Are we alone in the universe?

 Doubt it, there are 100 billion stars in our galaxy, and about every 5 of them have planets orbiting them. And there are many galaxies, numbers become meaningless, and there are many that are thousands of times larger than the Milky Way. But it is big, the distances are enormous, but I think it is unlikely that only a tiny fraction of one of them has intelligent life on it, although I question whether humans are as intelligent as we think, then we had not polluted our thin atmosphere with carbon that warms the climate to a limit that will soon be irreversible, so we wipe out the basis of life on the planet, for what, that a few people should have enormous incomes. Meaningless. So I sincerely hope that we are not alone, because then the universe will be sterile for intelligent beings in a short time.

What" would happen to the earth if humans suddenly disappeared?

 If humans vanished instantly, New York City's subways would flood in just 36 hours. The planet wouldn't fall silent; it would begin a chaotic process of reclaiming its space.

Within days, the fossil-fuel power plants that supply much of the global grid would run out of fuel. Wind turbines and solar panels would operate until their inverters failed or dust coated their surfaces. Nuclear power plants are programmed to shut down automatically, but their spent-fuel cooling pools rely on backup generators. Once those generators ran out of diesel, localized meltdowns could occur, releasing radiation into the surrounding environments.

Many major cities are engaged in a constant, hidden battle against water. Without people to run massive underground pumps, subterranean infrastructure would rapidly drown. Over the next few decades, the freeze-thaw cycle of water would crack pavements and building foundations. Weeds, vines, and eventually trees would take root in the concrete fissures, turning urban centers into dense forests.

Wildlife would experience a drastic shift. Domesticated animals would face immediate hardship, as millions of livestock and pets would lack food and water. Those that escaped would have to compete with wild predators. Highly specialized dog breeds would struggle to survive, but feral cats, pigs, and larger dogs might form packs, eventually interbreeding with wild counterparts. Abandoned cities would become new ecosystems, with skyscrapers serving as artificial cliffs for birds of prey to hunt proliferating rodent populations.

Fast forward a few centuries, and most modern architecture would be unrecognizable. Steel bridges would rust, snap, and collapse into rivers. Wooden structures would rot or burn from unsuppressed lightning strikes. The only enduring monuments would be massive stone structures like the Pyramids of Giza, Mount Rushmore, or the Hoover Dam.

Millions of years later, the human legacy would be reduced to a bizarre geological stratum—a thin layer of fossilized plastics, synthetic chemicals, and concentrated radioactive isotopes buried deep within the crust. Earth would eventually adapt and move on, erasing nearly all surface evidence that humans were ever there.

Which planet in our solar system would be the most difficult for humans to land on safely?

 To find the hardest planetary landing in our solar system, you have to choose between a world with no surface at all, or one where the air is hot enough to melt lead.

Attempting to land on a gas giant like Jupiter is a paradox because the planet lacks a solid surface.

Jupiter, as seen by the Cassini spacecraft in 2000. It lacks a solid surface, making a traditional landing impossible.

Jupiter is composed almost entirely of hydrogen and helium. A crewed spacecraft entering its atmosphere would experience a descent with no end. After passing through turbulent, ammonia-rich clouds and lethal radiation belts, the ship would face exponentially increasing pressures and temperatures. Eventually, the pressure becomes so immense that hydrogen gas compresses into a bizarre state called liquid metallic hydrogen. Any vessel would be crushed, melted, and assimilated into the planet's interior long before reaching the core. Because there is no rocky crust to park a vehicle on, a safe landing is physically impossible.

For a true planetary landing on a solid surface, Venus provides the most hostile environment for human survival.

An illustration of a heavily armored spacecraft descending through the thick, hostile sulfuric acid clouds of Venus.

From orbit, Venus looks like a serene, bright marble, but its atmosphere is a nightmare for aerospace engineering. The atmosphere is composed primarily of carbon dioxide, with thick clouds of sulfuric acid. The atmospheric pressure at the surface is roughly 90 times greater than that of Earth. Standing on the surface of Venus would feel akin to being submerged 900 meters (about 3,000 feet) underwater. Any unreinforced habitat or spacesuit would instantly implode from the weight of the air alone.

