November 30, 2021

The search for extraterrestrial life is a fascinating enterprise that has spurred the origination of a dedicated line of scientific inquiry, known as astrobiology.[1] Yet, at the time of the writing of this article the earth remains the only place in the universe where humans have been able to empirically detect life and its signatures. Why is it so? Is the earth special? Opinions on this differ widely.

Consider, for example, the views presented by the authors of three separate books that touch on the subject. In “The Privileged Planet,”[2] G. Gonzalez and J. Richards argue that the earth is exquisitely fit to support life, implying that our planet was designed for this purpose. In “Rare Earth,”[3] P. Ward and D. Brownlee refrain from appealing to the notion of design and present a naturalistic model of planetary formation and earth history. However, they do conclude that the earth is probably to be considered special, or “rare” as the title of their book suggests, inferring that complex life is less pervasive in the universe than commonly assumed. Finally, planetary scientist A. Ingersoll ends his book on planetary climates[4] with a statement that minimizes the earth’s uniqueness: “Earth seems to be a typical planet orbiting a typical star, with a typical collection of raw materials that constitute the building blocks of life. There is no reason to believe we are alone.”

The evaluation of such differing perspectives requires a basic understanding of some of the parameters that are often mentioned in the discussions of planetary habitability. This article aims at offering a synthetic review of several of these parameters, also incorporating some information gained from the study of exoplanets. Following this review, the second part of the article tries to relate the current scientific findings on planetary habitability with the Genesis account of creation, highlighting both areas of convergence and tension, and introducing a theological reflection on the original question about the earth being special or not.

Exoplanets: putting the earth into perspective

The question of how special the earth is can be addressed from a scientific point of view by comparing our planet and its characteristics with those of other planets. Until the early 1990s, the only other planets available for this comparison were the planets of the Solar System.[5] However, the discovery of exoplanets, which are planets located outside of the Solar System, opened new possibilities for a more robust comparison based on a much larger dataset.

There are several methods that are used to detect exoplanets, but the two most common are the radial velocity method and the transit method.[6] The radial velocity method is based on the fact that if a planet orbiting a star is quite massive, the center of mass of the rotational system will be a little off the star, meaning that the star too will be wobbling around the center of mass. To an observer placed in a radial direction from this rotating system, the electromagnetic waves coming from the star will be compressed or dilated as the star moves closer and further, respectively, in its wobbly motion. This shift in wavelength will be periodical, allowing to estimate a minimum mass for the orbiting planet and its distance from the star.

The transit method is based on the fact that a planet transiting in front of a star will block some of the light that is received from an observer more or less aligned along the orbital plane. The dip in the star’s luminosity will be greater if the planet’s radius is larger. Also, the dip in luminosity will be observed periodically as the planet keeps revolving around the star, allowing to calculate the duration of a full revolution and the distance of the planet from the star.

One implication of these detection methods is that they are biased towards finding planets that are massive and cause a greater radial velocity wobble, or orbit close to their star increasing the chance of a detectable transit. These biases could explain why, notwithstanding the increasing number of confirmed exoplanets with a recent count in excess of 4550,[7] the overlap with our planet in terms of orbital period and mass is still minimal. The confirmed exoplanets with mass similar or smaller than the earth’s are orbiting closer to their star, and the planets with orbital period similar or greater than the earth’s are more massive (Fig. 1). However, it is expected that the different regions of this diagram will continue to be populated with improvements in detection methods.

The discovery of exoplanets has expanded notions on the number and diversity of planets well beyond what was known from our Solar System. Most notably, the growing exoplanet census illustrates that a vast range of values even for basic planetary attributes, such as mass and orbital period. Therefore, earth-like conditions are not the only possible outcome or one of a few default outcomes required by a set pathway of planetary formation. What we experience in our planet is one combination among an extensive field of different planetary conditions.[8]

 

Planetary habitability: a mix of ingredients

The study of exoplanets has established that our galaxy is populated with a wide array of planets exhibiting different characteristics. Which of these planets would be a good place for complex organisms like us to live on? Astrobiologists who investigate this question try to identify some of the parameters that are necessary for a planet to be considered potentially habitable. There is no single definition of habitability, because the specifications chosen as requirements may vary.[9] However, what is clear is that habitability is not a function of a single variable, but lies at the intersection of multiple factors that have all to be present for a planet to be habitable. This narrows the field of habitability to a much smaller subset of the possible physico-chemical conditions present in the cosmos. What follows is a brief review of some of the parameters most commonly recognized as important for planetary habitability.

