Caltech scientists have recently shown that the presence of biology on a terrestrial planet acts naturally to more than double the lifetime (vs previous estimates) of the planet’s biosphere. In their paper (Proc. Natl. Acad. Scis., June 16, 2009) King-Fai Li et al. calculate that for the Earth, the predicted persistence of the photosynthetic biosphere 2.3 Ga into the future (plus the past 2.3 Ga of its existence) suggests that our planet will be remotely identifiable as an inhabited planet for nearly 1/2 of the Sun’s total lifetime.

The presence of life on Earth-like planets naturally delays the planet-killing effects of the Sun and CO2 much longer than previously believed. Click earth.Sun.

This has major implications for the search for extraterrestrial life because lengthening the total time a planet can support life makes it more likely that we will eventually detect them. Early searches for habitable extrasolar planets have already yielded interesting results (Tinetti et al., 2007). Indeed more successful searches are expected as we approach the 2015 Maslow Window, because the 2 key drivers of human expansion into the cosmos are 1) the discovery and exploration (and eventual settlement) of Earth-like planets and 2) the search for extraterrestrial life, including intelligent ETs.

Being an Earth-like planet is not easy — that’s why there are so few of them. One big problem is the Sun because standard evolutionary models for Sun-like stars show they increase in luminosity as they age. For example, the Sun was only 70% its current brightness when it was born, nearly 5 Ga ago. The “Faint Young Sun” should have nudged Earth’s temperatures out of the habitable zone, but it didn’t, except for a couple possible “Snowball Earth” hiccups. However much later in its life, the brilliant Sun on steroids ends habitability as the oceans evaporate and Earth is fried by a version of the “runaway greenhouse” that has already occurred on Venus.

This “Faint Young Sun” paradox requires that 3.5 Ga ago the partial pressure of CO2 was near 7000 Pa (about 7% of one atmosphere) producing a greenhouse that warmed the surface. However, the brightening Sun means that CO2 must have been continuously removed from the atmosphere somehow, and it was James Lovelock in the early 1980s — author of the Gaia Hypothesis — who suggested that “biologically enhanced silicate weathering” could do the trick. His model indicated that CO2 would drop to 15 Pa — the lower limit for photosynthesis — in only 100 Ma, thus terminating the biosphere.

Li et al. have lengthened estimates of a terrestrial planet’s habitable lifetime by recognizing that the total atmospheric pressure plays a key role in regulating its temperature. On Earth, nitrogen (78% by volume) is continuously removed from the atmosphere by “oceanic primary productivity and subsequent burial of organic sediments…” They estimate that it is possible to sequester about 1 atmosphere of nitrogen in the crust and mantle through biological processes. As the Sun’s brightness increases and nitrogen is biologically removed from the atmosphere, Li et al. calculate that CO2 becomes less able to trap outgoing infrared radiation due to standard pressure broadening effects on the CO2 absorption bands.

In essence, the biologically-triggered decline in nitrogen atmospheric abundance becomes a natural climate regulator by weakening the ability of the CO2 greenhouse to trap surging solar heat radiation. This keeps Earth — and any inhabited extrasolar terrestrial planets — cooler and habitable much longer than previously expected.

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