Here's a slightly awkward thought. Though we may feel ready to find other planets out there with life or even advanced civilizations, is it possible that they already spotted us, perhaps hundreds of millions of years in the past when our planet wasn't represented by humanity (flawed though we are) but giant, murderous reptiles? A new paper suggests that this could be the case, with Earth being much more detectable during the time of the dinosaurs than we currently are today.
The hunt for alien life, as we do so at the moment, is fairly elegant in its simplicity. As well as looking out for potential signals deliberately or unintentionally sent out into space by alien civilizations, we scan the stars for tiny dips in light that suggest an exoplanet has blocked our view of its light. Once we have located an exoplanet, we can look at factors such as where the planet is in its solar system to figure out if it is in a habitable zone.
Gases in planets' atmospheres block specific wavelengths of light, meaning that if we measure the spectra, we can get an idea of the chemical composition of the planet. As we've only ever found evidence of life on one planet (you're currently sitting on it) it makes sense to look for planets with a chemical makeup amenable to life on our own planet. But as a new paper from scientists at Cornell University suggests, in our current state we might not be at our most detectable.
“Modern Earth’s light fingerprint has been our template for identifying potentially habitable planets,” Lisa Kaltenegger, director of the Carl Sagan Institute, said in a press release, "But there was a time when this fingerprint was even more pronounced – better at showing signs of life."
The team looked at the last 540 million years of the Earth's evolution, and the chemical signatures that we would have given off at 100-million-year intervals. As stable (ish) as it might seem (if you ignore the hell out of the climate crisis), Earth's atmosphere has changed dramatically in our history.
For instance, for billions of years, there was no fire on Earth before cyanobacteria began to produce energy from sunlight, releasing oxygen into the atmosphere. Before then, methane would have been abundant thanks to bacterial life emerging on the planet.
The team focused on the last 540 million years as this is when the most interesting life emerged.
“The Phanerozoic is just the most recent 12% or so of Earth’s history, but it encompasses nearly all of the time in which life was more complex than microbes and sponges,” first author, astrobiologist and geologist Rebecca Payne said. “These light fingerprints are what you’d search for elsewhere, if you were looking for something more advanced than a single-celled organism.”
During this period, oxygen levels fluctuated from around 15 to 30 percent; we know this as under 13 percent, things will not burn, and at over 35 percent, matter will combust so well that forests would not be able to grow and sustain themselves. As dinosaurs began to thrive when oxygen peaked at around 30 percent, it could be an indication that if we find levels that high on another planet, it could be host to large creatures, like those that were once found on Earth.
Looking at the last 540 million years, the team found that two pairs of chemical signatures – oxygen and methane, and ozone and methane – were far more detectable as spectra to anyone looking our way around 100 to 300 million years ago than they are today. As well as telling us how our own planet would look to others, it could help us refine what to look for, as we find more exoplanets and refine how we can detect traces of them.
“This gives us hope that it might be just a little bit easier to find signs of life – even large, complex life – elsewhere in the cosmos," Kaltenegger added.
The study is published in Monthly Notices of the Royal Astronomical Society.