Readers discuss Pluto’s planetary status, balding black holes and more

Viral buzz cut

The coronavirus razes cells’ hairlike cilia, which protect a person’s respiratory tract from foreign objects, Erin Garcia de Jesús reported in “How COVID-19 sabotages cells” (SN: 8/28/21, p. 18).

Reader Ron Kern asked if the mowed-down cilia can grow back.

The cilia can eventually regrow, says viral immunologist Lisa Chakrabarti of the Pasteur Institute in Paris. While the coronavirus destroys cells’ cilia, the cells themselves seem to live on. Chakrabarti compares the regrowth to a forest recovering after a fire. “The roots are still there, and it will grow back.” By the time cilia regrow, though, the virus may have already invaded the lungs, potentially causing severe COVID-19, she says.

Clearing the air

Pluto became a dwarf planet 15 years ago when the International A­stronomical Union, or IAU, instituted a new definition for a planet, which requires a celestial body to clear other objects from its orbit, Lisa Grossman reported in “The definition of planet is still a sore point – especially among Pluto fans” (SN: 8/28/21, p. 20).

Several readers expressed confusion over why planets that share their orbits with asteroids, such as Jupiter and its Trojan asteroids, weren’t also demoted.

Such asteroids are effectively in the same category as satellites, Grossman says. “These cases don’t violate the IAU’s definition because the planets are still the most gravitationally significant objects around,” she says. Jupiter, for instance, controls the Trojans’ orbit. In contrast, Pluto’s gravity does not significantly affect the objects in its Kuiper Belt neighborhood.

Balding black holes

Black holes born with magnetic fields quickly shed those fields, Emily Conover reported in “Magnetized black holes go bald” (SN: 8/28/21, p. 18).

Reader Dave Foss wondered how a black hole could exhibit a magnetic field if nothing can escape its gravity.

“Scientists don’t expect a black hole itself to exhibit a magnetic field since anything inside the black hole that could generate that field would be cut off from the outside world,” Conover says. But if a massive object that already has a magnetic field, such as a neutron star, collapses to form a black hole, that field won’t disappear instantly, she says. In such cases, plasma surrounding the black hole maintains the magnetic field until the field blasts outward or falls into the black hole, simulations suggest.