How Dead Stars Make Planets

Rings of debris formed in the aftermath of stellar explosions could fuel the birth of new, rocky planets around dead stars. They could also provide an alternative way to make black holes, scientists said today.

Using NASA's Spitzer Space Telescope, researchers detected a cool disk of material glowing in infrared light around a young X-ray pulsar, a type of neutron star that sends out regular, directed pulses of radiation like a lighthouse beam. A neutron star is a dead star that has lost most of its material in an explosion.

New methods

Stars with about eight to 20 solar masses become neutron stars when they die. The stars run out of fuel as they age and their central cores collapse under their own immense weights.  Protons and electrons in the cores get compressed into a tight sphere of neutrons with about 1.5 solar masses all packed into a region the size of a city.

When infalling matter from the imploding star's outer layers reaches this neutron core, it bounces back and generates a powerful shockwave that blasts away the star's outer mantle in a stellar explosion called a supernova.

If material cast off from the explosion doesn't have enough velocity to escape the star's gravitational grasp, it will stall and fall back.

"It's like throwing a baseball straight up into the air," said study team-member Deepto Chakrabarty from the Massachusetts Institute of Technology (MIT). "Unless you're throwing it really, really fast, it's eventually going to fall back down on you."

This so-called "fallback" material can land back on the neutron star's surface or coalesce into a spinning debris disk around the star.

If the fallback material lands back onto the neutron star, it can cause the star to become a black hole. Scientists think this happens when a neutron star exceeds about three solar masses.

"Suppose you form a neutron star that is close to the upper limit. If enough stuff falls back, it'll push the star over this limit and a black hole will form," Chakrabarty told SPACE.com.

Black holes are typically thought to form from the gravitational collapse of stars that have more than about 20 solar masses. These stellar giants bypass the supernova explosions and the neutron star stage to immediately become black holes.

New worlds

If the fallback material instead forms a spinning disk around the neutron star, it can become fodder for the formation of new planets, the scientists say.

"This discovery demonstrates that the planet-creation process is a very robust and a very universal one," said Aleksander Wolszczan, an astrophysicist from Penn State University who was not involved in the finding.

In 1992, Wolszczan's team discovered a trio of rocky worlds around a fast-spinning pulsar. The finding was the first confirmed detection of planets beyond our solar system.

Planet formation around neutron stars would work similar to young stars, except that rocky planets would be favored over gas giants and the entire process would happen more quickly.

One reason for this is that material in the debris disk of neutron stars is more chemically evolved than material created in younger stars.

"The stuff that explodes from a supernova has all been processed through the nuclear engine in the middle of a star so you end up with lots of heavy elements," said study team member David Kaplan, also from MIT.