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Wolf1066
2012-Feb-21, 07:17 AM
I've been musing on what it would be like to be on a planet in the Alpha Centauri system with its two suns but it has rapidly become apparent that my grasp of physics is not up to the task.

Such things as where the habitable zone would be, how large, what the planet's orbit would be like (assuming roughly earth mass) how long the year would be, how the second star orbiting further out affects it and so forth confuse me greatly.

Could people please provide figures that would be possible/plausible for an Earth-like world (able to support human life if a ship ever reached it).

Would it still need a large moon to moderate the atmosphere? What would the tides be like on that world? What would be a realistic rotational period for the planet? How much light would it get from the secondary star? What would the moon's cycle look like?

Ideally, I'd like to be able to write up a computer program that models where the planet is in relation to both stars at any given time and thus get an idea of what it would look like if you were on the surface of such a world for one of its years.

Thanks.

astromark
2012-Feb-21, 07:38 AM
Good old fashioned ' I do not know.' and with a few points as to why... we do not yet know...It's a ?

The plain of debris of the planetary disk seems to contain a star. BOTH stars are orbiting about a central centre of mass.

It's Not edge on to us. We do not see any auscultation.. so we can not see any planets..

In the green zone of both stars could a extraterrestrial planet be.. maybe. just MAYBE.

Having a moon has played a roll but is NOT absolutely required for a life planet..

but good arguments show it may have helped here.. stability and tides.. the second star could provide that..there.

As to how it might look.. Its a fiction work but.. think of Jupiter as a second sun.. for half the year a bright daylight star..

and when viewed at night.. very bright.. spoiling the view of other stars..

and NO. I do not know these things.. but you asked.

novaderrik
2012-Feb-21, 08:05 AM
another thing to think about- if there were habitable planets orbiting around each star, how sophisticated would any life on either or both of them need to be before they noticed that there was life on the other planet, and what would be the odds of both species becoming aware of each other at around the same time?

swampyankee
2012-Feb-21, 12:11 PM
Determining the stability of the hypothetical planet in its orbit would be the first concern, but as this is non-trivial (there are ways of determining the stability of some systems without solving the ODE or PDE involved, but the papers I've read have stated these do not work for gravitationally-bound many systems), you may want to just use some rule of thumb.

Presuming stability, climate would be the second concern. For a planet orbiting a star like Alpha Centauri A, the influence of "B" would not be negligible -- it would vary from about 2% of the planet's energy input to about 0.1% -- and this would certainly have an interesting effect on climate.

antoniseb
2012-Feb-21, 12:54 PM
An Earth-like planet around the the Alpha star would have orbital perturbations similar to the ones we get from Jupiter, but their influence would be about 200 times stronger. I don't know if this means that the requirements for a stable orbit are too strict for any such planet to be likely. I suspect it means that when these stars were in the planet forming stage that their disk material had more distinct rings than ours did, and the resulting planets (from the same amount of initial material) would have been smaller and more plentiful... but that is a gut feeling, and not based on any simulation.

StupendousMan
2012-Feb-21, 01:31 PM
A recent paper on astro-ph investigates the possibility that a planet around Alpha Cen B might be habitable.

Oscillations in the Habitable Zone around Alpha Centauri B (http://arxiv.org/abs/1202.1265)

Abstract: The Alpha Centauri AB system is an attractive one for radial velocity observations to detect potential exoplanets. The high metallicity of both Alpha Centauri A and B suggest that they could have possessed circumstellar discs capable of forming planets. As the closest star system to the Sun, with well over a century of accurate astrometric measurements (and Alpha Centauri B exhibiting low chromospheric activity) high precision surveys of Alpha Centauri B's potential exoplanetary system are possible with relatively cheap instrumentation. Authors studying habitability in this system typically adopt habitable zones (HZs) based on global radiative balance models that neglect the radiative perturbations of Alpha Centauri A.
We investigate the habitability of planets around Alpha Centauri B using 1D latitudinal energy balance models (LEBMs), which fully incorporate the presence of Alpha Centauri A as a means of astronomically forcing terrestrial planet climates. We find that the extent of the HZ is relatively unchanged by the presence of Alpha Centauri A, but there are variations in fractional habitability for planets orbiting at the boundaries of the zone due to Alpha Centauri A, even in the case of zero eccentricity. Temperature oscillations of a few K can be observed at all planetary orbits, the strength of which varies with the planet's ocean fraction and obliquity.