Energy Source

For a planet to be habitable, energy must be available to sustain the metabolic activity of living organisms and to maintain water on the planet in a liquid state, which, as we will see in the next section, is a fundamental requirement for life. Light radiating from a star is the most efficient and relevant source of energy that can accomplish both functions and sustain complex life, to the point that C. McKay suggests that “a biosphere can have effects on a global scale, and hence be detectable over interstellar distances, only when it is powered by light.”[10]

The energy flux radiating from a star is one of the most important factors that controls the temperature on the surface of an orbiting planet. If the planet is orbiting too close to the star, temperatures will be too high for liquid water to be a stable phase. Conversely, temperatures will decrease with increasing distance from the star, to the point that at a certain distance, known as the frost line, temperatures will drop below the freezing point and liquid water will turn into ice. The distance range where temperatures are compatible with the presence of liquid water can be represented as a circular band in an orbital plane around a star, and it is called the Goldilocks zone or habitable zone (Figs. 2,3). The earth is obviously located within the habitable zone of the Solar System. For a star less massive than the Sun, the width and circumstellar distance of the habitable zone is reduced, because of the smaller energy flux radiating from the star (Figs. 2,3).

Stars also differ in terms of activity and instability of their magnetic field, and can emit flares of high-energy UV and X-rays with potentially catastrophic consequences for habitability.[11] Less massive M stars, which are much more abundant in the Milky Way Galaxy than more massive sun-like G stars,[12] exhibit higher flare activity persisting over a longer lifetime[13] (Fig. 3). Therefore, even if the abundance of M stars makes them an interesting target for localization of exoplanets within their narrow habitable zone, higher levels of damaging radiation may lower the chances of such exoplanets being truly habitable.

Finally, a significant fraction of stars consists of binary (or multiple) systems,[14] where two (or more) stars orbit around a common center of mass. Exoplanetary systems appear to be less frequently associated with binary stars than single stars,[15] further underscoring how stellar properties are an important factor for the establishment and preservation of planetary habitability.

Liquid Water

Why is there so much emphasis in astrobiology on the importance of the availability of liquid water as a prerequisite for life? We are all familiar with the fact that water is an important constituent of our bodies. What perhaps most do not realize is how crucial liquid water is for the existence of any living organism on the earth.[16] As pointed out by M. Chaplin, in an excellent review article,[17] “Liquid water is not a ‘bit player’ in the theatre of life — it’s the headline act.” Liquid water is essential for the folding, structure, stability and activity of proteins. It plays a part in both proton and electron transfer reactions, in the structure of DNA and the recognition of specific DNA sequences by proteins, and in the metabolic activity in the cell. In other words, liquid water is essential for the biochemistry of life. This explains why habitability is tied to the effects that different parameters, such as distance from the star or atmospheric composition, have on the availability of such precious liquid, which to us looks so deceivingly ordinary.[18]

Atmosphere

An atmosphere and its properties are essential for the subsistence of terrestrial life forms, like humans, who depend on breathing air for aerobic respiration. However, at a more fundamental level, an atmosphere is necessary for stabilizing the presence of liquid water on the surface of a planet and can play an important role in protecting life forms from the damaging effect of high-energy particles and radiation from space.[19] Additionally, an atmosphere controls climate, for example by trapping and distributing heat, and is an essential component of various geochemical cycles.[20] Therefore, a habitable planet needs both to be able to retain an atmosphere, and for that atmosphere to be of suitable composition and thickness to sustain life.

