Atmospherically Speaking

Today I have another Ask Tsana post.

Brookelin asked:

Hi again, Tsana.

I was wondering - in an alternate universe, what would it take for a species to survive on Mars?

I know that it has some atmosphere, but not a whole lot. With the pressure being below the Armstrong limit, could there feasibly be large creatures (between collie and bear size) that could survive would have higher thresholds and what would they need to do so?

If the water on a human's tongue boils in space, would an alien creature in these environments be able to have eyes and mouths?

What might these species' need to overcome the intense radiation caused by Mars' weak magnetosphere?

Could bio-genetically enhanced humans ever survive these conditions outside a space suit for periods of time upwards of an hour, but less than a day?

Are these too many questions? Do you know the answers to any of them, or is this more of a medical thing?

I don't have answers to all of these questions because, as Brookelin said, some are more medical/biological and that's not my area of expertise. I will say that what we generally know a lot about is life on Earth. There are some constraints that exist for life on other planets but there is nothing to say that it has to resemble Earth life. They could have eating and seeing organs completely different to what we're used to. Even on Earth there's a pretty wide variety. I'm not sure that merely genetically enhancing a human would be enough to let them walk around on Mars. Science fictions stories have gone there, but I'm not sure genetics is up to it. I could be wrong, I'm just guessing. Hopefully my comments below on atmospheres and life on smaller planets such as Mars will answer the rest of the questions, though.

Mars. Credit: NASA, ESA, and The Hubble Heritage Team
(STScI/AURA)

It's true that Mars has a very thin atmosphere; it's about 0.6% as dense as Earth's at their respective surfaces. Part of the reason for this is Mars's lower gravity. In general, gases will expand to evenly fill the container they're in. When the container is a planet's gravitational field, we get denser air closer to the ground and less dense air higher up. This is because the air higher up is pushing down on the lower air while having less air above it to push it down. More or less.

Air is made up of particles (atoms and molecules) which move around very quickly and bounce off each other. That's why a gas is a gas and not a liquid or solid: the particles in a liquid don't move quickly enough to completely overcome the forces attracting them to each other and the particles in a solid can't move more than vibrating on the spot because the forces holding them in place are so strong. The energy that makes the particles move, for all states of matter, depends on the temperature: the hotter, the faster. The other important consideration is particle mass. At the same temperature, oxygen and hydrogen molecules (O₂ and H₂) have the same energy. However, oxygen weighs sixteen times as much as hydrogen (because the atoms are larger and heavier) so it takes more energy to move oxygen molecules at the same speed as hydrogen molecules. The result is that at the same temperature, oxygen molecules move more slowly than hydrogen molecules. And it takes less energy for hydrogen molecules to reach escape velocity (the speed required to escape the gravitational pull of Earth/whatever planet) than oxygen. And that's why there is very little hydrogen in Earth's atmosphere despite it being the most abundant element on a cosmic scale — it escapes into space. It's also the reason only the gas giants, notably Jupiter and Saturn, have any significant about of hydrogen in their atmospheres — they have the strongest gravitational fields.

So, Mars. Mars is smaller than Earth, with about a third the acceleration due to gravity at its surface. Mars is made up of similar elements to Earth, most likely because they formed so closely together, so it's likely that the same sort of lighter elements could have made up Mars's atmosphere. However, due to the lower gravity, not only hydrogen but oxygen and nitrogen would also have escaped or never been captured by the planet. I would guess the main reason there's so much frozen carbon dioxide at the poles is because it has a relatively high melting point of -78ºC rather than the much colder melting points of oxygen (-219º C) and nitrogen (-210º C). For comparison, Mars's surface temperatures vary between -143º and +35º C. So basically, even if you imported or mined enough gas to raise the air pressure to human survivable levels, it would all be lost into space and would need constant replenishing which would get tedious and be difficult to sustain. You'd also, ideally, raise the surface temperature to more consistently human survivable levels — probably using some sort of greenhouse effect to trap more of the sun's energy — but that would just hasten the atmosphere's escape.

