Looking at the last term of the Drake Equation, we see that it relates to the lifetime of technological civilizations - how long they last as technological (meaning interstellar communicating) entities. The three biggest considerations for our civilization at the moment could be characterized as a) getting along with each other, b) getting along with the environment, and c) staying technologically alert for large-scale concerns from space. As an example of the last, the dinosaurs had over 200 million years to develop a comet deflector, but never did so. Some dinosaurs were bipedal, had opposable claws, and were pretty intelligent--so why didn't they, for example, invent space travel? Well, that's a topic for another essay. Meanwhile, let's stick to a few of the things we might want to deal with "out there" at various times in the future, from a few thousand to a few billion years from now.
Magnetic Field Reversal
The Earth's magnetic field is thought to be generated by a dynamo effect - that is, the movement of charged particles in its huge iron and nickel core as it spins. Other planets have magnetic fields also, and there seems to be some relationship between the strength of the magnetic field with the size and spin rate of the magnetic core. Jupiter, with a huge core and a 10-hour rotation period, generates a massive magnetic field, for example, and spacecraft sent there have to be specially built to withstand this intense field. You may remember the science fiction movie Outland with Sean Connery stranded in a mining colony on Io, one of Jupiter's moons. However, Io would be uninhabitable since the magnetic field of Jupiter causes a 5 million ampere electric current to flow through it.
However, magnetic fields can also be helpful. They can protect the inhabitants of the planet from high energy particles from, for example, the solar wind. When the high energy particles from the Sun encounter the Earth's magnetic field, they are deflected toward the poles, causing beautiful auroral "curtains" of color as they hit the atmosphere. Without the magnetic field of the Earth, these high energy particles could do damage to biology on Earth.
When rocks containing magnetite cool from volcanoes or are baked (as in clay pottery), they record the direction of the magnetic field of the Earth at the time of cooling. It turns out, from examining rocks of various ages, that the Earth has reversed its magnetic field many times - the last about 750,000 years ago (the average being about every few hundred thousand years). Recent measurements of ancient pottery and other evidence suggest that the Earth's magnetic field may be declining - perhaps getting ready for an overdue reversal. This could take place within the next couple of thousand years. If the Earth's magnetic field is just beginning to reverse, it would certainly be important for us to protect ourselves from the high energy particles of the solar wind and of space. It would not be as devastating an event as, for example, a comet impact, but it does indicate that we do not have the luxury of indulging in another Dark Age over the next thousand years or so. If civilization is to maintain itself, we need to be on our technological "toes" pretty much from now on.
Moon Stabilizes Earth's Rotation
The most popular theory for the origin of the moon is that it came from the Earth. We can calculate evolutionary histories of the moon's orbit as it moved away from the Earth after formation. (It is still moving away due to the Earth's tidal pull at about one inch per year. The majority of the tidal dragging comes from Earth's rotational slowdown, with most being caused by waters dragging over the fairly shallow Bering Sea.) In doing some of these kinds of calculations for Mars, it was discovered that the direction of Mars' rotational axis could flip rather suddenly. Now this is not the normal "precession" (as it is called) of a few degrees that changes, for example, our north star though the millennia. Mars was calculated to have flipped its rotation axis up to 90 degrees in as little as a couple of million years. This was a result of the orbital angular momentum, under certain circumstances, being transferred to the rotational angular momentum and causing a coupling that led to such a flip in rotation axis direction.
So why has this not occurred on Earth, whose axis has seemingly not flipped by more than a few degrees? The apparent explanation is that the Moon absorbs any transfer of orbital to rotational angular moment, preventing the flip.
Would such a flip be important? It could get very serious - like the time a couple of hundred million years ago when all the continents were combined into one big continent called "Pangaea"--if the Earth's rotation axis flipped such that this one big continent became a polar continent like Antarctica. So, it would appear that a moon is required for a stable planet with life.
This was perhaps surprising news to folks that would like to see habitable planets widespread in the galaxy requiring, as it does, both an earthlike planet in the circumstellar habitable zone as well as a fairly large satellite. This would seem to rule out habitable planets being very common. However, additional research into the rotational histories of the planets shows that the Earth used to spin a lot faster. If the earth spins faster, that also acts as a protection against flipping of the rotation axis. So, perhaps if the moon had not come off the Earth, our world would still be spinning fast enough to stabilize itself against flipping. Thus there may be many other habitable planets without a large moon, but the inhabitants will have even fewer hours in their day than we do.
The moon, of course, is now perfectly placed to exactly cover the solar disk during eclipses. This perfect fit has allowed, for example, a test of General Relativity, the uncovering of the element helium, and the discovery of the solar corona. And clearly the moon has been a great stimulus and practice ground for our first efforts at space travel. However, moving out at an inch a year, in about 1.6 billion years the moon will no longer be able to stabilize our planet's spin. We'll have to be ready for a climatologically wild ride by then unless we figure out what to do. Eventually the Earth will have the same rotation period as the moon's orbit (i.e., the day will equal the month) and then the moon may be expected to fall back toward the Earth, forming a ring perhaps not dissimilar to those around Saturn. It will, no doubt, be a great show.
Here Comes Andromeda
We could talk about many more interesting phenomena, but perhaps the most spectacular will be the merging of the Andromeda spiral galaxy with the Milky Way in about six billion years. Although no stars will likely touch (the spacing between stars is huge), this interaction will most certainly gravitationally affect every star in both galaxies. The Milky Way, in its 12 billion year history, has swallowed up many smaller galaxies, but such a merger will be a unique experience. Every star is thought to have a cloud (called the "Oort Cloud" in our solar system) that consists of about a trillion comets. As the two galaxies merge, these comet clouds will get scrambled, causing increased impacts onto each star's inner planets. Billions of stellar systems being tidally flung around may also cause instability in the orbits of the planets around them.
Nicolai Kardashev has suggested that there could be three classes of civilizations--Type I controlling the resources of its planet, Type II the resources of its star, and Type III the resources of its galaxy. At the moment we are estimated to be about type 0.1 or so. A Type II builds such things as Dyson spheres (structures encompassing the star so as to capture all of its energy). Clearly, a Type III civilization would be needed to deal with the merging of Andromeda with the Milky Way.
So, there we have it - some items on the agenda for the next few billion years. We have some time for planning, but it's important that we stay alert--we really can't afford to indulge in another extended Dark Age, for example. And who knows, we might make it to Type III before Andromeda gets here. If not, perhaps some other species in the galaxy may have gotten it together enough to help us out.
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Laurance Doyle is a principal investigator for the Center for the Study of Life in the Universe at the SETI Institute, where he has been since 1987, and is a member of the NASA Kepler Mission Science Team. Doyle’s research has focused on the formation and detection of extrasolar planets. He has also theorized how patterns in animal communication, like those of social cetaceans, relate to humans.