catloaf
2012-Feb-21, 03:41 PM
You don't need to write up a computer program because one already exists on Steam. Universe Sandbox and it's cheap too.

chornedsnorkack
2012-Feb-21, 03:51 PM
Such things as where the habitable zone would be,

Defined by square root of bolometric luminosity of the star. 1,23 AU for A, 0,71 AU for B

how large,
Unknown in Solar System, therefore unknown in Rigil Kentaurus.

what the planet's orbit would be like (assuming roughly earth mass)
The mass being small compared to star mass has small effect on orbit.

how long the year would be,
For A, roughly 1,3 years (475 days). For B, roughly 0,62 years (226 days - close to Venus).


how the second star orbiting further out affects it and so forth confuse me greatly.

Sun affects Moon. Expect rapid changes of orbital plane and apside line.


Would it still need a large moon to moderate the atmosphere?
It is not clear whether Earth does.


What would the tides be like on that world?
Lower bound is set by the tides due to star. Upper bound by tidal evolution of satellite, if any.


What would be a realistic rotational period for the planet?
Arbitrary.
Earth rotates in 24 hours. Venus in 243 days. Ceres in 9 hours. A habitable exoplenet can have anything in between, slightly faster, or far longer.

How much light would it get from the secondary star?
Here, the bolometric correstions of A and B relative to Sun will matter. At most (planet of B at periapse of A) about 1,5 % sunlight. At least (planet of A at periapse of B) about 0,04% sunlight.

What would the moon's cycle look like?

Depends on the moon. The orbital period of moon is constrained between Hill and Roche limits (actually, the limit is defined well inside of Hill limit).

catloaf
2012-Feb-21, 04:18 PM
I love you, chornedsnorkack.

transreality
2012-Feb-21, 10:07 PM
Just speculating.. I wonder if the planetary migration of any gas giant initially forming around one of the stars of the binary, would be outwards into an orbit surrounding both stars. I think this is the outcome of 'Kozai migration'. But such an orbit would be highly eccentric. Such an orbiting gas giant would be highly disruptive to any smaller planetismals and planets around either of the stars. Possibly meaning that if they formed they would be exposed to periodic and severe cometary bombardment. So they may be effectively uninhabitable, if they themselves are not flung out beyond the habitable zones themselves. Possibly an eccentric gas giant would prevent formation of planets, and disperse the planetismals into a widely scattered oort cloud. Such a gas giant may be detectable during the close part of its orbit, but if it is currently in a wide part of its orbit where it would be spending most time, it could be difficult to detect.

tony873004
2012-Feb-22, 01:28 AM
The orbits of Mercury, Venus, Earth, and Mars would do just fine if superimposed around stars A or B, provided that they orbit in roughly the same plane that A and B orbit each other. Otherwise the Kozai mechanism will heavily perturb their orbits. Here's a link to a discussion on this: http://www.orbitsimulator.com/cgi-bin/yabb/YaBB.pl?num=1162874858/

swampyankee
2012-Feb-22, 02:14 AM
You don't need to write up a computer program because one already exists on Steam. Universe Sandbox and it's cheap too.

...only if you're in the Windows world.

Wolf1066
2012-Feb-22, 06:32 AM
Thanks a lot for the answers, a lot food for thought here Especial thanks to chornedsnorkack for that very informative post.

chornedsnorkack
2012-Feb-22, 07:01 AM
Further analogy is Toliman vs. Errai.

Toliman AB orbital period 79,9+-0,1 years, masses 1,1 and 0,9 solar, eccentricity 0,518
Errai AB orbital period 68+-1 years, masses 1,4 and 0,4 solar, eccentricity 0,41

Errai definitely has Ab, period 2,5 years, mass at least 1,6 times jovian, eccentricity 0,1.

So a planet as far as Mars should be perfectly stable on either Toliman component unless its inclination allows Kozai mechanism.

We see Errai Ab, have since 1988. What are the current upper bounds on masses of Toliman planets?

catloaf
2012-Feb-22, 11:18 AM
...only if you're in the Windows world.