Titan's atmosphere as seen by Cassini. Credit: NASA

But all is not lost. Heavier molecules exist, particularly those made out of carbon. Titan, one of Saturn's moons, is smaller than Mars but has an atmospheric pressure greater than Earth's by about 45%. It's colder than Mars, which allows its atmosphere to condense a bit, but it's only got a surface gravity of around a seventh that of Earth's (less than half of Mars's). According to Wiki, its atmosphere is composed mainly of nitrogen (as is Earth's) and methane with some traces of heavier carbon molecules. It's a combination of the temperature, the distance from the sun, Saturn's magnetic field and some form of replenishing methane that keeps Titan's atmosphere thick and, well, full of methane. Distance from the sun is significant by itself because Titan is far enough that the ionising solar wind is weak enough to not completely ionise and destroy the top layers of its atmosphere. The same strategy probably wouldn't work on Mars to increase the atmospheric pressure permanently unless you could find some magically resistant to solar radiation molecule to populate the atmosphere with. There are two interesting theories for what keeps replenishing the methane on Titan (which should be destroyed even by the lowered energy it receives from the sun): cryovolcanoes — volcanoes shooting icy hydrocarbons instead of lava — or biological processes using/generating methane in place of water.

The high levels of ionising radiation on Mars are as much due to its lack of atmosphere as its lack of magnetic field. (Side note: there's evidence that there was a magnetic field on Mars in the past, though I don't think we know why it went away.) Earth's atmosphere absorbs a lot of the ionising and UV radiation the sun throws at us (part of the reason the ozone layer is important). Not all of it is deflected — and things like X-rays and gamma rays can't be deflected because they don't have an electric charge — especially near the magnetic poles where the aurorae are caused by charged particles, mostly from the sun, interacting with the atmosphere. However, giving Mars a magnetic field would definitely help. Earth's is generated by molten iron in its core so it's not outside the realm of over-dramatic science fiction to drill a hole into the centre and start the core spinning. Come to think of it, Hollywood's already done that, just with Earth not Mars. (For the record, the ridiculous issues with that movie include the structural integrity of the hole and the failure to correctly represent changes in gravity.) A more feasible way to avoid radiation on Mars would be to live underground so that the ground above you did the work of absorbing harmful radiation. The reason too much radiation is bad for all forms of life is that it destroys and changes molecules. In humans this is one of the causes of cancer. In microbial life, which might only have a few cells to begin with, it's more deadly. It's why sterilising things with UV light works.

So basically, the easiest way to get people living and wandering around on Mars is to have them live in airtight structures and give them suits for walking around outside it. The suits wouldn't have to be as extreme as space suits though, so that's something. I'm not saying it's completely impossible to walk around on the surface with less protection, just very difficult. And because someone will mention it in the comments if I don't, I've heard that Kim Stanley Robinson's Mars books, starting with Red Mars, do a good job of talking about the terraforming process, although I haven't read them. Ben Bova's Grand Tour of the solar system books (eg Mars or Saturn and Titan) explore alternative forms of life all over the solar system. If you can stomach a bit of sexism, some of them are worth a read.

Two links. Because I can

First, cells made out of metal, or at least, research heading in that direction. New Scientist has the story.

And here's a picture to capture your imagination: a supermassive black hole at the centre of a galaxy... stars flit around it in tight orbits. Some of those stars have planets. Some had planets but they were ripped from their orbits by the black hole's gravity. Starless planets, hurtling around erratically, if not being pulled apart, then smashing into each other. Their deaths leaving behind a dusty shroud. New Scientist. ArXiv.

It's posts like these that make me think that Tumblr might be a good idea. (Of course, my usual weekly posts are much less Tumblresque.)

Habitable Galaxies - Part 3: Galaxy Environments

This is part three in a series of posts about habitable galaxies. Post 1, covering types of galaxies and galaxy mergers, is here, last week's post 2 talking about active galaxies is here and this earlier post on the (most) habitable areas of our own galaxy is also relevant.

[Unrelated to the topic, but I wanted to say that I've been playing around with Blogger settings and made a favicon (the little icon that helps distinguish this tab from others) and an iOS home screen bookmark button. So now, if you're using Chrome or the latest version of Firefox (I suppose it should work for other versions of Firefox, but the second to most recent version failed for me) check out the little purple telescope on the white background. And if you're reading this on an iDevice, you can even see said telescope with a glossy Apple finish. What fun!]

So this week I'm talking about different galaxy environments. That is, the environment where a galaxy might be found, not environments within a galaxy (although I briefly covered that earlier). Let's start by looking at what sort of environments galaxies can be found in.

