For our planet to be habitable, a lot of things had to be just right. We needed to be the right size, have the right star, and be in the right orbit. Now it turns out that our position from the Sun had an even smaller window for habitability than we thought.
A new study has found that if we were just 15 percent further away, our planet would have entered an eternal ice age from which we would never have escaped. While pointing out how fortunate we are here on Earth, the research also has implications for finding life on other worlds.
“We find that planets with Earth-like characteristics orbiting a Sun-like star may never be able to escape from a glaciation era, if their orbital distance is greater than 1.27 Astronomical Units [AU, 1 AU is the distance from Earth to the Sun],” the researchers led by Sorbonne University in Paris wrote.
They add later that this figure could be as low as 1.15 AU, an increase of a mere 15 percent. The findings are published in Earth and Planetary Science Letters.
The reason is due to something called CO2 condensation. If a planet is too far from its star, then too much CO2 will be frozen as dry ice at the poles. This prevents a planet from experiencing a greenhouse effect, and so it never gets warm enough for deglaciation.
Note that this paper only looked at Earth-like worlds around Sun-like stars, ones with a similar size, mass, and orbit to our own planet. Other planets like Mars are mentioned but not focused on. Despite orbiting at 1.5 AU, we think it has been warm enough to support liquid water in the past.
As for Earth, this study really highlights just how lucky we are. Just a small increase in distance would give us permanent CO2 ice caps, limiting the amount of CO2 in the atmosphere.
If a planet has significant water cover, the results are even more dire. Within just 10,000 years, CO2 can be buried beneath water ice, being trapped forever and unable to contribute to deglaciation.
We’re starting to find quite a few worlds like ours, but we haven’t yet found many in exactly similar orbits with similar sizes. This study tells us that, when trying to find other habitable worlds, looking at worlds almost identical to ours will be key.