Well you can run this program and Steam in WINE. If you're on a Mac, why would you be running anything else other than Puzzle, Sherlock, or Photoshop anyway?

swampyankee
2012-Feb-22, 11:54 AM
Well you can run this program and Steam in WINE. If you're on a Mac, why would you be running anything else other than Puzzle, Sherlock, or Photoshop anyway?

Because it's a hand-me-up.

amensae
2012-Feb-23, 03:59 PM
On the issue of whether planets can maintain long-term stable orbits in a binary star system, the jury is still out; but a rough rule of thumb I have used is –
If the orbital period of the stars is less than a few days or more than 50-70 years, AND the stars’ mutual orbits not too elliptical, AND the planetary orbit is coplanar with the stars’ orbits (i.e. the angle of the orbital planes is not too oblique), then the perturbations are minor and the planetary orbit is stable.

eburacum45
2012-Feb-23, 09:02 PM
On the Orion's Arm worldbuilding site we are using a rough 'rule of three' guide; if the planet orbits only one star, then the other star must orbit at least three times as far away as the planet does.

If the planet orbits both stars, then the planet must be at least three times as far away as the mutual separation of the stars. We did have a larger figure of 3.5 for this latter value but the recent discovery of Kepler 16b has forced us to revise it downwards.
http://en.wikipedia.org/wiki/Kepler-16b
The planet orbits 3.1 times as far out as the separation between the two stars.

This works best for coplanar orbits with low eccentricity, by the way.

transreality
2012-Feb-25, 12:29 AM
It seems that gas giants (in particular) form close to their stars, and migrate out tossing any planetismals inwards. So any stable distant orbits have to be evolved towards during the systems early history. The separation of Alpha Centauri A and B is somewhere 11-37 AU, so the outer orbits wll be cold and uninhabitable. The inner orbits out to about 1.5AU, maybe 3-3.5AU at the most, may be possible, but any gas giant forming close would have had to migrate though these inner orbits. Then the orbit that such a migrating gas giant forms is going to highly eccentric with respect to either star or the mass centre of the system. And still be periodically close to either. If such a gas giant is present in the system, it may preclude any inhabitable planet. It may also be possible that the presence of such a gas giant is necessary to concentrate the planetismals into the inner systems where they can form a earthlike planet.

Wolf1066
2013-Jan-15, 03:41 AM
A bit of a thread necro, but I've finally gotten around to working on this project again.

Yes, yet another story set on a planet at Alpha Centauri, but it is a very plausible and nearby contender for having Earth-like planets in the HZ

So based on what was written above, I'm thinking:

Orbiting Alpha Centauri A at around 1.23AU with an orbital period of roughly 475 Earth days.
Orbit is close to circular, like Earth's, providing an average temperature difference of around 4degC between periapsis and apoapsis.

I've been looking at another thread here (http://cosmoquest.org/forum/showthread.php/93215-Weather-on-a-planet-with-no-axial-tilt) and was thinking of a 179 degree axial tilt (or 1 degree retrograde) based on the rotation of AC A and other planets in the system, which means weather's going to be mostly dependent on geography and the minor changes in temperature due to orbit.

So I was thinking of a moon in the same orbital plane plus or minus a degree to stabilise the axial tilt.

The planet I would be of similar density to Earth, having the same iron/nickel core, magma etc. but I was thinking of about 97% of the mass of Earth.

I freely admit that I gave up on physics years ago in favour of sciences that were less math-intensive.

Am I right in figuring that the gravity would be about .97g given the same density and 97% mass?

What size moon would be needed to provide stability of the planet's axis and what would its most likely orbit be?

I thought - to avoid too much "like Earth this and like Earth that" - that I'd have the rotational period of the planet being about 146.6 hours, or a little over 6 Earth days, then it occurred to me that the slower spin might affect the planet's ability to generate a life-protecting magnetic field.

Would that be the case? If so, what rotational period is required to cause a magnetic field powerful enough to protect Earthlike life (i.e. human colonists).

I'm naturally presuming copious amounts of liquid water and large enough bodies of both water and land to create climate differences, oxygen/nitrogen atmosphere.

