Over the last few decades, the existence of over 4,000 exoplanets has been confirmed by astronomers. We are yet to find a true Earth twin or anything truly Earth-like but plenty of exoplanets discovered are similar in size and may have a chance in hosting life – if other conditions are met. Among those conditions are volcanic and/or hydrothermal (signs of an ocean) activity, something that may have helped life to begin on Earth.
A team of researchers looked at 53 exoplanets with sizes the most similar to Earth – up to eight times the mass of Earth and twice the radius – and modeled how geologically active these worlds could be. Their modeling, reported in Publications of the Astronomical Society of the Pacific suggests that all of these worlds are likely to have volcanic activity and about a quarter might be "ocean worlds", harboring oceans under icy surfaces.
The team found intriguing possibilities between some exoplanets' potential properties and icy moons in the Solar System like Jupiter's moon Europa and Saturn's moon Enceladus, both considered ocean worlds and possible locations for life beyond Earth. If exoplanets exist that have ice exteriors, deep oceans, and geysers like these moons, then they might be a candidate for life too.
“If we see that a planet’s density is lower than Earth’s, that’s an indication that there might be more water there and not as much rock and iron. And if the planet’s temperature allows for liquid water, you’ve got an ocean world,” planetary scientist Lynnae Quick from the NASA’s Goddard Space Flight Cente, said in a statement. “But if a planet’s surface temperature is less than 32°F (0°C), where water is frozen then we have an icy ocean world, and the densities for those planets are even lower.”
The team started with basic considerations in terms of mass, volume, and distance from the parent star and the presence of other worlds to estimate how much internal heat these bodies should have. The models suggest that all these worlds have enough energy to have complex geological features such as volcanoes or hydrothermal sources at the bottom of water oceans.
These worlds might not look hospitable from a distance but the presence of life shouldn’t be discounted.
“Future missions to look for signs of life beyond the Solar System are focused on planets like ours that have a global biosphere that’s so abundant it’s changing the chemistry of the whole atmosphere,” says Aki Roberge, a NASA Goddard astrophysicist who collaborated with Quick on this analysis. “But in the Solar System, icy moons with oceans, which are far from the heat of the Sun, still have shown that they have the features we think are required for life.”
The upcoming James Webb Space Telescope and Extremely Large Telescope, among other in-the-works observatories, are expected to be able to study these distant worlds in more detail, and may be capable of discovering the tell-tale signs of volcanic activity.