Extrasolar Planets and Religious Responses

Joseph L. Spradley*

 Physics Department

Wheaton College
Wheaton, IL 60187-5593

From: Perspectives on Science and Christian Faith 51.1 (March 1999): 34-39.

 One of the most important discoveries of 1996 involved the first clear evidence for extrasolar planets orbiting several sun-like stars. More than a dozen candidates for planets around nearby stars have excited the imagination of astronomers and nonprofessionals alike. If it can be demonstrated that planetary systems are a common occurrence among the billions of stars in our galaxy, then the possibility of extraterrestrial life may take on greater credibility. Indeed, the idea that intelligent civilizations might exist on other planets could become more compelling. Early reactions to extrasolar planet discoveries jumped at the possibility that they might contain water and thus harbor life.1 However, a closer look at the accumulating data suggests that our solar system may be unique in its life-supporting planetary arrangement.

 Both the possibility of life in other planetary systems and the apparent uniqueness of our solar system have interesting religious implications. Even if the possibility of extraterrestrial life were to increase, Christian thinking should be able to accommodate such ideas with perhaps some theological adjustments concerning the singular nature of the Incarnation. Roman Catholic theologian Father Theodore Hesburgh states: "It is precisely because I believe theologically that there is a being called God, and that he is infinite in intelligence, freedom and power, that I cannot take it upon myself to limit what he might have done... Finding others than ourselves would mean knowing him better."2 Protestant astronomer Owen Gingerich of the Harvard-Smithsonian Center for Astrophysics agrees: "In Genesis there's a sacred story being told that focuses on us. But there is nothing that precludes intelligent life elsewhere in the universe."3 However, if the evidence continues to point toward the uniqueness of our solar system, many current assumptions of popular culture would be challenged.

For many scientific devotees, extraterrestrial intelligence has become an article of faith, following the postmodern trend to demote humans from any unique place of significance in the universe. Carl Sagan,who believed that every sun-like star would have planets,4 was one of the most vocal exponents of this view. The movie based on his novel Contact expresses this faith that something out there is "greater than ourselves" and "none of us are alone," repeatedly telling us, "if we are alone in the universe, there sure is a lot of wasted space." However, growing evidence suggests the possibility that the entire universe was necessary to produce the conditions for intelligent life on a single planet.5

Extrasolar Planet Background

The history of extrasolar planetary searches suggests that some caution is needed in assessing recent evidence. Early in the twentieth century, astrometric evidence from Barnard's star, a nearby red dwarf one-seventh the mass of the Sun, indicated a slight wobble which seemed to imply gravitational interaction by one or two Jupiter-mass planets in decade-long orbits. However, by 1980 further work showed that the wobble of Barnard's star was more likely the result of a companion star too small to observe.6

Double-star systems like Barnard's tend to rotate around each other in larger orbits than the tiny wobble of a star with a planetary system. The mass of an unseen companion is estimated from the amount of wobble detected from a visible star. Masses between about forty and eighty Jupiter masses usually qualify as brown-dwarf stars, defined as objects that form like other stars by gravitational collapse of a dust cloud rather than from a stellar disk, but are too small to sustain the nuclear fusion processes that energize most stars.

Planets condense from materials in the disk produced by the collapse and rotation of a newly forming star, and are believed to have masses less than about ten Jupiter masses. In standard theories of planetary formation, matter in the protoplanetary disk collides and coagulates into planetesimals ranging up to ten kilometers in size. The planetesimals attract each other by gravity to trigger a sequence of mergers that produces the inner rocky terrestrial planets, and the outer rock-and-ice cores that seed the giant gas planets.

Because giant planets require such a large amount of material, they should form only in regions several astronomical units (AU = Earth-Sun distance) from their host star. Only in these outer expanses of the disk (greater than about five AU) is it cool enough for ice to form out of water molecules, roughly tripling the amount of solid material available for planet making. When the ice-and-rock core reaches about ten Earth masses, it begins to attract huge amounts of hydrogen and helium gases and expands to about one Jupiter mass (318 Earth masses) until its gravity can begin to tear a gap in the disk that feeds it, thus stopping its growth. With this model, theorists were successful in accounting for the solar-system sequence of inner rocky planets (Mercury to Mars) and outer gas planets (Jupiter to Neptune) beyond five AU, but were completely surprised by the orbits of the new Jupiter-size extrasolar planets.7

Planet discoveries around sun-like stars began in 1995 and proliferated in 1996 with a new generation of computers and optical instruments. Since planets are about a billion times fainter than their host star, they are virtually impossible to see by direct methods. An indirect method involves searching for a tiny wobble in the motion of a star as it and any companions it may have orbit about their common center of mass. Although the gravitational interaction between a star and planet is too small to observe directly, the radial velocity (back and forth along the line of sight) alternately increases and decreases the wavelength of light from the star, causing an alternating Doppler shift toward the red and blue end of its spectrum.