Would it be implausible, given how much older the AC system is than our own solar system, that life on the planet would be only equivalent to around that of early Cenozoic? I figure a lot would depend on how long ago the planet gained sufficient liquid water (presumably from comets perturbed by AC B) and all the other bits fell into place to spark off life.

John Mendenhall
2013-Jan-15, 06:09 AM
Presuming the moon is the result of the impact of a Mars sized body, as Earth's Moon, then adjusting the time of the impact lets life be at any stage. Also takes care of many other problems, retrograde tilt, day length, surface composition, etc.

Well thought out idea for a story. Good luck!

Regards, John M.

chornedsnorkack
2013-Jan-15, 07:45 AM
The planet I would be of similar density to Earth, having the same iron/nickel core, magma etc. but I was thinking of about 97% of the mass of Earth.

I freely admit that I gave up on physics years ago in favour of sciences that were less math-intensive.

Am I right in figuring that the gravity would be about .97g given the same density and 97% mass?

No.
In case of same compressed density, obviously 0,99 g.
In case of same composition (same uncompressed density) less, but exactly how much less depends on the equation of state, which turns out to be not well known.


I thought - to avoid too much "like Earth this and like Earth that" - that I'd have the rotational period of the planet being about 146.6 hours, or a little over 6 Earth days, then it occurred to me that the slower spin might affect the planet's ability to generate a life-protecting magnetic field.

Would that be the case? If so, what rotational period is required to cause a magnetic field powerful enough to protect Earthlike life (i.e. human colonists).


Venus does not have magnetic field - but Mercury, rotating in 59 days, does.

Wolf1066
2013-Jan-15, 08:49 AM
Thanks, guys.

I'm trying to go for something plausible, so I don't have major WTF moments in any of the more-scientifically-inclined readers (I've read books that actually made it to print that made me cringe, and I'm not the most physics-oriented person, but I don't want to add to their numbers).

I don't want the place to be too inhospitable, like way too much or too little gravity but I don't want adapting to it to be "oh, yeah, and the day's like a whole forty minutes longer so I can cope with that."

It also needs to be some place that humans might believably want to colonise (which rules out anything too inhospitable).

For the purposes of my story, there needs to be carbon-based life that humans could conceivably eat (and conversely, could conceivably eat humans) which will mean local bacteria and viruses may find us suitable hosts.

The life might not look a lot like Earth life, but I would presume that there would be a stable ecology (at least until humans came along and set up colonies) with plant life, insects, aquatic creatures, amphibians, mammals, lizards, avians all occupying niches in the ecology. No doubt the diversity of mechanisms would be at least as wide as Earth's.

chornedsnorkack
2013-Jan-15, 09:14 AM
For the purposes of my story, there needs to be carbon-based life that humans could conceivably eat (and conversely, could conceivably eat humans) which will mean local bacteria and viruses may find us suitable hosts.

Very different things.

Viruses depend on the host genetic code. They depend on universality of genetic code on Earth.

There are reasonably good reasons why most of the 20 amino acids are these ones. No such reasons for genetic code-amino acid matches. Mitochondria, with their tiny genome, have been able to change the meaning of a few codons.

If a virus encounters a would-be host which consists of exactly the same 20 amino acids and where 60 codons have exactly the same meaning, but a few are differently assigned, then these few codons would when encountered screw up some vital virus protein - game over for the virus.

Whereas the bacteria could not care less about host genetic code - if the composition of host proteins, lipids and hydrocarbons are familiar, they are fine.


The life might not look a lot like Earth life, but I would presume that there would be a stable ecology (at least until humans came along and set up colonies) with plant life, insects, aquatic creatures, amphibians, mammals, lizards, avians all occupying niches in the ecology. No doubt the diversity of mechanisms would be at least as wide as Earth's.

Mammals, lizards and avians with their niches is peculiarity of Earth Cenozoic. The dinosaur extinction was, for one, nothing inevitable or logical.

Wolf1066
2013-Jan-15, 09:43 AM
Cheers, thanks for that.

So given local proteins etc that we can metabolise, otherworldly carnivores and bacteria aren't going to be too fussy when it comes to eating us but viruses may find us too alien?

Good point re the Cenozoic. So basically, I need to start with whatever spawned in the oceans and work out a logical progression from there, factoring in what might plausibly happen on the planet - given its different climate and conditions to ours - to work out what's roaming around on the planet by the time we discover the place billions of years later.

