So, You’re Still Interested In Astronomy?
As some of you will no doubt remember, last time out we took a look at exoplanets. So far, we’ve discovered more than two thousand of them. The trouble is, finding a place capable of supporting life as we know it seems to be rather difficult.
So, what makes the Earth so special? On this occasion I thought we’d look at 10 factors conducive to life. Then we’ll start to appreciate why finding an earthlike planet is much more difficult than we first realized.
Let’s start from the outside and work our way in.
- Unique Position in the Galaxy:
The Sun and our Solar System have been located in a steady orbit within our galaxy for the last 4.5 billion years. What helps is the fact that our orbit lies far from the center of our galaxy and between the spiral arms. The stability of our position is possible because the Sun is one of the rare stars that lie within something called the galactic co-rotation radius.
Sounds like something a doctor might look at, doesn’t it? But what does this mean?
Typically, the stars in our galaxy orbit its center at a speed that differs from the rate of the trailing spiral arms. Thus, most stars located between the spirals do not remain there for long, but would eventually be swept inside one arm or the other. It’s only at a certain sweet spot from the galaxy’s center – the co-rotation radius – that a star can remain in the empty place between two spiral arms.
2. So, why is it important that we are not in one of the actual spiral arms?
Our location is safer than anywhere else in the universe, because we are removed from the more densely occupied areas where stellar interactions can lead to a disruption of planetary orbits. In addition, we are farther from the deadly affects of supernovae explosions and all sorts of radiation that would make our planet unfit for human occupation.
3. The need for a Jupiter sized planet:
Large bodies like Jupiter are required for life to exist on the Earth. A recent study implicates all sorts of factors which I’ll simplify here:
Scientists have hypothesized that the presence of Jupiter sent large amounts of water-containing meteorites into the inner Solar System soon after it was forming. It’s also possible that Jupiter was responsible for sending the Mars-sized planet that is thought to have formed the Moon.
Now, what’s unique is that Jupiter-sized planets are not found as far out as 5 AU in other stellar systems. In fact, nearly all large planets have been found to be closer to their stars than the Earth is to the Sun (which would remove all rocky planets in the habitable zone from those systems). What’s incredible is the fact that, despite having been responsible for the shower of meteors that pelted the early Earth, Jupiter is now our great protector and is responsible for collecting and ejecting a large proportion of the comets that enter into orbit around the Sun. Yes, without Jupiter, life on Earth at this time would be difficult or impossible due to the large number of cosmic collisions.
Interesting snippet: There have been quite a few large planets found around other stars recently, but none are far enough away from their suns to stabilize the orbits of other, smaller bodies within the habitable zone, nor are they in the right spot that would mean they could protect those inner planets from cometary bombardment.
4.Stability of the Inner Solar System:
The continuing growth in the capabilities of sophisticated computer systems allows scientists to model the dynamics of the Solar System and ask “what if” questions regarding the location and size of planets. We already know that the presence of Jupiter is required to allow advanced life to exist on the Earth. However, Jupiter’s mass – along with the mass of the other gas giants – has a profound destabilizing effect upon the inner planets. How much? Listen to this…
If the Earth-Moon system were absent, the orbital period of Jupiter would create and increasing resonance over a period of 8 million years or so. This resonance would cause the orbits of Venus and Mercury to become highly eccentric, so much so, that they would come close enough to lead to a planetary ejection – most likely of Mercury – and an alteration of the orbit of Venus.
The simulations revealed that for the Earth-Moon relation to be effective, the mother planet needed at least the mass of Mars and it had to be situated within 10% of the distance of the Earth from the Sun. Incredible, because it means that Earth itself is at the precise spot needed to balance conflicting perturbations that would otherwise render the Solar System like every other one we’ve found so far…Lifeless!
5. Our orbit of the Sun:
The unique arrangement of large and small planetary bodies in the Solar System has helped to stabilize our environment greatly. But this is also true of our orbit around the Sun. It would appear the Earth is at the edge of the Goldilocks Zone for our star. Additionally, the Earth has one of the most stable orbits among all the planets discovered to date. The shape of our orbit is also a factor important to life. In fact, things like orbital eccentricity, seasonal variation, and axial tilt are essential for human life.
6.The presence of a large moon:
The Earth has a huge satellite orbiting around it. In fact, the Moon possesses the largest mass of any parent-satellite relationship in the system. Numerous lines of evidence indicate our Moon was derived as a result of a singular impact event around 4.25 billion years ago.
Here’s another incredible fact: scientists have calculated that the Moon must have formed just outside the Roche limit – the point at which an object would be torn apart by the Earth’s gravity. A collision which would have ejected material less than the Roche limit would have formed only rings around the Earth.
Now, while it would be brilliant to look into the night sky (and the day sky, come to that) and stare at all the pretty things floating around up there, a set of rings wouldn’t have helped contribute to the emergence of life as the Moon does.
Did you know: the regular rise and fall of sea level creates an environment experienced nowhere else in the solar system? You do now…
7. Van Allen Belts:
Another fortuitous result of our collision with a Mars-sized planet is the presence of the Earth’s heavy metallic core. In fact, the Earth has the highest density of any of the planets in our Solar System. This large nickel-iron core is responsible for the magnetic field. The magnetic field produces the Van-Allen radiation shield, a double-layered buffer (comprised of an inner level of protons and an outer stratum of electrons) which protects the Earth from plasma bombardment. If this shield were not present, life would not be possible on Earth.
Why is the Moon important to life? The collision of that small planet with Earth resulted in the ejection of the majority of the Earth’s primordial atmosphere. If this collision had not occurred, we would have had an atmosphere similar to that of Venus – which is 80 times thicker – with pressures equivalent to being a mile beneath the ocean. (Ouch!) Such a thick atmosphere on Venus resulted in a runaway greenhouse effect, and surface temperatures of 800˚F.
We would have suffered a similar fate.
Interesting fact: because the Earth is 20% more massive than Venus and further away from the Sun, our terrestrial atmosphere would have been even thicker than our sister planet. For some strange reason, we have a very thin atmosphere – just the right density to maintain the presence of liquid, solid and gaseous water necessary to life.
9. Axial Tilt and eccentricity:
The Earth is titled on its axis at an angle of 23.5˚. This is crucial to the development of our seasons. Now, two factors impact the progression of seasons. The most important is the location of land masses. Nearly all of Earth’s continental land mass is located in the Northern Hemisphere. Since land has a higher capacity to absorb the Sun’s energy, the Earth is much warmer when the Northern Hemisphere is pointing towards the Sun. This happens to be the point at which the Earth is farthest away – in the aphelion of its orbit. If the opposite were true, the seasons on the Earth would be much more severe, and we’d need to wear sun factor 1000 in summer, or fifty layers of clothing in winter!
10. Slow Rotation:
The Moon has had other beneficial effects on the Earth. How?
The Earth’s graceful rotation is one of the traits that allow life to thrive, as during the course of a year, our days are not only uniform, but allow vast swathes of the planet’s surface to be bathed in sunlight at regular intervals. However, it wasn’t always like this. Scientists now know that the Earth originally had a rotational period between 6 to 8 hours. However, the gravitational tug of the Moon over the last 4 or so billion years has modified that to 24 hours. Lucky for us, eh?
So there you go. An amazing dip into a much larger pond, which allows us to see what makes the Earth so distinct from other planets.
Next time, I thought it might be nice to delve into some of the information we’re gaining about the Solar System’s mysterious new member. New member? Oh yes, we haven’t found it yet, but we know it’s there because…
Ha! You’ll have to stop by again to find out.
See you then.