Long-Term Habitability Requires a Sun-Earth Twin

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 far are “habitable” in that they could conceivably have liquid water existing on some part of their surface for at least a brief time period.¹

There is a growing recognition, however, in the astrophysical research literature of the past several months that the possession of liquid water on a small part of a planet’s 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 balance 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 the part of their nuclear burning history where the brightness of the stars gradually and continuously increases as they age. 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, the planet 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 the 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 a planet’s surface for as long as Earth has possessed liquid water. The orbital features of the planet must also be carefully fine-tuned. In the words of Kadoya and Tajika,

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 multicellular life possible. Such exquisite and extensive fine-tuning is the hallmark of a superintelligent, superpowerful, supernatural Creator.