In the 15 years since the discovery of the first planet around a star other than our Sun, we have found more than 400 such exoplanets — but there is still no place like home. In How to Find a Habitable Planet, James Kasting extends knowledge of the planets in our Solar System to searches for distant Earth-like exoplanets that could harbour life.
Kasting is a visionary best known for his computer models that established the concept of 'habitable zones' around stars — regions where planets could host conditions conducive to life, such as liquid water. His research is wide ranging, from modelling ancient runaway glaciations on a 'Snowball Earth' to examining a distant future in which our planet might, like Venus, forever lose its precious oceans. The likelihood of discovering Earth-like planets around other stars in the next few years motivates Kasting to hunt for chemical signatures of life that might be detected in their atmospheres.
The search for extraterrestrial life is often equated with one project, the Search for Extra-Terrestrial Intelligence, or SETI. Its goal of detecting radio transmission from advanced technological civilizations is laudable — the cost is low and even a null result has an enormous pay-off in terms of understanding our place in the Universe. Whether the signal comes from potential predators, friends or the distant, unreachable echoes of other life forms, this is a story that humanity yearns to hear. However, it is a good bet that most life in the Universe will be simpler than us and mimic that on Earth, where single-celled organisms constitute the bulk of the biomass.
Discussing Earth's history, Kasting explains how prokaryotic cyanobacteria transformed the early anaerobic atmosphere on Earth into an oxygen-rich environment. These simple organisms paved the way for more complex species, but they also imprinted on our atmosphere a chemical signature of a living world: oxygen. Observations of exoplanet atmospheres will cast a wide net in the search for varied forms of life.
Another clue would be pooled water on the surface of a planet, which could provide a solvent for prebiotic chemistry. Kasting's habitable zone is often couched as a Goldilocks story: planets capable of hosting life must be neither too hot such that liquid water evaporates, nor too cold such that it freezes, but just right, allowing liquid water. In our Solar System, Earth sits snugly inside the inner boundary of Kasting's habitable zone, whereas Venus orbits too close to the Sun, dooming our near twin to be waterless and lifeless.
The case of Mars illustrates that there is more to habitability than a planet's distance from a star. Freezing temperatures render the red planet lifeless today. But there is evidence that liquid water once flowed on its surface, sculpting gullies and river beds. Impact craters from the heavy bombardment period of the Solar System overlay these twisting features, dating them to at least 3.8 billion years ago.
Kasting notes that this interpretation of the surface morphology on Mars is not universally accepted, but it is appealing. The possibility of a liquid-water phase in the planet's history raises the question of whether life might have had an early start there with a thicker atmosphere that trapped heat. Any surface water has long since evaporated because the planet was too puny to hang on to its atmosphere.
In recent years we have progressed from being able to detect only massive gas giants around other stars to discovering a handful of smaller rocky worlds a few times the mass of Earth. Kasting reviews the astronomical techniques involved. Ground-based telescopes can detect a planet through its influence on the motion of the parent star, revealed through Doppler velocity shifts and changes in the timing of pulsar flashes for neutron stars, or through brightness changes caused by gravitational microlensing and planetary transits.
From space, the Kepler mission, which launched in 2009, is now taking a census of stars to determine the fraction that have Earth-sized planets. The Hubble Space Telescope has detected abiotic atmospheric features on other planets, such as water and methane derived from physical processes, and the Spitzer Space Telescope has measured wind speeds in gas-giant exoplanets. Future missions will measure the mass and orbits of Earth-like planets and trace the motions of clouds.
Kasting deftly charts a course through planet formation, detection and geochemistry to introduce readers to what is likely to be the next big discovery: the detection of oxygen, water or methane in the atmospheres of exoplanets, signalling the possibility of life. Challenging the notion that life elsewhere is rare, he makes a strong case that simple life forms will contaminate atmospheres in ways that are spectroscopically detectable. But he cautions that abiotic sources of oxygen and methane can lead to false positives.
The key will be finding a convincing set of conditions. For Kasting, finding both liquid water on the surface and oxygen in the atmosphere would be enough. “I would be 99 per cent sure that life was present if I saw this combination of planetary characteristics,” he says. Such signatures of life could be detected in the coming decades.
How to Find a Habitable Planet will fascinate those who care about the evolution of Earth's climate and wonder about how we will eventually detect life — even simple cellular life — elsewhere.