The amount of a star's Doppler shift determines its velocity. The shift in the wavelength due to a Jupiter-size planet is only one part in ten million. In the Doppler shift, the periodic variation reveals the period of a planet's orbital motion. The velocity of the star and the period of its motion can be analyzed to determine the radius of the orbit (from Kepler's laws) and the minimum mass of the planet (from Newton's laws), but the unknown inclination of its orbit allows for a larger wobble than its apparent radial motion and, thus, a larger possible massˇup to a factor of about two. The shape of the periodic variation curve reveals the shape of the orbit. A circular orbit produces a perfect sine wave while an eccentric orbit produces an irregular variation which can be analyzed by computer to determine the orbital shape.

Extrasolar Planet Evidence

 Extrasolar planet discoveries around sun-like stars have revealed two new and unexpected types of planetary objects: hot-Jupiter planets with small circular orbits and eccentric-Jupiter planets with elongated orbits. In October 1995, Swiss astronomers, Michael Mayor and Didier Queloz, announced evidence of a companion object orbiting the star 51 Pegasi about forty light years away. New computer techniques revealed a periodic Doppler shifting of the light from the star, suggesting a tiny wobble of up to eighty m/s caused by a planet of at least 0.46 of Jupiter's mass and a period of only 4.23 days in a circular orbit of just 0.05 AU radius. At this tiny distance from the sun, the 51-Pegasi planet has a surface temperature of about 1800 deg. C. making it the first of several "hot-Jupiter" planets.8

During 1996, Geoffrey Marcy and Paul Butler of San Francisco State University announced the discovery of six new Jupiter-size planets in a survey of 120 nearby sun-like stars over a period of about ten years. Using a refined form of the method of Mayor and Queloz, they achieved a three-fold improvement in accuracy, detecting radial motions to about three m/s. Since Jupiter, which contains more mass than all the other planets combined, causes the Sun to move at speeds of up to 12.5 m/s, Jupiter-size planets can be readily detected. With these accuracies, however, Earth-size planets cannot be detected. Even Jupiter-size planets with periods of several years require that data be collected over a long enough time to determine their orbital periods.9

Three of Marcy and Butler's new planets were hot Jupiters with nearly circular orbits at distances of only 0.11 AU or less from their host stars (55 Cancri, Tau Bootis and Upsilon Andromedae), having periods less than fifteen days and minimum masses ranging from 0.68 to 3.87 Jupiters. Another planet discovered in 1997 around the star, Rho Corona Borealis, has a minimum mass of 1.1 Jupiters and a circular orbit at a distance of 0.23 AU and period of 39.6 days. Since it is much closer than Mercury to the Sun, it also appears to qualify as a hot Jupiter. These discoveries showed that the 51-Pegasi planet was not as unusual as it first seemed.

Marcy and Butler also found an eccentric-Jupiter planet around the star 70 Virginis with a 117-day elongated orbit ranging from 0.27 to 0.59 AU and a mass of at least 6.5 Jupiters. This discovery led David Latham at Harvard to suggest that an object found in 1988 with a mass of at least nine Jupiters orbiting the star HD 114762 in an 84-day elongated orbit that varies from 0.22 to 0.46 AU was also an eccentric-Jupiter planet rather than a small brown dwarf as first assumed. A third eccentric Jupiter was discovered in 1997 orbiting the star 16 Cygni B in a triple star system. It has a mass of at least 1.5 Jupiters and a 2.2 year orbit varying widely between 0.6 and 2.8 AU.

A few of the recent extrasolar planet discoveries appear to be a little more like Jupiter, but still rather puzzling. Two of Marcy and Butler's first six planets included a second one around 55 Cancri, having a period of about twenty years, an orbital radius of about five AU, and a mass of at least five Jupiters. Another one around 47 Ursae Majoris has a minimum mass of 2.3 Jupiters, a period of 3.0 years, and a nearly circular orbit at a distance of 2.1 AU, still less than the expected distance for a giant planet. At this distance it would have a surface temperature of about 85 Deg. C. low enough to allow for liquid water but with a huge surface gravity that would be problematic for life.

