People often joke about the certainty of death and taxes. Astronomers can add another certainty to that short list: Sooner or later someone will ask, “What do you think about the possibility of life out there?”
Most questioners are looking for a particular answer. Science fiction novels, The Planetary Society, and countless movies, from E.T. to Contact to Planet of the Apes, suggest that extraterrestrial life is a given and help conjure images of how that life looks. To answer questions about such life takes as much diplomacy as answering my wife when she asks, “How do I look?”
Experience suggests a strategy for handling both questions. Step one: Make a positive statement, such as “You look great!” or “That’s a great question!” Step two: Provide amplification. This part is trickier. It can make or break the interaction. If it lacks sincerity or includes the word but, (e.g., “You look great, but I thought you were going to wear the blue dress”), my wife may walk away feeling hurt and deflated. A better answer adds some specific feedback (e.g., “You look great, and I especially like the way that color goes with your eyes”).
In the case of the life-elsewhere question, an honest, fact-based amplification acknowledges the “great question” as opening the door to three fascinating topics: life on other planets, life on other astronomical bodies, and life other than “life as we know it.” Step-by-step discussion of these subjects can lead to opportunities for spiritually significant conversation.
Life on Other Planets
Technology and interdisciplinary research have enabled scientists to develop an extensive list of physical characteristics that must fall within limited ranges for a planet (or any other astronomical body) to be capable of life support. Those characteristics involve the planet’s star, moon(s), planetary companions, and galaxy, as well as the planet’s surface, interior, and atmospheric conditions. This list grows longer with every year. It started with two parameters in 1966,1 grew to eight by 1970, to twenty-three by 1980, to thirty by 1990, and to forty by 1995.2 Currently, the list includes more than 120 parameters and shows no signs of leveling off.3
The limits on some characteristics, especially on the essential-to-life features of a planet’s star, have been determined precisely. The limits on others, mostly on the features of the planet itself, presumably a terrestrial (rocky) planet, are less precisely known. Two reasons exist for this difference: First, trillions of stars are available for study while only 76 planets (9 in Earth’s solar system, 67 outside) have been discovered to date. Second, physical and chemical characteristics make stars, basically condensed balls of hot gas, much simpler systems than planets.
No one knows, of course, exactly how many planets exist. As recently as 1990, astronomers were divided between those who proposed that planets whirl around nearly every star and those who posited that the Sun alone possesses planets. Three research advances tilt the debate toward the latter scenario: (1) the availability of instruments and techniques capable of detecting and studying planets orbiting other stars; (2) the discovery that most, if not all, stars surrounded by disks of dust are young or still forming; and (3) the development of sophisticated theoretical models that explain how dust disks become planets.
Each of the 67 extrasolar planets discovered and studied to date orbits a relatively young, metal-rich star (a star rich in elements heavier than hydrogen and helium).4-8 This finding presents no surprise. The heavy elements needed to make planets and any type of life chemistry do not exist in sufficient quantity until at least two generations of stars have formed, burned out, and scattered their ashes, which then recycle to form more stars. Astronomers have learned that the longer a galaxy sustains star formation, the more metal rich its newly forming stars will be. In the case of the galaxy astronomers know best, the Milky Way galaxy (Earth’s own), only 2 percent of the stars possess metal richness adequate for planet formation.9
Of those Milky Way stars known to have planets, none formed as early as the Sun. The Sun benefited from a remarkable set of circumstances: it formed adjacent to two massive, star explosions (supernovae), each of which spewed out a different set of life-essential heavy elements.10-12 Those explosions occurred precisely at the right time and place for those heavy elements to be incorporated into the condensing solar nebula. Earth’s star may be the only star its age with an ensemble of both small rocky planets and gas giants. This finding implies that the probable number of life-site candidates falls far below 2 percent.
As for life-support planets in other galaxies, the odds look bleak. Astronomers have found that the Milky Way is exceptional for the longevity of its star formation processes. In 94 of every 100 galaxies, star formation shut down so long ago that few, if any, metal-rich stars reside there—hence few, if any, planets. The results of a Hubble Space Telescope (HST) study recently confirmed this conclusion. The HST searched for planets in an enormous cluster of old stars, 47 Tucanae, and found none.13
Observations indicate that the number of stars with planets, any kind or size of planets, adds up to only about 0.1 percent of all the stars in the cosmos. That number is at least a hundred times smaller than the estimate that launched the search for signals from extraterrestrial life.14 Small though that percentage may be, however, it still adds up to a lot of planets. If, for example, each star in that 0.1-percent group has ten planets around it, the number of planets would add up to a hundred million trillion (that is, 1020).
