When it comes to finding life on other planets, so far the score stands at Earth 1, Universe 0. It’s certainly not been for lack of trying. Over the past few decades NASA has launched a wide range of exploratory vehicles, from rovers like Curiosity to probes like Cassini. We’ve learned a tremendous amount about the planets, moons, and other bodies of the solar system as a result. At the same time, telescopes like Kepler have demonstrated the stars of the Milky Way galaxy commonly have planets, including planets of approximate Earth size that sit in the habitable zone of their stars, and might be capable of retaining liquid water on their surfaces.
But as the number of planets detected by Kepler stacks up, and new telescopes like the James Webb come online, it may be useful to turn the categorization of planets over to an artificial intelligence application capable of sorting them based on the likelihood of their supporting life. Researchers at the Centre for Robotics and Neural Systems at Plymouth University have created such an application using an advanced neural network, or ANN.
To build the ANN, the researchers trained it using the profiles of present-day Earth, the early Earth, Mars, Venus, or Saturn’s moon, Titan. Before we dive in further, let’s look at why these specific bodies were chosen.
Early Earth: The early Earth was significantly different than it is today. There’s evidence that life may have begun almost immediately. The Earth formed ~4.54 billion years ago, while evidence of microscopic fossils of up to 4.28B years old has been located. The oldest undisputed microbial remains are ~3.5B years old, from a time when Earth’s atmosphere contained very little oxygen — the Earth did not begin to transition to an oxygenated atmosphere until the Great Oxygenation Event ~2.45B years ago. Most of the life forms that had evolved prior to that event died. It’s therefore useful to consider an early Earth model — it captures a very different type of life.
Mars: We now know that Mars retained liquid water for a substantial period of time. It was probably most habitable during its Noachian period; that’s approximately the Hadean and Archean on Earth, roughly 3.7 to 4.1B years ago. This was partly because meteorite impact rates were an estimated 500x higher than they are today, releasing large amounts of energy as they impacted and leading to increased volcanic activity. A profile of Mars tells us what conditions are like on smaller planets that are farther away from the Sun — such a system, if glimpsed early in its evolution, could harbor life.
Venus: Venus is the very definition of a toxic hellstew, with surface pressures that would crush humans like a beer can stomped on by the Hulk and an 800C surface temperature. But there’s been some speculation that life might exist in the clouds of Venus, where conditions are more temperate, and Venus, like Mars, might have been far more habitable in its distant history.
This near-infrared, color mosaic from NASA’s Cassini spacecraft shows the sun glinting off of Titan’s north polar seas. This mirror-like reflection, known as the specular point, is in the south of Titan’s largest sea, Kraken Mare, just north of an island archipelago separating two separate parts of the sea. It is also the exact color of a Pan-galactic Gargle Blaster. Credit: NASA
Titan: Titan is one of the more interesting inclusions in this study. While we don’t know if life has arisen on Titan, this moon of Saturn is thought to resemble the early Earth at a much colder temperature. Titan is the only other place in the solar system known to have stable bodies of surface liquid (methane and ethane, in Titan’s case). NASA studies have indicated that complex organic molecules can form in Titan’s atmosphere. It’s theoretically possible that life could exist on Titan, with a liquid hydrocarbon standing in for water as a solvent.
The research team at Plymouth has taken this data and created an algorithm that rates the likelihood of a location nurturing life based on the one planet where we know life exists and the other locations where life either used to exist (early Earth), may have existed (Mars), might still exist in very difficult conditions (Venus) or might exist using a very different chemical chain (Titan). The lead author of the work, Christopher Bishop, shed some light on why his team undertook the project:
“We’re currently interested in these ANNs for prioritizing exploration for a hypothetical, intelligent, interstellar spacecraft scanning an exoplanet system at range,” Bishop said. “We’re also looking at the use of large area, deployable, planar Fresnel antennas to get data back to Earth from an interstellar probe at large distances. This would be needed if the technology is used in robotic spacecraft in the future.”
While the chances of discovering life outside the solar system will remain small in the next few decades, categorizing newly discovered planets on the likelihood of their having life could still let scientists better target research and exploration dollars and speed our search.