of astrophysics’ biggest puzzles.
Their current lack of buzz in the exoplanetary science community is largely because pulsar planets are not thought to be great candidates for life as we know it. And only a handful of such planets have been detected, while astronomers have already detected more than 5000 planets circling normal sunlike stars.
But lately, there’s been new interest in trying to learn more about these bizarre, neutron star-circling planets. After all, out of our Milky Way Galaxy’s estimated 200,000 pulsars, it’s likely that some small fraction will have planets that have simply yet to be detected.
The first known extrasolar planets were confirmed in 1992 by radio astronomers Alex Wolszczan of Penn State University and Dale Frail of the NRAO’s Very Large Array in New Mexico. Located some 2300 light-years away in the constellation of Virgo, the pulsar system PSR B1257+12 is now known to harbor three planets.
Two of the three are basically super-earths, with masses some four to five times that of earth, says The Netherlands Institute for Radio Astronomy (ASTRON).
Pulsars are neutron stars of only 10 to 30 km in diameter that have enormous magnetic fields; accrete matter and regularly burst out large amounts of x-rays and other energetic particles, ASTRON notes.
But their fast rotation rates are usually caused by the spin up from matter being drawn from a stellar companion.
Yet to detect planets around such objects?
Pulsar Timing
As Wolszczan told me in my 2001 book “Distant Wanderers: The Search for Planets beyond the Solar System,” the pulsar PSR B1257 +12 didn’t time right.
“There was this extra variability that was hard to explain,” Wolszczan told me. “The common thinking was that millisecond pulsars were rock solid clocks; the pulses go in a straight line with spikes, and we measure each pulse when it arrives.”
Yet as I note in my book, Wolszczan found that these predictable wavelength variations had to be due to the gravitational effects of planets orbiting the neutron star at the center of this system.
Today, there are five confirmed pulsar planet systems with seven planets in total, Julie Novakova, an astrobiologist at Charles University in Prague, Czechia, told me at a recent astrobiology conference in Copenhagen, Denmark. PSR B1257+12 is the only known multiple system with three planets, so searches suggest that pulsar planets are likely rare, she says.
Hoping For More
We can constrain their orbits and mass from timing the radio pulses from the millisecond pulsar, but we don’t know anything more, Novakova told me at the “Unique Species on a Unique Planet?” conference at the University of Copenhagen. It would be great to know about their composition and whether they have any atmospheres, and whether they have magnetic fields, she says. Any of that could tell us more about how they formed, says Novakova.
In a paper being submitted to Astronomy & Astrophysics, the authors put forth several possibilities for the origin of such planets. They write that it’s possible that pulsar planets simply formed in-situ from debris after the supernova explosion; or from remnants of a stellar companion that lost most its mass after the nearby supernova explosion. Or that the planets were even captured planets from a companion star.
But Novakova, the paper’s lead author, says such planets likely formed from infalling dust after the star exploded as a supernova.
A 2017 paper appearing in the journal Astronomy & Astrophysics investigated the potential for the survival of a pulsar planet’s atmosphere. Assuming planets possess an intrinsic magnetic field akin to earth’s or stronger, they could be able to hold onto their atmospheres for billions of years, the authors note.
It is theoretically possible that habitable planets exist around pulsars, says ASTRON. Such planets would need an enormous atmosphere that could convert the pulsar’s deadly x-rays and high energy particles into heat, ASTRON notes.
A Very Dark System
If standing on the surface of a planet at one earth-sun distance, the neutron star at the center of the pulsar system would only resemble a bright pinprick of light.
You can imagine a rocky or metallic planet composed of largely heavy radioactive elements, maybe with a faintly glowing magma ocean on the surface, says Novakova. If it had an atmosphere and a magnetic field, there could be aurorae illuminating the planet’s surface, but it would be nothing like the planets we have in our own system, she says.