The universe has environments now?

What we mean when we talk about galaxy environments is more or less talking about how many other galaxies are nearby. It's possible to get isolated galaxies or galaxies clustered together in groups of varying sizes. Our galaxy, the Milky Way, is part of a group creatively labelled the Local Group, which has about forty-five members. Of these, the Milky Way is the second largest (probably), with Andromeda the largest. Other members include the Triangulum Galaxy, the Large and Small Magellanic Clouds and a non-literal pile of dwarf galaxies.

As far as classifications go, smaller collections of galaxies are termed groups, while larger collections—containing upwards of fifty more densely-packed members—are called clusters. This may seem like a bit of an arbitrary distinctions (what makes the Local Group a group and not a cluster if it has almost fifty galaxies in it?) but it's important to remember that while we know the Local Group is full of dwarf galaxies, other groups and clusters are too far away for us to be able to see their smaller members. So when we say a cluster has fifty members, we mean that many medium to large galaxies.

Galaxy cluster Abel 2218, from APOD.  The cluster is also a lens, but that's another story.
Image credit: Andrew Fruchter (STScI) et al., WFPC2, HST, NASA

Spot the difference

So what does it matter where a galaxy is, anyway? Well, when it comes to life, most of what determines habitability is internal rather than external to the galaxy. The only external influence I can think of which could inhibit life (and jump in in the comments if you disagree!) would be a nearby AGN blasting at the galaxy. And even then, I don't think it would prohibit life everywhere in the targeted galaxy, just in the parts being most irradiated.

However, there are some properties of galaxies which are dependent on their environment. In a denser environment, where there are more galaxies, there had to have been initially more matter for those galaxies to form from. Because there was (by chance) more matter in that area, it was more strongly gravitationally attracted to itself and hence formed earlier compared with a lone galaxy in a sparser environment. Our current leading theory of galaxy formation and evolution is called hierarchical assembly and one of its tenets is that larger (more massive) objects form first. In a sparse environment, matter is by definition more spread out and hence, as well as being more weakly gravitationally attracted to itself, has further to travel before it can clump and collapse into a galaxy.

The corollary to this is that big galaxies in clusters are older and more evolved (because have gone through more mergers, partly thanks to there being more proximate galaxies), while isolated galaxies are younger and have undergone fewer interactions with other galaxies.

Living around

As far as life developing in other galaxies is concerned, it seems pretty trivial now to make the conclusion that life would have had the opportunity to arise earlier in cluster galaxies and later in isolated galaxies. Group galaxies such as our own would fall somewhere in the middle.

On the other hand, cluster galaxies would have undergone more mergers, have a greater chance of having been in the path of an AGN and are more likely to be elliptical. Those first two points are merely hazards to the development of life, but the latter also gives rise to different experience for that life compared with our Milky Way existence. What off Earth am I talking about? The night sky.

Our night sky is covered in stars with the disc of the Milky Way running through them. An elliptical galaxy, not having a disc component, would not have such a band of dust, gas and denser stars. In fact, they wouldn't have much dust or gas at all, which means no nebulae and significantly fewer hints as to where stars even come from. Depending on the exact placement of the planet, they would likely have a more or less uniform distribution of stars in the sky, maybe with a brighter patch in the direction of the galactic core. Life that evolved in an elliptical galaxy might not ever get to observe young stars in their vicinity, let alone star formation. How might their understanding of astronomy and, in particular, stellar astrophysics be shaped by this? I think it's an interesting question to explore.

On the other end of the scale, we have isolated galaxies which could also harbour life (if they're big enough to develop sufficient metallicity in sufficient time). But if it's truly isolated, it might be that it's not possible to observe external galaxies with the naked eye. (We can, but they sort of look like stars until you put a telescope to them.)  Maybe such a civilisation would skip past the part of astronomy that labelled Andromeda and the Magellanic Clouds as nebulae but who knows how they might interpret distant blobs and spirals in the sky once they had the telescopes to see them? Also remember that isolated galaxies are going to be smaller and form later. By the time life even evolved there, would there be many other spiral galaxies left? How much more powerful would dark energy be at that point? How quickly would all the other galaxies be retreating from them?

How empty would the sky be?

Some Links!

Some interesting sciencey links, presented in the order in which I came across them (which I think is the order they were published anyway).

