Jan 1, 20209 min read

Will the Sun engulf the Earth?

Updated: Jan 24, 2020

A visual artwork of a man looking at the sun through a space portal

Our Sun, the flaming ball in the sky, is about 93 million miles[1]away from us, and yet the right distance to make life thrive on our planet. At the dawn of Earth, the Sun offered just enough heat and light for photosynthesis, and life, be it unicellular or multicellular, was made possible under these conditions. And for the most part, it continues to offer the right ingredients, sometimes in variable amounts, ingredients that are imperative to all living things alike. But what if it didn’t anymore? What if the supply of heat increased? What if the Sun is getting hotter?

The evolution of Sun

The Sun, much like any other thing in the universe, has a lifespan. And the lifespan of stellar objects such as stars is longer and much more complicated. Stars are formed from interstellar clouds of dust and gases called nebulae[2]. Millions of years after their formation, most stars can start a process known as nuclear fusion, combining hydrogen into helium, under immense pressure and heat in their enriched cores. The stars are now in a state of equilibrium between gravity pulling inwards, and nuclear fusion pushing outwards. These stars are referred to as main-sequence stars[3], and our Sun falls into this category. But as eons pass, the stars like the Sun begins to run out of hydrogen to fuse, and this breaks the equilibrium, and gravity takes over. But when this happens, the temperature and pressure of the core increases, and hydrogen surrounding the core kickstart another round of nuclear fusion, which results in the expansion of layers outside this hydrogen shell. This converts the stars into giants, and once they cool down, they appear red, and such stars are given an appropriate name for their appearance, they are called Red giants[4]. And this is the fate of the Sun one day.

What of life?

As a star goes through its life, its luminosity will gradually increase. The luminosity[5] of a star is defined as the total energy it produces in a second, and this is correlated to the temperature and size of the star. For instance, the largest known star in the universe, RMC 136a1[6], is about 27,000 times larger than the Sun and has a temperature of about 53,000K. This makes it about 8.7 million times luminous as our Sun, and it radiates more energy in four seconds than the Sun does in an entire year!

The Sun isn’t nearly as big as the biggest stars in the universe. But much like any other main-sequence star, it has been going through the phases as well. The Sun is the oldest thing in the solar system and is almost 4.6 billion[7 ] years old now, but it wasn’t always this luminous, in its early years, it was only about 70% luminous[8] as it is today. And it’s luminosity will only increase from here. In three billion years, its luminosity is estimated to increase by 33%[9], and when it runs out of core hydrogen, the luminosity will be 67%[10] more than it is today. But after that stage, the Sun will begin to use the hydrogen surrounding the core, after which it will enter into the red giant phase, where its luminosity will increase until it reaches 121%[11] above its current value. And when this happens, things won’t look good for the things in its path.

But long before this happens, cataclysmic irreversible events will take place on Earth under the influence of the intensifying luminosity of the Sun. The rise in solar radiation will convert most of the carbon dioxide in the atmosphere to carbonates, and this will drop the atmospheric co2 abundance below 10ppm[12], the minimum amount needed to support plants. When the primary producers of Earth struggles, it will impact the whole ecosystem. However, some plants that comprise just about 5% of the Earth’s plant biomass[13], including crops like maize and sugar cane, are involved in a different kind of photosynthesis called the C4 pathway. The c4 carbon fixation is more efficient, and the plants that perform this can survive higher temperatures as well. But as the Sun’s luminosity increases further, these too will die off. And as the oxygen supply runs out, it will mark the end of most organisms, including us.

In the chapter planetary habitability on astronomical time scales from the book Heliophysics: Evolving Solar Activity and the Climates of Space and Earth [14], Donald E Brownlee, says that it’s possible that all animals could go extinct around 1 billion years from now. But it's hard to ascertain an exact time frame of mass extinction as such because evolution and natural selection might come into play under the harsh environments, and it could help some creatures to survive under the unforgiving Sun. But as time passes, the DO content[15]of water will cease to replenish, and as it gets reduced, the aquatic species will also start to go extinct one by one. And in about 1.3 billion years, it is estimated that all life forms, except for prokaryotes, will inevitably go extinct[16]. And Earth will seemingly be as it was around 3.5 billion years ago when the only forms of life were the simple celled microbes.