With regard to retaining an atmosphere, a tug of war between gravitational pull and kinetic energy of gas molecules means that atmospheric escape occurs more easily in celestial bodies of small mass and located close to a star. Proximity to a star not only can induce higher temperature and kinetic energy in atmospheric gases, but can also result in atmospheric erosion because of interaction with charged particles of the stellar wind. However, the latter process can be mitigated by the existence of a planetary magnetic field able to deflect the stellar wind, adding another possible factor of significance for planetary habitability.[21] Additionally, a range of variability in stellar activity exists between different types of stars and even in the course of the life of a single star, with important implications for long-term retention of an atmosphere.[22]

The crucial role played by atmospheric composition and thickness in determining very different conditions at the surface of terrestrial planets can be illustrated by a comparison between Venus and the earth. The two planets are relatively similar in mass, distance from the Sun, and bulk composition.[23] However, the atmospheric pressure on the surface of Venus is over 90 times higher than on the earth, with temperatures in excess of 450°C. Part of the reason for the more extreme Venusian environment is that most of the planet’s CO2 resides in its atmosphere, whereas on the earth it is dissolved in water and sequestered in carbonate rocks. CO2 is a powerful greenhouse gas, leading to increased surface temperatures, with the potential for a runaway greenhouse effect. This process consists of a feedback loop, where increasing temperatures lead to higher levels of water vapor (another potent greenhouse gas) in the atmosphere, which enhances the greenhouse effect, eventually causing complete loss of liquid water on the surface of the planet.

Rocky planet

The planets of the Solar System are typically divided in three groups based on their size and internal composition: the terrestrial (or rocky) planets (Mercury, Venus, Earth, and Mars); the gas giants (Jupiter and Saturn); and the ice giants (Uranus and Neptune) (Fig. 4). Although other possible types of planets have been detected through exoplanetary exploration (e.g., mini-Neptunes and super-Earths),[24] it is the terrestrial planets, composed mostly of rocks and metals and with a defined outer solid or liquid surface, that are considered a primary target for habitability. This is not just because of an anthropocentric bias of needing a solid substrate to walk on and avoiding being crushed under a supermassive atmosphere. Terrestrial planets expose various mineral assemblages on the surface of the planet, where liquid water also resides, including some heavier elements important for the metabolism of various life forms. This allows for a variety of biogeochemical reactions and geochemical cycles to take place, some of which act as a buffering system helping to preserve habitable conditions.[25]

On a broader level, variations in mass and composition of terrestrial planets can result in significant differences in internal structure and geodynamic activity.[26] For example, the earth is unique among the terrestrial planets of the Solar System by being the only one with active plate tectonics, mantle convection, and an outer liquid core directly responsible for the presence of a global geomagnetic field. There is no universal consensus yet on plate tectonics and an intrinsic magnetic field as necessary requirements for habitability,[27] but if they were, it would mean that only a subset of terrestrial planets could be considered habitable.

Finally, it should be noted that stellar nucleosynthesis in the Sun only produces helium, and not the other heavier elements that make up the earth. Where are then these heavy elements inherited from? Current models of formation of planetary systems suggest that a star and its planets form from condensation and accretion of material out of a gas nebula and protoplanetary disk. If this is the case, the protoplanetary disk must have the right initial composition to include heavy elements in sufficient abundances for the formation of terrestrial planets. Therefore, low-metallicity[28] stars are not a good target to look for potential systems hosting terrestrial planets.

Orbital parameters

The yearly, seasonal, and daily cycles, which punctuate the rhythm of our lives, are controlled by the rotation and revolution of our planet around the Sun. Each of these cycles can be used to illustrate how orbital parameters are important for habitability.

