Death and New Life
When the core of the former red giant has exhausted all of its fuel and shed all the gas it can, the remaining dense stellar cinder is called a white dwarf. The white dwarf is considered “dead” because atoms inside of it no longer fuse to give the star energy. But it still “shines” because it is so hot. Eventually, it will cool off and fade from view. Our Sun will reach this death about 8 billion years from now.
Observations of dusty debris belts around white dwarfs suggest that giant planets may hold small bodies in orbit around a white dwarf, and even fling them inward, to be chewed up by the dead star. This idea took root because of a variety of heavy elements detected in the atmospheres of white dwarfs -- so heavy that they should have sunk into the dense, dead star long ago.
Almost every star the universe eventually goes through this transition from red giant to white dwarf, although extremely low-mass stars will take longer than the present age of the universe to get there. However, if a star is extremely massive, it could follow a different path, expanding into a supergiant star and ultimately exploding as a supernova.
Q. Can planets be born in a supernova?
A. Yes!
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The unimaginably violent explosion of a supernova can trigger a whole new generation of stars, as well as planets. In fact, the first planets ever discovered outside our solar system are orbiting a pulsar, the crushingly dense remnant of a supernova that spins and pulses like a lighthouse. PSR B1257+12, discovered with the Arecibo radio telescope in Puerto Rico in 1992, hosts three planets. These planets would not have survived their star’s explosion. So they must have been created in the aftermath as debris formed a swirling cloud of gas and dust around the pulsar.
On the Shoulders of Supergiants
Supergiant stars contain many layers of different kinds of atoms fusing, creating an enormous output of energy.
To date, no planets have been found around supergiant stars that will explode one day. That doesn’t mean they aren’t there, however. Supergiants stars are very rare, and so bright that they would far outshine any orbiting bodies. It is possible that our technology is not yet advanced enough to find their planets.
Supergiants may be fleeting, but their explosions play an important role in this story. The shock wave from the supernova can trigger the formation of new stars, birthing new lives in the wake of death.
NEWS FLASH: Buried exoplanet treasure
+ EXPANDIn 1917, astronomer Adriaan van Maanen discovered what appeared to be a faint, fast-moving star. The same year, Walter Sydney Adams captured the star’s spectrum -- a chemical fingerprint. Little did they know that they had captured the very first evidence of a white dwarf surrounded by planetary debris -- and perhaps even the first indication of the existence of exoplanets.
The spectrum of the star showed calcium and other heavy elements, meaning the white dwarf had gobbled up material with these components relatively recently. The leading theory is that exoplanets could fling small rocky bodies toward the white dwarf, where they would be pulverized by the dead star’s gravity.
Today we call these dead stars with heavy elements in their photospheres "polluted white dwarfs." NASA's Spitzer Space Telescope has confirmed that about 40 white dwarfs have hot, dusty disks. Another handful emerged in data from NASA's Wide-field Infrared Survey Explorer (WISE). More than a century after Adams’ spectrum, the puzzle pieces are still coming together: In 2015, astronomers using NASA’s Kepler Space Telescope made the first discovery of a minor planet transiting a white dwarf, and in 2017 a different group found evidence of a white dwarf snacking on a possible comet-like body.
Want more? Learn about planets beyond our solar system at NASA's Exoplanet Exploration website.
