‘Alien Oceans’ argues the search for E.T. should include the outer solar system
Alien OceansKevin Peter HandPrinceton Univ., $27.95 Life as we know it requires liquid water. That’s why astronomers get excited when a planet is found in a star’s “habitable zone,” the Goldilocks region in which a planet is neither too close nor too far from its star to have liquid surface water. In Alien Oceans, NASA scientist Kevin Peter Hand argues that the notion of the habitable zone should be expanded. At least six moons in the outer solar system are likely to have oceans of water beneath icy facades. For these orbs, the sun isn’t the heat source that keeps oceans liquid. Instead, the decay of radioactive elements inside a moon’s rocky core might keep things warm. Another possibility is tidal flexing: If a moon follows an elongated orbit around a planet, ever-changing tides would create friction within the core and release heat. Similar conditions could allow for ice-covered oceans on planetary...