The fragmentation of planets that get extremely close to their stars could be the cause of mysterious cosmic bursts of radio waves.
Fast, millisecond-long radio bursts, or FRBs, shoot out from distant cosmic locations. Some of these bursts explode only once and others are repeated. A new computer calculation suggests the repetitive type could be due to the interaction of a planet with its magnetic host starresearchers report in the March 20 astrophysical journal.
FRBs are relatively new to astronomical research. Since the first was discovered in 2007, researchers have added hundreds to the account. Scientists have theorized dozens of ways the two different types of FRBs can occur, and almost all theories include compact magnetic stellar remnants known as neutron stars. Some ideas include powerful radio flares from magnetars, the most magnetic neutron stars imaginable (Serial number: 4/6/20). Others suggest a fast spinning neutron staror even asteroids interacting with magnetars (Serial number: 23/2/22).
“How fast the radio bursts are produced is still under debate,” says astronomer Yong-Feng Huang of Nanjing University in China.
Huang and his colleagues considered a new way to do the repetitive flares: the interactions between a neutron star and a planet in orbit (Serial number: 5/3/94). Such planets can get very close to these stars, so the team calculated what might happen to a planet in a highly elliptical orbit around a neutron star. When the planet rotates very close to its star, the star’s gravity pulls on the planet more than when the planet is at its farthest orbital point, stretching and distorting it. This “tidal tug,” says Huang, will rip some small groups off the planet. Each cluster in the team’s calculation is only a few kilometers wide and perhaps a millionth of the planet’s mass, he adds.
Then the fireworks begin. Neutron stars spew out a wind of radiation and particles, much like our own sun but more extreme. When one of these clusters passes through that stellar wind, the interaction “can produce really strong radio emissions,” says Huang. If that happens when the cluster appears to be passing in front of the star from Earth’s perspective, we might see it as a fast radio burst. Each burst in a repeating FRB signal could be caused by one of these clusters interacting with the neutron star’s wind during each close pass of the planet, he says. After that interaction, what’s left of the cluster orbits the star, but far from Earth’s perspective, so we’ll never see it again.
Comparison of calculated bursts with two known repeaters: the first discoveredrepeating roughly every 160 days, and a more recent finding repeating every 16 days, the team found that the fragmented-planet scenario could explain how often the bursts occurred and how bright they were (Serial number: 2/3/16).
The star’s strong gravitational pull on the planet during each close pass could change the planet’s orbit over time, says astrophysicist Wenbin Lu of Princeton University, who was not involved in this study but is investigating possible FRB scenarios. “Every orbit, there is some loss of energy from the system,” he says. “Due to tidal interactions between the planet and the star, the orbit shrinks very quickly.” So it’s possible that the orbit shrinks so fast that FRB signals don’t last long enough to be detected by chance, he says.
But the orbit change could also give astronomers a way to verify this scenario as an FRB source. Observing repeated FRBs over several years to track any changes in the time between bursts could narrow down whether this hypothesis could explain the observations, says Lu. “That may be a good lead.”