Possible Subterranean Life Means More Exoplanets Could Harbor Life

Artistic representation of the current five known potential habitable worlds. Will this list broaden under a new habitability model? Credit: The Planetary Habitability Laboratory (PHL)

When we think of life on other planets, we tend to imagine things (microbes, plant life and yes, humanoids) that exist on the surface. But Earth’s biosphere doesn’t stop at the planet’s surface, and neither would life on another world, says a new study that expands the so-called ‘Goldilocks Zone’ to include the possibility of subterranean habitable zones. This new model of habitability could vastly increase where we could expect to find life, as well as potentially increasing the number of habitable exoplanets.

We know that a large fraction of the Earth’s biomass is dwelling down below, and recently microbiologists discovered bacterial life, 1.4 kilometers below the sea floor in the North Atlantic, deeper in the Earth’s crust than ever before. This and other drilling projects have brought up evidence of hearty microbes thriving in deep rock sediments. Some derive energy from chemical reactions in rocks and others feed on organic seepage from life on the surface. But most life requires at least some form of water.

“Life ‘as we know it’ requires liquid water,” said Sean McMahon, a PhD student from the University of Aberdeen’s (Scotland) School of Geosciences. “Traditionally, planets have been considered ‘habitable’ if they are in the ‘Goldilocks zone’. They need to be not too close to their sun but also not too far away for liquid water to persist, rather than boiling or freezing, on the surface. However, we now know that many micro-organisms—perhaps half of all living things on Earth—reside deep in the rocky crust of the planet, not on the surface.”

Location in the night sky of the stars with potential habitable exoplanets (red circles). There are two in Gliese 581. Click the image for larger version. CREDIT: PHL @ UPR Arecibo and Jim Cornmell.

While suns warm planet surfaces, there’ also heat from the planets’ interiors. Crust temperature increases with depth so planets that are too cold for liquid water on the surface may be sufficiently warm underground to support life.

“We have developed a new model to show how ‘Goldilocks zones’ can be calculated for underground water and hence life,” McMahon said. “Our model shows that habitable planets could be much more widespread than previously thought.”

In the past, the Goldilocks zone has really been determined by a circumstellar habitable zone (CHZ), which is a range of distances from a star, and depending on the star’s characteristics, the zone varies. The consensus has been that planets that form from Earth-like materials within a star’s CHZ are able to maintain liquid water on their surfaces.
But McMahon and his professor, John Parnell, also from Aberdeen University who is leading the study now are introducing a new term: subsurface-habitability zone (SSHZ). This denote the range of distances from a star within which planets are habitable at any depth below their surfaces up to a certain maximum, for example, they mentioned a “SSHZ for 2 km depth”, within which planets can support liquid water 2 km or less underground.

If this notion catches on – which it should – it will have exoplanet hunters recalculating the amount of potentially habitable worlds.

The research was presented at the annual British Science Festival in Aberdeen.

Source: University of Aberdeen

See also: The Habitable Exoplanets Catalogue from the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo.

Exoplanet Gliese 581g Makes the Top 5

Exoplanet Gliese 581g is back, and “officially” ranking #1 on a list of potentially habitable worlds outside of our solar system thanks to new research from the team that originally announced its discovery in 2010.

Orbiting a star 20 light-years away, the super-Earth is now listed alongside other exoplanets Gliese 667Cc, Kepler-22b, HD85512 and Gliese 581d in the University of Puerto Rico at Arecibo’s Habitable Exoplanets Catalog as good places to look for Earthlike environments… and thus the possibility of life.

First announced in September 2010 by a team led by Steven S. Vogt of UC Santa Cruz, the presence of Gliese 581g was immediately challenged by other astronomers whose data didn’t support its existence. Vogt’s team conducted further analysis of the Gliese system in which it appeared that the orbits of the planets were circular, rather than elliptical, and it was in this type of scenario that a strong signal for Gliese 581g once again appeared.

Read: Could Chance For Life on Gliese 581g Actually Be “100%”?

“This signal has a False Alarm Probability of < 4% and is consistent with a planet of minimum mass 2.2M [Earth masses], orbiting squarely in the star’s Habitable Zone at 0.13 AU, where liquid water on planetary surfaces is a distinct possibility” said Vogt.

