How many tragedies of alien civilizations have we missed over the 13.8 billion years that elapsed since the Big Bang? Life must have been lost in many places before humans came onto the cosmic scene.
The graveyard of the Milky Way is full of stellar remnants surrounded by planets that used to be habitable long ago. We know that billions of Sun-like stars, born before the Sun in the Milky Way galaxy, had died by now and turned into compact metallic remnants called white dwarfs. The Sun is a late bloomer, formed in the last third of cosmic history. The paradise it created for life on Earth will eventually come to an end after it consumes its nuclear fuel and dies like many stars before.
At any moment in its evolution, a star has a “habitable zone” around it at a distance where liquid water could exist on the surface of rocky planets with an atmosphere. Also known as the Goldilocks zone, this is the distance range where the temperature is just right, not too cold for liquid water to solidify into ice, and not too hot for liquid water to vaporize. Given that the Sun nurtured life on Earth for the past 4 billion years, astrobiologists suspect that stars of its type should be prime targets in the search for life as we know it on Earth-like planets.
The nearest star to the Sun, Proxima Centauri, is 640 times fainter and so its habitable zone is about 25 times closer than the Earth’s distance from the Sun. Coincidentally, it has at least one Earth-mass planet in its habitable zone, Proxima b, but the planet’s atmosphere could have been removed by the stellar wind or ultraviolet flares from the closer star. If so, this would explain why we reside near a rare star like the Sun rather than a more common dwarf star like Proxima Centauri.
But even near the Sun, all good things must come to an end. Within a billion years, the Sun will brighten enough to boil off the oceans on Earth. The brightening trend will continue as the Sun will become a red giant. In 7.6 billion years, the luminosity of the Sun will rise by a factor of about 2,700, and its size increase by a factor of 250, engulfing the current orbital radius of the Earth. By then, the Sun would lose about a third of its mass, similar to my personal experience with a low-carb diet. The habitable zone of the red giant will shift from the Earth’s distance to the Kuiper belt at fifty times the current Earth-Sun separation.
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