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Durakken
2013-Oct-24, 02:24 AM
I need some help with some questions...

Background information...
I'm writing a sci-fi book, I want this to be closer possible than not, based on current knowledge.
The book will revolve around the colonization and eventual war with Tau Ceti f (possibly e)
From the articles I have read on these 2 planets. 1 is too hot (70 C) and the other is too cold (-40 C). I would like to make it roughly as earthlike as possible.

So questions:
#1. What are the min and max size of these planets based on their mass...
-> e = 4.30 ± 2.01 M
-> f = 6.67 ± 3.50 M

#2.
a) What would the atmosphere be like if the environment was like Earth on these planets? TCe I would think would have to have a thin atmosphere and TCf would have to have a denser atmosphere, but can you do that with just with changing the levels of the different elements in the air and what other things could you say about the world?

b) A prospective solution I was thinking is to say that the Atmosphere is pretty much very much like Earth, but with the increase in asteroid impacts, (10km asteroids roughly every 10 million years along with 1km astroids every 100,000 to 1 million years) would cause cycle of the planet being a relatively earthlike temperature where roughly every 10 million years you would get a spike up to Hotter climate and then fall slowly over the next 10 million years until it is hit with asteroid drops to real cold and then spikes back up to being heated. My thought is that the size of the planet would mitigate the extinctions and with the relatively frequency there might be something that would evolve that could tolerate this more easily and if not 10 million seems to be about the time needed to "recover" so I think this could work but I'm not sure...

#3. Tau Ceti is said to have low metallicity. What sort of effect would this have on planetary resources and such? Could I say that this planet's resources would be something like multiple by the relative size divided by 3 to come up with a rough estimate, like say the planet is 2x the earth and there is 300tons of gold on Earth so this planet would have 600tons divided by 3 for 200 tons... or would it mean something else?

#4. How would doing #2b's solution effect the surface and resources? For example would this mean there would be less water due to cataclysmic events or more due to more ice bodies striking? Likewise would there be more gold and platinum type resources due to those resources in sol largely being tied up in asteroids and such? Would there be any other oddities like certain elements being more or less bouyant than you'd expect on earth?

......................

The reason I chose Tau Ceti if you're curious is due to it being roughly 10 light years away. In this universe Earth has discovered warp that allows at the time of colonization 10x light speed. If you know any better system in and such around 10 light years away I'm open to hearing it. It's can't be too close though as too close as the distance is what causes too many people not to go.

For the same thing, but unrelated to the planet question... Due to events near the end of this story, I need information regarding 50 ly away from earth in all directions. Is there a good navigable source? Basically, at the end of the story Earth and this other planet cause a barrier 50ly around them and as time goes on they colonize as much as they can in this sphere. If I ever need to refer to stars/planets in this government I need to know what's in there... and since I don't like making stuff up when I don't have to or the information should be real and I know that there is a listing of some sort of all these stars, I need to know what's there that's easily accessible.
Also about how many stars is in that 100ly sphere?

Thank you for any help in advance...

eburacum45
2013-Oct-25, 07:47 AM
#1. What are the min and max size of these planets based on their mass...
-> e = 4.30 ± 2.01 M
-> f = 6.67 ± 3.50 M

Tau Ceti e is a superearth that orbits at or within the inner edge of the so-called habitable zone, and it is unlikely to be a very nice place to live for humans.

If the middle value of this range of masses is correct, i.e. 4.30 Earth masses, and assuming it has an Earth-like composition: the diameter would be 19,530km, 1.53 x Earth's, the density would be 6.6g/cm3, 1.2 x Earth's (because of compression) and the gravity would be 1.84 x Earth's.

If the upper value of this range of masses is correct, i.e. 6.31 Earth masses, and assuming it has an Earth-like composition: the diameter would be 21,770km, the density would be 7g/cm3, and the gravity would be 2.17x Earth's.

If the lower value of this range of masses is correct, 2.29 Earth masses, and assuming it has an Earth-like composition: the diameter would be 16,340km, the density would be 6g/cm3, and the gravity would be 1.4 x Earth's.

With such a high gravity the planet would probably have a thick atmosphere. If the smaller of these three options is correct, the density of the atmosphere might not be all that greatly different to Earth's. There are several other possibilities for this planet; it might be a waterworld, and so would have a larger diameter and a proportionally lower gravity. Or it might somehow have lost most of its volatiles in a cataclysmic collision, resulting in a planet with high gravity and an atmosphere about as thick as Earth's. However the local star, Tau Ceti, appears about 1.3 times as bright in the sky of this world as our sun does in ours, so this world is likely to be a hot world even if it does not have a thick atmosphere.

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Tau Ceti f is a superearth than orbits at the outer edge of the so-called habitable zone, but it is unlikely to be a very nice place to live for humans.