First up! This New Scientist article is about how the wakes of ships (the white "trail" they leave behind as they plough through the ocean) temporarily increases the albedo of the Earth—how much light from the sun it reflects. This would be good news for climate change (that is, enacting a more positive change and less the "oh gods we're all going to die in a tropical hell" sort) if not for all the greenhouse gases those ships also produce...

Next! This arXiv paper, "Kepler Exoplanet Candidate Host Stars are Preferentially Metal Rich", seems to back up some of the assumptions made in the paper I discussed a short while ago about the galactic habitable zone. In that paper they assumed that sufficient metallicity would be required to form planets and it looks like Kepler has confirmed that.

Penultimately! Forget about Martian meteorites containing fossilised life, simulations have shown that life from Earth may have made it off world and could be set to land on another planet. Could it be that when we finally get around to drilling through the Europan ice, we find Earthly extremophiles instead of aliens? Cosmos magazine have the full story.

Finally! National Geographic have an article about a newly discovered planet which has the potential to be Earth-like. Seems that it gets less energy from its sun than Venus does from Sol, but more than Earth receives. It's habitability thus depends on the appropriate weather conditions but with surface gravity 1.4 times Earth's it should be more or less possible to walk on the surface.

Habitable Galaxies - Part 2: Active galaxies

This is part two in a series of posts about habitable galaxies. Post 1, covering types of galaxies and galaxy mergers, is here and this earlier post on the (most) habitable areas of our own galaxy is also relevant.

A major attribute of galaxies is whether or not they are active. There are a few different things active can mean—actively star-forming, for example—but what I want to focus on today is whether they have active nuclei.

What's in a nucleus?

At the centre of our galaxy and most other large galaxies, there is supermassive black hole. I have briefly mentioned black holes in the past and I will eventually get around to writing a dedicated post on them. Honest. What you need to know to understand their role in galactic nuclei is as follows:

• They are very small and very dense.
• The supermassive part means that they range from around a hundred thousand times to the mass of the sun to billions of solar masses. The Milky Way's central black hole was calculated in 2008 (by this group) to be about 4.1 million times the mass of the sun. In kilograms that's about 8 x 10³⁶ or an 8 followed by thirty-six zeroes.
• As their name suggests, supermassive black holes are very massive. What massive really means (in any physics context, not just with regards to black holes) is that they exert a strong gravitational force.

Before you ask, we don't really know where these come from—there are theories, but no single one is yet the most accepted—but we do know that they must form early on in a galaxy's life (possibly even before the stars form, depending on which theory you subscribe to) and their evolution is closely tied with the host galaxy's.

Other things that can be found in the centres of galaxies include stars, dust and gas. Although following the orbits of stars in the centre of our own galaxy is what convinced us there was a supermassive black hole there (nothing else could be so massive and so small), most of what those stars do is simply orbit. (Yes, it is possible for one to fall into the black hole and yes, that would be very interesting and would generate a lot of energy but from what we've observed, the Milky Way's central stars seem to be in stable orbits. If you are interested in reading a (short and fairly uncomplicated) paper about S2, the star closest to our supermassive black hole, you can find it here.

When there is gas or dust in the vicinity of the black hole, it will tend to spiral inwards until it eventually passes the event horizon*. As this occurs, huge amounts of energy are released, outshining all the stars in the galaxy. This is what is called an active galactic nucleus. It is also, more or less, what causes quasars, the most distant objects we observe (because they're so bright we can see them very far away, you see).

Here is a nice NASA / ESA Hubble Space Telescope picture of jets coming off the nearby AGN, M87.

 * The point of no return.

Active life?

So the next question, the crux of this post, is can we have life in a galaxy with an AGN? The short answer is maybe. Of course, we have no concrete proof either way. Sagittarius A* (yes, that asterisk is part of the name), our central black hole, is not currently active** and we have even less evidence for life in other galaxies than we do for life on other planets within the Milky Way. The slightly longer answer is, it depends. There is evidence to suggest that the Milky Way was active in the past few million years and since there is still life on Earth, we can suppose that an AGN doesn't necessarily sterilise a galaxy.