Van Rijn
2013-Jan-15, 09:54 AM
Cheers, thanks for that.

So given local proteins etc that we can metabolise, otherworldly carnivores and bacteria aren't going to be too fussy when it comes to eating us but viruses may find us too alien?


Unless the life there had a common origin with Earth life, both are unlikely. But viruses that could affect us? Unlikely even if there had been a common origin (viruses have to evolve with their hosts).

Wolf1066
2013-Jan-15, 11:50 AM
So the only reliable option for food would be for the colonists to bring crops and livestock from Earth and let our own plants do the job of photosynthesising under AC A's light and turning atmospheric CO2 and nitrates and other minerals from the soil (presumeably compatible, mineral-wise, if the planet "fit the bill" for human habitation) into something we and our livestock could eat?

Aside from vast amounts of imported rations, what could colonists do to nourish themselves on an alien world given sufficient air, water, mineral resources. I'm presuming turning local trees into charcoal and synthesising carbohydrates wouldn't work.

swampyankee
2013-Jan-15, 12:48 PM
Aeroponic and hydroponic "farms." They would have had them on large space ships which required long travel times, and certainly in the non-terrestrial habitats that would have existed in the Solar System before any interstellar travel would (could) be attempted.

Nick Theodorakis
2013-Jan-15, 12:55 PM
It would be make sense for colonists to bring crops and livestock, but, as others have pointed out, if their proteins are based on the same amino acids ours are (or at least most of them), with same linkages, we could probably eat their protein. Alien carbohydrates may be a different story. Keep in mind we can't even eat most of the carbohydrate mass on earth.

Others have pointed out that viruses are too closely tied to the genetic machinery of their host to be a threat to us. By analogy, it would be like a computer virus designed for Windows 7 infecting a VAX computer. Or using a Mac Powerbook to bring down an alien computer.

Nick

Wolf1066
2013-Jan-15, 02:46 PM
Or using a Mac Powerbook to bring down an alien computer.
But... but... that like totes happened! :p

Based on the info here and the World Builders (http://www.world-builders.org/index.html) site, I've come up with the following:

Primary: Rigel Kentaurus A
Orbit: 1.23AU
Eccentricity: 0.0168 – Temperature variance approximately +4C at periapsis cf apoapsis*
Year: 475 Earth days.
Axial Tilt: 179/1 Retrograde
Mass: 0.803 Earth
Density: 5.51g/cm
Radius: 0.93 Earth – 5931.54km
Gravity: 0.912
Day: 146.6 Hours (6 days, 2.6 hours)

*Based on it being at the “Earth Equivalent” orbit and having a similarly small orbital eccentricity and a quoted orbit-related variation of 4C for Earth.

Moon:
Density: 3.28g/cm
Diameter: 3627km
Gravity: 0.169
Orbit radius: 365459.4 (57.3 Earth radii)
Inclination to ecliptic: 0 1
Orbital period: 28.235 Earth days

Any glaringly obvious foul-ups with my figures?

BioSci
2013-Jan-15, 05:16 PM
if their proteins are based on the same amino acids ours are (or at least most of them), with same linkages, we could probably eat their protein.

Nick

That would be a very big IF - there are many possible different amino acids - in fact there are also several amino acid analogues (non-protein amino acids) that are made by various plants that are rather toxic.

There is no reason that a separate life genesis would use the same amino acids to make proteins (if it even made 'proteins') and a very strong probability argument that they would not.

Of course, a large alien carnivore may still kill and eat humans and the really nasty alien "parasites" suck our blood :eek:- even if we are not digestible or are deadly poison.

If we wanted to eat the alien, significant processing or even hydrolysis and purification of constituents may be required.

Wolf1066
2013-Jan-15, 09:16 PM
This is why I love this place. So many different scientific fields meet here.

chornedsnorkack
2013-Jan-15, 09:38 PM
About the choice of the coded amino acids...

Informed guesses from 1981:

http://physwww.mcmaster.ca/~higgsp/3D03/WeberReasons.pdf.

BioSci
2013-Jan-16, 12:45 AM
About the choice of the coded amino acids...

Informed guesses from 1981:

http://physwww.mcmaster.ca/~higgsp/3D03/WeberReasons.pdf.

