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Two habitable planets around one star?
Posted: Posted December 17th, 2009 by chiarizio

Can anyone guide me through the astrophysics, celestial mechanics, geophysics and geochemistry, of a star-system containing two roughly Earthlike planets both in the "Goldilocks" zone ("just right" for temperature and insolation)?

If the star is pretty Sun-like, I don't think it's possible to have two such planets in stable orbits unless one is at the inner edge and the other at the outer edge of the "Goldilocks zone".

I think the inner one would have, from latitudes about 45 degrees or less from one of the poles (about 45 degrees North and northward and from about 45 degrees South and southward), a climate about like Earth's about 45 degrees or less from the Equator; its tropics would be even hotter.

I think the outer one would have, from latitudes about 45 degrees or less from its equator (between about 45 degrees North and about 45 degrees South), a climate about like Earth's about 45 degrees or less from the either pole; its polar regions would be even colder.

But, does anyone actually know for sure?

And, if the star's physics are changed -- if it's more massive and brighter and hotter, but not so much more that it wouldn't last long enough -- the star's "Goldilocks zone" might be wider, and maybe two Earthlike planets' orbits could fit in and still be stable.

Right or wrong?

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I don't know about the feasibility of two habitable planets in the goldilocks zone, so can't say one way or another. If the constraint was truly one planet, it might be interesting to put 2+ large habitable moons of a gas giant in the goldilocks orbit.

Posted December 17th, 2009 by Sakir
Sakir
 

Moons. Or one moon and one planet. Or two dwarf planets. (I'd LOVE to know what kind of celestial event would cause the latter).

a star with 25% the luminosity of the Sun will have a CHZ centered at about 0.50 AU and a star twice the Sun's luminosity will have a CHZ centered at about 1.4 AU.


Posted December 19th, 2009 by Xhin
Xhin
Nature is beautiful

I'm not an astrophysicist, but couldn't you force a planet beyond the habitable region to have a habitable temperature through greenhouse gasses in its atmosphere? If you trap enough infrared radiation from the sun that your surface temperature is roughly that of Earth's, and there's still enough Oxygen in the atmosphere, and you don't have one which is too dense, why couldn't earth life thrive there?

- Baalak called Thermal.

EDIT: Sorry for the double post. I took care of it.

Posted January 7th, 2010 by baalak
baalak
 

I'm not an astrophysicist, but couldn't you force a planet beyond the habitable region to have a habitable temperature through greenhouse gasses in its atmosphere? If you trap enough infrared radiation from the sun that your surface temperature is roughly that of Earth's, and there's still enough Oxygen in the atmosphere, and you don't have one which is too dense, why couldn't earth life thrive there?

- Baalak called Thermal.

EDIT: Sorry for the double post. I took care of it.
I don't know. The brighter the star, the more room in its "habitable zone", I guess; and if Mars had Venus's atmosphere maybe it'd be habitable.
But keeping such an atmosphere (or, for that matter, an atmosphere like Earth's) depends on plate tectonics (among other things, in Earth's case); (that is, it depends on there being plate tectonics, not so much on what the plates are actually doing). Mars is tectonically dead and "cooling off".
Really the idea of two roughly-Earth-sized moons of a gas-giant in the "habitable zone" is the easiest way to work it.
But I hope you're right, Baalak. 'Cause I'd like to see that.

Posted January 7th, 2010 by chiarizio

I don't know. The brighter the star, the more room in its "habitable zone", I guess; and if Mars had Venus's atmosphere maybe it'd be habitable.


Actually, scientists theorize that at one time, when Mars did have plate tectonics (and more important, a magnetic field that protected the atmosphere from being shot into space), Mars had a thicker atmosphere and was warm enough to have liquid water. Because of this, it would have been suitable for Earth-like life to live on. It would still be on the cool side, but it would be habitable.

However, because it had relatively small moons that didn't produce much of a tidal force (unlike Earth's, which creates enough of a tidal force that it actually produces friction in the Earth's core*), Mars cooled down much faster than Earth. This caused the core of Mars to stop spinning or to slow enough that it no longer created an electric field. In turn, this allowed the ionized solar winds and solar radiation to hit the Martian atmosphere, sending much of it off into space.

With the reduced atmospheric insulation, Mars became very cold with a thin atmosphere of mostly carbon-dioxide (which is heavy and harder to get rid of than lighter gases like hydrogen, helium etc). Most of the oxygen and water that had been on Mars reacted with iron in the soil, making Mars the pretty red color it is today., but also making the planet one large desert.

