(That’s a real-time screen shot from Celestia, a free space simulator. Highly recommended.)
Life On Other Planets?
Imagine, if you will …
Somewhere out there is a giant planet, larger than Jupiter, weaving a complex figure-eight orbit around and between two stars. Circling that planet is a moon that is just barely habitable. Barely.
Each Tuesday, the moon’s surface is scoured by hot gas. Wednesdays are Big Meteorite Impact Day. The inhabitants celebrate with barbeque, cooked the day before. Nice and crispy. Food should be a challenge, just like the rest of life.
(But don’t ask about Thursdays. It ain’t pretty.)
These creatures can chew through solid rock. They live underground and have a culture that makes the Klingon Empire look warm and fuzzy. They “see” with radar and “talk” with blue flame.
For them, technology is a means to an end: a means to END anything that annoys them (ba-dum tish).
One guy is working on a way to more efficiently vaporize an irritating neighbor. (This takes work; his people aren’t easy to kill.) He tinkers, makes a few adjustments —
He stops. Scans his workbench. Ponders. A smile that would terrify a velociraptor slowly creaks over his face.
Dude double-checks his math … then roars in delight. Blue flame flies everywhere. The children scatter, screaming in fear. He barely notices.
He’s figured out how to travel faster than light!
He builds a spaceship. (No welder needed; he crimps the metal together with his teeth.) He and his chums load it with food and with weapons that make our stuff look like pool noodles.
They gnaw a hole, drag the ship to the surface and head off into the void. Time to explore!
… and you do know that
we sent out maps so that
they could find us, right?
Sleep well tonight.
That’s actually not likely at all … and that’s the purpose of this section in my little e-tome.
That example was just for fun and I’ll admit that it was unfair. (A figure eight orbit around two stars would be hilariously unstable.) But what about a nice, Earth-like planet in the habitable zone around a stable star?
That’s a better candidate, but it’s still not that simple. All orbits are inherently unstable over the long term. For example, our Moon is drifting away from us at about 1.5″ per year. It also bobs and wobbles a bit in its orbit due to gravitational influences from other planets. (And asteroids.) (And your grossly obese Aunt Petunia, who thinks that Dairy Queen is one of the Essential Food Groups.)
The best you can hope for is one that’s stable enough, and a planet that remains habitable long enough (without annoyances like repeated bombardments or radiation), to give life and intelligence time to develop.
SETI (the Search for Extraterrestrial Intelligence) presents one side of the argument: there must be other alien civilizations “out there.” We’re finding Earth-like planets around other Sun-like stars all the time now; there are estimates that there could be millions of the things just in our region of the galaxy, much less the entire Cosmos. Some of those have to be “just right” for life. Don’t they?
The Rare Earth
The opposite argument has been called the Rare Earth hypothesis. This says that life might be common “out there,” but intelligent life is exceedingly rare.
John Gribbin would certainly agree. His book, Alone In The Universe, is based on the latest computer simulations and concludes that Earth is probably the only planet in our galaxy where intelligent (again: don’t miss that word) life could have arisen.
I’m not going to take a position on that here. I’ll freely admit that I do believe in God and that I believe that you and I are special creations, made in His image. But if we do someday discover Ewoks in a forest moon around another planet, I’ll be just as delighted as everyone else.
(It won’t affect what I believe, either. The Bible is completely silent on this.)
Remember, just having an Earth-like planet does not guarantee that intelligent life will result. I’m going to show you that a host of things worked together to make our Earth an ideal home for intelligence, advanced technology and multiple cultures.
He bathed once a year.
I have a weird sense of ‘yumor. (You may have noticed.) My fevered mind drew a strange picture (to the accompaniment of the “Looney Tunes” song) while reading Gribbin’s discussion of the computer models.
It doesn’t actually go like this, but … I imagine a bunch of scientists running a simulation, trying to model the formation of a solar system. They set it up, click “Run” … and a chorus of Joe Dirt-like voices says, “daaaang.” Failure. Try again; “daaaang.” Failure. Rinse, repeat, wipe hands on lab coat. Over and over.
On one run, a monstrous “hot jupiter” forms, then wallows and wobbles in toward the star, eating everything else in the system. You end up with one massively giant planet, smoking and bubbling and glowing in a close orbit. (We shall name it Aunt Petunia.) “Daaaang.” Next run: you get two stars that orbit each other like a pair of yo-yos doing parkour, eating and ejecting planets right and left. “Daaaang.” Even on those runs where they get planets, the orbits are elliptical and unstable over the long term.
What’s amazing, though, is what they discovered when they finally were able to model our solar system.