A hundred million trillion, then, is the number to which the data on various life-essential features must be applied. Some features fall within loose limits—others, within strict limits. Limits on the planet’s rotation period and its albedo (reflectivity) eliminate about 90 percent of the life-site candidates. Parameters such as the parent star’s mass and the planet’s distance from its parent star eliminate about 99.9 percent of all relevant candidates.
Dependency factors among certain of the parameters improve the odds somewhat, but many of these parameters must be kept within a specific range for long periods of time. Given how variable environments can be, this longevity requirement proves extremely limiting. The data demonstrate that the probability of finding even one planet with the capacity to support life falls short of one chance in 10140 (that number is 1 followed by 140 zeros).15
Life on Alternative Sites
The extreme improbability such a number indicates has driven some scientists to abandon the premise that life requires an Earth-like home. A satellite (moon) orbiting a giant planet that in turn orbits a star resembling Earth’s sun at the right distance could serve, they say, as a life site.16-18 The feasibility of such an alternative can be tested against a long list of recent findings.
None of the 67 “gas giant” planets found thus far outside Earth’s solar system orbit their stars in the zone life requires. Gas giants, which are many times larger than Earth, form under cold, low radiation conditions far from their stars. By gravitational interactions with interplanetary dust or with other planets and stars that pass by, most gas giants drift into the proximity of their stars. This drifting process drastically decreases their likelihood of retaining the nearly circular, stable orbit life demands.19-24 Of the known extrasolar gas giants, only two orbit anywhere near the life-habitable zone, and these two follow such an eccentric (i.e., elongated) orbital path as to make life on their satellites (moons), if they have any, impossible.25-28 The question remains unanswered as to whether or not giant planets can possibly retain the satellites during migration.
A satellite close enough to its planet to avoid enormous seasonal temperature fluctuations (caused by variations in the distance to the planet’s star, or heat source, as the satellite orbits its planet) becomes tidally locked to the planet—the same side always faces the planet. This tidal locking itself causes a host of life-destructive effects.
For example, tidal locking makes the satellite’s rotation period identical to the planet’s. Unless that period is short enough, day-to-night temperature differences become too extreme for life’s survival. However, the rotation period can only be that short if the satellite orbits closely. Within this sufficiently close range, however, another set of problems arises. For example, tidal forces generate drastic climatic and orbital instabilities (tidal torques force such a satellite to move farther and farther away from its planet), as well as massive and frequent volcanic eruptions (such as astronomers see on Jupiter’s moon Io).29 Any possible life-favorable conditions last briefly, at best.
A satellite with a highly improbable life-sustaining atmosphere most likely loses it in short order unless that satellite somehow possesses a strong magnetic field (similar to that of the Sun, Jupiter, and Earth). Otherwise, charged particles accelerated by the planet’s magnetosphere sputter away the satellite’s atmosphere. The magnetic field around Ganymede, the largest known planetary satellite and the only one with undisputed magnetism, measures less than 1 percent the strength of Earth’s.30-32
Another life risk for a satellite closely orbiting a large planet is that such a planet’s gravity significantly attracts asteroids, comets, and other debris passing nearby. This attraction increases the likelihood of bombardment, and such bombardment proves catastrophic to any possible life on the satellite.
A satellite cannot retain an adequate atmosphere for life unless its mass exceeds 12 percent of Earth’s mass. 33 At the same time, the satellite needs a mechanism to compensate for its nearby star’s increasing luminosity (brightness, thus light and heat radiation) as the star ages. The only known mechanism is the one seen on Earth, called the carbonate-silicate cycle. This cycle cannot operate, however, without lots of dry land (which eliminates ice-water environments such as Jupiter’s satellite Europa) and without a high level of plate tectonic activity.34, 35
Plate tectonics, in turn, require a certain minimum mass (0.23 Earth masses), and the demands of sustaining a carbonate-silicate cycle significantly increases that minimum. The best calculation to date sets the minimum mass of this hypothetical satellite at three times the mass of Mars, which is more than twelve times the mass of the solar system’s largest satellite. Of course plate tectonics also demand lots of liquid water (thus eliminating all dry satellites) and the precisely-timed introduction of just-right plant life in just-right amounts throughout the satellite’s history.36-37
More Radical Proposals
Sustaining the quest for other potential life sites, planetary scientist David Stevenson and origin-of-life researchers Jeffrey Bada and Christopher Wills go so far as to speculate that life might not require a home near a star.38-39 They suggest this scenario: A planet may be ejected from a normal planetary system before losing any of its light gases. If so, the planet may retain enough surface warmth (from interior radioactive decay) and a sufficiently heavy molecular hydrogen outer atmosphere (a heat-trapping blanket) to sustain life chemistry and metabolism.
