From the May 2009 issue

A dying star gives birth to planets

Astronomers have long known that planets form in dusty disks around newborn stars. Now, researchers suspect that planets also can develop in debris disks surrounding exploded stars.
By | Published: May 26, 2009 | Last updated on May 18, 2023
July 2009 dying star movie
Dying star.
NASA/JPL-Caltech/R. Hurt (SSC)
No one expected that the first planets discovered outside the solar system would be circling a stellar corpse. Yet that’s exactly what radio astronomers Aleksander Wolszczan and Dale Frail found in 1991 when they detected planets circling a pulsar designated PSR B1257+12. This pulsar — a neutron star that spins once every 6.22 milliseconds — formed when a star at least 8 times the Sun’s mass exploded. The crushed remnant contains about 1.5 solar masses and measures about 10 miles (16 kilometers) across.

How could such planets exist? After all, any pre-existing planets likely would have been destroyed by the explosion or ejected and left to roam the galaxy after their parent star lost most of its mass (and gravitational pull). Wolszczan suggested that the pulsar’s planets arose from debris left behind by the star’s explosion. The debris first would settle into a disk, where dust grains would start to coalesce. Eventually, collisions would build rocky objects of greater size until small planets would form.

Fast forward to 2006, when the Spitzer Space Telescope detected heat radiating from a dusty disk around the pulsar 4U 0142+61. The disk orbits the pulsar at a distance from roughly 1 million to 10 million miles (1.6 million to 16 million km) and contains enough material to make 10 Earth-mass planets. At last, astronomers had observational evidence that a debris disk could form around a pulsar.

This animation starts with a red supergiant star nearing the end of its life. The star weighs 15 solar masses and has a diameter some 40 times that of the Sun. A hypothetical gas giant planet orbits the star. When the red supergiant exhausts its fuel, its core collapses and generates a shock wave that destroys the rest of the star. This supernova explosion incinerates the existing planet. Meanwhile, the star’s corpse settles down as a neutron star. (Notice that the neutron star’s powerful gravity warps the light passing near it.)

Some of the supernova’s debris never reaches a high-enough velocity to escape the neutron star’s gravitational clutches. This material falls back toward the star and settles into a disk. The animation concludes with the debris coalescing into asteroid-sized bodies, the last step before full-fledged planet formation.

Video credit: NASA/JPL-Caltech/R. Hurt (SSC)