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Extrasolar Planets and Astrobiology

Caleb A. Scharf Columbia University


This book by Caleb Scharf offers a formal exposition about an exploding development throughout the world: the merging of multiple scientific disciplines to form a compelling new field, namely astrobiology. The most profound questions posed by ancient civilizations largely remain unanswered, left for religious leaders, philosophers, and the curious among us to ponder throughout the ages. Ancient Greek philosophers intensely debated the uniqueness of our Earth and the possibility of life elsewhere, but they and their academic successors made little progress for 2400 years. Suddenly and startlingly, we stand at the brink of answering those old questions. 

The answers are arriving from two directions: progress on the physical prerequisites for life, and hints of the first habitable worlds. Astronomers have found that the fundamental equations of gravity, electricity, and quantum physics (some yet to be fully understood) are the same everywhere throughout the spacetime of the universe. The atoms and molecules of which life is composed are also ubiquitous throughout the universe. The 92 naturally occurring atoms are seen in stars and galaxies by spectroscopic analysis of their light, filling the periodic table for most of the 13.7 billion year age of the universe. Those atoms combine into complex organic molecules such as alcohols and amino acids, which are found in comets, moons, and interstellar clouds. The legos of life are everywhere. Moreover, water, the great chemical cocktail mixer, is among the most abundant of substances in the universe, found on planets, moons, and comets. Water mobilizes, destroys, and recombines the organic molecules, to create uncountable molecular permutations of great size and complexity. Few can doubt that proteins and nucleotides will form given enough time. 

The energy required for life comes in many forms, including starlight, geothermal, tidal, and radioactive, offering the complex organics myriad ways to power further reactions. Chemical replication will surely occur by molecular precursors to RNA and DNA, leading to a competition for both the valuable molecular building blocks and for the free energy. The successful precursors will multiply and outcompete their neighbors, populating their environment. The blurry but inexorable transition from “prebiotic” to “living” clusters of organic molecules reveals life as a natural phenomenon, forged in the furnaces of stars. 

Meanwhile, astronomers have developed techniques to detect the first habitable abodes in the universe. The subsurface water in Mars, the oceans inside Europa and Ganymede, and the geysers of Enceladus all constitute rich chemistry labs, each one percolating for billions of years. The exploration of these new worlds seems as compelling as the most courageous transoceanic voyages in human history. Should robotic probes discover the first alien lifeforms, the ubiquity of life in the universe will be established and the diversity of life sampled. 

Beyond our Solar System, over 300 planets have now been discovered orbiting other stars. So far, only large ones similar in size to Jupiter, Saturn, and Neptune have been found. But new techniques are now in hand to detect terrestrial-sized planets. The Kepler mission will search for stars that dim periodically due to earths that cross in front, blocking only 1/10000 of the starlight. Astronomers will also make precise Doppler measurements of stars to detect their reflex motion in response to the gravitational tug by terrestrial planets. Some clever astronomers are surveying the smallest stars, the red dwarfs, to detect their dimming and reflex motion, to reveal earth-sized planets. 

For the future, NASA and the Jet Propulsion Laboratory have developed the Space Interferometry Mission that will use the interference of light waves gathered by a spaceborne pair of telescopes to detect earth-like planets, and measure their masses, around nearby stars. Just over the horizon are plans for a spaceborne telescope that blocks the glare of nearby stars, allowing us to take images of Earth-like planets and to determine their chemical composition from their spectra. Any worlds having oxygen atmospheres and surface oceans will smell fishy from 40 light years. This census of habitable earths will fill GoogleGalaxy with ports-of-call for our grandchildren who will send robotic probes and later themselves, at least those with extreme daring and patience. The urge to explore these new worlds comes from our anthropological roots at Olduvai Gorge two million years ago. What sets us apart from the stones and the stars is our insatiable desire to understand our kinship with both. 

Geoffrey W. Marcy 
University of California, Berkeley