The popular science news outlets are filled with articles claiming that our galaxy is teeming with habitable planets, as many as 40 billion by some estimates. These claims and estimates are based on astrobiologists noting that about 20 percent of the 3,480 planets discovered so far1 are “habitable” in that they conceivably could have liquid water existing on some part of their surface for at least a brief time period.

There is a growing recognition, however, in the astrophysical research literature of the past several months that a planet possessing liquid water on a small part of its surface for a brief period of time does not make that planet habitable. While life cannot exist without liquid water, it cannot even originate or survive unless the liquid water remains abundant for a long period of time. Also, there is now the broad acknowledgment by astrobiologists that life more advanced than unicellular or colonies of unicellular life-forms requires liquid water to continuously remain on a planet for at least a few billion years.

Getting a significant amount of liquid water to remain on a planet’s surface for a few billion years requires a delicate balancing between the increasing luminosity of the planet’s host star and the decreasing amount of greenhouse gases in the planet’s atmosphere. Two astronomers, Shintaro Kadoya and Eiichi Tajika, published a set of models in a recent issue of the Astrophysical Journal Letters, where they examined the long-term history of planet-star pairs, varying the mass of the star but keeping the planet Earth-like.2

Kadoya and Tajika took note that all stars with any conceivable possibility of hosting a habitable planet will be in that part of its nuclear burning history where the brightness of the star gradually and continuously increases as it ages. For example, during the history of life on Earth, the Sun’s luminosity has increased by 18–23 percent.3

For liquid water to remain on a planet’s surface for more than several million years, it must reduce the store of greenhouse gases in its atmosphere so that as its host star’s luminosity increases, its atmosphere traps less of its star’s heat. Kadoya and Tajika developed a detailed model for the decrease in the carbon dioxide degassing rate as a result of the cooling of an Earth-like planet’s interior and for the increase in the silicate erosion rate of its continental landmasses. This changing degassing rate and changing silicate erosion rate are the most significant factors affecting the reduction of greenhouse gases for an Earth-like planet.

Kadoya and Tajika’s models showed that Earth-like planets orbiting in the inner part of their host star’s water habitable zone became hotter with respect to time while Earth-like planets orbiting in the outer part of their host star’s water habitable zone became colder. In fact, planets orbiting in the outer water habitable zones became globally and permanently covered with ice, irrespective of the mass of the host star. Also, planets in the inner part of the water habitable zone typically get hotter at a rate that permanently transforms all the liquid water to water vapor.

Furthermore, Kadoya and Tajika determined that habitability longevity was critically dependent on duplicating Earth’s history of tectonic activity so as to produce the growth history of Earth’s continental landmasses and islands.

Kadoya and Tajika showed that the silicate weathering parameters in their model must be very carefully fine-tuned to maintain liquid water on its surface for as long as our Earth has possessed liquid water. The orbital features of the planet must also be carefully fine-tuned. In the words of Kadoya and Tajika,

“The orbital condition for maintaining the warm climate similar to the present Earth becomes very limited.”

Kadoya and Tajika’s research adds to the accumulating evidence that the liquid water  habitability condition is much more constraining than previously thought. Both the planet’s orbital features and its surface and interior features must be exquisitely fine-tuned for liquid water to remain on its surface for a long enough time period to make possible multicellular life. Such exquisite and extensive fine-tuning are the hallmarks of a super-intelligent, super-powerful, supernatural Creator.



  1.  “The Extrasolar Planet Encyclopedia,” accessed July 27, 2016,
  2. Shintaro Kadoya and Eiichi Tajika, “Evolutionary Tracks of the Climate of Earth-Like Planets around Different Mass Stars,” Astrophysical Journal Letters 825 (July 2016): id. L21.
  3. Hugh Ross, Improbable Planet: How Earth Became Humanity’s Home (Grand Rapids: Baker, 2016): 157. Pages 143–164 review the latest research on the Sun’s luminosity history and its impact on Earth’s life.
  4. Kadoya and Tajika, “Evolutionary Tracks,” 1.

Subjects: Exoplanets, Solar System Design

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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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