page_title: Rare Earth hypothesis
section_title: A large moon may neither be rare nor necessary
hierarchical_section_title: Rare Earth hypothesis / Criticism / A large moon may neither be rare nor necessary
caption_attribution_description: English: This artist's concept shows a celestial body about the size of our moon slamming at great speed into a body the size of Mercury. NASA's Spitzer Space Telescope found evidence that a high-speed collision of this sort occurred a few thousand years ago around a young star, called HD 172555, still in the early stages of planet formation. The star is about 100 light-years from Earth.
context_page_description: In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the origin of life and the evolution of biological complexity such as sexually reproducing, multicellular organisms on Earth required an improbable combination of astrophysical and geological events and circumstances.
  According to the hypothesis, complex extraterrestrial life is an improbable phenomenon and likely to be rare. The term "Rare Earth" originates from Rare Earth: Why Complex Life Is Uncommon in the Universe, a book by Peter Ward, a geologist and paleontologist, and Donald E. Brownlee, an astronomer and astrobiologist, both faculty members at the University of Washington.
  In the 1970s and 1980s, Carl Sagan and Frank Drake, among others, argued that Earth is a typical rocky planet in a typical planetary system, located in a non-exceptional region of a common barred-spiral galaxy. From the principle of mediocrity, they argued that we are typical, and the universe teems with complex life. However, Ward and Brownlee argue that planets, planetary systems, and galactic regions that are as friendly to complex life as the Earth, the Solar System, and our galactic region are rare.
context_section_description: The requirement of a large moon (Rare Earth equation factor ) has also been challenged. Even if it were required, such an occurrence may not be as unique as predicted by the Rare Earth Hypothesis. Recent work by Edward Belbruno and J. Richard Gott of Princeton University suggests that giant impactors such as those that may have formed the Moon can indeed form in planetary trojan points (L₄ or L₅ Lagrangian point) which means that similar circumstances may occur in other planetary systems.
  Rare Earth's assertion that the Moon's stabilization of Earth's obliquity and spin is a requirement for complex life has been questioned. Kasting argues that a moonless Earth would still possess habitats with climates suitable for complex life and questions whether the spin rate of a moonless Earth can be predicted. Although the giant impact theory posits that the impact forming the Moon increased Earth's rotational speed to make a day about 5 hours long, the Moon has slowly "stolen" much of this speed to reduce Earth's solar day since then to about 24 hours and continues to do so: in 100 million years Earth's solar day will be roughly 24 hours 38 minutes (the same as Mars's solar day); in 1 billion years, 30 hours 23 minutes. Larger secondary bodies would exert proportionally larger tidal forces that would in turn decelerate their primaries faster and potentially increase the solar day of a planet in all other respects like Earth to over 120 hours within a few billion years. This long solar day would make effective heat dissipation for organisms in the tropics and subtropics extremely difficult in a similar manner to tidal locking to a red dwarf star. Short days (high rotation speed) causes high wind speeds at ground level. Long days (slow rotation speed) cause the day and night temperatures to be too extreme.
  Many Rare Earth proponents argue that the Earth's plate tectonics would probably not exist if not for the tidal forces of the Moon. The hypothesis that the Moon's tidal influence initiated or sustained Earth's plate tectonics remains unproven, though at least one study implies a temporal correlation to the formation of the Moon. Evidence for the past existence of plate tectonics on planets like Mars which may never have had a large moon would counter this argument. Kasting argues that a large moon is not required to initiate plate tectonics.
caption: Collision between two planetary bodies (artist concept).
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