Two other Jupiter-like planets, both orbiting the star Lalande 21185, were announced in 1996 by George Gatewood of the University of Pittsburgh.10 Analyzing data from fifty years of photographic observations and eight years of photoelectric measurements, he detected one planet with about 0.9 of a Jupiter mass at 2.2 AU and a period of about 5.8 years, and another with about 1.1 Jupiter masses at eleven AU and a period of about thirty years.

An objection to extrasolar-planet interpretation of the Doppler evidence has been raised by David F. Gray, who claims that the perceived periodic motions of some stars may be the result of their pulsations rather than planetary interactions. Gray claims that oscillations in the star's atmosphere could reshape spectral lines by Doppler shifting and gives evidence that the spectral lines for 51 Pegasi vary in shape with a 4.23 day period. This objection has been countered by Mayor, Queloz, Marcy and Butler by pointing out that pulsations should change the brightness of 51 Pegasi, but it has a constant brightness to one part in five thousand. They also point out that only one period has been detected, with none of the other overtones or oscillation modes that should accompany pulsations.11

Extrasolar Planet Implications

The discovery of extrasolar planets around sun-like stars may at first seem to offer new hope for the existence of planetary systems like ours that would support extraterrestrial life. But the unexpected nature of these planets has raised new doubts about the possibility that any of them might harbor life. Hot-Jupiter planets and eccentric-Jupiter planets have initiated a new generation of theories about planetary formation and the uniqueness of our solar system. Evidence so far seems to indicate that our solar system is highly unusual in its life-supporting planetary arrangement.

The strangest of the new planets are the hot Jupiters with minimum masses ranging from 0.46 to 3.87 Jupiter masses and orbital radii less than 0.23 AU. They all are most likely gas planets with surface temperatures well above the boiling point of water. Revised theories suggest that they might have formed beyond five AU from their host stars in a dense protoplanetary disk, which then slowed them down and caused them to spiral inward. Such a process would obliterate any small, inner terrestrial planets congenial to life as we know it on Earth.12

The eccentric Jupiters have longer periods (84 days to 2.2 years) and larger orbits, but with huge eccentricities (0.35 to 0.67) and larger distances from their host stars out to as much as 2.8 AU. New theories suggest that two or more super-Jupiters forming from a dense protoplanetary disk might then interact with each other gravitationally, causing some to be thrown into eccentric orbits or even tossed free of the star (as seen in May 1998 in the first photo of a possible extrasolar planet). Such eccentric giants would gravitationally disturb and eventually collide with smaller inner planets, again precluding life-supporting planets like Earth.

The Jupiter-like planets are a little more like those in our solar system but still diverge from ideal conditions for life. Even though they have nearly circular orbits and are further from their host stars than the hot Jupiters, their huge mass (0.9 to 5 Jupiter masses) suggests that they are lifeless gas planets like Jupiter with stormy, violent winds and intense gravity. Those that are near the habitable zone (about two AU) would also tend to upset the orbital stability of any smaller Earth-like planets. In cases where systems of two planets have been identified (55 Cancri and Lalande 21185), the outer ones are most like Jupiter, but each has an inner hot Jupiter that would again preclude terrestrial planets.

 Although current methods can only detect Jupiter-size planets, the orbits detected so far appear to reduce the possibility of smaller life-supporting planets. Most of the 120 stars surveyed by Marcy and Butler do not appear to have Jupiter-like planets at all (in either size or period), but they could have smaller undetected planets. However, such Jupiter-like planets may be necessary for the development of complex life forms on smaller planets. Earth is struck by asteroids large enough to cause mass extinctions of species about once every fifty to a hundred million years. Computer simulations by George Wetherill show that without Jupiter in its present stable orbit beyond Earth, sweeping up most killer asteroids and comets (as seen with comet Shoemaker-Levy 9 in July 1994), this Earth-collision rate would be about a thousand times greater, too large to permit the development of higher forms of life, if any at all.13 Thus none of the 120 sun-like stars surveyed so far appear to offer much hope for life, greatly decreasing the probability factors that Sagan and others have presumed as the basis for the existence of extraterrestrial civilizations.

