Aliens studying Earth would see a habitable planet

As we continue to scour the night sky for potential holiday home planets, the outcome is generally pretty grim. Planets are either local but inhospitable or hugely promising but out of reach unless we radically increase our lifespan or improve our technology. How much would we need to increase our lifespan by? Ooh, a good 77,000 years should do the trick, give or take.

Putting that little dilemma to one side for a moment, what if the (galactically speaking) local planets we’ve discounted were perfectly welcoming, and our detection methods were just wrong? One way of testing this is to see how Earth looks on our sensors from space, and see whether we’d view it as a likely candidate as an outsider. And that’s exactly what scientists from the University of Arizona have done.

The good news: the readings show that Earth supports life. The bad news? That probably means we were right about our neighbours, too.

By measuring the absorption of light at different wavelengths, the researchers were able to detect high levels of methane, oxygen and ozone – all handy indicators that something is living and breathing down on the surface. Evidence of photosynthesis and water was also present and correct, though they didn’t find any ice, because the poles were out of view during the experiment.

Actually, we already knew this, and not just because we have Earth stamped on our intergalactic passport. This is the second time such an experiment has occurred. Back in 1993, Carl Sagan and his team used the launch of the Galileo spacecraft to examine the Earth from afar, and see what similarly teched-up aliens would make of Earth.

And that’s where another bit of bad news comes in: any alien considering Earth as an option for getting on the property ladder would find it considerably less appealing than it was back in 1993. Methane levels were 12% up on their 1993 levels, while carbon dioxide was also 14% higher. That’s not a surprise to the researchers – after all, in 1993 Earth was home to ‘just’ 5.5 billion humans, and that number has since swollen by an additional two billion.

“It’s a challenging intellectual enterprise,” Dante Lauretta, principal investigator of the mission, told Science. “I really tried to channel Carl Sagan.”

The experiment was done from the OSIRIS-REx, a probe launched by NASA last year. And just as Sagan’s 1993 version was a small part of a wider mission – checking out the atmosphere of Jupiter – OSIRIS-REx has bigger plans than just checking Earth is safe for humans. It’s on its way to examine the composition and trajectory of the Bennu asteroid, which has a 1 in 2,700 chance of hitting Earth. A long shot, maybe, but probably just as well our methodology for detecting hospitable planets is up to snuff, eh?