To be capable of life support, however, such a hypothetical site would require super-enrichment by radioactive elements, and no mechanism or scenario exists to bring this enrichment about—none that would accomplish the job without simultaneously destroying the molecular hydrogen outer atmosphere. If the planet somehow acquired this enrichment, it still faces a problem: heat from the radioactive decay would decline exponentially through time. So, while such a planet might serve as a brief stopover for primitive life, it could not stay within the life-support range of temperature and other conditions long enough to serve as any conceivable home for intelligent life.
If life claims a home anywhere in the vast cosmos, it must be on a planet like Earth orbiting a star like the Sun in a galaxy like the Milky Way. And, as ongoing studies shows, that possibility shrinks, rather than grows, as each year’s research adds to the harvest of data. Extraterrestrial life does indeed appear to be homeless—unless, of course, a transcendent, supernatural Being built that home. But that possibility points toward, rather than away from, belief in the biblical Creator.
Alternative Life Forms
One other possibility must still be addressed, a question that often hampers progress toward a realistic assessment of the chance for life elsewhere: To what degree might extraterrestrial life differ from “life as we know it”? At one time biologists speculated that extraterrestrial life might be based on exotic chemistry, something other than carbon.
So, biochemists went to work on the problem. Their research showed that only silicon and boron, besides carbon, can serve as the basis for adequately complex molecules—molecules capable of sustaining basic life functions, such as self-replication, metabolism, and information storage. This finding presents some significant problems, however. First, silicon can hold together a string of no more than a hundred amino acids—far too short a string to accommodate any conceivable life systems and processes. Second, throughout the universe boron is less abundant than carbon; so carbon always supersedes it. Third, concentrated boron is toxic to certain life-critical reactions.
The conclusion, published as early as 1961, still stands. Physicist Robert Dicke deduced at that time that if anyone wants physicists (or any other physical life forms, for that matter), carbon-based biochemistry is a must.40 The key word, here, is physical. What about life that is not physical?
The Spiritual Opportunity
Both science and the Bible offer helpful information on this topic of non-physical reality. Science points to the existence of a transcendent (beyond space and time), personal Creator, demonstrably the same Creator revealed in the pages of Scripture. The Bible, in turn, reveals the existence of life forms other than Earth life, other than physical life. This life may be described as spiritual life, and yet it possesses the capacity for at least some physical expression or manifestation.
The Bible calls these creatures (in English translations) “angels,” “ministering servants,” or “ministering spirits.” Three specific names are given in the text: Michael, Gabriel, and Lucifer. The latter, also called Satan, led a rebellion against God. Scripture refers to the angels who rebelled with him (about a third of the total number) as “evil spirits,” “devils,” or “demons.” The one reliable source of information about this other kind of life is the Bible, and further study is highly recommended.
The possibility for life elsewhere is in fact great, as great as the certainty that the Bible is a true, trustworthy, and relevant revelation from the Creator. Any question that leads to an opportunity to talk about the word of God as well as the work of God, the Creator, deserves to be called a great question.
- Iosef S. Shklovskii and Carl Sagan, Intelligent Life in the Universe (San Francisco: Holden-Day, 1966), 343-52.
- Hugh Ross, The Creator and the Cosmos, 2d ed. (Colorado Springs, CO: NavPress, 1995), 132-44.
- Hugh Ross, The Creator and the Cosmos, 3d ed. (Colorado Springs, CO: NavPress, 2001), 195-99.
- Guillermo Gonzalez, “The Stellar Metallicity-Giant Planet Connection,” Monthly Notices of the Royal Astronomical Society 285 (1997): 403-12.
- Guillermo Gonzalez, “Spectroscopic Analysis of the Parent Stars of Extrasolar Planetary System Candidates,” Astronomy and Astrophysics 334 (1998): 221-38.
- Guillermo Gonzalez, George Wallerstein, and Steven H. Saar, “Parent Stars of Extrasolar Planets. IV. 14 Herculis, HD 187123, and HD 210277,” Astrophysical Journal Letters 511 (1999): L111-14.
- Guillermo Gonzalez and Chris Laws, “Parent Stars of Extrasolar Planets. V. HD 75289,” Astronomical Journal 119 (2000): 390-96.
- Guillermo Gonzalez et al., “Parent Stars of Extrasolar Planets. VI. Abundance Analyses of 20 New Systems,” Astronomical Journal 121 (2001): 432-52.
