Is the universe really rife with life sites, as Carl Sagan asserted?1 Research data continues to shape an answer to this question, but the weight of evidence falls, thus far, on the negative side. The ever-optimistic naturalist must work harder and more imaginatively to keep up appearances—and hope.
The list of planets outside Earth’s solar system is growing. Not counting the rocky lumps near a couple of neutron stars, astronomers have discovered and confirmed sixty-seven planets in nearby regions of space.2, 3 At first glance, this finding may seem to support Sagan’s belief. A closer look, however, confirms what Peter Ward and Donald Brownlee’s recent book suggests: Earth is indeed a rare entity, perhaps utterly unique in its life-support characteristics.4
A team of twenty-four astronomers put Sagan’s hypothesis to the test. Using the Hubble Space Telescope’s (HST) detection capabilities, they focused on a “globular cluster” of stars called 47 Tucanae.5 (Globular clusters are the oldest pockets of star formation in any galaxy.) These stars provide an excellent test case because they are similar, physically and chemically, to virtually all the stars in most galaxies and to nearly 98% of the stars in the Milky Way galaxy.
If planets were as common in 47 Tucanae as they are among the stars in the Sun’s galactic neighborhood, the HST survey should have detected seventeen—the number mathematical modeling predicts. Instead, the research team found zero. 6 Not even one. This surprising finding lends support to the idea that special circumstances led to the formation of Earth’s solar system.
All sixty-seven of the planets discovered outside the solar system (and outside 47 Tucanae) are orbiting metal-rich stars (rich in elements heavier than hydrogen and helium) that are younger than the Sun.7-12 Astronomers anticipated this result, for they have long recognized that planets are comprised of “star ashes”—elements heavier than hydrogen and helium produced when stars burn out. In fact, to get enough of this heavy material to make planets requires at least two generations (or cycles) of star birth and star death. Additionally, most of that second cycle of star deaths must have occurred more recently than five billion years ago.
Astronomers now acknowledge that the Sun is exceptional. Less than five billion years ago, this star formed adjacent to two different supernovae (enormous star explosions or star deaths) that showered the Sun’s region of space with two different sets of metal-rich ashes. These events occurred just prior to, and just adjacent to, the condensation of the solar nebulae (the gas and dust cloud) from which Earth’s solar system formed.13-15 Amazing coincidence? Probability numbers suggest otherwise.
- Iosef S. Shklovskii and Carl Sagan, Intelligent Life in the Universe (San Francisco: Holden-Day, 1966), 343-50.
- Jean Schneider, Extra Solar Planets Catalog, a frequently updated Web site catalog at http://www.obspm.fr/encycl/catalog.html.
- S. H. Rhie et al., “On Planetary Companions to the MACHO 98-BLG-35 Microlens Star,” Astrophysical Journal 533 (2000): 378-91.
- Peter D. Ward and Donald Brownlee, Rare Earth (New York: Copernicus, 2000), 15-275.
- 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.
- Gilliland et al., L47, L50.
- 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, “Are Stars With Planets Anomalous?” Monthly Notices of the Royal Astronomical Society 308 (1999): 447-58.
- 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.
- 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.