Some months ago, some colleagues and I got into a discussion regarding whether a really bright AGN (even one unrealistically bright for the size of our galaxy and Sag A*) could wipe out life. We came to the conclusion that it would only do so if you were standing close enough to it. From memory, we estimated that if a planet somehow managed to find itself*** in an orbit a parsec from the active black hole, the black hole would through about as much light at it as the sun does. However, AGN emit much harder radiation than stars, meaning that a larger proportion of the energy would be at X-ray and gamma ray frequencies, both unfavourable to life. If we put the planet where Earth is, then even with nothing blocking the way we don't have a very high increase in dangerous radiation.

However, something is blocking the way: dust. As far as we know, dust near the black hole is requisite for turning on an AGN. But even ignoring that, there are many clumps of dust in the disc of the Milky Way. So many that we are unable to see through it all if we look along the disc. (Schlegel et al surveyed the dust in the galaxy and came up with this map. White bits have more dust, black bits have less.) In essence, as well as making it hard for us to notice supernovae near the centre of the galaxy, this would help shield us from AGN light. I wouldn't be surprised if we didn't immediately notice the AGN. Of course, closer in to the centre of the galaxy you have more problems and it starts to depend more on exact placement. Also, the dust actually only shields visible and UV light, so once you get too close the more concentrated X-rays and gamma rays become more of a problem.

On the other hand, a planet is less likely to form in the path of an AGN jet, simply because there are fewer stars in that direction. If it did, however, it would definitely not survive the experience.

Elliptical galaxies have significantly less gas in them (some might say no gas, but there would have to be some in the centre for the AGN to turn on, not to mention dust created by dying stars).  This would mean less shielding, making the AGN more noticeable. The bigger barrier to surviving the experience, however, would be the fact that elliptical galaxies are larger with with more massive central black hole which would generate a more energetic AGN (with more detrimental radiation). The final point to consider is that theories suggest AGN in elliptical galaxies are turned on thanks to dust being stirred up (into the black hole) from a merger. So the merger event could have some impact (see last week's post) on continued habitability. The dearth of dust also means that new planets would not be able to form in an elliptical galaxy.

So to summarise, an AGN wouldn't necessarily sterilise a galaxy, but might kill off life that had set up too close to the centre. Depending on an inhabited planet's placement in a galaxy, an AGN might not have a very large effect on daily life. There are a few additional problems for elliptical galaxies, but again, so long as the planet isn't too close to the centre (and its sun doesn't migrate to the centre of the galaxy too quickly), there's no reason for life to automatically be extinguished. Score 2 for extragalactic life.

Next time: the habitability of galaxies in different environments in the universe.


** Probably.
*** I personally really don't think a planet would be able to form in that region thanks to the density of stars and subsequent gravitational forces. I don't have any hard evidence to support this, but to me it makes sense.

The Galactic Habitable Zone

In the past, I've talked about the habitable zone around stars where water can exist as a liquid. This week, I'm going to talk about the galactic habitable zone which is the area in a galaxy where conditions are sufficiently hospitable for life to develop on planets which themselves are in an appropriate stellar habitable zone. Unlike the stellar habitable zone (also called circumstellar habitable zone), some aspects of the galactic habitable zone apply more broadly to theoretical life forms which might be completely different to the type of life we've encountered on Earth.

This post has been inspired by an article I came across on arXiv.org: A Model of Habitability Within the Milky Way Galaxy by Gowanlock, Patton & McConnell, which I will henceforth refer to as GPM. They constructed a few models of our galaxy and ran simulations to see which regions could be habitable.

What aspects should we care about?

The parent star
The sun of a potentially habitable planet orbits needs to be small enough that it survives for long enough for life to develop. Remember that more massive stars have shorter lifetimes and die explosive, sterilising deaths. The general consensus is that large, blue stars don't last long enough for complex life to develop. Even if a planet survives the actual supernova, its atmosphere would have been obliterated in the explosion and the corpse of the star—either a neutron star or a black hole, depending—wouldn't be very hospitable either.

Nearby stars
By a similar token, we don't want there to be a large short-lived star too close by either. A nearby star going supernova would also be quite bad for potential life harbouring planets. You don't have to be right next to a supernova for the gamma rays (and X-rays and cosmic rays) to do some serious molecule-killing sterilisation. However, if nearby supernovae happen early in the planet's history, there shouldn't be a problem with life developing later on (after the ozone layer heals).

