Scott Beadle FRAS explores the death of a star and the rebirth of planetary life.
In December I wrote a piece about a visual snapshot of the end of our Sun using the Helix Nebula as an example of the likely evolutionary track of our own star’s demise.
As their parent stars inevitably age and burn out, every inhabited planet across our galaxy faces the ultimate apocalypse – the destruction of its system and the likelihood of being reduced to a cinder.
Yet the emerging view of what happens to planets at the end of a star’s life has more complexity than scientists once thought. What’s more, the ever-shifting fortunes of survivability in the universe suggest one planets apocalypse may be another’s genesis.
Roughly 5 billion years from now the sun will go through a late life crisis. Fusion reactions will migrate to a hydrogen-rich shell surrounding an inert core of helium. The Suns internal temperature will spike, and our star will expand into a red giant. The Sun’s outer layers will reach escape velocity and peel off into space. As the Sun loses mass, the planets’ orbits will widen to conserve the solar system’s angular momentum. Earth will migrate out to Mars’ current orbit and the red planet will move proportionately farther out.
Unfortunately, Earth and Mars will not become some benign tropical paradises at some new but perfectly balanced, habitable distance from our now seriously bloated star, they will already have been reduced to cinders before entering their final death spirals as their orbital velocities slow and the gravitational tug and friction from the tenuous gases in the Sun’s ballooning atmosphere drag them back.
So, that’s it then, a sun-like star’s evolution into a red giant would render the inner region of the solar system uninhabitable. Yet all hope may not be lost. The habitable zone will expand along with the star. This will warm once-frozen planets and their moons, bringing a brief springtime after a 10-billion-year winter. Under the warm glare of the awakening giant star, a frozen moon’s icy top layer will melt quickly into liquid water. Ancient craters will dissolve into the warming seas. After being in hibernation for all of its star’s main sequence lifetime, the moon’s newborn oceans, laden with a carbon-rich broth, could spring to life. This zone could remain habitable for 1 billion years.
According to some theories, life on our planet may have begun within a few hundred million years after Earth cooled down from its fiery birth. This suggests newly inhabited planets could exist around many red giant stars. Once frozen, carbon rich moons like Saturn’s Titan would thaw out and become incubators for the first stages of life.
Observations of dusty discs surrounding young stars as well as detailed theoretical simulations have strengthened the conventional wisdom about how planets form around newborn stars.
First tiny grains of interstellar dust settle into a disk whirling around the star’s equator. The grains then coagulate to form rocks, which coalesce under the relentless pull of gravity to form asteroid-sized bodies. These planet embryos quickly sweep up the remaining dust along their orbital paths. As long as gas is available, the more massive embryos bulk up and become Jupiter-like worlds. Smaller or slower forming embryos end up as terrestrial bodies like Earth, Mars, and Venus.
But the planet formation process in our galaxy appears so robust that a dust disc around a star at the end of its life can also produce planets. In fact the first exoplanets discovered (beyond our solar system) were second-generation planets in orbit around the pulsar PSR B1257+12! Astronomers have also predicted that late-in-life planets could be born from the mergers of white dwarfs an inevitable outcome of the evolution in some binary systems. Scientists at NASA’s Goddard Space Flight Centre propose that Earth-sized planets could form out of the shredded white dwarf debris. They would be tough little planets with tarry surfaces of carbides or even diamond. The atmosphere of a carbon planet might include nitrogen, ammonia, carbon monoxide, methane, and other hydrocarbons. Oceans of ammonia mixed with metals and organic material.
However first, second, third generation of stars and planets all face ultimate destruction.
In Arthur C Clarkes 1955 short story The Star he describes a voyage to
a supernova remnant where they discover a lone surviving planet far from the exploded star. An intelligent race, foreseeing the coming apocalypse, creates
a vast archive of its civilization for future visitors to find.
Perhaps such is the fate of extra-terrestrial civilizations scattered across the galaxy.
The conditions for planetary birth, survival, and rebirth seem common throughout a cosmos that ironically, is indifferent to maintaining planetary paradises for very long. Never the less, the fundamental physics of the universe seems adept at rebuilding entire planets from the ashes of a stars death seems adept at rebuilding entire planets from the ashes of a stars death.