The year is defined as the time it takes for the earth to complete a full revolution around the Sun. The trajectory of this orbital path is nearly circular, unlike the more eccentric orbits of many of the exoplanets that have been discovered so far. A highly eccentric orbit could impact habitability by periodically moving a planet out of the habitable zone. Furthermore, all the planets of the Solar System also have low eccentricity orbits, ensuring that potentially destabilizing gravitational interactions created by elliptical orbits are avoided. Recent analyses have concluded that low eccentricities are typical of systems with multiple planets, and that only ~1% of exoplanetary systems have eight or more planets.[29] Therefore, one could say that our Solar System is uncommon not so much because of the low-eccentricity orbits of its planets, but because it has eight planets. This suggests that the overall architecture of a planetary system may be another important factor to consider for habitability.[30]

The seasonal cycle is controlled by the tilt of earth’s rotational axis. Its inclination at about 23.5 degrees allows for a good distribution of insulation over the yearly cycle even at high latitudes. Lower or higher tilt could result in thermal extremes across latitudes or through the year.[31] Moreover, the large mass of the Moon, stabilizes the earth’s axial tilt over time.[32] It is thought that without the moon the earth’s tilt could swing widely with dramatic consequences, for example, for its ability to retain an atmosphere.[33] How crucial the moderate inclination and stability of the axial tilt is for habitability is still being debated,[34] but it is recognized as an important climate-controlling factor.

A day is defined as the time it takes for the earth to complete a full rotation around its axis. It is a common experience for us to observe the daily rising and setting of the sun and alternance of day and night. However, we experience this daily rhythm because the period for a full rotation of the earth (the day) is much shorter than the period for its full revolution around the Sun (the year). It is possible for orbiting objects to attain a condition, known as synchronous tidal locking, where the rotation and revolution periods are the same. This is well illustrated by the Moon. An observer from the earth always sees the same face of the Moon, not because the Moon does not rotate but because the time it takes to rotate around its axis, about 29.5 days, is the same it takes to revolve around the earth. Similarly, a planet in synchronous tidal locking with its star would have one hemisphere always exposed to the light of the star and the other always in darkness, with the potential for dramatic temperature contrasts.[35] In a paper on tidal locking of potentially habitable exoplanets, R. Barnes concludes that “tidal locking is possible for most planets in the habitable zones of GKM dwarf stars.”[36]

Planetary habitability and Genesis 1

Genesis 1 provides an account of God’s creative activity that comprises the organization of our world and the creation of different life forms that inhabit it. This account invites interaction with the wealth of scientific information acquired in the study of planetary habitability.

At its most fundamental level, we can affirm that Genesis 1 is extremely modern in its understanding of the connection between environmental prerequisites for life (i.e., habitability) and life itself. In the text, the initial conditions of the earth are presented as tohu wa bohu or “unformed and unfilled” (Gen 1:2), followed by a symmetric account of the organization of different spaces and their filling with living creatures.[37] Therefore, Genesis 1 articulates the “forming” of the planet as essential and integrally connected to its “filling.”

The text is also exceptionally relatable to the modern understanding of habitability in presenting the sequence of steps God took to make the planet habitable. The first days of creation could very well provide the chapter index for a textbook on planetary habitability, starting with light - the energy source (day 1), liquid water and atmosphere (day 2),[38] the terrestrial nature of our planet and the emergence of dry land (day 3),[39] and the establishment of orbital parameters that control the yearly, seasonal, and daily cycles (day 4).

There are, however, also areas of tension between the Genesis account and alternative cosmological models accounting for planetary habitability. The biblical text adamantly portrays our world as the result of God’s intentional plan and direct intervention. However, mechanistic accounts of origins tend to characterize natural processes as either undirected or coincidental. In speaking of planetary habitability, sometimes a language of “chance” is used that excludes the involvement of any foresight. Other times, habitability is presented almost as a statistical inevitability among the many possible configurations in a very large universe.[40]

Another area of tension has to do with time. Current modeling of many astrophysical and geological processes, from planetary formation to crustal differentiation, involve time scales that are orders of magnitude greater than the biblical timeline of a recent creation week. One possible way to alleviate this time discrepancy is to adopt a passive gap or “two-stage creation” interpretation of Genesis 1.[41] This view suggests the earth was already present, in an unformed and unfilled state, before creation week, potentially experiencing physical processes over a preceding undetermined amount of time. However, this interpretative approach does not solve all the issues, because the creation week account still includes significant planetary reorganization (in terms of atmosphere, oceans, continents, and orbital parameters) that are not typically understood as involving timescales of a few days. Other possible approaches suggest that God’s young creation included some “appearance of age,”[42] that God creates through accelerated process,[43] or that we should simply profess ignorance before the most incredible act of divine intervention recorded in Biblical history.

