Have you ever wondered why we see only one hemisphere of the Moon? The reason why is because the Moon is tidally locked to Earth. The Moon is close enough to Earth that the gravity Earth exerts on the near side of the Moon is substantially greater than the gravity Earth exerts on the far side. The difference in these two gravitational exertions gradually forces the Moon’s rotation period around its axis to equal the Moon’s period of revolution about Earth. Today, the Moon’s revolution and rotation periods both equal about 29 days, which means that observers on Earth’s surface see only one hemisphere of the Moon (see video clip in the figure below).
Figure: Tidal locking of the Moon to Earth means that only one side of the Moon faces Earth.
Tidal locking takes time. The greater the difference between a body’s initial rotation period and its revolutionary period, the more time it takes to become tidally locked. The greater the distance between the two bodies, the more time it takes for tidal locking to occur. (The force of the tidal locking effect declines with the fourth power of the distance between the two bodies.) The smaller the mass ratio between the two bodies, the longer it takes for the smaller of the two bodies to become tidally locked.
In the case of the Moon, we do not have an accurate estimate of its initial rotation period. However, the Moon probably became tidally locked to Earth within about a billion years.
Tidal locking works in both ways. The Moon’s gravity is gradually slowing down Earth’s rotation rate. A few hundred million years ago, Earth’s rotation rate was just 20 hours. Astronomers calculate that 40 billion years from now Earth will be tidally locked to the Moon. However, both the Moon and Earth will be incinerated by the Sun in 4–5 billion years from now.
In a paper that has been accepted for publication in the journal Celestial Mechanics and Dynamical Astronomy, astronomer Rory Barnes provides calculations that establish that most extrasolar planets astronomers have labeled as “habitable” are very likely to be tidally locked.1 For a number of reasons, tidally locked planets are very unlikely to be habitable. I discuss several reasons why in my book Improbable Planet.2 An obvious reason is that one hemisphere of the planet always faces its star, thereby making that hemisphere blazing hot while the opposite hemisphere of the planet never faces its host star, thereby making that hemisphere freezing cold. While a thick planetary atmosphere might make the twilight zone between light and dark on the planet a suitable temperature for liquid water, atmospheric transport quickly moves all the planet’s surface water from its starlit side to its dark side where it becomes permanently frozen.
Barnes first shows that many planets as distant from their host stars as Earth is from the Sun could be tidally locked. For example, if Earth’s initial rotation period was 72 hours and it had no moon, it would be tidally locked to the Sun within 4.5 billion years (Earth’s present age = 4.57 billion years).
Barnes then shows that all planets orbiting stars as massive or less massive than the Sun that are at the appropriate distance from their host stars where liquid water on the surfaces of the planets is a possibility could be tidally locked. Planets orbiting stars more massive than the Sun are unlikely to be habitable for the following reasons:
- They burn up their nuclear fuel supply much more rapidly (a star’s brightness increases with the fourth power of its mass)
- They radiate much more deadly ultraviolet and X-ray radiation
- Their brightness changes at a much more rapid rate
- Their luminosity at any given time is not as stable
Planets orbiting stars less massive than the Sun need to be closer to their host stars to be in the zone where liquid water on their surfaces is possible. Such planets have a much higher probability of being tidally locked. Such planets dominate the population of extrasolar planets. Fifty of the sixty nearest stars are less than half the Earth’s mass. Thirty are less than 0.15 times Earth’s mass.
Barnes calculates that virtually all planets orbiting stars less than 0.15 Earth’s mass that have the possibility of surface liquid water will be tidally locked within one billion years. For planets in the liquid water zone of stars less than half Earth’s mass, Barnes determines that about one-half will be tidally locked in less than one billion years.
Astronomers’ main tool for discovering extrasolar planets (exoplanets) has been the Kepler Space Telescope (KST). Kepler has found 1,013 confirmed exoplanets and an additional 3,199 unconfirmed exoplanet candidates. The KST has finished its mission and is due to be replaced by the Transiting Exoplanet Survey Satellite (TESS). The launch date for TESS is June 2018. It will survey 500,000 stars for planets, which is 3.5 times as many stars as surveyed by Kepler. Astronomers expect TESS to discover at least 3,000 more exoplanets.
The rationale for both KST and TESS has been to find potentially habitable planets. Barnes’s calculations reveal, however, that about half of KST’s 3,199 unconfirmed exoplanet candidates, assuming they are actual planets, will be tidally locked. As for the exoplanets likely to be discovered by TESS, Barnes shows that the vast majority will approach tidal locking in less than one billion years. Barnes ends the abstract to his paper with this sentence: “These results suggest that the process of tidal locking is a major factor in the evolution of most of the potentially habitable exoplanets to be discovered in the near future.”3
Barnes’s paper shows that even if one only uses liquid water as a criterion for habitability, the possibility of finding such a planet is relatively remote. If one takes into account all nine known habitable zones,4 nothing less than miraculous interventions from the Creator God of the Bible could explain why the universe contains one planet on which diverse life thrives.
Featured Image: Artist’s rendition of the Neptune-sized planet, Gliese 436-b that is tidally locked to its host star. Image credit: NASA
- Rory Barnes, “Tidal Locking of Habitable Exoplanets,” Celestial Mechanics and Dynamical Astronomy, (accepted August 2017), eprint: arXiv:1708.02981.
- Hugh Ross, Improbable Planet: How Earth Became Humanity’s Home (Grand Rapids: Baker, 2016): 80, 88–90.
- Barnes, “Tidal Locking.”
- Hugh Ross, “‘Electric Wind’ Becomes 9th Habitable Zone,” Today’s New Reason to Believe (blog), Reasons to Believe, July 4, 2016, https://www.reasons.org/blogs/todays-new-reason-to-believe/electric-wind-becomes-9th-habitable-zone; Hugh Ross, “Astrosphere Habitable Zones Display Fine-Tuned Characteristics,” Today’s New Reason to Believe (blog), Reasons to Believe, July 7, 2014, https://www.reasons.org/articles/astrosphere-habitable-zones-display-fine-tuned-characteristics; Ross, Improbable Planet, 78–93.