At this time of year, Christmas carols and songs are in full swing. One song, “Baby, It’s Cold Outside,” won an Academy Award in 1949 and we hear it often in December because it’s the month when we who live above the Tropic of Cancer all notice that it’s getting a lot colder.
Now, a research paper explains why the surfaces of Earth-twin planets orbiting other stars are very likely to get a lot colder. For such planets Winter is coming, and there is no hope of spring.
By contrast, we live in an amazing planetary system. Our star has gotten 18–22 percent brighter over the past 3.8 billion years of life’s history on Earth (see figure).1
Figure: Sun’s Luminosity throughout Its History
Image credit: Hugh Ross
During that same period, geological and biological processes have combined to continuously cool Earth’s surface at rates that nearly perfectly compensate for the increased heating due to the Sun’s brightening. Those processes include:
- gradually falling rates of volcanic outgassing of greenhouse gases (predominantly carbon dioxide and methane);
- falling rates of continental growth and tectonic plate spreading, which lessens the rates at which carbon that is subducted into Earth’s mantle is returned to the atmosphere; and
- increasing biomass and biodiversity, which results in more carbon dioxide being removed from Earth’s atmosphere through photosynthesis and thereby converted into organic carbon, much of which gets buried and subsumed into Earth’s crust and mantle.
Earth’s geological and biological processes have induced more cooling through the gradual removal of greenhouse gas from Earth’s atmosphere and the gradual increase of Earth’s albedo (reflectivity). The latter results in increasing proportions of solar radiation being reflected into interplanetary space.
This long-enduring delicate balance between the Sun’s brightening and resultant cooling from geological and biological processes has allowed a diverse and super-abundant ecosystem of microbes over the course of 3.2 billion years to chemically transform Earth’s atmosphere, oceans, and continents so that animals could exist and thrive. With the balance extended for another 0.6 billion years, Earth’s atmosphere, oceans, and continents became further transformed so that human beings could exist and thrive.
In a recent issue of the journal Astrobiology, British planetary astronomer David Waltham explains why the enduring delicate balance between the Sun’s brightening and cooling from geological and biological processes is unlikely to occur on any other star-planet system.2 He shows that for the delicate balance to be sustained the star-planet system must be comprised of a star that is exactly like the Sun and a planet that is exactly like Earth.
Problem of Alternate Planets
As I explain in my book Improbable Planet, it takes extraordinary fine-tuning of Earth’s photosynthetic life and Earth’s plate tectonics to sustain tectonic plate subduction at rates sufficient to continuously remove greenhouse gases from Earth’s atmosphere so as to compensate for the Sun’s brightening.3 In two recent blogs on Earth’s carbon cycle (Part 1, Part 2), I describe how sixty different features and processes on Earth must be fine-tuned in different ways and at different rates for life to be continuously sustained throughout the past 3.8 billion years. As geophysicist Robert Stern observed, “Earth is the only known planet with subduction zones and plate tectonics, and this fact demonstrates that special conditions are required for this mode of planetary heat loss.”4
Microbial life may be possible on Earth-like planets but only for a short time period. Unless an Earth-like planet possesses the sixty different features and processes that I describe in the blogs, it will not be able to continuously compensate for the brightening of its host star. Such a planet will never be able to sustain animals or the equivalent of human beings.
Problem for Stars More Massive Than the Sun
The mass of a star determines how rapidly it brightens during that part of its history when its nuclear furnace is fusing hydrogen into helium. Stars more massive than the Sun brighten at a much more rapid rate. This enhanced brightening implies that the maximum possible time window for life to be sustained on one of its planets will be too short for the equivalent of humans to ever exist.
On Earth, the combination of geological and biological processes has compensated for the Sun’s increasing luminosity by progressively taking greenhouse gases from Earth’s atmosphere, chemically transforming them into carbon minerals, and depositing those minerals into Earth’s crust and mantle. However, over time these processes could remove too much carbon dioxide from the atmosphere. Photosynthesis requires a minimum level of carbon dioxide in the atmosphere. Photosynthesis rates plummet when the carbon dioxide level falls below 200 parts per million, and it ceases altogether in all plant species responsible for producing food for human consumption when the carbon dioxide level falls below 150 parts per million.
Within just a few tens of millions of years or less the Sun will be so bright (see figure) that atmospheric carbon dioxide levels will have to fall below 150 parts per million to keep Earth’s surface temperature cool enough for advanced life to survive. Hence, advanced life on Earth is doomed. Within a few tens of millions of years or less either the Sun will be too bright or there will be too little atmospheric carbon dioxide.5 To put it another way, we humans are living in the last half percent of the maximum time window for life on Earth.
The luminosity of a star correlates with the fourth power of its mass. The rate at which a star consumes its hydrogen available for fusion into helium rises roughly with the third power of its mass. Therefore, a star just 1 percent more massive than the Sun will either become too bright or its planet’s atmosphere will possess too low of an abundance of greenhouse gases by the time primitive life is able to transform the planet to make it a fit place for advanced life.
Problem for Stars Less Massive Than the Sun
Waltham devotes most of his Astrobiology paper to addressing the problem of stars less massive than the Sun. He explains how such stars experience luminosity increases that occur at rates much slower than the Sun’s. For planets with the geological and biological processes at levels adequate to make possible a long history of life, the removal of greenhouse gases from their atmospheres will occur at rates that overcompensate for the increases in their host stars’ luminosities. Therefore, Waltham writes, “Their climates will cool at a faster rate than is compensated by the relatively slow evolution of their smaller stars.”6
Planets orbiting stars less massive than the Sun will become permanently globally glaciated (baby, it’s cold outside) before any conceivable primitive life can chemically transform the planets’ surfaces so as to make the existence of advanced life possible.
No Problem for Our Planetary System
More than 90 percent of all stars are less massive than the Sun. More than 9 percent of all stars are too massive for advanced life to possibly exist on any of their planets. As explained in my two recent blogs (Part 1, Part 2) on Earth’s carbon cycles, much, much less than one Earth-like planet out of a million will possess the necessary geological and biological processes required for the existence of advanced life. Both the Sun and Earth are rare indeed.
In this way, scientific advance reveals results consistent with a science-faith integration model. Waltham’s research has provided yet more evidence that only the God who created and designed the universe for humanity’s specific benefit possesses the power, knowledge, intellect, and care to design the Sun and Earth so that we humans can live and thrive.