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:

  1. They burn up their nuclear fuel supply much more rapidly (a star’s brightness increases with the fourth power of its mass)
  2. They radiate much more deadly ultraviolet and X-ray radiation
  3. Their brightness changes at a much more rapid rate
  4. 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

Endnotes

  1. Rory Barnes, “Tidal Locking of Habitable Exoplanets,” Celestial Mechanics and Dynamical Astronomy, (accepted August 2017), eprint: arXiv:1708.02981.
  2. Hugh Ross, Improbable Planet: How Earth Became Humanity’s Home (Grand Rapids: Baker, 2016): 80, 88–90.
  3. Barnes, “Tidal Locking.”
  4. Hugh Ross, “‘Electric Wind’ Becomes 9th Habitable Zone,” Today’s New Reason to Believe (blog), Reasons to Believe, July 4, 2016, http://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, http://www.reasons.org/articles/astrosphere-habitable-zones-display-fine-tuned-characteristics; Ross, Improbable Planet, 78–93.

Subjects: Exoplanets, Moon & Its Formation, Sun, Extrasolar Planets, Life on Other Planets, Solar System Design

Check out more from Dr. Hugh Ross @Reasons.org

About The Author

Dr. Hugh Ross

Reasons to Believe emerged from my passion to research, develop, and proclaim the most powerful new reasons to believe in Christ as Creator, Lord, and Savior and to use those new reasons to reach people for Christ. I also am eager to equip Christians to engage, rather than withdraw from or attack, educated non-Christians. One of the approaches I’ve developed, with the help of my RTB colleagues, is a biblical creation model that is testable, falsifiable, and predictive. I enjoy constructively integrating all 66 books of the Bible with all the science disciplines as a way to discover and apply deeper truths. 1 Peter 3:15–16 sets my ministry goal, "Always be prepared to give an answer to everyone who asks you to give the reason for the hope that you have. But do this with gentleness and respect, keeping a clear conscience." Hugh Ross launched his career at age seven when he went to the library to find out why stars are hot. Physics and astronomy captured his curiosity and never let go. At age seventeen he became the youngest person ever to serve as director of observations for Vancouver's Royal Astronomical Society. With the help of a provincial scholarship and a National Research Council (NRC) of Canada fellowship, he completed his undergraduate degree in physics (University of British Columbia) and graduate degrees in astronomy (University of Toronto). The NRC also sent him to the United States for postdoctoral studies. At Caltech he researched quasi-stellar objects, or "quasars," some of the most distant and ancient objects in the universe. Not all of Hugh's discoveries involved astrophysics. Prompted by curiosity, he studied the world’s religions and "holy books" and found only one book that proved scientifically and historically accurate: the Bible. Hugh started at religious "ground zero" and through scientific and historical reality-testing became convinced that the Bible is truly the Word of God! When he went on to describe for others his journey to faith in Jesus Christ, he was surprised to discover how many people believed or disbelieved without checking the evidence. Hugh's unshakable confidence that God's revelations in Scripture and nature do not, will not, and cannot contradict became his unique message. Wholeheartedly encouraged by family and friends, communicating that message as broadly and clearly as possible became his mission. Thus, in 1986, he founded science-faith think tank Reasons to Believe (RTB). He and his colleagues at RTB keep tabs on the frontiers of research to share with scientists and nonscientists alike the thrilling news of what's being discovered and how it connects with biblical theology. In this realm, he has written many books, including: The Fingerprint of God, The Creator and the Cosmos, Beyond the Cosmos, A Matter of Days, Creation as Science, Why the Universe Is the Way It Is, and More Than a Theory. Between writing books and articles, recording podcasts, and taking interviews, Hugh travels the world challenging students and faculty, churches and professional groups, to consider what they believe and why. He presents a persuasive case for Christianity without applying pressure. Because he treats people's questions and comments with respect, he is in great demand as a speaker and as a talk-radio and television guest. Having grown up amid the splendor of Canada's mountains, wildlife, and waterways, Hugh loves the outdoors. Hiking, trail running, and photography are among his favorite recreational pursuits - in addition to stargazing. Hugh lives in Southern California with his wife, Kathy, and two sons.



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