GPM come to the conclusion that, depending on the type of supernova, it could sterilise planets within a range of 2-27 parsecs (6.5-88 light-years). The range is so broad because supernovae come in a variety if flavours from the dying stars I mentioned earlier, which can be of all different masses, to binary stars where the larger one throws off its outer layers first, turns into a white dwarf, cannibalises its partner and then explodes from over-eating. The latter are more bright and will on-average sterilise planets within 18 parsecs (59 light-years) whereas an average star-dying supernova will sterilise within 8 parsecs (26 light years). For a bit of perspective, the our galaxy is about 30 000 parsecs or 100 000 light-years in diameter.

As an aside, I should also mention that there is a theory that some past mass extinction events on Earth were caused by supernovae. Googling "extinction supernova" brought up a lot of hits for different extinctions. Here is one of the top hits, chosen a bit arbitrarily.

Finally, GPM conclude that there is no where outside of the central 2500 parsecs of the galaxy (which they didn't consider in detail) where there are always going to be too many supernovae for life to develop, where they've defined the time taken for complex life to develop as four billion years. That's four billion years either from the time the planet forms or from the time it gets sterilised by a nearby supernova.

Metallicity
This is sort of a less obvious one. Most of the universe is made of hydrogen and helium and a little bit of other rubbish. Although chemists define metals in a fairly specific way, astrophysicists tend to lump anything heavier than helium (that is, elements whose atoms have more than two protons in their nuclei) into the “metal” category.

Rocky planets are made out of, well, rocks rather than hydrogen or helium and if there are no heavier elements around, we'll only get gas giants forming. Heavier elements are produced when stars die, either in a supernova or in the more mundane red giant phase that our sun will eventually go through. Therefore, rocky planets can only form in areas where there have been enough stellar deaths to seed the interstellar medium with heavier elements. How big stars are depends mainly on how much gas there was around when they formed. Consequently, bigger stars are able to form closer to the centre of the galaxy (in the most dense environment) earlier, die explosively and leave metal-enriched dust behind. Then, when later generations of stars form, there is more chance of rocky planets forming around them.

The most metal-poor areas of the galaxy are the outer edge and the halo which is the spherical and sparsely populated area surrounding the disk of our galaxy. The spiral arms, where we are (if you're wondering, we're about two-thirds of the way out from the centre, close to the middle of the stellar disk). The other thing to note about metallicity is that it increases over the lifetime of the galaxy.

GPM looked at stars with lifetimes longer than four billion years whose planets escaped being irradiated by a nearby supernova for at least that period of time as well. Most of the habitable planets exist close to the centre of the galaxy, with half of them between 2500 parsecs and about 4000 parsecs, but life was still possible (though much sparser) up to the edge of the galaxy. (Remember, Earth is around 8500 parsecs from the centre.)

Other planets
I'm only going to cover this one briefly. The presence of other planets in any given system with a habitable planet could stuff around with our habitable planet. Our searches for extrasolar planets have found a lot of “hot Jupiters”—gas giants very close to the star—and our current theories of planetary formation suggest that these got there by migrating in after forming much further out. Such a migration would almost certainly spell terminal trouble for the previously habitable planet.

So where is this galactic habitable zone of which you speak?

Previous studies had defined the galactic habitable zone as an annulus (or flat doughnut, for those of you more culinarily and less mathematically inclined), with the inner rim defined by the radius at which there are too many hazards to life (for example from supernovae in the densely starred inner regions), and the outer rim determined by metallicity or lack thereof. In general, this region has in the past been calculated to be centred on our location in the galaxy, extending inwards and outwards by only 1000 or so parsecs.

On the other hand, GPM found that the whole galaxy (minus the inner region which they ignored but will study in a later paper) was habitable but the areas most amenable to life were close to the centre and a bit above and below the main concentration of stars in disk. The former for reasons of metallicity and the latter because those areas had the same metallicity as the main disk but there were fewer nearby stars to go boom and sterilise them. Our Earth, for comparison, is fairly close to the centre of the disk.

There is a bit more I'd like to say about galactic habitability, but I think I'll leave it for a future blog post. This post only covers habitability without our own galaxy, but stay tuned for more!

Lifting life off Earth

A friend suggested I blog about getting a biosphere off Earth and onto another planet (within the solar system). This is a bit of a challenge for me since, if I am anything, then I am not a biologist. As a result, I am going to focus on the transportation logistics more than what, specifically, we would need to get to said other planet.