If the middle value of this range of masses is correct, i.e. 6.67 Earth masses, and assuming it has an Earth-like composition: the diameter would be 22,070km, 1.73 x Earth's, the density would be 7.1g/cm3, and the gravity would be 2.24 x Earth's.

If the upper value of this range of masses is correct, i.e. 10.17 Earth masses, and assuming it has an Earth-like composition: the diameter would be 24,620km, the density would be 7.8 g/cm3, and the gravity would be 2.74 x Earth's.

If the lower value of this range of masses is correct, 3.17 Earth masses, and assuming it has an Earth-like composition: the diameter would be 17,920km, the density would be 6.3g/cm3, and the gravity would be 1.61 x Earth's.

With such a high gravity the planet would probably have a thick atmosphere, although it may have lost much of its volatiles in a series of catastrophic collisions. The local star, Tau Ceti, would appear about 0.23 times as bright in the sky of this world as our sun, but with a very thick atmosphere and a high greenhouse effect this planet might sustain a temperature range similar to that of Earth. This planet is more likely to hold life than the other one, but if the atmosphere is mostly CO2 it will be toxic to most Earth life, and the high gravity would make it uninviting for anything resembling humans.

(I've used the following calculators to arrive at these figures;
http://www.transhuman.talktalk.net/iw/Geosync.htm
http://web.archive.org/web/20030218125318/www.geocities.com/Area51/Corridor/8611/mseqstar.htm )

eburacum45
2013-Oct-25, 07:53 AM
This list we've made for the Orion's Arm project contains most, but not all, of the known stars within 50 ly; (the colonies are, of course, fictional)
http://www.orionsarm.com/eg-article/49fc193234b06
there may be many unknown red and brown dwarfs in this volume, but most of these haven't been identified yet.

lpetrich
2013-Oct-25, 08:57 AM
I found a lot of nice stuff on this question:

Exoplanet Mass and Radius and the Physics of Planetary Interiors (http://www.mpa-garching.mpg.de/lectures/Biermann_10/Lecture2.pdf) - presentation: page 8 contains a diagram showing masses and radii, both calculated and observed. MgSiO3 in it is an approximation of typical undifferentiated Solar-System rocks, including the Earth's mantle.

Mass-Radius Relationships for Solid Exoplanets (http://seagerexoplanets.mit.edu/ftp/Papers/Seager2007.pdf) - journal paper

NASA ADS: The Mass-Radius Relation for Cold Spheres of Low Mass (http://adsabs.harvard.edu/abs/1969ApJ...158..809Z) -- the largest possible planet has around Jupiter's radius and 3 times Jupiter's mass

[astro-ph/0511150] Internal Structure of Massive Terrestrial Planets (http://arxiv.org/abs/astro-ph/0511150)

Inevitability of Plate Tectonics on Super-Earths - Abstract - The Astrophysical Journal Letters - IOPscience (http://iopscience.iop.org/1538-4357/670/1/L45?rel=ref&relno=6) - continental drift on large Earthlike planets

eburacum45
2013-Oct-25, 10:05 AM
...the largest possible planet has around Jupiter's radius and 3 times Jupiter's mass Of course, several detected planets are apparently significantly larger than this, because they are not cold spheres, but are 'puffed-up' by thermal expansion.
see
http://en.wikipedia.org/wiki/WASP-17b for instance

Durakken
2013-Oct-27, 06:19 AM
@eburacum45, Thanks for the links, though it does look like it'd be a good idea for me if I download the source of the planet generator so that I can alter it so that I can specify if I wan't, say, Earth gravity, I can alter that param and have it display the info. ^.^

@everyone, so no info on the whole using asteroid impacts to alter the climate thing?

eburacum45
2013-Oct-27, 10:40 AM
I don't think the effects of an asteroid strike would last very long, a couple of years at most, unless you are suggesting some periodicity in the asteroid strikes caused by an invisible brown dwarf companion or something.

Durakken
2013-Oct-27, 11:05 AM
I don't think the effects of an asteroid strike would last very long, a couple of years at most, unless you are suggesting some periodicity in the asteroid strikes caused by an invisible brown dwarf companion or something.

wikipedia says that more recent discovers suggest that there is a significantly larger dust cloud or whatever around the solar system that results in impacts at a 10x frequency. This would place 10kilometer, mass extinction, asteroids as happening roughly every 10 million years rather than every 100 million. From what I have read 10km asteroids when they strike earth, cause a nuclear winter of sorts for a few years after impact and then cause higher than normal temperatures due to dust in the atmosphere... and recent research has found that it takes roughly 10 million years to recover from said impact. This would mean that there would be a rough cycle of ten million years of higher than normal temps that slowly falls, till the next impact which then spikes down.

I'm not sure what the effects would be, but assuming gravity is 1g, the perimeter would be around 2.56x that of earth, then maybe the result would have to be reduced by the same factor of the size difference...making the strike take roughly 3 million years to recover from.