12. Black Hole
A black hole is a collapsed star from which not even light can escape. Of course, this is somewhat of a misconception; light moves at 186,282 miles per second in a vacuum, and the point at which the escape velocity of a black hole equals the speed of light is known as the event horizon, a realm where some very funky physics takes place. An errant black hole passing by would spell a very bad day for our solar system. Astronomers might first notice its effects tugging on the outer planets and the Pioneer and Voyager spacecraft. A passing black hole would only have to pull us out of our current orbit to do us in.
11. Alien Invasion
This scenario is fraught with uncertainties. How common is life? How easy is interstellar space travel? Would a species millions of years more advanced be benign or hostile? It’s interesting to note that science fiction aliens have gone from hostile in the age of H.G. Wells and War of the Worlds to friendly during the Cold War to hostile again. One camp says that any civilization that survived its technological adolescence must be peaceful. Another mode of thinking says that an intelligent civilization probably evolved from a top predator species, just like us. In either case, even peaceful contact between civilizations here on Earth rarely bodes well for the more primitive of the two.
10. Planetary Resonance
Newtonian mechanics is great for predicting the position of planetary bodies in the near term, but things get a bit messier when you look millions of years in the future. This is the classic three-body problem, and those little tugs from other bodies do add up. Researchers have proposed an interesting scenario in which the planet Mercury either gets ejected or even impacts the Earth, although it’s unlikely to occur prior to the death of our sun.
9. Gamma-Ray Burst
Gamma-ray bursters (GRBs) occur in two varieties; the death of a hypernova star or the collision of a binary pulsar pair. The amount of energy released is enormous, equal to the output of our sun in its entire lifetime. A GRB is observed every few days; fortunately, they’re located at extra-galactic distances and can do no harm. Two objects within an 8,000-light-year radius are a bit more troubling: WR 104 and Eta Carinae. For a blast of deadly cosmic radiation to occur, the poles of either object would have to be oriented nearly straight at us. Such a burst would ionize our upper atmosphere, strip away our ozone layer, and expose our biosphere to radiation.
A nearby supernova would likewise expose us to a bath of ionizing radiation. A recent stir has been made about the red giant star Betelgeuse, which could go supernova today or 10 million years from now. But as astronomer Phil Plait points out, Betelgeuse is about 650 light years away, well out of the 25-light-year “kill zone…” Betelgeuse will simply give us a good light show, as the familiar constellation Orion takes on a drastically different appearance.
7. Killer Comet
Large comets can and do strike the Earth on occasion. The world was made aware of this on the morning of June 30 1908, when a fragment of a comet struck the Siberian tundra in what has become known as the Tunguska event. Doubtless a large, once-in-100 million-years comet is out there in the Oort cloud waiting to plunge into the inner solar system intent on causing an extinction-level event here on Earth.
6. Near-Earth Asteroid
Another plot device for bad made-for-TV movies, the threat of Near-Earth Asteroids (NEAs) is a very real concern. Every so often, we hear in the news that a large rock just missed us, in astronomical terms. To this end, astronomers have devised what’s known as the Torino Scale, a way to assign potential risk to an incoming body. Mitigation will be two-fold; first will be survey and identification, and second will be removal of the threat, which is much tougher. Missions to NEAs will be the name of the game for NASA over the next decade.
5. Super Solar Flare
Our sun puts out massive flares which can do critical damage to our increasingly technologically reliant civilization. Such an event was witnessed during the Carrington super flare of 1859. The good news is that our sun doesn’t spew out the type of life-extinguishing flares we observe on certain adolescent type stars; the bad news is our sun goes into overdrive every 11 years, and a large Carrington-style event will definitely happen again.
4. Galactic Oscillation
As we orbit the core of our galaxy every 250 million years, our solar system bobs up and down above the galactic plane. Curiously, this resonance correlates with several mass extinctions, and there are theories to explain this phenomenon. There are also galactic habitability zones, areas where there’s not too much radiation or wild stellar activity (see nos. 5, 8, and 9 above) that can do life in. As the sun moves about the galaxy, those nasties may come within range in the future.
Albedo is the reflectivity of a given material; snow and cloud tops reflect light back into space, land and vegetation tend to absorb proportionately more light and heat. Complicating the current climate controversy is a lesser-known effect called global dimming. Author James Kasting in his book How to Build a Habitable Planet calculates that the Earth was an iceball in its geological past and a large runaway albedo change could be the trigger for massive ice ages. Likewise, a loss of our polar ice sheets could accelerate a thermal exchange, with equally disastrous consequences.
2. Red Giant Sun
Our understanding of stellar evolution tells us that our sun is at about middle age in its hydrogen fusion lifespan and will expand in about 4 billion years and perhaps engulf the Earth as it begins fusion of heavier elements. The good news for our far off descendants is that icy places such as Saturn’s moon Titan may be balmy vacation spots for 100 million years or more.
Cosmologists Fred Adams and Gregory Laughlin point out in The Five Ages of the Universe that we live in the stelliferous era, a time when stars shine and create those elements that life needs to exist. But that won’t always be the case. In an estimated 1014 years, the final stars will collapse or wink out. The cycle of supernova explosions that seed new stars will come to an end, and even old white dwarfs will cool to well below the infrared. On stupendously long time scales, even the remaining black holes will evaporate via Hawking radiation, as our expanding universe ticks on into infinity. It’s sobering to think that in time, even protons themselves may decay, as we’re left with a universe filled with radiation stretched to absurdly long wavelengths, and infinity to go.
There is one more interesting thing that science has to say about the items on this list: The reason that we’ve had hundreds of millions of years to evolve here is that our little corner of the universe, is, well, boring. The principal of mediocrity states that planet-demolishing black holes or gamma-ray bursts simply aren’t that common in our neck of the woods, and the fact that we’re even here stands as a testimony to this. You’re much more likely to sustain a personal end of your universe every time you get in an automobile, or eat one cheeseburger too many. We can and should study these cosmic events as we hopefully become wiser and more “cosmopolitan” as a species. It’s all a matter of cosmic perspective.