Religious Responses

 The possibility of extraterrestrial civilizations has become almost an article of faith for many contemporary scientists, despite the lack of any evidence for their existence, and the discovery of extrasolar planets has not added much hope. Just as ancient civilizations looked to the skies for their deities, many modern materialists hope for radio signals from space to confirm their faith in higher intelligences on extrasolar planets. Such blind enthusiasm offers a naturalistic substitute for faith in God, and can easily lead to alternative religious innovations. Such influence is evident in three, nineteenth-century religious movements that incorporated extraterrestrial life into their religious thought: the New Jerusalem or Swedenborgian Church, the Mormon Church, and the Seventh-Day Adventist Church.

The Swedish scientist and sage Emmanuel Swedenborg (1688˝1772) claimed to have had conversations with extraterrestrials and worked out a theology that included them. The Swedenborgian Church was organized in London (1787) and now has about 40,000 members. Besides the Book of Mormon, Joseph Smith (1805˝44) provided his Church of Jesus Christ of Latter-Day Saints with other Scriptures, including Doctrine and Covenants and The Pearl of Great Price, which taught that there are many inhabited worlds in the universe. This doctrine is given considerable emphasis in the theology of the Mormons, who now number some eight million members. Beginning in 1846, Ellen G. White (1827˝1915) began having visions involving extraterrestrials. When she and her associates founded the Seventh-Day Adventist Church in 1863, White developed a theology involving extraterrestrials in which sin occurred only on Earth. In The Story of Patriarchs and Prophets, she taught that Christ passed from star to star to superintend the sinless intelligences of other worlds. This cosmic conception of Christianity has spread to a worldwide membership of about 4.4 million.14

Most scientific interest in extraterrestrial intelligence today is based on naturalistic arguments concerning the probability of life arising on extrasolar planets, but it is often closely associated with religious themes, such as a yearning for meaning, wisdom, and even immortality which is presumably possessed by extraterrestrial higher intelligences.15 Although plausible arguments are used to further this faith, no evidence has yet been found to support it. If the evidence for the lack of Earth-like extrasolar planets that can support intelligent life continues to accumulate, the only saving hope for many naturalists will fail. It is too early to say for sure, but the most important lesson that might emerge from such evidence is the uniqueness of our solar system with its life-sustaining planetary arrangement as a special gift from God to his creatures on Earth.



1R. Cowen, "Two Extrasolar Planets May Hold Water," Science News 149 (January 27, 1996): 52.

2Quoted by Frank Drake and Dava Sobel in Is Anyone Out There? The Scientific Search for Extraterrestrial Intelligence (New York: Delacorte Press, 1992), 191.

3Quoted by Kenneth Woodward in "A Vindication of God," Newsweek (August 19, 1996): 58.

4I. Shklovskii and C. Sagan, Intelligent Life in the Universe (San Francisco: Holden-Day, 1966), 410˝413.

5For a summary of the evidence before 1993, see Hugh Ross, The Creator and the Cosmos (Colorado Springs: NavPress, 1993), 123˝35.

6D. Mammana and D. McCarthy, Other Suns: Other Worlds? (New York: St. Martin's Press, 1996), 43˝59.

7New planet formation theories are outlined by James Glanz, "Worlds Around Other Stars Shake Planet Birth Theory," Science 276 (30 May 1997): 1336˝9.

8M. Mayor and D. Queloz, "A Jupiter-Mass Companion to a Solar-Type Star," Nature 378 (1995): 355.

9A good summary of recent discoveries is by Robert Naeye, "The Strange New Planetary Zoo," Astronomy (April, 1997): 42˝9. Marcy and Butler maintain a good website for current information on extrasolar planet discoveries at http://cannon.sfsu.edu/~williams/planetsearch/planetsearch.html.

10G. Gatewood, "Lalande 21185," Bulletin of the American Astronomical Society 28 (1996): 885.

11D. Gray, "Absence of a Planetary Signature in the Spectra of the Star 51 Pegasi," Nature 385 (1997): 795 and Joshua Roth, "Does 51 Pegasi's Planet Really Exist?" Sky and Telescope 93 (May 1997): 24˝5.

12D. Mammana and D. McCarthy, Other Suns: Other Worlds?

13G. Wetherill, "How Special is Jupiter?" Nature 373 (1995): 470 and Ken Croswell, Planet Quest (New York: The Free Press, 1997), 161˝73.

14M. Crowe, "A History of the Extraterrestrial Life Debate," Zygon 32 (June 1997): 147˝62.

15See, for example, Drake and Sobel, Is Anyone Out There? 159˝62.