Sean McMahon

For the first time in history, new planets are regularly discovered around other stars. Could they support life? In a new paper in Planetary and Space Science, my colleagues and I show that life may exist far outside the traditional “habitable zone”.

Life on Earth requires liquid water, an accessible source of energy and a host of chemical elements including carbon, nitrogen, oxygen and phosphorous. Where else in the universe are these requirements met?

Answers to this question usually sketch a narrow “habitable zone” around each star, in which any orbiting Earth-like planet could sustain liquid water on its surface. Any closer to the star, the greenhouse effect would spiral out of control and boil off the oceans; any further out, the climate would shift too far the other way, freezing the planet into a snowball.

There are good reasons to celebrate when planets are found in the habitable zones of distant stars. Such planets, if they are similar in chemical composition to the Earth, could support complex biospheres powered by sunlight. But apart from the Earth, the planets and moons in our own solar system most likely to be habitable (such as Mars and Europa) lie outside the conventional habitable zone. Their surfaces receive too little heat from the sun for water to remain liquid. Nevertheless, deep under the ice and rock, life could thrive in liquid water maintained and circulated by geothermal heat. We know that anaerobic microbes miles below the Earth’s surface derive nutrients and energy from minerals and hydrothermal fluids. In fact, the Earth could still support life even if it were removed to several times its current orbital distance, far beyond the habitable zone.

These observations seem to limit the application of conventional habitable zones, which exclude planets with frozen surfaces but habitable subsurface environments. Last year, my colleagues and I announced at the British Science Festival in Aberdeen that we were working to extend habitable zones to include these planets (see media coverage here and here). The results of this project are now available.

We began with the observation that the deeper one digs under the surface of a planet, the hotter it gets; the temperature at any particular depth is controlled by the balance of solar energy transferred from above and geothermal energy transferred from below. By considering these sources of energy and the way they are transferred as heat through rocks and atmospheres, we derived a computer model to estimate the temperature at any depth below the surface of a hypothetical rocky planet orbiting its sun at any distance. This allowed us to extend the habitable zone by calculating the distance from the sun where water would freeze – not just on the surface of a planet – but at any given depth below its surface. We used this distance as the outer edge of a new, extended habitable zone, which we call a subsurface-habitability zone (SSHZ). For example, one can calculate the SSHZ for planets able to support liquid water at depths of, say, five km or less. Here’s how SSHZs look compared to the standard habitable zone for an Earth-like planet orbiting our Sun (click to enlarge):

Habitable zones for deep biospheres are evidently much wider. This means that, as you might expect, many more planets and star systems can be considered habitable once the possibility of deep biospheres is taken into account. If these habitable environments are commonly inhabited, then deep biospheres may even be more common in the universe than surface biospheres – especially if life can get started in deep subsurface environments, independently of conditions at the surface. On the other hand, they may be much more difficult to detect.