An image of the sun as seen from earth in a distant future — Visual artwork of a future sun as viewed from Earth; Original image from Vitalii Tkachenko @Unsplash, see photo link under sources

As the Sun's luminosity increases further, the rate of evaporation will begin to increase, causing our oceans to evaporate uncontrollably. When water moves into the upper layers of the atmosphere, UV radiation can cleave a water molecule, and let the free hydrogen escape into space. Now, this is a process that happens normally, but slowly due to the lack of water vapor in the atmosphere. When the temperature on Earth shoots up under the unforgiving sun, the abundance of water vapor in the atmosphere will become so high that this process of hydrogen removal begins to speed up[17]. More greenhouse gases in the atmosphere only make the condition worse, and will soon lead to a scenario where liquid water can no longer exist. By this time, the Runaway greenhouse effect[18] will have made Earth into a present-day Venus where temperatures get scorchingly hot. These processes will likely render the planet barren and devoid of liquid water in around 3 billion years. Without oceans, plate tectonics stops, and with that, Earth becomes no longer special in the solar system. But extremophilic[19] microbes could well exist even when Sun enters into the red giant phase!

What of the planet?

Suppose one fine day, the sun vanished. For eight minutes or so, we wouldn’t notice any funny business. But when the light ceases, and the panic ensues, the earth will have become a projectile shot out of the solar system. This is because Physics teaches us that matter attracts matter, and this is what we ever so nonchalantly refer to as gravity. But essentially, speaking from an Einsteinian point of view, matter bends the fabric of spacetime, and other matter just falls toward it like beads or tennis balls being attracted towards a bowling ball kept on a trampoline. This is similar to the working of the solar system with the sun (the bowling ball) tugging on the planets(tennis balls). And it is this gravitational tug that makes our planet fall into orbit around the sun. Without the sun, the orbits simply won’t work as they do.

Bearing this in mind, consider the cosmic future when the sun transitions into a red giant. When a star does this, it will lose mass rapidly, and the sun won’t be an exception. As this happens, one would think that the gravitational tug of the sun on all the celestial bodies in its backyard would diminish, thereby expanding the orbits of the planets that revolve around it. An alien watching from a safe distance might see our planet running away from the raging Sun. But it is not a simple estimation as that. As far as our neighbors, Mercury, and Venus are concerned, calculations have proved that they won’t be outrunning the enlarging Sun. But Earth’s future under the circumstances is still up for debate. Most of the articles on the internet offer a hazy answer to what might happen to our planet. However, in recent times, the debate has inclined to doomsday for the earth, as scientists predict that our planet will meet its demise when the Sun throws its penultimate tantrum before it becomes a white dwarf.

A good read about the subject is an article called the Distant future of Sun and Earth revisited[20], on the Monthly Notices of the Royal astronomical society journal. The researchers Klaus-Peter Schröder of the University of Guanajuato in Mexico and Robert Smith of the University of Sussex look into the scenario, and their calculations estimate a grim end for the planet. The Sun Will Eventually Engulf Earth—Maybe on scientific American[21], also sheds light on the same research done by Klaus-Peter Schröder and Robert Smith and writes how this worst-case scenario will play out.

According to the researchers, when the Sun reaches its largest radius, it will tidally lock Earth, similar to the way Earth locks the Moon, and consequently, the angular momentum of the planet will reduce, and without capable of resisting these entities, our planet too shall get devoured by Sun. Earth has to be currently orbiting at about 1.15 AU away from the Sun for it to survive. So, yes, bad news, but on the bright side, at least we won’t be aboard the mothership when it happens.

Will life find a way like it always does?

Firstly, there is a possibility, however distant, that we will become a multi-planetary species. If that were to be the case, then the question is trivial. But, if this doesn’t happen, we will be amongst the long line of species that would have gone extinct under the hostility of our star. But life is tricky. Because we are still debating on how it came into being in the first place, it’s hard to tell what will become of it. But as the Sun becomes a fully grown red giant, and if Earth still lives to see it, it will become the new Mercury, and Mars will become the new Venus. Which means that Jupiter will be the new Earth? That’s right. Around stars like our Sun, there is a habitable region called the goldilocks zone[22], where the conditions are just right for life. That’s where Earth and Mars are at right now. But when the Sun transforms into a red giant, this habitable zone will shift outwards, and the likes of Jupiter, Saturn, and Neptune, and their Moons will fall into this zone. Jupiter and Saturn might not have a solid surface, but their Moons, especially Europa[23] and Titan[24] are good contenders for future life. Scientists believe that there is an ocean beneath Europa’s icy surface, and when the Sun melts this icy sheath, it will probably be the best place to host life!

Will the Sun engulf the Earth?

The evolution of Sun

What of life?

What of the planet?

Will life find a way like it always does?

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