Do you know that terrible catastrophe might happen at any time! Some cataclysms would terminate life on Earth, while others might destroy the entire universe almost momentarily. Only one-planet based civilization doomed to perish because in the space exists wide range of the cosmic threats. What they are? Common existential risks typically includes such as: nearby massive star goes into supernova, deadly sweeping of the gamma-ray burst, tremendous asteroid collision and alien invasion.
Medium-size asteroid of just few tens kilometers across must be unstoppable, because we haven't possess the technology to destroy it or just alter its course to avoid collision. Incoming object of such diameter would be virtually immune to any applied influence. It can easily withstand directed impact or even nuclear explosion. Once it hit the Earth and devastations going to happen on the global scale. There should be severely fractured crust, that will results in catastrophic magma eruption. In few hours time, entire human population will go into oblivion, just as did so dinosaurs seventy million years ago.
Looming asteroid might be found well in advance, but approaching mini black hole impossible to see even through professional telescope. Strayed micro-black hole would pass through the Earth until it met the center, and devour whole planet in mere half a minute time. Pending disaster should be hardly detectable by means of modern technology and thus unpreventable, but advanced humanity would easily survive on the remaining planets. How many micro black holes is hanging around our solar neighborhood? Currently, the exact figures is unknown but some estimates put them very high. From the beginning of the universe there must be widespread swarms of those exotic objects. Thus, human ultimate fate is directly depending on the chances of luck.
The next doomsday scenario is a almost incomprehensible supernova cataclysm. At the end of stellar evolution every supergiant undergoes through rapid phase of so-called gamma-ray burst. Astronomic demise is so violent that resulting lethal beams would sterilize vast parts of the Milky Way galaxy. Close supernova explosion would strip Earth's atmosphere off, and leave all present life forms to die. Intense flow of hard radiation should kill all living creatures far beyond of any chances of natural recovery. Perhaps, that would perfectly explain long-standing Fermi Paradox. The answer is simple: unfortunate aliens were exterminated by unsurmountable force of nature.
What did your hear about quantum instability? Perhaps, nothing. Then I explain to you that this is highly improbable, but still possible event that may happen under some peculiar circumstances. This could lead to the quantum state, when all cosmic matter is being instantly annihilated. If that apocalyptic scenario would ever take place, then whole universe will cease to exist. No familiar stars, no known planets, no galaxies... just absolutely empty space. Did this nasty thing is really going to happen? Certainly, yes! This is just a matter of time. But would it be an instant occurrence throughout universe, or some sort of spherical, bound outward propagation? Nobody knows for sure, but string theory suggest superluminal changing should be probably involved. To understand the exact nature of such event farther research is needed.
Final conclusion. We must take decisive steps to ensure human survival as dominating species. That could be only done by spreading ourselves to the entire Solar System, including the Moon, Mercury, Venus, Mars, major asteroids and some outer planets. Multi-planet, spacefaring civilization is our only option, otherwise we going eventually to die. Single habitat is highly susceptible to various kinds of man-made or natural disasters. Instead, numerous space settlements scattered across the Solar System effectively ward off any astronomical threats. Demise of one local habitat would have no or little influence on the entire sophisticated society.
What can we do to guarantee our long-term survival? It is quite obvious that single-planet civilization will disappear with destruction of the Earth itself. This can be avoided if we choose to spread ourselves throughout Solar System and beyond. That means a simple thing: large-scale space colonization. At the first stage of the implementation it would require from us to make some great efforts. But soon afterwards, ever-growing demand for interplanetary space travel should drive the global economy up. If we start to do that today, then whole business might be accomplished within one century time. The money for such grandiose undertaking can be painlessly taken from the military budgets of world's leading powers.