A Solution to the Faint-Sun Paradox Reveals a Narrow Window for Life

 

Our sun is getting brighter. If you could travel back in time to the dawn of the solar system 4.5 billion years ago, you’d find a star that was about 30% dimmer than it is now. Over the subsequent eons, it has shone more and more brilliantly — a function of the nuclear fusion process that takes place in the cores of stars like our own — and it will continue to do so until the end of its life, roughly 5 billion years from now.

 

That original faint sun should have led to disaster here on Earth. If our modern Earth were placed under that sun, temperatures would average about −7 degrees Celsius — too cold for liquid water to flow. “The planet should have been completely frozen,” said Toby Tyrrell, an earth system scientist at the University of Southampton. “It shouldn’t have been possible for life to develop.”

 

And yet it did. We know that our planet had liquid water on its surface as early as 4.4 billion years ago, and maybe even earlier, as water vapor condensed out of the atmosphere. Single-celled life seems to have sprung up shortly thereafter. And both the planet’s water and its life have persisted — despite a few close calls — to concoct the relative oasis we inhabit today.

 

If the sun was so faint, how was this possible?

 

The faint young sun paradox, as it’s known, has vexed scientists for decades. But recent work has led many researchers to think that we now have a solid hold on the problem. Old ideas have been refined, while new ideas — including one involving an unnervingly close moon that sculpted tidal waves the size of skyscrapers — have helped to fill the few remaining gaps in our understanding.

 

Scientists have also come to realize that the faint young sun paradox carries important implications not only for Earth, but for our understanding of how life in general might come to be. Did our world, even in its optimal location around a relatively sedate star, manage to produce life by only the slimmest of margins? What might that mean for the prospect of life elsewhere? “It’s a really fundamental question concerning the habitability of Earth over all its history,” said Benjamin Charnay, a planetary scientist at the Paris Observatory, “and has strong implications concerning the habitability of exoplanets.”

 

In exploring the mysteries of the faint young sun problem, we have opened up the history of our world like never before. In doing so, we’re discovering that what was once a paradox may in fact reveal the very reason for our existence.

 

The Paradox

 

In the mid-20th century, scientists started to understand the mechanics that drive the evolution of stars like our sun. Deep in a star’s core, hydrogen fuses into helium, producing energy. As the amount of hydrogen decreases, the core shrinks, which in turn boosts the fusion rate. The star gets brighter over time

 

 

 

 In 1958, the German American astrophysicist Martin Schwarzschild and the British astronomer Fred Hoyle used this knowledge to separately arrive at the same conclusion: When our sun first formed, it must have had only about 70% of the luminosity it has now. “The very first models of stellar evolution predicted that,” said James Kasting, a geoscientist at Pennsylvania State University. In the 1960s, however, scientists began to find evidence of water on Earth dating back 4 billion years. This appeared to contradict the solar models; Earth should not have been warm enough under the faint young sun to possess liquid water. One paper in 1965, in an effort to solve the discrepancy, suggested that either the sun was older than we thought, or the model of our sun’s evolution “may require some modification to permit higher luminosities.” 

 

The American astronomers Carl Sagan and George Mullen made a more substantial attempt to resolve the paradox in 1972, performing the first detailed analysis of the faint young sun problem. They suggested that a thicker atmosphere on the early Earth might have been able to trap more heat, keeping the planet warm enough to support liquid water. The greenhouse gas they put forward was ammonia.

 

Ammonia did not last long as a potential solution to the paradox, however. “It’s destroyed by solar ultraviolet radiation,” said Georg Feulner, a climate researcher at the Potsdam Institute for Climate Impact Research. “People realized later on that it just doesn’t work out.” In the late 1970s, scientists turned to another greenhouse gas — carbon dioxide — as a possible solution. “Carbon dioxide is a lot less problematic,” said Feulner. “There’s a lot of carbon in the Earth’s system early on, so it’s a reasonable assumption you can produce significant amounts of carbon dioxide in the atmosphere.”

 

Kasting and his colleagues explored carbon dioxide’s possible effects  in 1981, noting that volcanoes could have released enough of it to overcome the faint young sun problem. Even if Earth did manage to freeze over — which appears to have happened on at least three occasions — volcanoes poking their noses through the ice might reverse the process. “The resultant large greenhouse effect should melt the ice cover in a geologically short period of time,” Kasting and his colleagues wrote.

 

 

 

Source: Quanta Magazine