For someone like me, who fully accepts the revelation of Scripture that God recently fashioned the earth and filled it with life in six days, resting on the seventh, there remains an important question: How do we relate with modeling and observations that reconstruct the origin of our planet from a long chronology perspective? Should we avoid getting involved in areas of study that appear to challenge our biblical understanding of origins? Although dealing with tension is often hard, consecrated students, who seek God’s guidance in every aspect of their life, will benefit from engaging more deeply with these complex subjects, for several reasons: 1) because models of planetary formation may accurately describe what is happening in the cosmos now, irrespective of its past history. In the same way in which we can distinguish between the punctual creative act of God and His sustainment of creation, we can build on the distinction between operational and historical science and highlight the value of understanding the regular workings of the natural world we are part of; 2) because the process of scientific testing of hypotheses spurs the acquisition of new data, making us better acquainted with nature and its facts. Continual discovery and increase in knowledge teach us a certain humbleness and can lead to awe and reverence for the Creator of the universe; 3) because we learn to better evaluate the nature of data and their limitations, with the potential to help develop alternative models that ease some of the tension; 4) because we gain a better understanding of the secular mindset and learn how to effectively engage with it; 5) and because faith is truly tested when we do not have all the answers.

Conclusion

What can we then say to the original question of this article? Is the earth special? Perhaps, from a numerical perspective, it’s too early to say. Recent published estimates[44] suggest that the probability for a G star like the Sun to have an Earth-sized rocky planet orbiting within its habitable zone is less than 18%, which would translate into a maximum of about 6 billion planets in the Milky Way Galaxy. However, as we have briefly seen in this article, there is much more to habitability than just planetary radius, distance from the star, and type of star, so that truly earth-like planets would be just a tiny fraction of this upper probability limit.[45] Many or few, what really matters is that we live in a universe where the earth is habitable and there is life on it. That is the most amazing fact confirmed every day by empirical, observational science. You and I are part of that miracle.

Scientific insight helps us open the black box of planetary habitability and appreciate the beauty and complexity of life and how, among a myriad of possible combinations, this is a marvelously suitable space for us to live our lives.

The Bible cuts through the noise and reveals in straightforward terms why this is so: because God wanted it and did it. The Lord created the heavens; He fashioned and made the earth, He founded it; He did not create it to be empty, but formed it to be inhabited (cf. Isaiah 45:18).

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Ronny Nalin, PhD
Geoscience Research Institute

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References

[1]Astrobiology is a relatively new discipline. For example, the NASA Astrobiology Institute was established in 1998 and the first issue of the journal Astrobiology was published in 2001. However, scientific interest and research efforts aimed at discovering extraterrestrial life certainly predated adoption of the term astrobiology, as exemplified by the Search for Extraterrestrial Intelligence (SETI) program. In the article “The signature of life: Designing the astrobiological imagination” (Grey Room 23, Spring 2006, p. 68), S. Helmreich makes the interesting observation that the move to the term astrobiology involves a “retreat from the category of ‘intelligence’,” included in the term SETI, lowering the bar with the use of a generic reference to life.

[2]Gonzalez, G. and Richards, J.W., 2004. The Privileged Planet: How Our Place in the Cosmos Is Designed for Discovery. Washington, DC: Regnery Publishing, 464 pp.

[3]Ward, P.D. and Brownlee, D., 2000. Rare Earth: Why Complex Life Is Uncommon in the Universe. New York, NY: Copernicus, 333 pp.

[4]Ingersoll, A.P., 2013. Princeton Primers in Climate: Planetary Climates. Princeton, NJ: Princeton University Press, p. 246.