In addition to the crushing pressure, Venus experiences a runaway greenhouse effect. The surface temperature averages around 475 degrees Celsius (887 degrees Fahrenheit). This is hot enough to melt lead, zinc, and tin. The extreme heat would instantly cook conventional life-support systems, melt electronics, and rapidly degrade spacecraft seals.

A panoramic view of the surface of Venus captured by the Soviet Venera 13 lander in 1982.

Despite these apocalyptic conditions, Soviet engineers managed to land robotic probes on Venus during the Cold War. Because the atmosphere is so incredibly dense, the later Venera landers did not even need parachutes for the final stages of their descent; they used aerodynamic braking discs to drift down through the thick, fluid-like air.

Once on the ground, the engineering challenge shifted entirely to survival. The Venera 13 robotic lander, which touched down in 1982, holds the record for the longest survival time on the Venusian surface. It lasted for just 127 minutes before the immense heat and pressure breached its protective titanium hull and destroyed its instruments. Designing a crewed lander to keep humans alive long-term on Venus would require specialized high-temperature refrigeration, massive titanium pressure hulls, and materials that push the boundaries of materials science.

While Mars presents difficulties with its thin atmosphere and dust storms, aerospace engineers plan to eventually send humans to visit and inhabit it. On Venus, achieving even a few hours of robotic surface time represents an engineering marvel, making it the single hardest planetary surface in the solar system for humans to reach and survive on.

What will the last humans see and think when the Sun melts the Earth?

Surprisingly, the existence of humans, or their biological or robotic descendants, is projected to extend Earth's lifespan before it is melted by the aging Sun. This disregards any human efforts to extend the habitability of our planet by building shades or altering Earth’s orbit.

Our system is already 4.5 billion years old, and it’s roughly halfway through its life before it becomes a tiny white dwarf. Before it happens, it will expand and become a red giant. It will then absorb Mercury and Venus. The fate of Earth is less certain because, as the Sun expands, it will also lose mass, causing the orbits of the remaining planets to expand. Most likely, Earth will be devoured in a manner similar to that of Venus. There is a small chance that it will become a burnt ember of what it was if it's spared this sorry fate.

In the run-up to this, many natural systems that make our planet habitable will fail one by one. For decades, we thought we might have only 600 to 800 million years before Earth becomes uninhabitable, and many answers on Quora still repeat this, but recently, we have made some progress in knowing with greater certainty the real fate of our planet. There is even a chance that we might have up to 1.5 billion years before it becomes uninhabitable, assuming that humans or our biological or robotic descendants are gone.

It is projected that as hundreds of millions of years pass from the present, the atmosphere will become increasingly devoid of carbon dioxide, which is used by plants. This will happen because our planet will receive more heat from the Sun, accelerating rock weathering and increasing the rate of carbon dioxide absorption. Also, the plate tectonics, which recycles nutrients over geological timescales, might end.

We currently produce copious amounts of this gas as a byproduct of industrialization. If our descendants still exist, it’s safe to assume that we will still be supplying the atmosphere with this gas. Hopefully, it will not be ruined by the speed at which we are releasing these gases now.

If the lowering of carbon dioxide concentration can be avoided, the next dangerous step will be the increase in the energy Earth will receive from the aging Sun. If our descendants don’t step up and either build a shade or move Earth’s orbit away, then the upper limit of Earth’s habitability will be around 2 billion years.

It will then become so hot that the oceans will boil away, and we will not be able to live on Earth as it melts under the Sun's expansion. This will happen on timescales of another two billion years or so. Although Earth’s orbit would tend to expand as the Sun loses mass, tidal effects and the Sun’s enormous growth may still lead to engulfment.

At some point, its rocks will melt, and a global magma ocean will cover the surface, erasing the entire history of our planet, the good and the bad. When Sun’s surface approaches really close, Earth will at some point start to be dragged by the Sun’s atmosphere, and it will spiral into its plasma and literally melt. This will only be possible to witness from a distance and not from the surface.