The first discovered extrasolar planet was found in 1995, orbiting the nearby star 51 Pegasi.1 This finding persuaded many astronomers and millions of lay people to conclude that Carl Sagan was right—our Milky Way Galaxy (MWG) was filled with billions of planets, most of which would prove to be analogues to the planets in our solar system. Sagan concluded that this, in turn, would imply that life would be present on several million of them. The subsequent discovery of dozens more extrasolar planets before the end of the twentieth century further stirred the hope that we soon would be able to “explore strange new worlds, to seek out new life and new civilizations, to boldly go where no man has gone before.”
Teams of astronomers from all over the world continue to search for strange new worlds. As of December 11, 2009, these groups have found a total of 407 planets. Yet not a single one is an analogue to any of our solar system’s planets. None of the newly discovered planetary systems permit the existence of a planet like Earth. The exuberant vision imparted by Sagan has, for nontheists, turned into a dirge. In his book God: The Failed Hypothesis, atheist and particle physicist Victor Stenger laments that Earth, “a tiny blue speck in a vast universe,” is alone, the only locale where advanced life might exist.2
Now, an international team of astronomers, led by the Peruvian astronomer Jorge Meléndez, has discovered at least one reason why the Sun’s planets are proving unique.3 Meléndez has devoted much of his career to the quest to find a star that would qualify as a true twin of the Sun, in the sense that the star would possess all the characteristics necessary to make possible the existence of advanced life on a planet orbiting it. In 2006, he wrote, “Despite the observational effort carried out in the last few decades, no perfect solar twin has been found.”4
In an ongoing effort to understand why the Sun appears unique among all the stars in the MWG, Meléndez joined forces with three other astronomers to study the Sun’s composition relative to stars closest to being its analogues or twins. A solar analogue is a star that roughly approximates the mass, age, and spectral type of the Sun, what astronomers would indentify as a main sequence dwarf between spectral type G0 and G5 (the Sun spectral type is G2). A solar twin is a star whose characteristics are “almost identical to the Sun’s.”
Meléndez’s team performed “a differential elemental abundance analysis of unprecedented accuracy (~0.01 dex) of the Sun relative to 11 solar twins from the Hipparcos catalog and 10 solar analogs from planet searches.”5 In other words, they compared the abundances of different atomic elements in the Sun with abundances of the same elements seen in the most solar-similar stars to a precision never before achieved.
They found that the Sun possesses a startling unique elemental signature. Compared to the best solar twins, the Sun has about a 20 percent depletion of refractory elements relative to volatile elements. The refractory elements include aluminum, calcium, magnesium, and silicon that figure prominently in interplanetary dust, meteorites, and rocky planets like Earth. The volatiles are elements like carbon, nitrogen, and oxygen that factor into the gas molecules that make up the planetary atmospheres.
The only rational explanation Meléndez’s team could discern for the Sun’s elemental abundances was the unique system of planets orbiting the Sun. A few relatively small gas giant planets (volatile-rich planets) orbit the Sun at large distances, and some relatively medium-sized rocky planets (refractory-rich planets) orbit at close-in distances from the Sun. Meléndez’s team concluded that the peculiar solar elemental composition “would imply that solar-like stars with planetary systems similar to our own are a relatively rare occurrence.”6
Are the Sun and its system of planets so unusual that their rarity forces the conclusion that both were supernaturally designed for the specific benefit of the human species (known as the anthropic principle)? The evidence accumulated so far looks that way. However, the evidence comes from a database of only 21 stars. Before drawing any firm anthropic principle conclusions, pro or con, Meléndez argues that detailed comparisons with many more stars and planets are necessary.7 We agree and look forward to even more stringent tests of competing explanatory models for the universe and its stars and planets.
1. M. Mayor and D. Queloz, “A Jupiter-Mass Companion to a Solar-Type Star,” Nature 378 (November 23, 1995): 355.
2. Victor J. Stenger, God: The Failed Hypothesis (Amherst, New York: Prometheus Books, 2007): 154–62. The quote is on page 160.
3. J. Meléndez et al., “The Peculiar Solar Composition and Its Possible Relation to Planet Formation,” Astrophysical Journal Letters 704 (October 10, 2009): L66–L70.
4. Jorge Meléndez, Katie Dodds-Eden, and José A. Robles, “HD 98618: A Star Closely Resembling Our Sun,” Astrophysical Journal Letters 641 (April 20, 2006): L133.
5. J. Meléndez et al., L66.
6. J. Meléndez et al., L69.
7. J. Meléndez et al., L69–L70; Jorge Meléndez and Iván Ramírez, “HIP 56948: A Solar Twin with a Low Lithium Abundance,” Astrophysical Journal Letters 669 (November 10, 2007): L92.