We think of Earth as our standard for habitability, and thus the goal of finding an ‘Earth 2.0’ is to identify living worlds like ours orbiting similar Sun-like stars. But maybe Earth isn’t the best standard. Are there ways planets can be more habitable than our own, and if so where would we find them? That’s the tantalizing question posed in a paper by Iva Vilović (Technische Universität Berlin), René Heller (Max-Planck-Institut für Sonnensystemforschung) and colleagues in Germany and India. Heller has previously worked this issue in a significant paper with John Armstrong (citation below); see as well The Best of All Possible Worlds, which ran here in 2020.

The term for the kind of world we are looking for is ‘superhabitable,’ and the aim of this study is to extend the discussion of K-class stars as hosts by modeling the atmospheres we may find on planets there. While much attention has focused on M-class red dwarfs, the high degree of flare activity coupled with long pre-main sequence lifetimes makes K-class stars the more attractive choice, although less susceptible to near-term evaluation, as the paper shows in its sections on observability. It’s intriguing, for example, to realize that K-class stars are expected to live significantly longer than the Sun, as much as 100 billion years, and because they are cooler and less luminous than G-class stars, their habitable zone planets produce more frequent transits.

Image: This infographic compares the characteristics of three classes of stars in our galaxy: Sunlike stars are classified as G-stars; stars less massive and cooler than our Sun are K-dwarfs; and even fainter and cooler stars are the reddish M-dwarfs. The graphic compares the stars in terms of several important variables. The habitable zones, potentially capable of hosting life-bearing planets, are wider for hotter stars. The longevity for red dwarf M-stars can exceed 100 billion years. K-dwarf ages can range from 50 to 100 billion years. And, our Sun only lasts for 10 billion years. The relative amount of harmful radiation (to life as we know it) that stars emit can be 80 to 500 times more intense for M-dwarfs relative to our Sun, but only 5 to 25 times more intense for the orange K-dwarfs. Red dwarfs make up the bulk of the Milky Way’s population, about 73%. Sunlike stars are merely 8% of the population, and K-dwarfs are at 13%. When these four variables are balanced, the most suitable stars for potentially hosting advanced life forms are K-dwarfs. Image credit: NASA, ESA, and Z. Levy (STScI).

Let’s dig into this a little further. The contrast in brightness between star and planet is enhanced around K-dwarfs, and spectroscopic studies are aided by lower levels of stellar activity, which also enhances the habitability of planets. While an M-dwarf may be in a pre-Main Sequence phase for up to a billion years, K stars take about a tenth of this. They emit lower levels of X-rays than G-type stars and are also more abundant, making up about 13 percent of the galactic population as opposed to 8% for G-stars. With luminosity as low as one-tenth of a star like the Sun, they offer better conditions for direct imaging and their planets are far enough from the host to avoid tidal lock.

So we have an interesting area for investigation, as earlier studies have shown that photosynthesis works well under simulated K-dwarf radiation conditions. The authors go so far as to call these ‘Goldilocks stars’ for life-bearing planets, and there are about 1,000 such stars within 100 light years of the Sun, Thus modeling superhabitable atmospheres to support future observations stands as a valuable contribution.

The authors model these atmospheres by drawing on Earth’s own history as well as astrophysical parameters, finding that a superhabitable planet would be somewhat more massive than Earth so as to retain a thicker atmosphere to support a more extensive biosphere. Plate tectonics and a strong magnetic field are assumed, as are elevated oxygen levels that would “enable more extensive metabolic networks and support larger organisms.” Surface temperatures are some 5 degrees C warmer than present day Earth and increased atmospheric humidity supports the ecosystem.

The paper continues:

In terms of the atmospheric composition, key organisms and biological sources affecting Earth’s biosphere and their atmospheric signatures are considered. A superhabitable atmosphere would have increased levels of methane (CH4) and nitrous oxide (N2O) due to heightened production by methanogenic microbes, as well as denitrifying bacteria and fungi, respectively (Averill and Tiedje 1982, Wen et al. 2017). Furthermore, it would have decreased levels of molecular hydrogen (H2) due to higher enzyme consumption (Lane et al. 2010, Greening and Boyd 2020). Lastly…molecular oxygen (O2) levels could increase from present-day 21% by volume on Earth to 25% to reflect a thriving photosynthetic biosphere (Schirrmeister et al. 2015).

Given these factors, the authors deploy simulations using three different modeling tools (Atmos, POSEIDON and PandExo, the latter two to examine observability of transiting planets). Using Atmos, they simulate three pairs of superhabitable planets in differing locations in K-dwarf habitable zones, varying stellar radii and masses and star age. They focused on organisms and biological sources that had influenced Earth’s biosphere, including O2, H2, CH4, N2O and CO2 at a variety of surface temperatures.

The results offer what the authors consider the first simulated data on superhabitable atmospheres and assessments of the observability of such life. What stands out here is the optimum positioning of a superhabitable world around its star. Note this:

We find that planets positioned at the midpoint between the inner edge and center of the habitable zone, where they receive 80% of Earth’s solar flux, are more conducive to life. This contrasts with previous suggestions that planets at the center of the habitable zone—where our study shows they receive about 60% of Earth’s solar flux—are the most favorable for life (Heller and Armstrong 2014). Planets at the midpoint between the center and the inner edge need less CO2 for temperate climates and are more observable due to their warmer atmospheric temperatures and larger atmospheric scale heights. We conclude that a superhabitable planet orbiting a 4300K star with 80% of the solar flux offers the best balance of observability and habitability.

Image: An artist’s concept of a planet orbiting in the habitable zone of a K-type star. Image credit: NASA Ames/JPL-Caltech/Tim Pyle.

Observability presents a major challenge. Using the James Webb Space Telescope, a biosignature detection at 30 parsecs requires 150 transits (43 years of observation time) as compared to 1700 transaits (1699 years) for an Earth-like planet around a G-class star. That would be a mark in favor of K-stars but it also underlines the fact that studies of that length are impractical even with the anticipated Habitable Worlds Observatory. The JWST is working wonders, but clearly we are talking about next-generation telescopes – or the generation after that – when it comes to biosignature detection on potential superhabitable planets.

So what we have is encouraging in terms of the chances for life around K-class stars but a clear notice that observing the biosignatures of these planets is going to be a much harder task than doing the same for nearby M-class dwarfs, where extremely close habitable zones also give us a much larger number of transits over time.

The paper is Vilović et al., “Superhabitable Planets Around Mid-Type K Dwarf Stars Enhance Simulated JWST Observability and Surface Habitability,” accepted at Astronomical Notes and now available as a preprint. The earlier Heller and Armstrong paper is “Superhabitable Worlds,” Astrobiology Vol. 14, No. 1 (2014). Abstract. Another key text is Schulze-Makuch, Heller & Guinan, “In Search for a Planet Better than Earth: Top Contenders for a Superhabitable World,” Astrobiology 18 September 2020 (full text), which looks at candidates. Cuntz & Guinan, “About Exobiology: The Case for Dwarf K Stars,” Astrophysical Journal Vol. 827, No. 1 10 August 2016 (full text) should also be in your quiver.