The following is my interpretation of Gribbin’s book, which is based on these models. As usual, I will leave out a bunch of arcane details and take major liberties, especially with the chronology.
But imagine, if you will …
The Primordial Stellar Group
Once again, let’s do a quick recap and pick up where we left off.
8-9 billion years have passed since the Big Bang. Galaxies have formed. The Interstellar Medium (ISM) is mostly hydrogen and helium gas, but repeated supernovas have slowly enriched it with a small percentage (about 2%) of heavier elements.
A cloud of gas and dust in the Orion Arm of our galaxy begins to coalesce. An open cluster forms with a few thousand hot young stars. These types of clusters are quite common (the Pleiades are a well known example). They’ll form, last for a while, then slowly dissipate and spread apart over millions of years.
This cluster includes a giant star with around 30 times the mass of our Sun.
Remember Betelgeuse? We discussed him previously. This star is much more massive. It will explode as a supernova … but the precise location and the timing may be the happiest “accident” in history.
Astronomers see NGC 346, an open
cluster in the Small Magellanic Cloud.
Your Aunt Petunia sees a Dairy Queen.
A Supernova And A Special PPD
This giant star first threw off an outer shell. Some of that dust traveled about 1/10th of a lightyear; a “curdle” formed. It rotated and slowly flattened into a protoplanetary disk, or PPD.
In the center of that spinning disk, our baby Sun formed. It pulled in gas from the disk and grew more massive. Eventually, fusion started. The Sun celebrated with fireworks: powerful (and beautiful) streamers from its poles called Herbig-Haro objects (see below).
Then that nearby supernova exploded, scattering a generous dose of heavy elements, including some important radioactives, into our baby solar system.
Gribbin says that the timing is quite critical. Had that supernova exploded sooner, the disk would have been ripped apart. If it had happened later, we wouldn’t have received that unusually high (compared to other star systems about the same age as ours) dose of heavier elements.
Either way, you and I probably wouldn’t be here.
Herbig-Haro objects. Detailed images
from the Hubble arrived about the same
time as the explosion of strange multi-
colored hairdos among rebellious teens.
Is this a coincidence? You decide.
The Planets Form
If we speed this up and watch from high above, the protoplanetary disk might remind you of a bunch of debris floating in a spinning pool of dirty water. (Hold that thought.) Tiny particles slowly clump together into larger rocks.
These are mostly traveling in the same direction, so they’re not slamming into each other at high speeds. It’s chaotic, though: some combine and form even larger rocks, some bob around in erratic, elliptical orbits and some are ejected from the system entirely.
Close to the Sun, we get the rocky (or “terrestrial”) planets, including Earth. Farther out, past the “frost line,” the gas and ice giants (like Jupiter) form. See the image below, which shows the planets, in order, scaled according to size — but not distance.
If you want to get an idea of the distances, mark a straight line about 24 feet long. Place a basketball at one end and a cue ball from a billiards table at the other. That’s a rough scale model of the Earth and Moon. Not perfect, but it’ll help you visualize it.
Want to add Mars to your scale model? Stand next to that basketball and imagine a baseball almost a mile away. That’s Mars at its closest approach to Earth. The Sun? That would be a giant ball roughly 10-11 stories high, about 1-3/4 miles away.
Space B huge, y’all.
The Giant Impact
In the middle of the chaos something interesting happens: two planets form in the same orbit. One is our baby Earth; the other is a Mars-sized object that trails behind … and its orbit is highly unstable.
Imagine a pendulum hooked to the Sun, swinging back and forth; toward Earth, then away from it. Unlike the pendulum in Aunt Petunia’s grandfather clock, though, each swing gets “wider.” It comes a bit closer to Earth … then farther … then closer …
It crashes into our planet. Earth’s crust is shattered, throwing debris out into space. The collider is destroyed, but the Earth has become molten; the iron-nickel core from that collider “sinks” into our planet and joins the metal already there. We gots lots of metal in the middle now.
Most of the crustal debris around the Earth would be drawn together by gravity to form our Moon — some of the models say this would only have taken a month or two!
The Moon would have been much closer to Earth at first, causing tides that literally warped Earth’s surface, keeping it molten. But this gravitational energy had a cost: the Moon migrated outward to its present position, about 240,000 miles away.
“That’s A Big Moon!”
One common storyline in science fiction is that aliens are surprised by how large our Moon is, compared to the size of the Earth. Some astronomers even consider Earth/Moon to be a dual-planet system.
