When it comes to finding alien life, there’s been a big paradigm shift in the last few decades. Thanks to the work of thousands of astronomers worldwide, we now have a lot more targets for potential life. From the oceans beneath Enceladus to temperate planets around nearby stars, there could be life both near and far away. But how can we spot it? Well, NASA is making plans.
The space agency wants to understand which objects are most likely to exhibit signs of life, and what these signs would actually be. The starting point for this endeavor is to look at Earth, the only object we know to support life. NASA has also started the Nexus for Exoplanet System Science (NExSS) to help coordinate the efforts.
"Before we go looking for life, we're trying to figure out what kinds of planets could have a climate that's conducive to life," Tony del Genio, one of the three co-leaders of NExSS, said in a statement. "We're using the same climate models that we use to project 21st-century climate change on Earth to do simulations of specific exoplanets that have been discovered, and hypothetical ones."
Scientists are aware that life out there might take some bizarre forms, but a project like NExSS can't be built solely on speculations. "We have to go with the kind of life we know," del Genio added. And as far as we know, life needs water, so that’s the current focus for astronomers.
In the Solar System, the most interesting targets are two icy moons, Europa and Enceladus, which orbit Jupiter and Saturn, respectively. Both moons host vast oceans underneath their icy exteriors. Just this year, we got confirmation of hydrothermal activity beneath the ice of Enceladus. Water, heat, and lots of molecules make interesting chemistry, but maybe not life.
There are a few missions (proposed and approved) that will visit Europa, and scientists are using what we see in the Arctic and Antarctic to find the most interesting locations on the surface of the moon. "When we visit Europa, we want to go to very young places, where material from that ocean is being expressed on the surface," researcher Morgan Cable, from NASA’s Jet Propulsion Laboratory, added. "Anywhere like that, the chances of finding evidence of life goes up – if they're there."
Other teams are instead looking beyond the Solar System. We have found many worlds that orbit the habitable zone of their stars. Liquid water could exist on those planets. But if we're looking at our own system from far away, we would have three planets in the habitable zone (Venus, Earth, and Mars) and only one suitable for life. Taking this into account, many are working to simulate what our planet would look like if it was indeed far away.
It is possible to reconstruct Earth's rotation and work out that there are continents just from a handful of pixelated images, but we can’t yet do that for exoplanets. Nevertheless, we will soon, and when our tech catches up with our ambitions, this research will be very important.
"I think that in 20 years we will have found one candidate that might be it," said del Genio. Research scientist Andrew Rushby, of NASA's Ames Research Center, added: "It's been 20 years away for the last 50 years. I do think it's on the scale of decades. If I were a betting man, which I'm not, I'd go for Europa or Enceladus."
The researchers consider a healthy dose of skepticism to be very important. They are committed to anticipating all possible false flags that we might find along the way.
"Life has to be the hypothesis of last resort," Cable said. "You must eliminate all other explanations."