Yes, that paper reflects the early ideas regarding abiogenic amino acid selection from a pool of organic soup...unfortunately much of this early work was not based on hard data but more of an 'adaptive storytelling' - that while plausible is not really supported by any real data or biochemical insight.

More recent thinking and further data on alternative possible ways for life to occur (such as RNA-world, replication first, of combination of replication and chemistry evolving together) has suggested that the current set of 20 amino acids is partly a simple contingent selection of amino acids with useful chemical properties (most likely selected for simple molecules and from related biochemical pathways).

see: "On the evolution of the standard amino-acid alphabet" Genome Biology 2006, 7:102

But numerous lines of evidence, from abiotic chemistry to protein engineering, combine to indicate that this alphabet could potentially have consisted of fewer, more, or just plain different amino acids..

There have been other more recent papers to consider these aspects - unfortunately it is difficult to formulate strong tests of these various hypothesis and we are essentially left with plausibility arguments.

Wolf1066
2013-Jan-17, 10:02 AM
How much of a stretch would it be for plant and animal life on an Earth-like exoplanet to be both edible and nutritious to humans? What is the likelihood, say percentage chance, that life evolving elsewhere would be able to provide us with the appropriate carbohydrates/proteins?

Van Rijn
2013-Jan-17, 10:17 AM
How much of a stretch would it be for plant and animal life on an Earth-like exoplanet to be both edible and nutritious to humans? What is the likelihood, say percentage chance, that life evolving elsewhere would be able to provide us with the appropriate carbohydrates/proteins?

But with the same origin? I'd think it would be extremely unlikely. Even with the same origin, it would evolve on a different world. We have evolved to be able to eat a wide range of food on Earth, but there is still quite a bit of stuff we can't do much with, and other stuff that is downright dangerous. This would be life that we never evolved to eat. MAYBE you could find or adjust some bacteria to do something with it.

If it didn't have the same origin, bacteria may have trouble . . . though if there is an energy source, they might manage something.

Wolf1066
2013-Jan-18, 01:36 AM
OK, so given they have some means of growing their own food and raising stock - presumeably under artificial conditions given the planet's long night-time periods - what would colonists living on that planet experience?

I'm presuming that the moon orbiting in the same plane as the planet would result in regular monthly eclipses (both stellar and lunar), which due to the planet's negligible tilt will be always full in the equatorial region and always partial as you increase/decrease latitude.

Temperature-wise, it'd tend to cooler towards the poles and warmer towards the equator, which no doubt would create climate, as would the difference in absorbtion by land and sea (assume roughly the same ratio as Earth give or take about 5%) and the temperature differences between day and night sides.

Am I right in presuming that the longer day (which, given the tilt is going to have equal-length light and dark periods), is going to cause colder temperatures at night due to the longer period (around 3 Earth days) in darkness? With variations depending on cloud cover, latitude etc?

Weather factors: Day/Night temperature differences, latitude differences, sea/land differences, terrain, ocean currents, periapsis/apoapsis differences.

Anything I've missed? It seems from other things I've read that AC B would have negligible effect even at closest approach.

I'm guessing weather would probably seem to "come out of nowhere" to humans used to regular seasons and follow no easily calculated patterns.

Twilight. I suspect there would be very little variation of twilight time throughout the planet's year. How would I calculate the amount of time for twilight at various latitudes? I'm presuming it's likely to be a fair bit longer than for comparable latitudes on Earth.

chornedsnorkack
2013-Jan-18, 11:57 AM
Note that defining a "habitable zone" is speculation. It is not actually known in Solar System how much less or more illuminated a planet could be compared to Earth and still be habitable.

Estimating Earth equivalent illumination is on much firmer ground:

1) Take square root of bolometric luminosity. That gives the distance to Earth equivalent illumination.
2) Raise the distance to cube, divide by the mass of the primary, take square root of result. This gives the orbital period.

Wolf1066
2013-Jan-18, 01:35 PM
Thanks for that.

I'd already found the MP^2=a^3 formula. It was the luminosity equation I couldn't find.

The formulae I found for HZ seemed to define it as where there would be liquid ground water - between the temperature extremes of 0 and 100 degrees Celsius.

