Gravity is a property of matter, anything with mass. This includes the Moon, Earth, Uranus, the Sun, and even you and your body. In fact, if you are attracted to somebody, have them stand 3 quarters of a millimetre away from you. At this precise distance, every atom in your body and every atom in their body will draw you two together with roughly the same gravitational force that the Sun is exerting on you right now. Obviously, we don't feel either of those forces because compared to the gravitational influence of the Earth, they are almost nothing. You don't need to concern yourself with the gravitational attraction between you and someone you're hugging, or, your individual body and the Sun, 150 million kilometres away. In fact, the Sun may as well not exist in this scenario. What if it didn't? What if the sun disappeared?
To put your worries at ease, it's not going to happen. The Sun will die billions of years from now by expanding, boiling off our oceans until our beloved planet is a barren wasteland and then engulfing the Earth whole. The Sun is not going to simply vanish. Matter and energy can't vanish, according to Lavoisier's law of conservation of mass. But let's answer this question anyway, as a thought experiment. What can the Earth do without the help of our nearest star?
At the exact moment that the Sun disappeared, we would have no idea due to the fact that it takes light from the Sun 8 minutes and 20 seconds to reach Earth. So, for a little over 8 minutes after the Sun disappeared, we would have no idea. Theoretically, the sun could have disappeared at the very second you read these words, but you wouldn't come to realise until roughly 8 minutes time. There's some food for thought. Once we did discover our skies' lack of a sun, confusion and mass panic would ensue. The Sun's gravitational clutch on our planet would also take 8 minutes and 20 seconds to end. This is because gravity waves propagate at the speed of light. So, the very moment we saw our Sun disappear, we would also lose its gravitational influence. Our Earth would fly out in a straight line, tangent to wherever it was in its orbit.The finite speed of light and gravity mean that as panic and fear spread across the Earth at the loss of our Sun, we could still, for a while, look up into the sky and see our fellow planets, further out, continuing to operate as if nothing had happened.For example, Jupiter would continue orbiting and reflecting light from a Sun that no longer existed for about 30 minutes after the Earth already knew the Sun was gone. And, depending on where Jupiter was in its orbit, it would take another 30 minutes to an hour for us to watch the reflected light of Jupiter fade.
With no moonlight or sunlight, the universe itself would be our only source of visible light from space. In 2004, Abdul Ahad calculated that the Milky Way contributes about as much light as 0.03% of a full Moon. So there would be barely enough light from space for us to see a bit. However, we would still have access to multiple non-renewable energy sources, so for some time, we could used man-made light to guide us around our cities and towns. It could just be like any other night; but it would always be night.
On the other hand, Photosynthesis would stop immediately, and this would be detrimental to our lives.It is estimated that 99.9% of the natural productivity on Earth is done by photosynthesis, which requires the Sun. Without the Sun, plants would no longer be able to receive carbon dioxide and output oxygen, which is the element required for us animals to breathe. But, don't worry. Collectively, all of the humans on earth, all 8 billion of us, breathe in about 6 trillion (6,000,000,000,000) kilos of oxygen every year. But our atmosphere contains a comfortable 1.1 quintillion (1,100,000,000,000,000,000) kilos of oxygen. So, even without photosynthesis, and, including all the other animals, insects, and microorganisms that consume oxygen, it would take us thousands of years to run out, making it pretty low down on OUR list of worries, but that can't be said for the plants. The poor plants themselves would be much worse off. Without the Sun, most of them would die within days or weeks. Except for large plants. Giants trees, for instance, contain enough sugar for energy that they could hypothetically live in the dark for years. Their problem is going to be that the Earth will get quite cold. In fact, mighty trees would freeze to death. Their blood, the water and sap inside of them, would solidify way before they died of starvation.
Currently, with our Sun, the average surface temperature on Earth, hot places, cold places and different seasons considered, is an arguably comfortable 14-15°c. Without the Sun to add energy, the Earth would radiate away heat exponentially, meaning it would go fast at first and then happen more and more slowly. By the end of the first week without the Sun, the average surface temperature across Earth would be freezing. 0°c, 32°F. Not quite a summer holiday in Barbados, but still tolerable. Panic and fear and anarchy aside, for the first few weeks or possibly months, we could get by. But by the end of the first year without the Sun, the average global surface temperature would deteriorate all the way to -73°c, -100°F.
The best option, really, would be to move to geothermal areas like The Blue Lagoon or Námaskarð in Iceland. These places would be the few safe havens for human life after the vanishing of the Sun. Nearly all life on Earth exists because of, and is dependent upon, extraterrestrial energy: the Sun. But the Earth produces its own heat. Despite floating in the coldness of space for billions of years, below the crust the Earth is very, very hot. 20% of this warmth came from the fact that when the Earth formed, mass crushed so tightly in the middle that the pressure liquified rock. The other 80% of Earth's internal heat comes from that fact that deep in its core, radioactive elements decay (mainly uranium, thorium and potassium), providing the energy needed to keep the Earth's core at 5,000°c. Anyone who failed to secure a position within one of these bastions of warmth, would likely be dead within the first year of sunless earth.
In the next 10 or 20 years things would start to get wet with dew. But not with water droplets. Instead, droplets of liquid air. The air would literally become cold enough for the gasses that make it up to condense, or form clouds and precipitate, first as rain, and then as it got colder and colder, eventually, as snow. A year or so after the sun disappeared, Earth's oceans will have frozen over. Ice all the way across. The colour of our planet would go from pure blue and a vibrant green to a dull, lifeless icy pale blue and the brown of dying plants. But ice is less dense than liquid water, which means that ice floats. And ice is a relatively effective insulator. So, for billions of years after the Sun disappeared, liquid water could still exist at the bottom of our oceans, protected and insulated from space by miles of ice above it, and warmed by vents on the ocean floor that spew water out, super heated by Earth's interior.This would continue on Earth for a very, very long time - Sun or no Sun.
Instead of becoming frozen and lifeless, extremophiles, like microbes, that live around hydrothermal vents deep in the ocean, would be fine. They live deep in the ocean, far below the point where sunlight can penetrate, and they make energy not through photosynthesis, but via chemosynthesis - converting heat, methane and sulphur into the energy they need. They are then eaten by other, bigger organisms, like clams and tubeworms. Extremophiles deposit minerals back into the vents, meaning that their food chain is complete, it's a circle. It's independent of the Sun. Earthlings like them would thrive if the Sun were to disappear. They would live just fine without ever knowing that the Sun was gone. Or without ever even knowing it existed in the first place.
By some incredible circumstances, life, here on Earth, all alone, flying through space with no Sun, would kind of be fine. The human race would be almost completely extinct and many, many species would not make it through just the first 100 years, but its far from becoming a frozen dead rock. And now you can't get sunburnt anymore, eh? Not just negatives in the no-sun life.
If the Sun disappeared, spaceship Earth would fly out in a straight line covering about 30 kilometres every single second. After just 1 billion years, it will have covered 900 quadrillion (900,000,000,000,000,000) kilometres, or about 100,000 lightyears - a trip that could potentially take it all the way across our galaxy, near thousands of stars. And nothing's to say it couldn't fall into orbit around a new star, kick-starting life on earth once again and allow its still-living extremophiles to proliferate life on Earth all over again; maybe one day developing life intelligent enough to uncover whatever was left of our lives.
So, despite the highly probable extinction of humans, loss of our wildlife, covering of our oceans, and almost all life being reduced to zero, there would still be a happy ending for the 3rd closest planet to the su- oh wait.