TL;DR version: Mars + MOAR ATMUSFEER = coolish side of comfy for humans.

Posted January 8th, 2010 by bloodb4roses

But I hope you're right, Baalak. 'Cause I'd like to see that.


So would I. That would make the concept of an ice age on my conworld easier to work with. That, and it sounds like a great plot device.

Doesn't Venus sort of support this theory too? Even though it is farther from our sun than Murcury is, its surface temperature is hotter because of heavy atmospheric insulation. So what if we could move Venus even farther away from the sun? Its temperature might cool down but maybe not to the extent that a planet with a lighter atmosphere...

Posted January 9th, 2010 by Cerne
Cerne
 

It's not just the atmospheric density, but its composition. Carbon Dioxide is very effective at trapping infrared radiation. An atmosphere of a different chemical, even with the same density, wouldn't necessarily be so hot. You'd likely need to reduce the sunlight reaching Venus by quite a lot before its temperature decreased.

The problem with mars, as I understand it, is that it's not massive enough to hold onto a thick atmosphere. Back when it used to have a magnetic field it could probably hold onto more, yes, but that hasn't been the case in probably two billion years, maybe more.

You could have a planet revolving around a distant sun remain warm because of its atmospheric insulation , or you could have a planet around a small star be habitable for similar reasons. It's all in the mix.

If I'm wrong about this, I'll be surprised. I'm no expert, but all the information I've seen supports this line of reasoning. If I'm incorrect, I'd love to be told why.

- Baalak called Insulated.

Posted January 10th, 2010 by baalak
baalak
 

That all sounds reasonably plausible to me. I just don't want to go with something without being criticized for it while it was in its formative stage, then be criticized afterward by my public without a single reason behind the criticism. And the public will do that - they don't owe you any explanation at all, they're your audience. It's always better to get criticism before then because your work isn't officially published yet and you can demand reasoning behind the person's criticism. My thoughts, anyway. I was planning on having a smaller star for my planet's sun anyway. Regarding what you posted, this should make things more workable now.

I think with Mars it was several things, not just density. As bloodb4roses said, Mars probably needed a magnetic field created by plate tectonics as well as a denser atmosphere and a different atmospheric composition. To get the atmospheric density, you need a reasonable mass in relation to size for the planet. And for that, from what I know, you need a working planet core which should in turn give you working plate tectonics, and that gives you the magnetic field. I think it's the result of a bunch of different but inter-connected things working together. And Mars did have liquid water at one time so it was doing something right.

I had also heard that maintaining an atmosphere had to do in part with planetary rotation, but maybe I heard wrong.

Posted January 11th, 2010 by Cerne
Cerne
 

And Mars did have liquid water at one time so it was doing something right.


Well, Mars DID at one point have enough core heat to make everything else work, and a thicker atmosphere, it just cooled internally a lot faster than Earth did. Hopefully, the Earth won't cool to that point before we either kill ourselves off or learn to live in space colonies.

Posted January 11th, 2010 by bloodb4roses

Hopefully, the Earth won't cool to that point before we either kill ourselves off or learn to live in space colonies.
Don't worry, the sun will swell into a red giant and swallow Earth's orbit, incinerating everything, long before our core solidifies.

- Baalak called Optimist.

Posted January 14th, 2010 by baalak
baalak
 

Well, Mars DID at one point have enough core heat to make everything else work, ..., it just cooled internally a lot faster than Earth did.
As I understand it, a meteor strike that busted through Mars's crust and rang its mantle's bell, had something to do with bollixing up the plate-tectonics beyond any hope of recovery. Or so someone has theorized; I doubt any eye-witnesses survive.
Such an impact could have a similar effect on Earth. But the impacting object would be bigger than the K-T dinosaur-killer (the Chixculub event?).

Posted January 15th, 2010 by chiarizio

Might be worth a look:[quote:0a376ee4c5="http://en.wikipedia.org/wiki/Epimetheus (moon)"[11] the next will be in 2010. At that time, Janus's orbital radius will increase by ~20 km, while Epimetheus's decreases by ~80 km; Janus's orbit is less affected because it is 4 times more massive than Epimetheus. As far as it is currently known, this arrangement is unique in the solar system.


Posted January 1st, 2011 by intermundi

The timing and magnitude of the momentum exchange is such that the moons "trade" orbits, never approaching closer than about 10,000 km.


Oh, WOW 😮 😯 😲 :shock: :o :!: , intermundi!!!

Posted December 30th, 2018 by chiarizio
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