And, located near the center of its star’s habitable “Goldilocks” zone and receiving about the same relative amount of light as Earth does, Gliese 581 g isn’t just on the list… it’s now considered the best candidate for being an Earthlike world — knocking previous favorite Gliese 667Cc into second place.

Read: Billions of Habitable Worlds Likely in the Milky Way

The announcement was made on the PHL’s press site earlier today by Professor Abel Méndez, Director of the PHL at UPR Arecibo.

Diagram of the Gliese system. The green area is the habitable zone, where liquid water can exist on a planet’s surface. (PHL @ UPR Arecibo)

“The controversy around Gliese 581g will continue and we decided to include it to our main catalog based on the new significant evidence presented, and until more is known about the architecture of this interesting stellar system”

– Prof. Abel Méndez, UPR Arecibo

Coming To A Solar System Near You… Super-Earth!


It is our general understanding of solar system composition that planets fall into two categories: gas giants like Jupiter, Saturn, Neptune and Uranus… and rocky bodies that support some type of atmosphere like Earth, Mars and Venus. However, as we reach further into space we’re beginning to realize the Solar System is pretty unique because it doesn’t have a planetary structure which meets in the middle. But just because we don’t have one doesn’t mean they don’t exist. As a matter of fact, astronomers have found more than 30 of them and they call this new class of planet a “Super-Earth”.

“Super-Earths, a class of planetary bodies with masses ranging from a few Earth-masses to slightly smaller than Uranus, have recently found a special place in the exoplanetary science.” says Nader Haghighipour of the Institute for Astronomy and NASA Astrobiology Institute, University of Hawaii. “Being slightly larger than a typical terrestrial planet, super-Earths may have physical and dynamical characteristics similar to those of Earth whereas unlike terrestrial planets, they are relatively easier to detect.”

Having a super-Earth in the neighborhood opens the avenue towards habitability. Chances are planets of this type have a dynamic core and are able to maintain a type of atmosphere. When combined with being within the habitable zone of a host star, this raises the bar towards possible life on other planets.

“It is important to note that the notion of habitability is defined based on the life as we know it. Since Earth is the only habitable planet known to humankind, the orbital and physical characteristics of Earth are used to define a habitable planet.” says Haghighipour. “In other words, habitability is the characteristic of an environment which has similar properties as those of Earth, and the capability of developing and sustaining Earthly life.”

But being a super-Earth means that there is a lot more going on than just being in the zone. To qualify it must meet three requirements: its composition, the manifestation of plate tectonics, and the presence of a magnetic field. For the first, the presence of liquid water is a high priority. In order to determine this possibility the values of its mass and radius have to be known. To date, two super-Earth planets for which these values have been determined – CoRoT-7b and GJ 1214b – have given us fascinating numerical modeling to help us better understand their composition. Plate tectonics also plays a role through geophysical evolution – just as the presence of a magnetic field has been considered essential for habitability.

“Whether and how magnetic fields are developed around super-Earths is an active topic of research.” notes Haghighipour. “In general, in order for a magnetic field to be in place around an Earth-like planet, a dynamo action has to exist in the planet’s core.”

Last, but not least, comes an atmosphere – the “presence of which has profound effects on its capability in developing and maintaining life.” From its chemical properties we can derive the “planet’s possible biosignatures” as well as the chemicals which formed it. Atmosphere means environment and all of this leads back to being within a habitable zone and of sufficient gravity to keep atmospheric molecules from escaping. Says Haghighipour, “It would not be unrealistic to assume that super-Earths carry gaseous envelopes. Around low-mass stars, some of such atmosphere-bearing super-Earths may even have stable orbits in the habitable zones of their host stars.”

Has a super-Earth been detected? You betcha’… and studied right down to its spectral signature. “The recently detected super-Earth GL 581 g with its possible atmospheric circulation in the habitable zone of its host star may in fact be one of such planets.” says Haghighipour. “More advanced telescopes are needed to identify the biosignatures of these bodies and the physical and compositional characteristics of their atmospheres.”

Further Reading: Super-Earths: A New Class of Planetary Bodies.