Gribbin says that, without a similar giant impact and large moon, it’s unlikely that those aliens could have ever evolved the intelligence to come here in the first place. Why? Here are a few reasons.
The collision shattered the crust, giving us plate tectonics and continents. Without it, Earth would have become a smooth, round “water world” with a thick crust, but little or no dry land. That impact also gave us our 23 degree axial tilt, resulting in the four seasons.
Earth’s unusually large iron-nickel core (thanks to the giant impact), which is molten (thanks to the radioactives from that supernova explosion, mentioned earlier), acts as a dynamo, giving us a strong magnetic field. This shields us from extraterrestrial radiation. How important is this? The magnetic field weakens and then “flips” periodically. There’s a correlation between these reversals and several extinction events.
Finally, of course, our Moon has significant tidal effects on the Earth. Millions of years ago, the Moon’s tides would have “washed” life out of the oceans and onto dry land. The presence of our large Moon also helps stabilize the Earth’s axial tilt.
That’s no Death Star, OB1. It’s
Mimas, one of Saturn’s moons.
There’s more evidence that our Moon, and the impact that created it, are critical to our existence.
Venus is a near-twin to Earth. It’s about the same size and for years, was believed to be just a hotter version of our planet. (It’s closer to the Sun.) But many folks imagined a steamy, tropical planet with rain forests and exotic life — until the Mariner (USA) and Venera (Soviet) missions took a closer look.
The surface of Venus is way hotter than that — hotter than Mercury, which is even closer to the Sun. The reason is a runaway greenhouse effect, combined with a thick, poisonous atmosphere with a surface pressure 90 times higher than that of Earth’s. It traps the solar radiation, keeping the surface temperature above 860 degrees.
The planet’s surface is also much smoother than Earth’s. Venus has no moon; there was no giant impact to create plate tectonics. Venus has a weak magnetic field, because it didn’t “swallow” a spare iron-nickel core. Solar radiation is able to attack the top layers of the atmosphere, ripping apart water molecules and causing constant, violent lightning.
Read Gribbin’s book for more details. But tonight, if you spot Venus in the sky, look at it and say, there, but for the grace of God, goes Earth.
Back to the formation of our solar system. Gribbin says that the successful models have to give special treatment to the orbits of Jupiter, Saturn, Uranus and Neptune — the “giants.” For one thing, Jupiter and Saturn have helped Earth maintain a relatively smooth and circular orbit for billions of years, plenty of time for intelligent life.
Jupiter and Saturn would also have “plucked” asteroids and comets from the icy debris field, sending them crashing into the rocky planets around the Sun.
This heavy bombardment ended about 4 billion years ago. It doesn’t sound very pleasant, but you should be very glad that it happened. That’s where much of our air, water and most of the minerals that we can mine (including gold and iron) came from. (See the NASA/Spitzer image below.)
As already mentioned, the giant impact that formed our Moon meant that we would have dry land here on Earth. Venus probably was a water world at one time, but became so hot, it evaporated and was destroyed in the upper atmosphere by solar radiation.
(Mars and Mercury are also dry, but for different reasons; I won’t get into them here.)
One Last Look At Venus
Man, I’ve left out a ton of stuff. But to quickly finish the story: not long after the heavy bombardment ended, life appeared on Earth. It changed very slowly before the Cambrian Explosion (about 500-600 million years ago) when all of a sudden, multiple complex lifeforms appeared in rapid succession (“sudden” and “rapid” in the geological sense, of course: 5-20 million years).
The surface of Venus appears to be “young,” only 600-700 million years old. The best answer is that it was impacted by a large object around that time. Magma flowed through the cracked crust and “re-smoothed” the surface. It also rotates “backwards” from the other planets, which is strong evidence for a massive impact. Not as massive as the giant impact that formed our Moon, but large enough to change Venus dramatically.
Gribbin says that scientists can calculate the rough size of this impactor. If it was a large, icy cometary object from beyond the “frost line,” it would have “shed” icy crystals on its way in to the Solar System. The ice crystals would have blanketed our planet, blocking the Sun and causing a “snowball Earth.” Could the Cambrian have marked the survivor’s emergence from this “snowball?” The timing is intriguing.
This hasn’t been proven, of course; Gribbin is speculating. But the coincidences are striking. It’s known that Earth has suffered from “snowball” periods in the past; this might explain it. The strike on Venus would help with the timing.
But I’ll leave it at that; get Gribbin’s book and read it (especially chapter 7, which details the evidence for the impact and the “snowball Earth”).
Enceladus, another moon of Saturn.
It’s a “snowball,” but so far, no one
has tried to build a snowman there.
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