As far as types of planets go, our solar system is fairly diverse.
We have two gas giants, one with a prominent ring system, two ice giants, and even a planet
close enough to the sun as to be outside the sun’s habitable zone for being too close,
and of course three other rocky worlds including earth, all of which at some time had liquid
water on their surfaces.
And that does not get into the moons, where liquid water is also apparently rather abundant
locked under shells of ice.
Studying these worlds close up is important to exobiology because it tells us what some
of these exoplanets we’re finding could be like.
There are probably other Venuses out there nearly indistinguishable from our own suffering
the same runaway greenhouse effect, as there are likely lots of Jupiter and Saturn analogues
and so on.
But the most interesting of the planets, Earth, is the planet that we’d most like to see
an analogue of as it could well be inhabited.
But there are also types of planets that we do not have, the most important of which is
a super earth, which could also be habitable or at least certain ones.
Super earths are defined somewhat ambiguously though; the concept is based on the mass of
the planet rather than its surface conditions, which can be anything from rocky, like earth,
to a sort of mini-Neptune that’s nothing like earth.
There is no consensus yet how to subdivide these types of planets up, nor is there a
consensus on the size of planet needed to even be called a super earth.
I tend to favor restricting the term super earth to earth-like rocky planets, since it
doesn’t really make much sense using the term “earth” when discussing a miniature
Uranus.
As to size, all you can really say is larger than earth, and no bigger then ten earth masses.
We’re most interested in the earth analogues, ones that have a rocky surface and atmosphere
something like our own planet, just larger.
What would such a planet be like?
How would those differences affect life that might develop on those worlds?
The short answer is that it depends on just how big the planet is.
One of the leading solutions to the Fermi Paradox is called the rare earth hypothesis.
This concept suggests that earth is extremely special and possesses a number of attributes
that are just so that complex life, and a civilization, can arise here.
Without those attributes, which range from everything from possessing a moon and tides
to plate tectonics, life might not have been able to get much past the microbial state
and would suggest that intelligence is rare in the universe because the conditions of
earth are rare.
While somewhat of a bummer of a solution for those hoping for a galaxy inhabited by many
different civilizations, it can also go the opposite way.
There may be worlds in this galaxy that are better at producing complex life and civilizations
than Earth is.
This is where the super earths come in.
It’s thought that if you had a planet twice the mass of Earth, but similar in all other
ways with liquid water and so on, it might actually be a better environment for life
to arise.
There are several reasons for this.
The higher gravity of such a planet would cause it to retain a thicker atmosphere.
This is important because planetary atmospheres help shield the surface from harmful cosmic
rays.
But there’s another advantage.
A larger planet would also generate, assuming it’s internal structure is similar to Earth,
a more powerful magnetic field.
Life on earth could not exist as we know it without our magnetic field because it shields
us from the solar wind.
If you have a stronger magnetic field, you have more shielding.
Increased gravity would also do other things.
It would cause the topography of the planet to be significantly flatter, overall, than
earth’s.
This would be compounded by higher surface erosion rates.
Imagine such a planet as an ocean world, with waters that are a bit calmer than here, possibly
with small continents or groups of islands that are low in profile, but ideal for life
to use, as it happened here.
And there’s another advantage.
A planet twice the size of earth would retain more heat internally, which could drive plate
tectonics significantly longer than it will last here on earth.
We don’t know how long that will be for earth; the earth’s interior is not completely
understood in that regard, and in fact there’s some evidence that suggests that in the past
the plates ground to a complete halt for a time, and may do so again in the future only
to start up again when something gives.
That doesn’t sound good.
But I digress, let’s assume our hypothetical planet is like earth where plate tectonics
are vital to complex life and atmospheric composition through the carbon cycle.
Time is good for life, especially intelligent life, so if you had just such a super earth
orbiting a long-lived, stable star such as an orange dwarf, within the habitable zone
where surface liquid water is possible, all providing stability for developing life then
you might just outdo earth in conduciveness for the process.
Well, we’re proof that the magic of intelligence can happen on a perhaps imperfect world, so
who knows what else might be out there.
One can only imagine what life on a planet twice the mass of earth might look like.
Would the higher gravity produce squat, but very muscular and stocky beings?
Would the lower light levels of the orange dwarf make for very big eyes?
It’s fun to think about and imagine, but really nothing is known about what alien life
would look like.
You can only conjecture and make a lot of assumptions when doing so.
But we do know this.
Gravity eventually stops being an advantage.
The bigger you make your earth-like planet, the less earth-like it becomes.
You get thicker and thicker atmospheres with crushingly high atmospheric pressures and
gravity so strong that very high energy is needed to even move.
And many other factors would come into play, your planet may no longer allow for land or
islands, and instead favor a full water world with a shallow, calm ocean covering the entire
planet.
This is not good for an intelligence should it arise on such a world; too much water could
be just as bad as no water at all in regards to the development of a civilization.
But I’ll save that subject for a future video.
Thanks for listening!
I am science fiction author and futurist John Michael Godier and be sure to check out my
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explorations into the interesting, weird and unknown aspects of this amazing universe in
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