Habitable Planet Definition?

Factual Questions
1

I’ve been interested in following the stories regarding the exoplanet discoveries being made recently. Better equipment is allowing us to get deeper looks into neaby star systems.

One of the common quotes is that it’s just a matter of time before we find an “earth-like” planet or a “habitable” planet soon.

Given what we know about our own planet, I’ve been wondering if we’ve figured out the ranges that would make a planet habitable for humans? For example, given Earth’s sea level atmosperic pressure at 1 atmosphere and given that humans can survive at very high elevations (assuming proper conditioning), what is the range of atmospheric pressures that would be acceptable?

Earth’s atmosphere is primarily nitrogen and oxygen (78% and 21% respectively). What are the ranges of mixes and percentages that would be breathable? For example, we already use hydrogen/oxygen and helium/oxygen mixes for scuba diving so I’m assuming given the right conditions, humans could survive on those mixes.

I don’t know that I’ve ever read anything that defines what “habitable” could mean and I’m looking for some more information.

2

Mainly when they describe a planet as being habitable they mean roughly the same mass as earth and roughly within the habitable range around the star in question.

You start with the star itself. Typically that would be F, G or K class stars (the sun is a G class). Anything hotter (say an O, B or A star) would be overly bright and not last long enough and M and cooler stars would require the planet to be very close and possibly tidally locked to the star.

Once you do that then you nail down the distance from the star that water can exist. This habitable zone makes any planet within itself a candidate for potential life.

Then you check to see what kind of planetary body exists there. For life as we know it we’re looking for a rocky body not much larger than earth. Now if a large gas giant is present in the zone then we’d have to start looking for rocky moons since the gas giant will have ejected any other planets in its sphere of influence.

So medium class stars, habitable zone where water could be found and an earth mass rocky body. After that they we’d need to figure out atmospheric composition.

3

There’s actually a fairly well defined set of criteria defining the habitable zone. In short, is the area around a given star where an earth-like planet (small and rocky) would have liquid water, and thus could might support some sort of life as we know it. Right now astronomers really can’t learn much about an exoplanet other than it’s mass, size and orbit, so we don’t know the compositions of any exoplanets in detail. We can see that a planet is small and dense, which implies that it’s rocky, or that it’s large and not dense and probably a gas giant. Nobody can yet say whether or not some planet has a 20% oxygen atmosphere and water, for example.

4

Missed the edit window:

Apparently I spoke too soon. There are a few preliminary observations of exoplanet atmospheres, but it’s still too early to get really detailed information about its composition. Here’s an article on the latest such observation. Still, it seems like the best we can say is “yep, there’s water vapor… in some quantity”.

5

I knew about the Habitable Zone for planets. I guess what I was looking for was assuming we find a rocky planet in the Habitable Zone, what range of conditions could exist on that planet and still make it livable for humans? Going on my examples above, I’m assuming that a planet with an atmospheric pressure of 2 bar and a mix of hydrogen/oxygen would be livable (given the fact that scuba divers can breathe that and still live). So what are the atmospheric range of conditions that would be acceptable?

6

Until someone lights a match, that is :wink:

7

That’s harder to say for sure. But we can estimate some upper and lower bounds for given gases. For oxygen, we’d need a certain minimum – the highest altitude cities where people can live are around 15,000 feet, and there the partial pressure of oxygen is about half of what it is at sea level. So humans might be able to scrape by with about 10% oxygen at 1 atm, 5% oxygen at 2 atm, etc. Too much CO2 can get toxic: 1% causes drowsiness (at 1 atm), and concentrations above 5% can cause some pretty serious effects.

8

See if you can find the book “Habitable Planets For Man” by (IIRC) Stephen Dole, publ. Rand Corp. Probably long out of print, but in you local good big library?

He goes through the criteria that would make a planet habitable. FWIW:
Oxygen is 21% at 14psi; or about 2.8psi partial pressure. generally, anything up 8,000 to10,000 feet is “livable”. So that would be about 3/4 sea level? 1/2 bar is at 18,000 feet. Oxygen for pilots is required, IIRC, at 12,500ft?

Odds are that a planet with less than about 0.6G has lost too much atmosphere over a geological time. As pointed out above, hydrogen and oxygen do not last long as separate gasses. H2 boils away first if free. Odds are about 1.4G is the limit of what we would tolerate as a livable heavy planet.

Nitrogen narcosis is a problem. Not sure what the limit is, but IIRC 200 feet for any length of time is a dive limit; 33 feet per bar, so that’s 6 atm. I have this theory that oxygen partial pressure in any atmosphere is pretty self-limiting; if it weren’t for plants constantly refereshing it, ours would all combine with something - carbon, calcium, iron - and disappear. Too high an oxygen concentration and even the wet jungle would burn. Just for fun get an chem major to demonstrate - safely - what happens when you light a match near pure oxygen at 1 bar. Sci Fi authors compensate by saying Argon or some other inert gas is a significant portion of their fictional thick atmosphere. (It’s what, 1% or so of ours?)

As mentioned, there’s a habitable zone that fits just right around certain classes of stars. the stars are classes OBAFGKM(Q) ("Oh Be A Fine Girl Kiss Me Quick) in order of size, heat, and age. The bigger OBA stars are thought too young for an earth-like planet to evolve. Smaller K and M stars may be too small. Dole posits that if the planet is too close, it locks like the moon to face its sun; one theory says, scifi aside, that configuration cannot be habitable. A large moon (or the “planet” is a moon) or a binary planet may be habitable closer in and so get full sunshine. Regardless, since most stars are KM and small, maybe 10% of stars could harbour habitable planets.

then there’s awhole mss of other factors; a planet like Jupiter is surrounded by such a strong magnetic field that the radiation is to intense for a habitable planet inside that zone. A planet needs the big internal magnetic field (Mars lacks) to generate the protection against solar and cosmic radiation. etc.

