It's a well-known fact that Earth will not last forever. Our Sun is a middle-aged star, and it will inevitably run out of fuel, which will cause it to evolve into a red-giant star. From there, it will shed its outer layers of gas, and its innermost core will begin to collapse, causing it to shed its gaseous layers while the core will eventually become a white dwarf - the collapse stopped by electron degeneracy. Earth will likely be consumed in the aftermath - or rendered uninhabitable long before, due to the unpredictable energy output of the failing Sun and the increased temperatures.
There are, however, other interesting places we could potentially settle on before or after the Sun meets its eventual end. We've gathered some of the most interesting propositions.
Cloud Cities on Venus
It has long been believed that Venus is one of the most Earth-like planets in our solar system. It has a rocky body, with a solid surface, an atmosphere, a molten core, weather, and it's not a frozen void like Mars, or a gaseous body like Jupiter, Saturn, or the other gas giants in our solar system.
We did a lot of guessing about the conditions that exist on the surface of Venus and beyond, and many of them were proven true when the Soviet Union (now Russia) sent 10 probes to explore the second planet from the Sun. What they found is still shocking.
The surface is much more volatile than expected. Many of the probes were crushed by Venus' extreme atmosphere within minutes. The one that survived the longest hung in for a little over 2 hours. The mission was called Venera, and it was the first to ever successfully send probes to the surface of Venus. The project extended from 1961 to 1984, and the probe that lasted the longest managed to send back the very first pictures from the surface of another planet.
The features that make Venus Earth-like also set it apart. Its atmosphere is the densest and most deadly of all the terrestrial planets, composed almost entirely of carbon dioxide, a bit of nitrogen, and scant amounts of water vapor. Additionally, it is known to be that it sometimes rains sulfuric acid, and due to the effects of runaway greenhouse gases, temperatures are hot enough to melt lead.
You'd think it would be a total nonstarter if Earth were rendered uninhabitable, but it has been suggested that humanity could flee to Earth's 'evil twin' and live on cloud cities far above the planet's surface. The key to survival would be avoiding the hellish landscape below, where temperatures regularly reach 842 degrees Fahrenheit (450 degrees Celsius), and the pressures are high enough to crush almost anything unlucky enough to venture below.
The farther you go up in Venus' atmosphere, the more temperature and pressure normalize. Eventually. you'd come to that sweet spot where the temperatures are quite warm, but not too inhospitable for life, and the atmospheric pressures are similar to that of Earth. Still yet, the cloud cover would provide sufficient protection from space debris and toxic exposure to the Sun's harsh radiation.
The BBC helps explain how these cloud cities work: "There’s still the problem of staying afloat in a suffocating atmosphere dotted with clouds of drain cleaner. But the solution is perhaps the happiest coincidence behind the entire audacious scheme. CO2 is heavier than air on Earth – which means a balloon on Venus filled with an Earth-like atmosphere of nitrogen and oxygen is lighter than the Venusian air. Fill a Venusian balloon with Earth air, and it will fly into the sky like a helium balloon."
"To live on Venus, then, just fill a balloon with nitrogen and oxygen, and live inside the balloon. A big enough balloon will have enough lifting power to support you and your supplies – and a really big balloon could do even more. “A one-kilometer diameter spherical [balloon] will lift 700,000 tons – two Empire State Buildings. A two-kilometer diameter [balloon] would lift six million tons,” says Geoffrey Landis - the NASA scientists who played a big part in popularizing the idea. “The result would be an environment as spacious as a typical city.”
Inside or on Asteroids
One problem a cloud city would face would be getting access to the raw minerals we would need to survive, so, why not venture somewhere that has easy and abundant access to any mineral you can imagine - like an asteroid in the asteroid belt? There are definitely upsides and downsides to traversing the inner solar system and traveling to the asteroid belt, which is situated between Mars and Jupiter. Some astronomers have also suggested we are ignoring two potential key pieces of real estate: the strange asteroid-like objects which orbit Mars as natural satellites, called Deimos and Phobos.
