More livable than Earth? New index sizes up the habitability of alien exoplanets

Image: James Webb Space Telescope

Researchers at the University of Washington’s Virtual Planetary Laboratory have devised a new habitability index for judging how suitable alien planets might be for life, and the top prospects on their list are an Earthlike world called Kepler-442b and a yet-to-be confirmed planet known as KOI 3456.02.

Those worlds both score higher than our own planet on the index: 0.955 for KOI 3456.02 and 0.836 for Kepler-442b, compared with 0.829 for Earth and 0.422 for Mars. The point of the exercise is to help scientists prioritize future targets for close-ups from NASA’s yet-to-be-launched James Webb Space Telescope and other instruments.

Astronomers have detected more than 1,000 confirmed planets and almost 5,000 candidates beyond our solar system, with most of them found by NASA’s Kepler Space Telescope. More than 100 of those have been characterized as potentially habitable, and hundreds more are thought to be waiting in the wings. The Webb telescope is expected to start taking a closer look soon after its scheduled launch in 2018.

“Basically, we’ve devised a way to take all the observational data that are available and develop a prioritization scheme,” UW astronomer Rory Barnes said Monday in a news release, “so that as we move into a time when there are hundreds of targets available, we might be able to say, ‘OK, that’s the one we want to start with.'”

This isn’t the first habitability index to be devised. Traditionally, astronomers focus on how close a particular exoplanet’s mass is to Earth’s, and whether its orbit is in a “Goldilocks zone” where water could exist in liquid form. But in a paper accepted for publication in the Astrophysical Journal, Barnes and his colleagues say their scheme includes other factors such as a planet’s estimated rockiness and the eccentricity of its orbit.

The formula could be tweaked even further in the future. “The power of the habitability index will grow as we learn more about exoplanets from both observations and theory,” said study co-author Victoria Meadows.

Barnes, Meadows and UW research assistant Nicole Evans are the authors of “Comparative Habitability of Transiting Exoplanets.” The study was funded by the NASA Astrobiology Institute.

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.

Planetary Habitability Index Proposes A Less “Earth-Centric” View In Search Of Life

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It’s a given. It won’t be long until human technology will expand our repertoire of cataloged exoplanets to astronomical levels. Of these, a huge number will be considered within the “habitable zone”. However, isn’t it a bit egotistical of mankind to assume that life should be “as we know it”? Now astrobiologists/scientists like Dirk Schulze-Makuch with the Washington State University School of Earth and Environmental Sciences and Abel Mendez from the University of Puerto Rico at Aricebo are suggesting we take a less limited point of view.

“In the next few years, the number of catalogued exoplanets will be counted in the thousands. This will vastly expand the number of potentially habitable worlds and lead to a systematic assessment of their astrobiological potential. Here, we suggest a two-tiered classification scheme of exoplanet habitability.” says Schulze-Makuch (et al). “The first tier consists of an Earth Similarity Index (ESI), which allows worlds to be screened with regard to their similarity to Earth, the only known inhabited planet at this time.”

Right now, an international science team representing NASA, SETI,the German Aerospace Center, and four universities are ready to propose two major questions dealing with our quest for life – both as we assume and and alternate. According to the WSU news release:

“The first question is whether Earth-like conditions can be found on other worlds, since we know empirically that those conditions could harbor life,” Schulze-Makuch said. “The second question is whether conditions exist on exoplanets that suggest the possibility of other forms of life, whether known to us or not.”

Within the next couple of weeks, Schulze-Makuch and his nine co-authors will publish a paper in the Astrobiology journal outlining their future plans for exoplanet classification. The double approach will consist of an Earth Similarity Index (ESI), which will place these newly found worlds within our known parameters – and a Planetary Habitability Index (PHI), that will account for more extreme conditions which could support surrogate subsistence.

“The ESI is based on data available or potentially available for most exoplanets such as mass, radius, and temperature.” explains the team. “For the second tier of the classification scheme we propose a Planetary Habitability Index (PHI) based on the presence of a stable substrate, available energy, appropriate chemistry, and the potential for holding a liquid solvent. The PHI has been designed to minimize the biased search for life as we know it and to take into account life that might exist under more exotic conditions.”

Assuming that life could only exist on Earth-like planets is simply narrow-minded thinking, and the team’s proposal and modeling efforts will allow them to judiciously filter new discoveries with speed and high level of probability. It will allow science to take a broader look at what’s out there – without being confined to assumptions.

“Habitability in a wider sense is not necessarily restricted to water as a solvent or to a planet circling a star,” the paper’s authors write. “For example, the hydrocarbon lakes on Titan could host a different form of life. Analog studies in hydrocarbon environments on Earth, in fact, clearly indicate that these environments are habitable in principle. Orphan planets wandering free of any central star could likewise conceivably feature conditions suitable for some form of life.”

Of course, the team admits an alien diversity is surely a questionable endeavor – but why risk the chance of discovery simply on the basis that it might not happen? Why put a choke-hold on creative thinking?

“Our proposed PHI is informed by chemical and physical parameters that are conducive to life in general,” they write. “It relies on factors that, in principle, could be detected at the distance of exoplanets from Earth, given currently planned future (space) instrumentation.”

Original News Source: WSU News. For Further Reading: A Two-Tiered Approach to Assessing the Habitability of Exoplanets.