When a bunch of leading scientists got together last week to discuss the latest in big thinking, there was no shortage of doomsday predictions. In particular, Earth’s fate was painted in three shades of grim.
Sometime in the next few billion years, according to new studies presented at the annual meeting of the American Association for the Advancement of Science, the third rock from the sun will either freeze or fry. Unless things simply dry up much sooner.
The expanding furnace
While Earth’s fate is not entirely sealed, predictions of the death of the sun are widely accepted.
The life-giving, aging star we orbit is using up its fuel supply and will collapse within 7 billion years. Before that, though, there will be an agonizing period of repeated swelling, as the sun grows into a red giant. How giant?
“Earth will end up in the sun, vaporizing and blending its material with that of the sun,” said Iowa State University’s Lee Anne Willson. “That part of the sun then blows away into space, so one might say Earth is cremated and the ashes are scattered into interstellar space.”
Willson and her colleague George Bowen studied other red giants, medium-sized stars like our sun that are near death, and used their findings to calculate the fate of Earth.
Artist’s conception of the view of a hypothetical planet around a distant red giant star.
As the sun burns its core of hydrogen, gravity will force a collapse. When compacted, the sun will heat up and burn the small amount of hydrogen that remains in a shell wrapped around the star’s core. This will force the sun to expand into a red giant. Eventually, the core will heat up enough to burn stored helium and the sun will fluctuate in size before collapsing into a white dwarf.
“Earth will get scorched as part of the process the sun will go through as it transforms from being a red giant into a white dwarf,” Willson said.
Out from the frying pan and into the frost
There are two possible paths to salvation, though both involve a frigid end.
“If the sun loses mass before it gets too big, then Earth moves into a larger orbit and escapes,” Willson told SPACE.com. “The sun would need to lose 20 percent of its mass earlier in its evolution, and this is not what we expect to happen.”
Fred Adams, a University of Michigan physicist, has for a few years been modeling the fate of the entire universe. He said his work agrees with Willson’s.
“If Earth stays in its present orbit, its fate is to be fried,” Adams said in a telephone interview. “That is the most likely fate.”
Meanwhile, Adams has modeled a second possible method of escape.
A less bad scenario
Other scientists have learned that planets around other stars often follow odd-shaped orbits, indicating their paths might have been disrupted by the gravity of a passing star. Adams and a colleague got to wondering whether some future passing star or star system might, in similar fashion, kick Earth into the cosmic hinterlands.
So he and Gregory Laughlin, of NASA’s Ames Research Laboratory, simulated many possible encounters with passing stars over the next 3.5 billion years — assuming Earth would support life at least that long. The odds of the planet being ejected from the solar system, they determined, are one-in-100,000.
“Life on Earth would actually continue longer if Earth is sent out of the solar system than if it stays.”
“These aren’t real good odds,” Adams points out, “but they’re greater than the odds of winning the lottery, so they’re worth considering.”
A report on the work will be published in an upcoming issue of the journal Icarus.
Adams figures if Earth is sent off into some cold cosmic corner, the oceans would freeze solid after about a million years. But some forms of life, supported by hydrothermal vents or other internal energy sources, might continue for up to 30 billion years, he estimates.
“Life on Earth would actually continue longer if Earth is sent out of the solar system than if it stays,” he said.
Or, we might just dry up and die
Before Earth’s oceans ever have a chance to freeze or fry, they might have already dried up and evaporated into space, said James Kasting, a Penn State professor of meteorology and geosciences. Kasting estimates his version of the end is a mere 1 billion years away.
“The sun is getting brighter with time and that affects the Earth’s climate,” Kasting said. “Eventually temperatures will become high enough so that the oceans evaporate.”
And, Kasting said, a cataclysmic finale may come even sooner. As Earth becomes a global desert, carbon dioxide levels are expected to drop. At a certain level, which he and his colleagues say might be achieved in half a billion years, there would not be enough carbon dioxide to support photosynthesis, and most plants would die.
Remaining plants would not be sufficient to support a biosphere, Kasting contends. So while the entire planet might incinerated in a few billion years, or cast off into a deep freeze, it’s possible that life on Earth is already in the sunset years.
“If we calculated correctly, Earth has been habitable for 4.5 billion years and only has a half-billion years left,” Kasting said.