What would the average terraformable planets look like?

Question

Let’s say that a "terraformable planet" is a planet who :

• is in the goldilock zone of its star(s)
• has around the same gravity as Earth
• has a solid surface covered with minerals not too toxic for humans
• has enough water for plants and/or algae to grow
• has an atmosphere that can become breathable (even if it takes hundreds of thousands of years following the introduction of Earth plants)
• is mostly safe from meteor impacts

(Criteria chosen for simplicity sake)

The Earth is a blue sky / yellow sun / single moon / 20-ish hours day kind of planet.

But what about terraformable planets ?
What would be the most common sky colors ? Star colors ? Number of moons ? Approximate day lengths ?

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Note : I can accept something like “We can deduce a probable answer regarding this one aspect but it’s completely impossible for the other ones”, if it’s justified.

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Edit (based on answers) :

The criterias above are completely arbitrary, but please take them into account when answering.
I added them to avoid the vague "Once you have the technology you can terraform anything you want" answer.

More details on this universe (not needed to answer) :

The story I'm working on is not set in the distant future, and in it people do not have the technology to terraform any celestial object they want.
It doesn't really matter how they do it. They can only terraform planets corresponding to these criteria (and a couple other that don't affect answers validity).

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My english isn't perfect, please tell me if something isn't clear.

Answer 1

Most stars are red dwarfs, so most planets in the goldilocks zone will be orbiting close to a red dwarf. The sun will be a bright red colour and appear larger in the sky than the sun does.

If the planet has the same surface gravity as Earth and a solid surface it must have about the same mass and density as Earth, at least not so different to be significant.

The planet probably won't be the right temperature. Since it will have an atmosphere with significant amounts of CO2, CH4 and H2O (and N2) and no free Oxygen, surface temperatures will be high. Part of the terraforming process will be to remove CO2 and CH4 from the atmosphere, and significantly cool the planet. If your planet has a CO2 rich atmosphere and the right temperature now, terraforming will just send it into an ice age. There would be seas, but they may be substantially smaller than on Earth.

An alternative is a planet with little atmosphere, but substantial amounts of water frozen in the crust. Here the terraformers would need to create an atmosphere to warm the planet. It may be easier to thaw out a frozen world than to cool a world with a runaway greenhouse.

Temperature control will be the big problem, since you are going to change the atmosphere, the temperature the planet is now won't be the same as the temperature the planet will be after you have done with it. The Earth was terraformed nice and slowly by cyanobacteria, and they nearly sent the world into a permanent snowball Earth state.

The sky would be either Blue or Cloudy. The colour of the sky comes from dispersion, and all gasses act the same. The rocks would be the same sort of browns and greys that exist on Earth. Most rocks are chemically pretty inert, and chemically inert = non-toxic.

Meteor strikes are no an issue to any planet with an atmosphere, and a sufficiently mature planetary system.

Having a moon is useful for stabilising the planet's axis of inclination. However if the planet is orbiting close to a red dwarf, moons are unlikely (the Earth is probably unusual in this respect)

Answer 2

What the most common sky colors for terraformable panets are really depends on what the most common sky colors are for any planet at all. If we can terraform the planet we can alter the atmospherical composition, so really all colors are terraformable.

The same goes for the color of the star, any given star has a habitable zone so any star could be host to a terraformable planet.

The number of moons doesn't influence terraformability at all.

Day length is the factor that is the most important in determining wether a planet is terraformable or not (of those you listed). It doesn't influence any of your points for determining wether a planet is terraformable or not, but if the terraforming process can't change it and the planet has to be able to support human life on it after the process it can't vary too much.

If human life has to be comfortable, you can't deviate more than an hour from the 24-hour scheme, as too short or long days compared to our natural rythm cause a variety of problems in the human body.

If it just has to be possible to live there, no matter the consequences, the day/night cycle has to support continuous life on the entire planet.

I can't really say exactly how much that would be, but making a (very) wild estimate based on the assumption that you'll introduce life seeded from earth (or no more than lightly modified), I'd say that the day can't be much longer than 100 hours, although there's no lower limit.

If local life can adapt to the extreme light cycle and the humans have the ability to travel if require for survival I'd say there isn't really an upper limit either, this question has some answers on how life could be sustained on a planet with long day/night cycles, so long as humans can just follow life they should be able to survive everywhere.

Answer 3

The absolute minimum would be a planet which has enough mass to hold an atmosphere, and the ability to deliver enough energy to the planet to drive the ecosphere. With sufficiently advanced technology, you could terraform the Moon (although without intervention the atmosphere would leak away in @ 10,000 years), and Mars is commonly used as a target for terraforming in science fiction and informed speculation.

Since criteria one is generally set by the gravitational field of the planet, we are looking at plants as small as Mars, and conceivably to "Super Earths", although the immense gravity fields would make getting around difficult.

For energy, we can fall back on the magician's trick and say it's done with mirrors. Platoons of sufficiently large mirrors orbiting the planet could deliver enough energy to warm the planet and drive the ecosphere, even as far out as Titan, in the orbit of Saturn. A different trick would be to manipulate the solar photosphere to create a laser of immense power and illuminate the target planet, meaning you cold grow potatoes as far out as Triton in the orbit of Neptune, and conceivably out to the Kuiper Belt.

For very small planets, it may even be possible to erect a giant "bubble" around them to hold and maintain an atmosphere for geological ages.

So the real answer is "how long do you want to take and what sort of resources are you willing to devote to the project?"

Answer 4

What would the average terraformable planets look like?

It will look like space body with $\small \approx$1g gravity on surface.
Or like Gas giant - like Saturn, Uranus, Neptune, Jupiter.

That's all. Or you work it to be the way you like it to be, or you out of terraforming business.

Day-night is to short-long - place orbital mirrors, make it as you wish.
Need magnetic field - make one.
Do not like atmosphere - replace it.
Low on water - add more.
Gravity not strong enough to keep atmosphere - encapsulate that dwarf planet with shell.
Gas Giant - cover it with shell.
Do not like light spectrum - Fresnel lenses, mirrors, graphene lenses, diffraction grating
Not enough light, you guessed it, orbital mirrors, thermonuclear reactors, sun orbit mirrors, lenses.
As worst case scenario shelters with everything it needs to be pleasant.

Surface is't good enough - remove it, rework it - you have automation for that, you have equipment, you have algorithms, you have programmers, you have energy.

I truly do not understand it. Yes in 1930x - no computers at all, all work is done by humans with tools. 1960 - no computers, 99.9999999% work is done by humans with tools. 2016 - computers everywhere. I do not know how much in percentage work is done by tools without humans - but it's not something unusual.

All terraforming is just matter of energy and program for it.

Gravity is only problem (kinda if you do not have enough material to make it bigger)
Centrifugal force is not problem, gravity is, centrifugal force not.

If someone, wish terraform a planet or even planet's, lights years away from home it have to be prepared for that action. In first place because they already used pretty good tech to get there.

If someone talks about terraforming planes - thinks more space, be more space, looks at least at today technologies. Most important from them all at the moment is our ability to automatize tasks. It is a way to not linearly scale amount of work done per one human. One human is capable to press button to terraform star system.