[5]The existence of exoplanets had been postulated earlier, but the first confirmed detection was published in 1992: Wolszczan, A. and Frail, D., 1992. A planetary system around the millisecond pulsar PSR1257 + 12. Nature 355, 145–147.

[6]For an excellent webpage that illustrates different detection methods with simple graphic animations, visit https://exoplanets.nasa.gov/alien-worlds/ways-to-find-a-planet/ accessed on 10/28/2021

[7]Based on https://exoplanets.nasa.gov/discovery/exoplanet-catalog/, accessed on 11/1/2021.

[8]Planetary scientist Sara Seager conveyed the idea of a vast set of possibilities with this statement: “Any planet one can imagine probably exists, out there, somewhere, as long as it fits within the laws of physics and chemistry.” Jet Propulsion Laboratory news release, June 11. 2015: Helium-shrouded planets may be common in our galaxy. https://www.jpl.nasa.gov/news/helium-shrouded-planets-may-be-common-in-our-galaxy, accessed on 10/5/2021.

[9]A representative definition of planetary habitability is the one given by Barnes et al.: “We define a ‘potentially habitable’ planet to be one that is mostly rock, with a small (≤100 bar), high molecular weight atmosphere, and with energy sources and an internal structure such that the surface temperature and pressure permit liquid water for geological timescales.” (Barnes, R., Meadows, V.S. and Evans, N., 2015. Comparative habitability of transiting exoplanets. The Astrophysical Journal, 814(91), p.2, doi:10.1088/0004-637X/814/2/91).

[10]C.P. McKay, 2014. Requirements and limits for life in the context of exoplanets. PNAS, 111:35, p. 12630. McKay goes on to say that “Life based on geothermally derived chemical energy would, by dint of energy restrictions, always remain small and globally insignificant.” In his book Astrobiology: Understanding Life in the Universe (2020. Hoboken, NJ: John Wiley and Sons Ltd, pp. 363-382), C.S. Cockell explains how aerobic respiration generates up to 10 times more energy than anaerobic modes of metabolism. Complex multicellular organisms have high energy requirements, which explains why almost all of them are powered by aerobic respiration. Oxygenic photosynthesis is the main pathway for increasing the concentration of oxygen then used in aerobic respiration, establishing light as the energy source for complex life. In some celestial bodies - such as Enceladus, one of Saturn’s moons - liquid water is known to exist in subsurface oceans under an icy crust. However, it is thought that complex life is unlikely to be sustainable in these dark subsurface oceans because of the impossibility of performing light-dependent photosynthetic reactions.

[11]Segura A., 2018. Star-planet interactions and habitability: Radiative effects. In: Deeg H., Belmonte J. (eds) Handbook of Exoplanets. Springer, Cham. https://doi.org/10.1007/978-3-319-30648-3_73-1.

[12]Chabrier G., 2003. Galactic stellar and substellar initial mass function. Publications of the Astronomical Society of the Pacific, 115(809), 763-795.

[13]Davenport J.R., Covey K.R., Clarke R.W., Boeck A.C., Cornet J. and Hawley S.L., 2019. The evolution of flare activity with stellar age. The Astrophysical Journal, 871(241), 13 pp., https://doi.org/10.3847/1538-4357/aafb76.

[14]Gao, S., Liu, C., Zhang, X., Justham, S., Deng, L., and Yang, M., 2014. The binarity of Milky Way F, G, K stars as a function of effective temperature and metallicity. The Astrophysical Journal Letters, 788(2), L37.

[15]This is particularly true as the distance between the stellar companions decreases. Wang, J., Fischer, D. A., Xie, J. W., and Ciardi, D. R., 2014. Influence of stellar multiplicity on planet formation. II. Planets are less common in multiple-star systems with separations smaller than 1500 AU. The Astrophysical Journal, 791(111), 16 pp; Hirsch, L.A., Rosenthal, L., Fulton, B.J., Howard, A.W., Ciardi, D.R., Marcy, G.W., Nielsen, E., Petigura, E.A., de Rosa, R.J., Isaacson, H. and Weiss, L.M., 2021. Understanding the impacts of stellar companions on planet formation and evolution: A survey of stellar and planetary companions within 25 pc. The Astronomical Journal, 161(3), p.134.

