What makes Earth ‘just right’ for life?

Within a year, we have been able to see our solar system as never before. In November 2014, the Philae Probe of the Rosetta spacecraft landed on the halter-shaped Comet 67P/Churyumov-Gerasimenko. In April 2015, the Dawn spacecraft entered orbit around the largest of the asteroids, Ceres (590 miles in diameter), orbiting between Mars and Jupiter. And in July, the New Horizons mission made the first flyby of the dwarf planet Pluto, making it the most distant solar-system object to be visited. Other spacecraft continue to investigate other planets, for example, Cassini (whose Huygens probe landed on Titan) studies Saturn and its moons, and Orbiters and Rovers are exploring Mars.

The cameras on such spacecraft return fascinatingly beautiful images (see, for example, NASA’s Solar System Exploration). But they show a deadly beauty, hostile to life as we know it. No planet in the solar system other than Earth is able to support human life, unless humans bring oxygen, food and shelter with them, and even then it would only be for short, dangerous visits. Among the many exoplanets discovered outside our solar system, some may be Earth-like, but most clearly aren’t.

What makes Earth ‘just right’ for life? Part of the answer is that the environment has shaped life just as life has shaped its environment. In other words, Earth itself was ‘terraformed’ by the life it supported over time. For example, the large fraction of pure oxygen in Earth’s atmosphere wasn’t a condition for life to develop on this planet, but a consequence of it. Photosynthesis, first by cyanobacteria and later by plants, started in a nitrogen-carbon dioxide atmosphere at least some 2.4 billion years ago, ultimately resulted in 21% molecular oxygen by volume.

It appears that what was essential to life on Earth from the outset was the abundance of liquid water. And that has to do with the irradiation of the Earth by sunlight; closer to the Sun would be too warm, further too cold. A somewhat heavier Sun would not have provided stable irradiation for almost five billion years. A lighter Sun would have its ‘habitable zone’ so close that magnetically-driven ‘weather in space’ and gravitational tides would likely pose severe hazards to life.

The abundance of water in the Earth’s oceans and atmosphere caused ‘geochemical weathering’ of rocks and soil, a chemical process that removes carbon dioxide from the atmosphere and stores it in sediments. This process removed much of the carbon dioxide in Earth’s early atmosphere, but continued sequestration would ultimately remove so much that plants couldn’t survive. As plants ‘breathe in’ carbon dioxide to grow, this would mean no food, and ultimately no oxygen for mammals, including humans. Carbon dioxide is, however, brought back into the atmosphere by volcanic eruptions (with fossil fuel burning as a modern-day additional source). Volcanism works because of the high temperature inside the Earth, which drives plate tectonics and its associated volcanic eruptions. It also happens to drive the magnetic dynamo that shields life on Earth from hazardous cosmic rays from the Sun and throughout the Galaxy.

That Earth still has an active dynamo and plate tectonics is a consequence of its size. Had Earth been smaller, like Mars or like the Moon, its interior would have cooled off so much by now that neither of these processes would work any more. It is also likely a consequence of the presence of water, which works to promote plate tectonics, and removing water from a planet can help stop its plate tectonics. This appears to have happened to Venus, being closer to the Sun than Earth.

However there are other factors associated with the Earth being ‘just right’ for the life that it supports. The sciences that look into this are astrobiology (focusing on the biological aspects), geology (focusing on planetary properties), and heliophysics (looking at the physics of the sun-space environment). Exploring the universe around us, they enable an ecological understanding of why we live in our exact environment, and where else in the Galaxy we might look for similar conditions. As we have no means of interstellar travel, and as interplanetary travel doesn’t get us to another suitable environment, we have only this one oasis, Earth, to call home.

Featured image credit:The Earth seen from Apollo 17 by NASA/Apollo 17 crew; taken by either Harrison Schmitt or Ron Evans. Public domain, via Wikimedia Commons.