Of course, the possibility of liquid water under the frozen surface of more distant moons is outside of that rather simplified definition...

And 0-100 degrees is a lot of territory. Life might well exist between those extremes, but I personally would not like to live near either for any great length of time. Especially the top end.

chornedsnorkack
2013-Jan-18, 10:23 PM
The formulae I found for HZ seemed to define it as where there would be liquid ground water - between the temperature extremes of 0 and 100 degrees Celsius.


But the temperature is impossible to derive from illumination, because it also depends on albedo and greenhouse effect. Earth temperature notoriously has changed in a large range in last 20 000 years, while illumination was constant.

Wolf1066
2013-Jan-19, 02:44 AM
The pages I found for HZ were:

http://www.planetarybiology.com/calculating_habitable_zone.htm

http://www.astronomynotes.com/lifezone/s2.htm

The latter of which acknowledges it's an imprecise calculation that does not factor greenhouse, albedo or the possibility of tidal heating of Jupiter's moons.


Does anyone have any thoughts on post #37?

I presume that the moon's orbit needs to be prograde with respect to the planet's rotation (which itself is retrograde compared with the rotation of AC A and other bodies orbiting AC A) to avoid problems with tidal effects.

Disinfo Agent
2013-Jan-19, 06:42 AM
Curiously, in the year since you asked your original question, a planet was discovered around Alpha Centauri B (http://blogs.discovermagazine.com/badastronomy/2012/10/16/alpha-centauri-has-a-planet/#.UPo-4Ge8GSo).

Wolf1066
2013-Jan-19, 08:04 AM
Curiously, in the year since you asked your original question, a planet was discovered around Alpha Centauri B (http://blogs.discovermagazine.com/badastronomy/2012/10/16/alpha-centauri-has-a-planet/#.UPo-4Ge8GSo).
Which means that my prediction of future events was almost as inaccurate as the Mayans' :p

Disinfo Agent
2013-Jan-19, 03:49 PM
Take it as a badge of honour. All good science fiction authors made predictions that didn't pan out - and they're still fun to read. Being right all the time about the future is for amateurs like film critics, economists, and cult leaders. ;)

Wolf1066
2013-Jan-20, 03:09 AM
As Agent 86 was so fond of saying, "missed it by that much."

"That much", in this case being around 24AU and orbiting a different body in the system.

The news does, however, fill me with hopes for the plausibility of the in-story time-scale, as I was planning on having the Earth-sized world around AC A discovered in 2015 (incidentally, a few months after the confirmation of (extremophile) life on Enceladus).

The discovery of an Earth-sized planet over four light years away within the last year has removed my one concern about the timing - that 2015 might be "too optimistic" a date for the discovery of a planet that size over that great a distance.

In my story, 2015's going to be a great year for astronomical discoveries.

CaptainToonces
2013-Jan-20, 05:25 AM
Does this mean that the orbital plane of this planet is head-on with our telescopes on Earth? How close to head-on is it? Is that known? Is it the same as B's plane of rotation?

How does the plane of orbit relate to the plane of orbit that A & B orbit each other on?

Wolf1066
2013-Jan-20, 06:49 AM
I'm not sure of any of those things, that's why I'm here to learn.

I was thinking that it'd be roughly in the same plane that B orbits A, but if that doesn't work, that's the sort of thing I'm trying to learn.

That's why I put up what I've got so far - so those of a more physics-minded nature can pick holes in what I've got and say useful things like "Nah, that'd never work, the moon'd have to be blah blah blah to keep it stable" or "it needs to be so-and-so to be detected from Earth" or whatever.

I'm presuming that it's at an angle to us that is detectable given improved telescopes circa 2014/2015. Perhaps another space-borne telescope.

Van Rijn
2013-Jan-20, 10:45 AM
Which means that my prediction of future events was almost as inaccurate as the Mayans' :p

Well, I wouldn't bet too much on the current planet candidate. The claim is based on one group's data, and they claim they noticed a periodicity that's buried deep in noise. I'd really like to see some solid confirmation on this. I wouldn't say it's well established.

See this post, for instance, and the summary link.

http://cosmoquest.org/forum/showthread.php/138824-Wunderland!!!!-Alpha-Centauri-planet?p=2074248#post2074248