Compared to exoplanets, we know a lot about the Earth. It’s our home planet and the one that shapes our expectations when we study other worlds. Planetary science evolved out of Earth science. Exoplanet models started out as Earth climate models. Yet research on Earth throughout its history shows that the climate has been very different at different times. The young Earth may as well be an exoplanet. But the young Earth still had life—a different kind of life than what dominates the biosphere today. So what do we really mean when we say we’re looking for “habitable” and “Earth-like” planets? What kind of biosignatures would we expect from a planet like the early Earth, rather than today’s Earth?
The period from about 4 to 2.5 billion years ago, called the Archaean age, was a critical one in Earth’s history. A couple of things happened at the beginning of the Archaean. One, the Earth cooled down enough for continents to form. (Some Archaean rock is still around today, including in Scotland (https://en.wikipedia.org/wiki/Lewisian_complex)!) Two, the first life forms appeared: single-celled prokaryotes and bacteria. These life forms didn’t photosynthesise and produce oxygen as a by-product, so the Earth’s atmosphere contained little to none of it at this time. Any astrobiologists looking for signs of life on Archaean Earth wouldn’t find traces of oxygen. As a potential biosignature, it’s out.
Some scientists believe Archaean Earth had a hazy atmosphere similar to that of Saturn’s moon Titan today. Titan, visited in recent years by the Cassini and Huygens missions, is a fascinating world with wildly different atmospheric chemistry from modern Earth. Like Earth’s, the atmosphere is mostly nitrogen, but unlike on Earth, the next most common element is methane, a molecule composed of hydrogen and carbon atoms.
When photons from the Sun and high-energy particles accelerated at Titan by Saturn’s magnetic field hit the upper atmosphere, they break apart the methane into its individual parts. This initiates a whole domino chain of complex chemical reactions that creates a huge variety of hydrocarbons—molecules that consists of chains of varying numbers of hydrogen and carbon atoms. The hydrocarbons are usually called “organic,” although they aren’t alive and don’t say anything about whether there is life present. They float around in the atmosphere, giving it the hazy orange appearance seen in images like this (https://www.sarahhorst.com/titan.jpg).
Why do scientists believe Archaean Earth might have had a Titan-like “organic” haze? In brief, rocks from some parts of the Archaean period have a certain ratio of sulphur isotopes that could only be achieved by ultraviolet photons breaking up sulphur gases in an oxygen-less atmosphere. Variations in the sulphur isotope ratio are correlated with cooler temperatures and with evidence of greater methane in the atmosphere. The theory is that in periods with a larger amount of methane, photons bombarding the atmosphere caused Titan-like hydrocarbon hazes to form. The hazes absorbed UV photons, affecting the sulphur isotope ratio and cooling the planet at the same time. On top of that, laboratory experiments have shown that bombarding gases in conditions similar to the Archaean environment leads to the production of Titan-style hazes.
Why is this relevant to life? Methane gas is created by organisms. It can be—and is—also created by other, non-biological processes like volcanoes. That’s why we generally don’t think the methane on Titan is a sign of life. The temperature on Titan is about -180 Celsius/-290 Fahrenheit and there’s no liquid water. Mars is a little more controversial… there’s a mystery source of methane on Mars. Mars rovers and orbiters have detected temporary spikes of methane, which quickly disappear again. Lakes of liquid water have also been detected under the Martian ice caps. How would methane emitted by organisms in these lakes get into the atmosphere? That, as well as the existence of life on Mars now or in the past, remain open questions. But back to the Earth.
Most of the methane on Earth, now and in the past, comes from biological sources. A haze of methane-derived hydrocarbons in the Archaean period could be considered a biosignature: an indirect chemical sign of life. Not only would an organic haze potentially be a sign of life, it could also support the existence of life. A Titan-style haze would absorb UV radiation, protecting life forms on the surface from its harmful effects. (Today, ozone shields us from UV light, but ozone is a by-product of oxygen and wouldn’t exist in an oxygen-less atmosphere.)
Now back to exoplanets. Early observations of exoplanets have shown that a lot of them have clouds or hazes or generally some kind of stuff floating around in the atmosphere. This makes sense from both a physics and a chemistry perspective. So imagining that exoplanets could have organic hazes like Titan and the early Earth is attractive for multiple reasons. One, we can detect those hazes! Two, in the right context—based on the other atmospheric chemistry, the planet’s temperature, the star’s activity, and other factors—these hydrocarbon hazes could be evidence of life. And third, they could protect life from the harsh UV radiation emitted by the M-class stars which many exoplanets we’re interested in orbit.
The downside is that the chemistry behind hydrocarbon hazes is extremely complex. We certainly don’t understand it all or have a complete list of the kinds of molecules formed by disassembling methane and reassembling its bits, Lego-style, into new creations. We don’t know all the effects these hazes would have on either the environment of a planet or our observations of it.
But an advantage of the fact that Archaean Earth may have had hazes—of treating our own planet’s history like an alien planet—is that we have a lot more information about Earth’s past climates. We’re not limited to a few photons collected from light-years away. There are rocks still left over from the Archaean, the geological record, fossil evidence of early bacteria. We have some idea of what the atmospheric chemistry was like and we know what kind of photons the young Sun 4 billion years ago was producing. Exoplanet science isn’t just the study of far-away objects we’ll never visit: it’s entwined with the study of our own planet.