Thirty years ago, astronomers discovered the first exoplanets, a trio of small rocky planets orbiting a dead star acting as a cosmic lighthouse. A new survey of hundreds of these lighthouses, or pulsars, revealed that the existence of those planets is the exception rather than the rule. Fewer than 0.5% of pulsars are likely to host Earth-sized or larger exoplanets, which deepens the mystery of how any planets exist in those systems at all.
“One of the main things about pulsar planets is we don’t actually know how to get a planet around a pulsar,” said Iuliana Nițu, a doctoral student at the University of Manchester in the United Kingdom and lead researcher on the new survey. “It’s circular: You need to study the population to learn more about and constrain your models, which then tell you more about populations.”
Planets of the Dead
A pulsar is born when a massive star dies. The star reaches the end of its atom-fusing life, goes supernova, and leaves behind a small, dense ball of neutrons spinning a thousand times per second. The neutron star continues to shed energy through energetic beams, and if the star is oriented just right, one of those beams will sweep across our field of view on Earth and appear to “pulse” at regular intervals (see video below).
If another object—a neutron star, a white dwarf, a black hole, or, in rare cases, a planet—orbits the pulsar, the object’s gravitational pull can subtly change the timing of the pulsing. The three Earth-mass planets around PSR B1257+12 and the five other pulsar planets discovered since were all found through pulsar timing variations that they induced.
Astronomers want to know how those planets can survive the violent deaths of their stars to begin with, and then continue to orbit the stars’ leftovers. “The discovery of the first pulsar planet, and the first exoplanet, around the millisecond pulsar B1257+12 sparked some really interesting research on solar system evolution and just how much planets can take and still hang around,” said pulsar astronomer Matthew Kerr of the U.S. Naval Research Laboratory in Washington, D.C. Kerr was not involved with this research.
The planets might have been there pre-supernova and survived, they might have formed afterward out of debris, or they might have been captured as they wandered past. The only way to narrow down the options, Nițu explained, is to find more pulsar planets and see whether they have anything in common that could point to how they formed. “There’s not really been any unbiased and big searches until now,” she said.
To initiate the search, the team used archival observations from the Jodrell Bank Observatory in the United Kingdom and analyzed the timing regularity of nearly 800 pulsars. After searching for timing variations from objects with a wide range of masses and at a wide range of orbits, their computer algorithm flagged just 15 pulsars with irregular timing, most of which were previously known to be irregular for non-planet-related reasons.
The researchers discovered one pulsar, PSR B0144+59, with timing variations that could be consistent with an exoplanet but would need follow-up observations to confirm. On the basis of their results, they calculated that fewer than 0.5% of pulsars are likely to host exoplanets of 4 Earth-masses or larger, but that Moon-mass planets might still be possible but undetectable. (In contrast, Sun-like stars are likely to have at least one planet, on average.) The team published its results in Monthly Notices of the Royal Astronomical Society in March and presented them at the Royal Astronomical Society’s National Astronomy Meeting 2022 in July.
Out of the Ashes
For all their regularity, a pulsar’s pulses can actually be quite noisy and mimic timing variations that aren’t really there. “That’s a bit of a spanner in the works,” Nițu said, “because how do you differentiate between the pulsars that are doing something weird and something orbiting around the pulsar?”
Despite these challenges, this is the largest survey of pulsar timing to date, and “I think the work and results are robust,” Kerr said. “In particular, because [the researchers’] sample is large and the type of pulsar is varied, I think it’s reasonable to treat their results as representative of the ‘true’ pulsar population, and thus safely conclude that terrestrial-mass planets around pulsars are quite rare, and Jupiter-mass planets are strongly excluded.”
According to Kerr, some recent supernova research has suggested that disks of rocky and dusty debris could last long enough for a new set of planets to form from the ashes of dead stars. That might help explain a preference for small pulsar planets. “Identifying planets from such debris disks is a cool way to constrain those models…. And I think once some of the new telescopes—CHIME, MeerKAT, FAST, all of which have superb sensitivity—start to build up long data sets, we’ll be in a great place to find some planets, even if they are rare.”
—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer