Just inside the Goldilocks Zone

Let’s say we have an Earthlike planet, but it’s JUST BARELY too close to its parent star to be inside the Goldilocks Zone. (Like, oh, say, Earth, in a billion years when the sun warms up. :eek: ) [EDIT: For the uninitiated, the Goldilocks Zone – also called the Comfort Zone – is the range of distances from a star at which an earthlike planet could support life on its surface. The usual criterion is the ability to have liquid water on the surface.]

Is it possible, given that probably has some kind of an atmosphere, for the tropics to be too hot for liquid water, but NOT the upper latitudes? I.e. could we have a baked desert belt around the equator but oceans farther north and south?

Or would being too close for liquid water ANYWHERE on the surface also mean that the water vapor would be heated to the point where it reached escape velocity, thereby eventually draining the planet of ALL water, liquid or otherwise?

This, I think. Though it may have had a few million years when life could have been sputtering along before then.

It’s not quite the same thing, but the Permian-Triassic extinction event 250 million years ago is thought by many to be a very similar scenario. Basically, global warming made the area around the equator too hot to support life, and the only things that survived lived near the poles. The equator region wasn’t so hot that it boiled water, though. The temperatures I’ve seen in articles tend to be closer to around 60 deg C. Still pretty hot, but not quite to water boiling temperatures. The oceans weren’t dried up around the equator, but the land in the “dead zone” was certainly a baked desert.

Nobody knows for sure, it’s all speculation. One theory said a lower-gravity planet is likely to bleed off its water in that way at a faster rate, so bigger planets will be more oceanic and smaller ones more desert.

For the “baked Alaska” scenario, it depends on topography. One could speculate on a configuration like earth, where the water pours into the tropics, turns to steam, and we end up with a highly reflective cloud cover and a tropic that’s more steamy amazonian jungle than sahara desert… The cloud cover would also moderate the tropic temperatures…

there was an episode many eons ago where the siberian plateau exploded in volcanoes; the carbon dioxide and resulting greenhouse effect killed over 90% of the life on earth, the oceans mostly died from excessive carbonate acidity, the tropics were too hot for most life.

AKA The Great Dying.

I suppose if the equatorial regions were all very high latitude so that there wouldn’t have been any oceans there even if the temperature was cooler you might have two separate temperate regions that could keep their water because water wouldn’t be entering those regions to evaporate.

You might then also have two different types of life in the two regions that didn’t interact/compete.

Too late to edit. read altitude there not latitude.

Piggy-back question - what about a tidally-locked planet, where the daylight side was too hot, the dark side was too cold, but the twilight band was a ‘Goldilocks’ temperature? Could it support life long term?

Microbial, sure. Underwater? Maybe. Land? Seems unlikely. The weather on that planet would have to be ferocious.

I had assumed there was a distinction between water evaporating from equatorial oceans, and water actually escaping from the atmosphere.

Is the inner boundary of the Goldilocks Zone supposed to be the point at which the surface temperature is too hot for liquid water, or is it supposed to be the point at which water vapor would escape into interplanetary space out of the atmosphere entirely?
EDIT: I realized, after typing this, that “too hot for liquid water” depends entirely on the atmospheric pressure. I’d guess a world with Venus-like atmospheric pressure (90 times Earth’s sea level pressure at the surface) could have liquid water at MUCH higher temperatures than we could here.

There was a book that talked briefly about this - either What If the Earth Had Two Moons, or What If the Earth Had No Moon/What If the Moon Didn’t Exist (depending on the printing).

The basic gist of it was that the tidal pull of the sun (or of a planet, for a tidally-locked moon suited for life - think Pandora from Avatar) would make a significantly raised band of really friggin steep mountains and ice around the whole world at the “edges” of the area facing the larger body. You can see a minor example of it on Iapetus, actually. He explained why, but I don’t remember the technical parts. The gist was that if there was water, it was all iced into that band, BUT, even if it was a tidally locked situation, there’s still a bit of long-term “wobble” that might make the band somewhat (really miserable but possibly) habitable due to sublimation of the ice and the shifting of area that was in the sun or not.

The question with a tidally locked planet was whether the atmospher would eventuially come to the dark side (sorry) and being below the liquid/solid temperature of the atmosphere, it would all condense and freeze there… or would atmospheric circulation from warmer areas provide enough heat to prevent a total atmospheric freeze? Would glaciers (ice or air) growing out from the dark side squeeze into the goldilocks zone, on into the sunlight and melt and evaporate, providing a passable environment?

Allegedly ato one point, the arctic was warm enough that dinosaurs roamed the shores up from Montana to Alberta and Alsaska, where huge forests grew, despite 6 months of dark up north. So maybe the proper circulation will keep things moderately warm.

There are a couple of problems with a planet tidally locked to a star.

The star would have to be a red dwarf, otherwise a planet roughly the size of Earth would be too far out to be tidally locked. And red dwarfs tend to be a lot more variable, so you’d get extreme cold periods and extreme hot periods and massive solar flares depending on the solar weather.

Also, red dwarfs are a lot less energetic than main sequence stars, so a planet with roughly the average temperature of Earth would have a lot less solar energy for photosynthesis. And if life is confined to the terminator, then that light is going to be at an extreme oblique angle, resulting in even less energy.

But the biggest problem is that the atmosphere would superheat on the hot side, and then condense as solids on the cold side. Not just water, but oxygen and nitrogen and CO2 and methane. The cold side is really cold, as cold as Pluto. So even in a situation with glaciers on the dark side that gradually slump into the light side and get melted, there would probably be almost no atmosphere since the vast majority of volatiles will be sequestered on the dark side.

Really? Aren’t you instead going to have tremendous circulation between the two hemispheres? I would think you would have a ridiculous convection current flowing from the cold side to the hot side and then back, at hurricane force.

But what happens to volatiles on the dark side? They freeze. And when the freeze, they precipitate. And then what happens? They sit there forever.

So when the planet first becomes tidally locked, you’d have a lot of moving volatiles. But the volatiles all get baked off the hot side and frozen onto the cold side. And then, no more atmosphere, and therefore no more storms.

Assuming that the hurricane-force convection currents don’t carry enough heat to the dark side to keep it above freezing.

Except the ridge on Iapetus is equatorial – that is, it isn’t at the edge between the Jupiter-facing side and the far side, it’s running right down the middle of one side.

Yep, and it’s not that high, and it only goes halfway round anyway. The article I read most recently suggests that the current going theory is that a moon of Iapetus or a captured asteroid basically slammed itself into the surface along its trajectory after its orbit deteriorated.

I was (sloppily) referring to it more for the *shape *than for any other reason, as (at least to me when I first encountered the concept) a huge-ass raised band around a planetoid was a little hard to picture. Apologies for any resulting confusion.

I can’t add much here, but thought some of you, who seem to enjoy speculating about planetary configurations would like a book called Medea: Harlan’s World.

It came from a set of seminars Harlan Ellison was asked to give at UCLA in the '70s. He invited other writers to join in and they all, with the students, designed a world they could all base some stories in.

The planet they invented was quite weird, but seemed to make sense. The stories the writers wrote there weren’t really great, but the worldbuilding parts of the book were pretty cool. I wonder if their assumptions hold up to today’s science.

I can see this happening to water, but not air; after all, Earth’s atmosphere doesn’t precipitate out over the polar regions, even Antarctica, and in this case, the atmospheric circulation also inhibits heat transfer (I doubt a polar vortex would form over half of a tidally-locked planet since there is no rotation). It also depends on how dense the atmosphere is, but too dense and the greenhouse effect on the sunlit side would be too strong and cause the atmosphere to escape until it fell.