Defining the Habitable Zone

TheoA wrote:

They do mention that complex life of any kind is unlikely.

The same was said about planetary systems arround other stars
I rather keep an open mind about what an habitable zone is. One thing we should always keep in mind is that defining habitable zone as the zone where the water can be in liquid form due to the star calorific contribution is misleading. There can be other sources of heat (like tides , radiactive decay ) or other factors that should be accounted for , as the fact that water rarely is found pure. A mixed water/amonnia ocean could remain liquid at very cold temperatures. Runaway greenhouse effect is another way to extend the habitable zone. More exotic mix could be devised to more extreme neviroments

Happy New 2010 !

Yes, if Earth, for example, had air pressure of 5 or 10 atmospheres and higher concentration of CO2, maybe it could orbit the sun in the same position of Mars, and maybe Earth and Mars forming a trinary system, Earth and Mars orbiting each other in a central barycenter, and the Moon orbiting both.

Due to the presence of Earth and Moon, Mars would be active and maybe complex life could exist in its surface. Due to volcanism and life, it would have a thicker atmosphere.

Water being very salty could allow life to live at lower temperatures, while Earth, even with high air pressure, would have the same temperature due to being in the same distance as Mars from the sun.

Earth, due to being close to Mars, would have higher volcanic activity and have a thicker atmosphere.

So there will be planets that even being a bit far from the habitable zone, will have complex life due to greenhouse effect. Or maybe life living in low temperatures in planets that have very salty water or water mixed with ammonia and maybe in pure ammonia. Or maybe even in ethane seas.

And maybe there will be planets more close to the star, but due to high albedo due to extensive cloud cover, reflecting most radiation a planet receive from the star, it could be habitable, even being close, but not too close, or it will have slow rotation like Venus. Such a planet would have to have a fast rotation. It would be or in the same position of Venus or between Earth and Venus.

Remember, if Earth was colder, it would have less clouds. If Earth was hotter, it would have more clouds, serving as a negative feedback to avoid excessive heat.

Diakonov wrote:

Earth, due to being close to Mars, would have higher volcanic activity and have a thicker atmosphere.

What is your reasoning behind this?

Diakonov wrote:

So there will be planets that even being a bit far from the habitable zone, will have complex life due to greenhouse effect. Or maybe life living in low temperatures in planets that have very salty water or water mixed with ammonia and maybe in pure ammonia. Or maybe even in ethane seas.

This is all well and good but it is pure speculation. Habitable Zone is based on Earth like surface with liquid water.

I think we underestimate just how stable and benign conditions on our planet & system has been.

In any case, such flights of fancy should not result in our scientists calling every other planet they find,
in every weird place, as being 'potentially' in the habitable zone.

Diakonov wrote:

And maybe there will be planets more close to the star, but due to high
albedo due to extensive cloud cover, reflecting most radiation a planet
receive from the star, it could be habitable, even being close, but not
too close, or it will have slow rotation like Venus. Such a planet
would have to have a fast rotation. It would be or in the same position
of Venus or between Earth and Venus.

Question is how long could it remain stable. Before the probes got to Venus the very same was assumed about Venus, no. Thick clouds also means almost no light on the surface. Even with Earths wispy cloud layer a marginally cloudy day cuts surface isolation to 10% or less of a normal day.

Diakonov wrote:

Remember, if Earth was
colder, it would have less clouds. If Earth was hotter, it would have
more clouds, serving as a negative feedback to avoid excessive heat.

If the Earth were colder there would also be more ice. Enough to lock the planet into long ice ages. If the Earth were hotter more ice would again melt, forming dark, heat absorbing oceans. There are far more runaway feedback's than there are stabilizing ones.

But in the case of ice melting, it would have no much effect, since the ice in oceans are only on poles. If the ice was in equator and if it melt, it would have a much higher effect. And also a thick cloud cover would make less insolation go to surface, keeping it cooler.

To me, if Venus had almost the same rotation speed of Earth, oceans and stable life, with a thick cloud cover (in the entire atmosphere, if clouds were of liquid water, not sulfuric acid) and air pressure similar to Earth it would even be colder than Earth, possibly having a long ice age, despite the shorter distance to the sun. The cloud cover would block most sun light. It would be an anti-greenhouse effect, like on Titan.

The cloud cover would be a very effective mirror against the excess of sun light, keeping Venus colder for longer periods, if Venus was like Earth.

