Â鶹ÒùÔº

With the discovery of ever more exoplanets——scientists, of course, want to know if they are habitable for life. (At least, life as we know it.) But assessing habitability is a difficult task, as information about an exoplanet can be scarce. Now a new metric developed by researchers in England could help.

The habitable zone around a star is the region where the global mean surface temperature of a planet allows liquid water on its surface. The sun's "Goldilocks Zone" 0.95 AU () to 1.67 AU.

If Earth had been just 7.5 million kilometers closer to the sun (5%) it would be uninhabitable now, with, like Venus, its water having boiled away. This is because the sun's luminosity increases, now, by about 1% every 150 million years; this value has been increasing with time and will keep increasing. So, both Venus and the closer Earth could have once held water, but as the sun's luminosity increased, that water went into the atmosphere and then into space.

On Earth, life can exist in habitats with a : temperature, pressure, salinity, pH and even exposure to radiation.

Life has been found a dozen kilometers or more below the land and ocean bottom. Tardigrades the vacuum and radiation of space. But generally, multicellular life prefers moderate temperatures, -20°C and 122°C. Only recently have scientists introduced models using combinations of climatological parameters to describe surface habitability across Earth and across Earth-like bodies in an extrasolar system.

"However," exoplanetary climatologist Hannah L. Woodward from the University of London, and co-authors in The Planetary Science Journal, "none of these definitions have yet been validated against how life is observed to be spatially distributed across Earth."

So, the scientists set out to provide a "more nuanced" understanding of what constitutes surface habitability, using a global climate model and applying it to remote observations of Earth's surface.

They analyzed different climatological variables for goodness-of-fit: surface air temperature, precipitation, evaporation, sea ice concentration, an aridity index and combinations, with two temperature ranges representing the limits for microbial life and complex life. (Complex life, a subset of microbial life, includes animals as well as flora such as plants, fungi, algae, etc.) Liquid water availability is taken into account by the precipitation and evaporation fluxes, both measured as volume per unit time.

For Earth, they utilized the ERA5 . Reanalysis is a combining of incomplete observed data and model results to create a complete dataset of climatological variables that accords with the laws of physics. The reanalysis data are observed or calculated on (here) a two-dimensional surface grid a quarter of a degree on a side for both latitude and longitude.

Microbial life can exist between -20°C and 122°C, and complex life between 0°C and 50°C. The wider range for microbial life reflects what's seen on Earth, while acknowledging that planets with different surface pressures may have liquid water in a temperature domain other than 0-100°C.

Water availability for life requires that precipitation be greater than evaporation and also that except for limited cases of life. If either condition—for temperature or water availability—is not satisfied, then the habitability of the exoplanet is classified as "limited."

In particular, the group looked at life that depends on photosynthesis as a proxy for the abundance and distribution of surface life on Earth. Plants are the dominant form of life on Earth, making up 80% of global biomass. They create food from inorganic substances, not by consuming other animals. (Shout out to the Venus flytrap.)

Moreover, most photosynthetic life uses a pigment that produces a unique reflectance spectrum that remote sensing systems can use to generate a global map of photosynthetic productivity. Finally, the scientists assume that wherever photosynthetic life can thrive, so can life that consumes other organic matter (called heterotrophic life).

The group wanted to compare their climate model's metrics of surface habitability to the observed surface habitability of modern Earth as given by remote satellite data.

A good agreement would allow astronomical observers of exoplanets, who only have access to remote sensing data, to make some reasonable guesses about the habitability of the planet itself. The model calculated the distribution of Earth's surface habitability using reanalyzed climate variables for 2003-2018, and compared it to the observations.

They considered several types of surface conditions, such as warm and humid with a dew point less than 25°C (which they labeled W25), an oppressively humid environment, and different combinations of sea ice fraction and aridity.

They found that a metric defined using surface air temperature, aridity index or sea ice concentration alone "is not sufficient in capturing the observed patterns of habitability," they wrote.

The combination of surface temperature and sea ice concentration did better at higher latitudes, "but incorrectly label lower-latitude regions of observed limited habitability regions as habitable, resulting in an overestimation in the fractional habitability." Using the aridity index gave better results at lower latitudes, "but exhibit poor predictive skill overall."

But the habitability of the W25 world, defined using surface temperature, precipitation and evaporation, "most closely aligns with the observed patterns both qualitatively and quantitatively," better than surface conditions defined by ice-free area, water availability and other combinations of temperature and humidity.

The group then constructed their own definition of surface habitability, based on a metric that includes surface temperature, precipitation and evaporation. This aligned most qualitatively and quantitively closely with observed habitability patterns. In fact, this metric performed best of all, showing accuracies (proportion of grids correct) of 0.67 and 0.70 (on a scale from 0 to 1) for microbial and complex habitability respectively, and it performed especially well on land with respective accuracies of 0.77 and 0.80.

They conclude that their new metric "offers a good representation of the observed surface habitability of modern Earth, with a dependence upon only three parameters commonly output by GCMs [general circulation models] that are closely tied to the global atmospheric circulation and enable a first-order approximation of surface habitability upon exoplanets."