Julian Date: 2459239.18
2020-2021: LXXVII
THE DAILY ASTRONOMER
Monday, January 24, 2o21
Exploratorium XII: Sweltering Earth
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Our apologies. We'll be off tomorrow,
but back on Wednesday.
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Location
Earth
Time
1.1 billion years in the future
How much time do we truly have left on this verdant sphere? Well, here I am not referring to just us, but to all life on Earth. At some level we are all connected: from kangaroos to grasshoppers to hyacinths and coral reefs. The one trait that ultimately binds us all together is our reliance on the star Sol, affectionately known as the Sun. Were the Sun to somehow disappear, life would soon follow. So, the question about our remaining time on Earth is truly an issue pertaining to the Sun's lifetime. Or, is it?
Stellar astrophysicists are quite confident about the Sun's energy source: thermonuclear fusion. Presently, the Sun is converting hydrogen into helium within its unfathomably hot core (27 million degrees F). During this process, some of the initial hydrogen is converted into pure energy. Earth receives about one one billionth of this solar output. Although that percentage is minute, it is sufficient to sustain life on this planet. Astrophysicists are also convinced that the Sun possesses enough hydrogen reserves to maintain these core fusion reactions for the next five billion years. After exhausting this supply, the Sun will expand into the red giant stage, in the process of which it will incinerate the inner two planets and, perhaps, Earth, as well. Even if our home planet is not consumed, Earth will become so hot its surface shall become molten again. Based on this scenario, one would think that we Sun-dependent life forms will have a mere five billion years to frolic about on Terra Firma before doomsday. It turns out that we have far less time than that, about eighty percent less., hence our presence here on Earth more than one billion years in the future.
We came here at night so we wouldn't be able to see any life forms. After all, we have no way of knowing what forms life will assume in such a distant future. We also made it cloudy because we can also not know the stellar configurations in our sky. The solar system has revolved around the galaxy more than four times since our home time period. Many of the stars currently adorning our sky have perished. Others are milling about in far distant reaches of the Milky Way. The continents are completely different, though how different remains highly speculative. Pangea Proxima, the next supercontinent to form after our home epoch, took shape hundreds of millions of years ago and has already long since fragmented. Their arrangement 1.1 billion years remains unknown.
We do know, however, that the Sun is 10 percent more luminous than it was during our home time period. As the Sun fuses hydrogen, it accumulates greater and greater helium stores. This helium builds in the core and, being a more massive material, increases the core temperature and pressure. These increases will cause the fusion reactions to "burn" hotter.
Another consequence of this fusion acceleration relates to the Sun's size. As we can see in the above chart, the Sun's size will slowly increase over time. The increased size counterbalances the temperature increase so that the Sun's effective temperature remains stable for the next few billion years even as the luminosity increases as a consequence of the Sun's expansion.
As the solar luminosity increases, so, too, will the solar constant, defined as the amount of solar energy the planet receives per unit area. The higher this value, the warmer the planet. In 1.1 billion years, the heat energy Earth receives will cause the atmosphere to moisten and perhaps the oceans, themselves, will evaporate. Life will be slowly but inexorably expunged from the planet.
So, one might think it is rather a pointless expedition for us to venture 1.1 billion years in the future when we can't know any of the constellations, or the continental configuration or even what life forms, if any, are loitering about on Earth. It is a dark and stormy night and we are mired in darkness on this sweltering Earth. Is there any point to this exploration?
Well, it's so much warmer here than it is at home.
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