[16]Alpert, P., 2005. Sharing the secrets of life without water. Integrative and Comparative Biology, 45 (5), 683–684.

[17]Chaplin, M., 2006. Do we underestimate the importance of water in cell biology? Nature Reviews Molecular Cell Biology, 7, 861–866.

[18]For a short essay on the remarkable properties of water, see Schaeffer, R., 2021. Wonderful Water. In: Gibson, L.J., Nalin, R., and Rasi, M.H. (eds) Design and Catastrophe: 51 Scientists Explore Evidence in Nature. Berrien Springs, MI: Andrews University Press, 17-19.

[19]See the introduction in Dong, C., Jin, M., and Lingam, M., 2020. Atmospheric escape from TOI-700 d: Venus versus Earth analogs. The Astrophysical Journal Letters, 896:L24, 6pp.

[20]In addition to the classic hydrological cycle, one should certainly mention the carbon cycle. For the linkage between atmospheric pCO2 and the biosphere, hydrosphere, and lithosphere, see for example Sundquist, E.T., 1985. Geological perspectives on carbon dioxide and the carbon cycle. In: Sundquist, E.T., Broecker, W.S. (eds.) The Carbon Cycle and Atmospheric CO2: Natural Variations Archean to Present. American Geophysical Union, Geophysical Monograph Series 32, 5-59.

[21]For example, it is commonly thought that Mars may have experienced significant erosion of a once thicker atmosphere because of its current lack of a global intrinsic magnetosphere. See, for example, Tasker, E., 2017. The Planet Factory: Exoplanets and the Search for a Second Earth. London, UK: Bloomsbury Publishing Plc, pp. 237-238.

[22]For example, stars of intermediate mass like the Sun are thought to go through an early stage of heightened activity, known as the T-Tauri phase, with the potential of stripping off the primordial atmosphere of inner planets. See Tasker, op. cit., pp. 34, 64-65.

[23]Information for this comparison between Venus and the earth is based on Ingersoll, op. cit., pp. 7-25.

[24]On planetary types, see this informative webpage on NASA’s exoplanet exploration website: https://exoplanets.nasa.gov/what-is-an-exoplanet/planet-types/overview/ accessed on 10/28/2021.

[25]For example, the carbonate-silicate cycle. See Cockell, op. cit., pp. 411-413.

[26]For example, differences in the Mg/Si ratio or in the abundance of radioactive elements would impact viscosity, heat flow, and the internal structure of the planet. See Tasker, op. cit., pp. 128-130; Waltham, D., 2019. Is Earth special? Earth-Science Reviews, 192, 445-470.

[27]See discussion in Waltham, op.cit.

[28]In astronomical language, all elements heavier than He are considered metals.

[29]Bach-Møller, N., and Jørgensen, U.G., 2021. Orbital eccentricity–multiplicity correlation for planetary systems and comparison to the Solar system. Monthly Notices of the Royal Astronomical Society, 500, 1313–1322.

[30]This line of reasoning is reflected in the interest in understanding how the presence and positioning of a gas giant like Jupiter affects the stability the earth’s orbital and spin parameters. See, for example, Horner, J., Vervoort, P., Kane, S.R., Ceja, A.Y., Waltham, D., Gilmore, J., and Turner, S.K., 2019. Quantifying the influence of Jupiter on the Earth’s orbital cycles. The Astronomical Journal, 159(10), 16 pp.

[31]Ward and Brownlee, op. cit., pp. 223-226.

[32]Laskar, J., Joutel, F., and Robutel, P., 1993. Stabilization of the Earth's obliquity by the Moon. Nature, 361(6413), 615-617.

