Older Alien Worlds Might Still Be Stuck in the Slime Age

There’s an active debate in the astrobiology community of where to use our best telescopes to look for our best chance of seeing alien life. That debate has been going on for decades, and will continue for some time to come, but now there’s another option up for consideration. A new paper in the International Journal of Astrobiology by Dr. Christopher Doughty of Northern Arizona University talks about how the total amount of carbon a planet is able to bring into the service of biology is a good indication of how evolved life on it might be. Typically, astrobiologists have used the age of a planet’s star as a proxy for its chances of harboring intelligent life. The logic is simple - if a star has been around for longer than our Sun, its planets have likely been around longer than Earth. And therefore, they have had even longer than Earth has to develop complex, intelligent life. Longer time to evolve equals higher chance for intelligence. That makes logical sense, but not all planets are created equal. In fact, not even all parts of the same planet are created equal in biological terms. Earth is a great example of that. The tropics, where it is consistently warm, with abundant moisture and ample sunlight, churn out new species and evolutionary adaptations much faster than the frigid poles of our own planet. Fraser talks about the future of exoplanet hunting. Biologists call this the Evolutionary Speed Hypothesis (ESH), and it is also relatively easy to understand. More biological activity fuels larger populations, and therefore more generations of life. More generations lead to higher rates of genetic mutations, and therefore to faster diversification of life and a higher chance for intelligence to evolve. Dr. Doughty and his co-authors take that logic a step further. On a planetary scale, they apply what is known as the Net Primary Production (NPP) - a measure of how much carbon photosynthetic organisms turn into organic matter every year. And they argue that the NPP of an exoplanet can serve as a rough “proxy” for how many “lives” have been lived on the planet. Turning back to Earth allows them to lay out some useful benchmarks. For three billion years, the only life on Earth was single-celled organisms that dominated our oceans, and fixed carbon at relatively low rates. In total, before the Cambrian explosion around 500 million years ago, the total amount of carbon fixed in biological systems on Earth was around 2.4x10^25 grams. Fraser talks about the possibility of "superhabitable" planets. After the Cambrian explosion, however, the amount of carbon capture skyrocketed, leading up to the advent of modern humans. By the time our ancestors showed up, which the authors argue is a good point for the advent of “intelligence”, the amount of carbon captured by biological processes had grown to 9.4x10^25 grams. Using those benchmarks, the authors then turned to see how exoplanets would stack up against our own pale blue dot. One particular exoplanet candidate stood out - TRAPPIST-1e. This has been a focal point of much astrobiological research, including an entire project to develop climate models. But it appears that life evolving on this potential candidate would be severely throttled in terms of its complexity. TRAPPIST-1e has three main downsides. First, it is tidally locked to its star, a red dwarf that only illuminates half of the planet. That means, on the other side of the world, literally no photosynthesis can happen at all. Second, red dwarfs mainly output energy in infrared, and while plants can theoretically evolve to use that to drive photosynthesis, they aren’t currently equipped to do so. What’s more, infrared wavelengths do not reach very far down into water, meaning microbes that develop in the ocean would have a hard time making use of it. Finally, TRAPPIST-1e seems to have relatively limited precipitation according to those climate models, limiting the availability of another crucial component of biology as we know it - water. All these problems combine to make this particular exoplanet not a great candidate for advanced life. Fraser talks about TESS, one of our main planet hunting telescopes. That’s not to say that other exoplanets in our neighborhood aren’t. The authors looked at 29 confirmed rocky exoplanets and used this three-zone scale (single-cell, multi-cell, and intelligent) to calculate where they believe each of them might be. And two of them fell into the “intelligent” category. K2-3d, which is located about 143 light years away, ranked the highest, as it is slightly larger than Earth and noticeably warmer, it came in at an NPP of 1.74 times that of Earth. Another great candidate is GJ 1061c, located just 12 light years away. It receives around 45% more stellar energy than Earth, and is also around 7 billion years old, allowing it to eke out Earth’s NPP with a value of 1.03x our own planet’s captured carbon. To be clear, this does not mean there are advanced alien civilizations on either of those planets, and the authors are very explicit about that. There are also some big assumptions in the methodology itself. For example, should tidally locked planets be eliminated entirely because planets need a “sleep” cycle - which they do on Earth for complex biochemical repair. And most paleontologists view evolutionary transitions as a series of sudden breakthroughs, like the Cambrian explosion, rather than a steady, incremental progression. This calls into question whether a gradual marker like NPP is really indicative of evolutionary stages at all. What the paper does do is add another topic to consider when figuring out where to point those advanced telescopes. At the very least, this new framework can help inform the debate on how we can find out if we’re truly alone in the universe or not. Learn More: NAU - Is a nearby advanced alien civilization out there waiting to pounce on Earth? Unlikely, says new study C.E. Doughty et al - Calculating potential cumulative carbon fixed and evolutionary stage for Earthlike planets in our solar neighborhood UT - Exoplanets Without Lots of Water Can't Maintain Their Carbon Cycles UT - Future Telescopes Could Detect Life Managing their Planet Atmospheres