Always fun to speculate…

Our sun is a G, Alpha Centauri A and B are F and K, IIRC - conveniently close and about right. Epsilon Eriandii IIRC is a G star at 10 light years…

9

It’s hard to predict the sort of gravity that humans can tolerate. We really only have two data points: 1g is fine for everyone (duh), and 0g causes problems that become progressively more serious after a few weeks. It could very well turn out that a very fit human can survive just fine at 2 or 3g (as long as she’s in really good shape), or it might turn out that there are horrible physiological problems anywhere outside 0.9-1.1 g. And that’s not even considering the potential for developmental problems. All life as we know it has been at exactly 1g for billions of years.

There have been a few experiments where human subjects would live in a giant centrifuge at slightly increased gravity for long periods of time, but the only conclusions that I’m aware of are that humans get dizzy and nauseous in centrifuges, due to centripetal forces and different apparent gravity between the head and feet.

10

As always, NASA has a good bit of info on what we know about humans and higher gravity.

11

So in sum, we need (a) a partial pressure of more than two PSI of oxygen, with enough “inert” gas to make breathing possible and no (or only trace) toxic or asphyxiating gases in it, a temperature range of less than 40 C and sufficiently high to permit liquid water for at least part of the time, a gravity range that is as yet unknown but appears to be not too far from 1.0 G. I think we might need to add dry land being present – a planetary ocean with no land would not be considered ‘habitable’. Presumably adequate insolation is needed too.

12

The number I heard was that if the “wheel” was less than 300 ft diameter, the coriolis effects would cause disorientation problems. Every time you turn your hear, you would feel weird. This effect is best felt on that carnival ride (spinner?) where you all are standing against the edge of the drum and then it spins really fast. Shake your head side to side and you’ll see the effect. This is for 1G. I assume the problem at 2G requires twice as bif a wheel.

Also, IIRC, the original astronauts before the shuttle flew in high oxygen-low pressure. 7PSI, if I recall. This made the Apollo 1 fire that much more devastating, and also meant the pressure differencetial made it impossible to escape by opening the hatch.

13

I’ve never heard of spectral class Q. The classes after M are L and T, both for brown dwarfs with different temperature ranges.

Alpha Centauri A is class G; epsilon Eridani is class K.

14

I don’t know about that–I can imagine a society of extraterrestrial human colonists who basically live their entire lives on boats and larger “floating cities”; I’ve certainly seen the idea used in fiction. (A pure ocean planet evolving a native species of tool-using intelligences probably would be tricky; even if we imagine non-streamlined or partially streamlined squid or octopus like creatures with tentacles, it’s still hard to imagine them ever getting out of their Stone Age if they live their entire lives in the sea. No fire, no metallurgy, etc.)

15

There are zones on earth that are greater than 40C and still habitable (for example, the average temperature difference between Alaska in winter and the Sahara in summer).

With conditioning, I also have to believe that humans could tolerate larger differences in gravity. For example, let’s assume that on the multi-year trip to the nearest star system to visit our newly discovered planet, the gravity on the ship were increased .1g each year to get to 1.5g. I find it hard to believe we wouldn’t adapt to that fairly easily. Decreasing g gets trickier but I bet we could easily tolerate .7 or .8g with no ill effects.

16

Water worlds: I had read that there might be planets with an H2O “mantle” hundreds of kilometers deep, with a silicate/iron core. One problem with life on those is, would small but vital amounts of minerals remain available in solution, or would they tend to sink down so far as to become biologically unavailable? Even on Earth the availability of iron is an important limiting factor on life in the oceans, and a water-mantle planet would be even worse.

Dim suns: in addition to the tidal locking problem, another problem is the sheer narrowness of the habitable zone. Although the suitability of red dwarf systems has recently been reevaluated more favorably. One advantage is that even if conditions are marginal for life, a dimmer sun could last for hundreds of billions of years, giving life a longer chance to evolve. And the large number of red dwarfs might make up in quantity what they lack in quality.

High gravity: anyone with a bad back or flat feet can tell you that even 1G can be a problem. People regularly die just from tripping and falling. Maybe humans could adapt to higher gravity but remember that it isn’t just how heavy everything is, it’s how fast everything falls. At 3G you would need spectacularly good reflexes.

17

Thanks. TLTG (Too lazy to google). When I went to school many years ago, they said the stars after M were “Q” but the classification was considered obsolete. I assume with Q/T we are getting to the brown dwarf stage?

Jack Vance’s The Blue World is an example of a water-only world where humans live on giant lily pads…

In Pournelle’s “War World” series, the planet is very small with a thin atmosphere; part of the problem is that what we can tolerate as healthy adults or for short times may not work for example, for children. Outside of the deeper valleys, IIRC, children need to be born in pressurized rooms.

18

Nobody can seem to agree on what, if anything, comes after M in the spectral sequence. I’ve also heard R, N, and S after M. Part of the problem seems to be that, while the sequence from O through M is a one-dimensional one dependent only on temperature, after that other factors like composition start making a difference, so it’s not a simple one-dimensional sequence any more.

19

R and N were types of Carbon stars. Class S has zirconium oxide in its atmosphere. All three types are redder than ordinary M type stars, so if you’re going to sort the spectral classes by color, then they go after M. Temperature-wise, they belong in class M, though.

However, going by color is not the usual practice any more. As you say, temperature is the key variable. Classes L and T are based on temperature and are cooler than M.

20

Correct me if I’m wrong, but doesn’t molecular oxygen only come from respiration, a distinctly biological process?

50/50 chance such an exoplanet would be defended and not habitable by humans.