We've long known they are anomalies as far as "moons" go. For starters, we have no idea whether Deimos and its fellow companion Phobos were zooming past Mars and were captured by its gravitational pull, or if they are somehow byproducts from the planet's formation. Neither are spherical, like most traditional moons, yet they have an almost circular orbit around Mars. Back in the 1950s and 1960s, there was even some speculation as to whether one or both were hollow or not due to their unusual characteristics, and today it is thought possible that Phobos may contain caverns. Phobos is the largest of the two, coming in at a diameter of 14 miles (22 kilometers), while Deimos has a diameter of just 8 miles (13 kilometers) - meaning these are some of the smallest moons in our entire solar system.
We may not be able to settle on either of the satellites, but they could prove decent bases for tracking out to the asteroid belt and finding a suitable candidate for relocation - plus, the asteroid belt is full of all sorts of materials we would need to build an Expanse-like colony in the outer solar system once Earth becomes uninhabitable.
Phobos and Deimos themselves are ticking time bombs. Take Phobos, for example. It orbits Mars from the closest distance of any "moon" in the solar system -- just 3,700 miles (6,000 kilometers). It's drawn into Mars' gravitational pull by 6.6 feet (2 meters) every hundred years. Therefore, astronomers expect it will be torn apart by Martian gravity in 30 to 50 million years,
Deimos might fare better, as it orbits Mars from a distance of 14,576 miles (23,458 km), but other factors, such as its strange tilt and orbit, may rule it out. No worries, there are plenty of great candidates in the asteroid belt.
This brings us to actual...
It's believed that if Venus or Mars fail us, several moons circling the distant icy outer planets might become prime residential real estate. Arguments can be made for several different moons but there are two that many astrobiologists agree are great contenders for hosting transplanted Earth life, and those are Titan and Europa.
Titan, the shining beacon of the great ringed planet Saturn, is probably the most Earth-like place in our solar system. It has mountains, valleys, shorelines, a thick, nitrogen-rich atmosphere, and it even has liquid floating on its surface but, totally not the kind you'd want to drink, as it is composed of hydrocarbons - like methane and ethane. It has even been confirmed by the Cassini mission that Titan is geologically active. It would certainly take a lot of terraforming, but this amazing moon could be a great contender to migrate to once Earth starts to die.
Europa is another moon on our list. This Galilean moon of Jupiter doesn't have a protective atmospheric shield like Titan, but it does have a large series of cracks on its surface and it's believed that beneath its icy shell may be home to large reservoirs of water-ice.
Given Jupiter's far distance from the Sun, you'd think it would be a cold, barren wasteland, but tidal stresses between Jupiter and Europa keep the small moon from becoming completely iced over.
Per NASA, "Tidal heating could be powering a system that cycles water and nutrients between the moon's rocky interior, ice shell, and ocean, creating a watery environment rich with chemistry conducive to life."
"This is why studying Europa's chemistry – on the surface and within the suspected ocean – is important for understanding its habitability, because living things extract energy from their environments by chemical reactions."
"For Europa to be potentially habitable, it would need to have the essential chemical ingredients for the chemistry of life. These include carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur, which are common elements, and scientists think it's likely they were present on Europa as it formed. Later on, asteroids and comets impacted the moon and would have deposited even more organic, or carbon-containing materials."
The big downside is that Europa has an extremely tenuous atmosphere, and is subjected to harsh radiation from Jupiter itself. This is bad for the life present on Earth, but there may be ways around this, and as a bonus, the radiation could very well help produce oxygen, which is a must for our survival.
"The radiation splits apart water molecules (H2O, made of oxygen and hydrogen) in Europa's extremely tenuous atmosphere. The hydrogen floats away and the oxygen stays behind. Oxygen is a very reactive element, and potentially could be used in chemical reactions that release energy, which lifeforms could exploit. If the oxygen somehow makes its way to the ocean, it could possibly provide chemical energy for microbial life."
We need more information on both moons to decide which one is most adaptable to human life. An artificial space station (think Xenon or Elysium) might be our best bet.
While either one of these options could be crucial to our ultimate getaway plan once the Earth is experiencing its last hours, we still need a large amount of research to do. And for the time being, our primary focus should be on saving or prolonging the life of the planet we already reside on.