THE LIFE OF SUPER-EARTHS

Since astronomers discovered the first planet circling another star in 1995, they’ve found hundreds; predictably, this has energized the debate on whether life exists beyond Earth. Sasselov (Astronomy/Harvard Univ.) reviews the hard evidence in favor (not much) before proceeding to explain discoveries and simulations that suggest we are not alone.

No telescope has directly observed an extra-solar planet, but the author delivers a clear explanation of how instruments and, since 2009, a satellite are detecting subtle changes in a star’s light or movement that reveal not only the presence of planets (600 so far) but their size, orbits and a hint of their composition. Sasselov maintains that the minority of “super-earths” possess conditions favorable to life: proper temperature, protective atmosphere, volcanism and tectonic movements. These are rocky, watery planets from one to 10 times the mass of Earth, which barely makes the cut. The author reminds readers that life is not fussy. Microbes thrive inside Antarctic ice sheets and in hot rock miles beneath us. Near boiling vents at the sea bottom, far beyond the reach of sunlight, they feed on hydrogen sulfide or other toxic chemicals that spew out and support a dense ecosystem of higher life forms. Life has existed for four billion years, a time comparable to the age of the universe (13 billion), so it may be a normal cosmic process along with planet formation.

As short, cogent and stimulating as John Gribbin’s Alone in the Universe (2011), but far more optimistic. Readers should check out both.