As you may recall from a recent column, the final fate of Earth depends on the last stages of the sun’s life, its red-giant phase, as it dies in six or seven billion years.
During its red-giant phase, our sun will use up most of the hydrogen at its core by fusing it into helium and then helium into carbon. As a result, our day star will begin a series of expansions and contractions that will force its outer extremities farther outward into the solar system.
Earth will fry from the intense heat of the sun’s expanding perimeter. Its surface rocks will melt. Its atmosphere will be blown into space. Its oceans will boil away.
Will Earth survive its close encounter with the sun? Will its orbit slow from friction with the sun’s gases? Will it then plunge toward the sun’s core and be vaporized?
Or will Earth migrate outward and become a cold, dead hunk of rock and metal in orbit around a dead star?
No real consensus exists among astronomers about Earth’s final condition. However, they generally agree about the sun’s future after its red-giant phase.
After several expansions and contractions, the sun’s thermonuclear reaction will cease entirely, and our magnificent day star will become a puny white dwarf.
Its core will collapse to perhaps the size of Earth. A once-proud, million-mile-wide hydrogen bomb will become a dense ball of carbon, a dirty diamond only 8,000 miles wide, shining with a surface temperature as high as 180,000 degrees F.
What happens to Earth depends on factors contemporary astronomers are still determining.
At the minimum prediction of the sun’s expansion, the sun’s expanding outer shell will reach near Earth, and our planet will deep-fry at about 6,000 degrees F.
At its maximum predicted expansion, Earth will be inside the sun’s outer shell. Consequently, Earth must plow through the sun’s thin, hot gasses. Our planet’s momentum through space will slow.
Earth will spiral slowly toward the sun’s center for a century or longer and eventually vaporize into hot gasses. Earth’s gassy remnants will quickly dissipate into the raging inferno surrounding them.
Astronomers calculate that a sun-like star loses about half its mass as it expels its hot gases during its expansion and contraction phase. Earth may survive the sun’s expansion because the sun’s loss of mass will reduce its gravitational oomph. The sun’s lower gravity allows our planet to drift into an orbit farther from the sun than it currently occupies.
That possibility is potentially verifiable with observation. However, such observations are limited by the difficulty of studying tiny Earth-like planets so close to their parent stars, even relatively inconspicuous white dwarfs.
To overcome that difficulty, astronomers have begun to use a new technique called gravitational microlensing.
Briefly, astronomers can now observe the light from distant stars as that light passes around a closer star in the same line of sight. The gravity of the nearer star acts like the lens of an enormous telescope. The farther star’s light bends around the nearer star, thus magnifying the image of the farther star.
Recently, astronomers from the University of California, Berkley, used the Keck telescope in Hawaii to gravitationally microlens a white-dwarf star system 4,200 light-years from Earth. (One light-year equals about six trillion miles.)
The white dwarf’s mass is about half our sun’s current mass. That suggests that the white dwarf was similar in size to our sun before its expansion-contraction phase caused it to lose half its mass.
Significantly, the Berkley astronomers discovered a planet in Earth’s physical ballpark, 1.9 times Earth’s mass and about twice Earth’s distance from the sun, orbiting the white dwarf.
Their computer model suggests that the planet was originally around the distance from its star that Earth is from the sun. During its expansion-contraction phase, the star lost about half of its mass into space, causing the planet to migrate to a more distant orbit.
Thus, a planet first blasted by high temperatures and then frozen by the cold depths of space survived the chaos of stellar death by drifting outward.
Of course, one observation of a stellar system we think resembles what the sun will look like in six billion years does not prove that Earth will survive. But it does prove that survival is at least possible.
Only time and hundreds of similar observations will tell the tale.
Tom Burns is the former director of the Perkins Observatory in Delaware.
