What Our Dead Earth Will Look Like from Space

Earth isn't long for this universe, cosmologically speaking. According to some of the most recent estimates, life here on the blue marble probably has some 1.75 billion years left to go before the sun collapses in on itself, turns into a white dwarf, and goes about pulling us inexorably towards it. Of course, by then humans will have either moved on to colonize other planets or torched themselves into nuclear or methane-choked oblivion.

Regardless, astrobiologists recognize that on a planetary scale, life comes and goes within a relatively small window. That goes for alien life, too, obviously.

"If life does exist anywhere else in the universe, it may only be fleeting," Charles Q. Choi writes in Astrobiology Magazine. "Now scientists are researching how signs of life might look on dying planets."

See, humans have been dreaming of discovering extraterrestrial life since we realized we were mere organ-filled specks on a big spinning rock hurling through space. We've spent considerably less time dreaming of discovering interstellar corpses. But in a new study published in Astrobiology, scientists with Cornell University insist that we should broaden our horizons.

Many life-supporting planets may have already closed their biological windows, after all. And we should know what to send our satellites out to look for if we really are intent on proving that life exists outside our tiny little planet—or finding prime new real estate for humanity when Earth bottoms out. So, the scientists reason, we'll have to examine what Earth will look like from about 1.1 to 2.8 billion years from now—the window in which life will decline and ultimately perish—and extrapolate. In other words, we'll have to figure out what our world will look like from space after it's dead.

The astrobiologists explain that "the death of the biosphere as we know it today begins with the extinction of higher plant species." Then, rising temperatures will erode our planet's silicates, and they'll start sucking CO2 out of the skies (millions of years too late, naturally). Eventually, there won't be enough carbon dioxide to sustain plants, and they'll all die. Then the animals will, too. Eventually, microbes will again inherit the Earth, living deep underground or at extremely high altitudes.

"This biosphere would favour unicellular, anaerobic (non-oxygen-dependent) organisms with a tolerance for one or more extreme conditions," the Cornell scientists write.

"The final survivors of Earth could persist either in caves, deep underground, or in relatively cool refuges at high altitudes until roughly 2.8 billion years from now, when the Sun will probably make the planet too hot for astronomers to detect any life from a distance," Choi adds. That is, until about 2.8 billion years from now, when we get sucked close enough that the White Dwarfed sun cooks everything into oblivion.

So, throughout this period, Earth's effectively dead, what might it look like? In short, it's probably covered in clouds, and loaded with methane, ethane, or methanethiol—biosignatures a satellite could detect from afar.

"The death of plants and animals would also generate large amounts of decaying matter that would release compounds such as methanethiol into the atmosphere," Choi notes. "Methane could also be a biomarker when all other biomarker gases become undetectable in a dying planet's atmosphere. In fact, far-future levels of methane in Earth's atmosphere could be 10 times higher than the present — methane-producing bacteria get more of the carbon dioxide they need as fuel because plants are no longer there to remove the carbon dioxide."

The researchers think dead Earth might be cloudy, too. Microbes may take to the more hospitable skies, and airborne microorganisms could lead to larger clouds. There are in other words, a number of available biomarkers that would make post-intelligent-life Earth detectable to intelligent life elsewhere. As such, we should expect to look for the same signs in dying worlds elsewhere in the cosmos. Every planet gets its own apocalypse, after all—you just have to know what to look for.