- Guillermo Gonzalez, private communication, 1991. The 2 percent figure was determined from the minimum metal richness observed in stars with planets and the maximum age of stars with planets. Interestingly, Carl Sagan came up with the same figure in 1966 (Shklovskii and Sagan, 344).
- S. Sahipal et al., “A Stellar Origin for the Short-Lived Nuclides in the Early Solar System,” Nature 391 (1998), 559-661.
- G. J. Wasserburg, R. Gallino, and M. Busso, “A Test of the Supernova Trigger Hypothesis with 60Fe and 26Al,” Astrophysical Journal Letters 500 (1998): L189-93.
- Peter Hoppe et al., “Type II Supernova Matter in a Silicon Carbide Grain from the Murchison Meteorite,” Science 272 (1996): 1314-16.
- Ronald L. Gilliland et al., “A Lack of Planets in 47 Tucanae from a Hubble Space Telescope Search,” Astrophysical Journal Letters 545 (2000): L47-51.
- Shklovskii and Sagan, 343-50.
- Ross, The Creator and the Cosmos, 3d, 187-99.
- J. F. Kasting, D. P. Whitmire, and R. T. Reynolds, “Habitable Zones around Main Sequence Stars,” Icarus 101 (1993): 108-28.
- Darren M. Williams, James F. Kasting, and Richard Wade, “Habitable Moons around Extrasolar Giant Planets,” Nature 385 (January 1997), 234-36.
- Darren M. Williams, “Habitable Moons around Extrasolar Giant Planets,” in The Stability of Habitable Planetary Environments (Ph.D. thesis, Pennsylvania State University, 1998), 111-20.
- Frederic A. Rasio and Eric B. Ford, “Dynamical Instabilities and the Formation of Extrasolar Planetary Systems,” Science 274 (November 1996): 954-58.
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- D. N. C. Lin, P. Bodenheimer, and D. C. Richardson, “Orbital Migration of the Planetary Companion of 51 Pegasi to Its Present Location,” Nature 380 (April 1996), 606-7.
- Stuart J. Widenschilling and Francesco Marsari, “Gravitational Scattering as a Possible Origin for Giant Planets at Small Stellar Distances,” Nature 384 (December 1996), 619-21.
- Stuart Ross Taylor, Destiny or Chance: Our Solar System and Its Place in the Cosmos (Cambridge, UK: Cambridge University Press, 1998).
- Jean Schneider, Extra Solar Planets Catalog www.obspm.fr/encycl/catalog.html.
- S. Vogt et al., “Six New Planets from the Keck Precision Velocity Survey,” Astrophysical Journal 536 (2000): 902-14.
- Gozalez, Wallerstein, and Saar, L111-14.
- Ing-Guey Jiang and Wing-Huen Ip, “The Planetary System of Upsilon Andromedae,” Astronomy and Astrophysics 367 (2001): 943-48.
- Jean Schneider, Extra Solar Planets Catalog. wwwusr.obspm.fr/departement/darc/planets/encycl.html.
- William B. McKinnon, “Galileo at Jupiter—Meetings with Remarkable Moons,” Nature 391 (1997), 23-26.
- Guillermo Gonzalez, “New Planets Hurt Chances for ETI,” Facts & Faith 12, no. 4 (1998), 2-4.
- D. A. Gurnett et al., “Evidence for a Magnetosphere at Ganymede from Plasma-wave Observations by the Galileo Spacecraft,” Nature 384 (December 1996), 535-37.
- M. G. Kivelson et al., “Discovery of Ganymede’s Magnetic Field by the Galileo Spacecraft,” Nature 384 (December 1996), 537-41.
- Kivelson et al., 541.
- Hugh Ross, The Creator and the Cosmos, 3d ed. (Colorado Springs, CO: NavPress, 2001), 180-83.
- Darren M. Williams, thesis, 115-17.
- Katherine L. Moulton and Robert A. Berner, “Quantification of the Effect of Plants on Weathering: Studies in Iceland,” Geology 26 (October, 1998): 895-98.
- Tyler Volk and David Schwartzman, “Biotic Enhancement of Weathering and the Habitability of Earth,” Nature 340 (1989), 457-60.
- David Stevenson, “Life-Sustaining Planets in Interstellar Space?” Nature 400 (1999), 32.
- Christopher Wills and Jeffrey Bada, The Spark of Life (Cambridge, MA: Perseus Publishing, 2000), 250-52.
- Robert H. Dicke, “Dirac’s Cosmology and Mach’s Principle,” Nature 192 (1961), 440.