[33]Mars is typically presented as an example of a planet subject to significant changes in obliquity over time, with important consequences for its atmosphere and climate. Nakamura, T., and Tajika, E. 2003. Climate change of Mars‐like planets due to obliquity variations: implications for Mars. Geophysical Research Letters, 30(13), 1685, doi:10.1029/2002GL016725.

[34]For a popular news piece discussing this topic, see “The earth’s tilt is key to life,” June 22, 2018, NNY360, available at https://www.nny360.com/news/the-earth-s-tilt-is-key-to-life/article_1778e00c-58e8-5d25-b560-7831dd6ff394.html, accessed on 10/29/2021. For a paper proposing that, in some circumstances, variations in axial tilt may actually be favorable for habitability, see Armstrong, J.C., Barnes, R., Domagal-Goldman, S., Breiner, J., Quinn, T.R., and Meadows, V.S., 2014. Effects of extreme obliquity variations on the habitability of exoplanets. Astrobiology, 14(4), 277-291.

[35]It is intuitive how this could have negative implications for habitability. However, there are suggestions that in favorable circumstances atmospheric circulation and ocean heat transport could mitigate the thermal contrast and preserve habitability. See, for example, Hu, Y., and Yang, J., 2014. Role of ocean heat transport in climates of tidally locked exoplanets around M dwarf stars. Proceedings of the National Academy of Sciences, 111(2), 629-634.

[36]Barnes, R. (2017). Tidal locking of habitable exoplanets. Celestial Mechanics and Dynamical Astronomy, 129(4), p. 509.

[37]R.M. Davidson (2015) also identifies the “unformed and unfilled” theme as a key organizational structure of the Genesis 1 account, in his chapter “The Genesis account of origins.” In: Klingbeil, G.A. (ed) The Genesis Creation Account and Its Reverberations in the Old Testament. Berrien Springs, MI: Andrews University Press, 59-129. Available online at https://www.grisda.org/the-genesis-account-of-origins, accessed on 10/29/21.

[38]With an implicit acknowledgment of their interconnectedness in the separation of “waters above” and “waters below” that is reminiscent of the multiple geochemical cycles between atmosphere, hydrosphere, and lithosphere.

[39]The appearance of dry land is at a minimum an affirmation of the rocky nature of our planet, but a geologist could read into it a hint to tectonism and differentiation of the continental crust. It is also interesting to note that the creation of vegetation on day 3 fits into the “forming” rather than “filling” stage of creation. This is because plants provide the basic nourishment for living creatures (Gen 1:30) and in that sense are a necessary prerequisite for life. However, from a planetary perspective, an astrobiologist would not miss the implication of the appearance of oxygenic photosynthesis, which is another key topic typically discussed in reviews of Earth’s habitability.

[40]To this last point, it is important to remark that habitability is necessary but not sufficient for life. In other words, even if a perfect environment existed for the sustainment of life, the origination of a living organism is a completely separate event that would entail its distinct set of requirements and “coincidences.”

[41]See Davidson, op. cit., pp. 87-102.

[42]Essentially positing that in order for God to create a richly textured world that allows for exploration and prediction into the future, context and continuity must be provided also into the past. A blank slate does not give the opportunity to detect patterns, hence the necessity to create “history” by fiat rather than true process.

[43]In other words, our modeling and understanding of processes that appear to require long timescales is not incorrect, except that when God acted, He accelerated the sequence of steps and reached the end product in a much faster way.

[44]Kunimoto, M., and Matthews, J.M., 2020. Searching the entirety of Kepler data. II. Occurrence rate estimates for FGK stars. The Astronomical Journal, 159(248), 27 pp.

[45]Ward and Brownlee (op. cit., p. 37) try to convey the sheer amount of “nearly irreproducible circumstances” that make the earth “rare,” with this illustration: “If some god-like being could be given the opportunity to plan a sequence of events with the express goal of duplicating our “Garden of Eden,” that power would face a formidable task. With the best intentions, but limited by natural laws and materials, it is unlikely that the Earth could ever be truly replicated. Too many processes in its formation involved sheer luck. Earth-like planets could certainly be made, but each outcome would differ in a critical way.”