If nature is left to its own devices, about 7.59 billion years from now Earth will be dragged from its orbit by an engorged red Sun and spiral to a rapid vaporous death. That is the forecast according to new calculations by a pair of astronomers, Klaus-Peter Schroeder of the University of Guanajuato in Mexico and Robert Connon Smith of the University of Sussex in England.
Their report, to be published in the Monthly Notices of the Royal Astronomical Society, is the latest and gloomiest installment yet in a long-running debate about the ultimate fate of our planet. Only last year, the discovery of a giant planet orbiting the faint burned-out cinder of a star in Pegasus had suggested that Earth could survive the Sun’s death.
Dr. Smith called the new result “a touch depressing” in a series of e-mail messages. But “looked at another way,” he added, “it is an incentive to do something about finding ways to leave our planet and colonize other areas in the galaxy.”
As for sentimental attachment to any of the geographic features we might have come to know and love, Dr. Smith said, “I should add that the Himalayas are a passing thought anyway. They didn’t even exist until India smashed into Asia less than 60 million years ago — the blink of an eye compared with the billions of years we are discussing.”
While he does not expect the argument to end, Dr. Smith said in an e-mail message that, if anything, in the new calculations he and Dr. Schroeder had underestimated the forces that would be dragging the Earth down toward the Sun.
“So,” he said, “I would be surprised if anyone were able to rescue the Earth again in a future paper.”
Roberto Silviotti of the Capodimonte Observatory in Naples, Italy, who found the planet around that dead star in Pegasus, said it was not surprising that people were interested in the fate of the earth, adding, “I think that the point is not only that this is our planet but also that the we know the solar system and the Sun much better than any other planetary system and therefore we should be able, potentially, to make much better forecasts.”
Earth’s basic problem is that the Sun will gradually get larger and more luminous as it goes through life, according to widely held theories of stellar evolution. In its first 4.5 billion years, according to the models, the Sun has already grown about 40 percent brighter.
Over the coming eons, life on Earth will become muggier and more uncomfortable and finally impossible.
“Even if the Earth were to marginally escape being engulfed,” said Mario Livio, an astronomer at the Space Telescope Science Institute, “it would still be scorched, and life on Earth would be destroyed.”
About a billion years from now, the Sun will be 10 percent brighter. Oceans on Earth will boil away. The Sun will run out of hydrogen fuel in its core about 5.5 billion years from now and start burning hydrogen in the surrounding layers. As a result, the core will shrink and the outer layers will rapidly expand as the Sun transforms itself into a red giant.
The heat from this death rattle will transform the solar system; it will briefly be springtime in the Kuiper Belt out beyond Neptune. Mercury and Venus will surely be swallowed, but the Earth’s fate has always been more uncertain.
The reason is that in the course of ballooning outward, the Sun will blow off a substantial share of its mass. Thus, the Sun’s gravitational grip on its planets will be weakened, and they will retreat to more distant orbits. The Earth will wind up about where Mars is now, “on the border line between being engulfed or escaping engulfment,” as Dr. Livio put it.
Whether or not the Earth is engulfed depends on which of two effects wins out. At the same time that the Earth is retreating to a safer position, tidal forces between it and the expanding Sun will try to drag the planet inward and downward. In 2001, an analysis of these opposing forces by Kacper Rybicki of the Polish Institute of Geophysics and Carlos Denis of the University of Liege concluded that it looked bad but that the Earth might have a chance of surviving.
According to Dr. Smith and Dr. Schroeder, that chance is nil. One key to their work is a new way of calculating how much mass the Sun loses during its cataclysmic expansion, and, thus, how big it gets and how far the Earth eventually moves outward. The more mass lost, paradoxically, the bigger the Sun swells, like a balloon whose elastic weakens when it is stretched. Using a new technique, developed by Dr. Schroeder and Manfred Cuntz of the University of Texas in Arlington, the authors calculated that the lost mass would amount to a third of the Sun’s original mass, compared with previous estimates of a quarter.
As a result, the red giant version of the Sun — at its maximum — will be 256 times as big across as the star is today and 2,730 times as luminous.
Skimming over the flame tops of this giant, the bare, burned Earth would produce a bulge in the Sun. But friction would cause the bulge to lag as it tried to follow the Earth. The gravitational tug from the bulge would slow the Earth and would cause it to spiral inward, where friction from gases in the Sun’s expanded atmosphere would slow it even more.
Then it would go down.
After a period of burning helium and shrinking and expanding and then finally shrinking again, the Sun will wind up as tiny cinder known as a white dwarf, fading away for the rest of time.
Is there any way out of this fiery end for the robots or cockroaches or whoever will be running the Earth in a billion years?
One option is to leave for another planet or another star system.
Another option, Dr. Smith said, is to engage in some large-scale high-stakes engineering.
In the same way that space probes can get a trajectory boost by playing gravitational billiards with Venus or Jupiter to gain speed and get farther out in space, so the Earth could engineer regular encounters with a comet or asteroid, thus raising its orbit and getting farther from the Sun, according to a paper in 2001 by Don Korycansky and Gregory Laughlin of the University of California, Santa Cruz, and Fred Adams of the University of Michigan.
Dr. Laughlin said that when their paper first came out, they were praised by the radio host Rush Limbaugh and other conservatives for forward thinking.
But Dr. Laughlin said they were actually not advocating the orbit-shifting project, noting that a miscalculation could lead to the comet’s hitting the Earth.
“There are profound ethical issues involved,” he wrote in an e-mail message, “and the cost of failure (an Earth-sterilizing impact) is unacceptably high.”
Anyway, such a maneuver would prolong the viability of the Earth for only a few billion years. After that, the planet would be stranded in the cold and dim.
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