Or maybe within some billion years, when the sun will increase its size, making Earth hotter. Ice would melt, oceans will be darker, but the hotter temperatures will (maybe) cause the entire atmosphere to have thicker clouds, blocking most of sun light, keeping Earth at cool temperatures. But with time, the sun will continue to grow in size, until the excess of heat will heat too much the cloud tops, removing the cloud layer and Earth will be really hot, like Venus. And men will have to move to Mars. But maybe at this time Mars will be also very hot, and humans would only live in Mars poles, or maybe in the moons of Jupiter, mostly in Callisto.

When the sun reach red giant stage, humans will have to move to Titan, where it, in the future, will be hotter. But the heat will make it to lose slowly its atmosphere, ending like Ganymede with time. And within that time, humans will have to find another planet in other system... Or move to Triton or Pluto, where it may have enough resource for human to use as energy for some time. But at that time we would be in another planet of other system.

Remember that Venus has the atmosphere it does because it lacks oceans to store all the carbon in. It doesn't seem that a lot of water made it to Venus, or if it did, it fled the planet long ago.

In case of Venus, I didn't said it had much water in the past. Or if I said, it was just a speculation of mine. I was only talking about if Venus was more like Earth, with a lot of deep waters.

If Venus had lots of water, it would only take more time to Venus turn itself into that hot world, because a planet with lots of water and hot enough would produce too much water clouds, blocking almost all visible light to the surface. Clouds in excess serving as an anti-greenhouse agent, even if it had higher air pressure. CO2 would be stored in the oceans, not in air.

Venus would be almost a true mirror like Enceladus, and because of that it would take longer time to heat up its surface. If Venus had water clouds, deep oceans and tolerable temperatures today, even if the sun increased a bit its size (and that is what will happen in the next billion year) Venus would still have tolerable temperature, if it continued to have such very reflective clouds.

Venus would still be in the habitable zone, but only due to thick clouds avoiding much light going to surface. It would be very cold compared to the proximity to sun. But only if it was like Earth, not today dry Venus with yellow sulfuric acid clouds. A shame Venus is not a water rich planet!!! If it was that, it would certainly had rich aquatic life.

Ah, I forgot something. The hipothetical Venus I'm talking about would have 90% water covering the surface, but most of that ocean would be shallow, with maximum depth of about 1.5km. And some parts being very deep like Earth's oceans.

It's all just speculation.

Imagine astronomers that found an Earth-like planet next to the considered habitable zone in a G2V star. Calculating the distance, astronomers will say that planet is out the habitable zone, and that water would be impossible to be liquid on the surface. But if that planet had a very high albedo, being mostly covered by clouds like the hypothetical Venus I talked about, with clouds reflecting almost all the star radiation? The surface would be very cold to the estimated temperature. And such planet is mostly covered by water, having only small islands as land. And in that case, scientints would be wrong by measuring the temperature only by distance or mass. Is it possible to measure the albedo of planets with our telescopes?

Let's say they estimated temperature by about 350°C, but the true temperature on the surface is about 50°C or even 25°C, depending on the albedo produced by clouds. And such world would have a higher air pressure, allowing it to be hotter and still having oceans even if it was hot. In that case, it would take a long time to turn into a world like our Venus.

Hi! Sorry if this is off-topic. But, I'm so curious.
As we know, we can calculate HZ in a single star with this formula:
d = sqrt(L_star/L_sun)
The question is what is the formula of habitable in binary or more complex star system, either circumbinary or not?

@Pastro: in the general case, this is not a simple question.

First off how to define the habitable zone? The formula you are using defines it as the region of space where the stellar flux density is the same as for Earth.

It is therefore possible to expand this definition to a binary star system (basically add the fluxes from the stars), but bear in mind that you lose the spherical symmetry (so the region is now a more complicated shape than a sphere), and because the stars are moving along their orbits the region is time-varying as well.

For a few cases you can use simple approximations: e.g. if you have a wide binary where you can neglect the radiation from the secondary star (binary orbit much wider than the planetary orbit), or if you have a circumbinary orbit around a close binary (binary orbit much smaller than the planetary orbit) you can treat the situation as being a single star with the combined luminosity of the two individual components.

Yeah, I have asked in other astronomy group, but the answers were almost the same.
I thought calculating HZ of binary or more complex star system is as easy as single one.

On the subject of habitable zones in binary star systems, this paper recently appeared on the arXiv:

An Analytic Method to determine Habitable Zones for S-Type Planetary Orbits in Binary Star Systems