Is Earth special? A large fraction of the scientific community doesn’t think so. In fact, most have adopted the Copernican principle, believing that Earth’s capacity to support life is commonplace. However, a number of factors indicate that Earth may be rare (or possibly unique) in its capacity to support life—even among the 100 sextillion terrestrial planets in the observable universe, according to a recent paper.
In two decades, the exoplanet catalog has grown to over 2,000 known exoplanets. Using data from those planets and host stars, astronomers have developed models to determine information about planets not yet discovered. Based on those models, astronomers have estimated that the observable universe contains around 1020 terrestrial planets!1 For comparison, somewhere between 1022–1024 stars exist in the observable universe, so roughly one in a hundred stars have rocky planets. These models also allow astronomers to compare terrestrial exoplanets to Earth. Amidst the comparisons, Earth stands out in at least three ways.
1. Age: Earth is younger.
While most terrestrial exoplanets are between 8–8.4 billion years old, Earth is much younger, only 4.5 billion years old. Why is such a young planet habitable? This is probably because older planets (that formed earlier in the history of the universe) are subject to dynamical and radiation effects that diminish the possibility of hosting life.
2. Galaxy type: Most planets reside in the wrong galaxies.
The number of planets per star remains largely constant with galaxy size; so, most terrestrial planets reside in galaxies about twice the size of the Milky Way. However, the vast majority of galaxies this large are not spiral but elliptical. Consequently, most of the terrestrial planets in the universe reside in ellipticals, but research suggests that truly habitable planets must orbit stars in a spiral galaxy—such as the Milky Way.
3. “Dangerous” neighbors: Earth has none.
Most planets that orbit otherwise life-friendly stars might have any hypothetical life exterminated due to radiation from nearby supernovae, gamma-ray bursts, active galactic nuclei, or dark matter annihilation regions. Dynamical encounters with interstellar gas clouds or dark matter clumps could also disrupt the stability of potentially habitable planets.
One theological point warrants discussion. The Bible gives much information about God’s activity to bring about human life here on Earth, but it says nothing about whether He performed similar work somewhere else in the universe. Except for angelic beings (they have no physical body), the Bible leaves open the question of whether life exists elsewhere in the universe. However, it emphatically states that all things exist because of His divine action (see John 1:1–3).
It seems likely scientific discoveries will continue to provide a growing body of evidence that Earth’s habitability is the exception instead of the rule. Astronomers have much work to do before they have the capacity to determine whether life exists beyond Earth, but the search is interesting from both a theological and scientific perspective.
Food for Thought
Would finding life on a planet outside our solar system diminish the case for God?
- Erik Zackrisson et al., “Terrestrial Planets across Space and Time,” Astrophysical Journal, preprint, submitted February 1, 2016, http://arxiv.org/abs/1602.00690.