However, if you are interested in the what and how to maintain it once there, I strongly suggest reading these two posts by Patty Jansen: So you want to be a space farmer (part 1) and Growing crops in space (part 2). She has a background in agricultural science and hence is significantly more knowledgeable than I on the matter.

Getting off the ground

Lifting anything off Earth into orbit requires a large chunk of energy. Exactly how much depends mainly on the weight (and a little bit on the size, in the sense of how big—and hence heavy—the spaceship doing the lifting needs to be). To get something off Earth (pretty much to anywhere further away than the moon, though it's not that different for the moon either), we need to give it enough energy to overcome the energy of Earth's gravitational pull.

Some basic terminology first:

• Kinetic energy is the energy something has due to its movement. It mostly depends on how fast the object is moving, but also on its mass. A faster object will have more kinetic energy, but of two objects moving at the same speed, the heavier one will have more kinetic energy. The formula for kinetic energy is

• Potential energy is stored energy that has the potential to turn into a more directly useful form of energy. For example, if you lift a brick off the ground, you're giving it the potential to turn gain kinetic energy when you drop it. In fact, thanks to conservation of energy, the amount of potential energy you add to it when you lift it up will be equal to the kinetic energy it gains as it falls and just before it hits the ground. What we are interested in is gravitational potential energy. You can have other types, like spring potential energy, which is the energy stored in a spring when it is stretched or compressed. The general formula for gravitational potential energy is

• Conservation of energy is the law that says energy cannot be created or destroyed but can only change forms. Hence, potential energy can be transformed into kinetic energy and vice versa, but neither can appear out of nothing. (On a macroscopic level. Things get a little bit more complicated on a quantum scale, but that's not relevant here.)
• Escape velocity is the velocity required to get something off the surface of a planet. It's basically the amount of kinetic energy required to overcome the potential energy stored between your object and the planet. It's found by equating the kinetic and potential energies above (ignore the minus sign, it's just a convention). Doing that, the mass for the object, m, cancels out and we find a common escape velocity depending only on the mass of the planet, M, and the radius of the planet, R. (This is assuming we're trying to get off the surface. For other distances, replace radius with distance from the centre of the planet.) Incidentally, Earth's escape velocity is about 11 kilometres per second (more than 40 000 km/h). The general formula for escape velocity is

OK, so that's the basics. To get something off the ground and into space, we need to make it go pretty fast to overcome Earth's gravitational pull. However, we don't do it all in one go; it's just not logistically a brilliant idea. Among other things, the faster you go within Earth's atmosphere, the greater air resistance (the friction air exerts on the spaceship) is. This is why rockets usually have stages. The space shuttles, for instance, had two initial boosters to get it off the ground, another large rocket to get them out of the atmosphere, then some small rockets which stay attached to the shuttle (the others are discarded when the fuel is used up) for orbital manoeuvring and coming back down to Earth. Here is an infographic from Wiki.

The tricky thing, when we're talking about getting the elements of an ecosystem off the ground, is how much they weigh. I don't know of any reason why crops wouldn't be transported as seeds which, compared with plants weigh a lot less and take up a lot less room. Depending on where you're taking them, giving them enough water and the right kind of soil is likely to be much more of a problem. In most cases, I think it would be best to mine the necessary water from whatever nearby source you can (the rings of Saturn, mayhaps?) and possibly ditto with the minerals needed for soil, but see Patty's posts linked to up top because I'm far from an expert.

Animals, however, would be a lot harder. With our current technology, we can't really transport a bunch of animals in foetus form and then grow them when we get to wherever like we can with plant seeds. Animals weigh a lot and eat a lot and produce a lot of waste products. That sort of thing (while making good fertiliser for our off-world plants) would be very difficult to transport.

To put this in a bit of perspective, let's look at the weight and lifting capacity of the space shuttle (which have almost all been decommissioned now with only Atlantis having one mission left). According to Wiki, an empty space shuttle weighs close to 70 000 kg, has a maximum payload weight of 25 000 kg and the payload bay is 4.6 times 18.3 metres (doesn't say how tall, but let's assume tall enough for cows). Twenty-five thousand kilograms may seem like a lot, but remember that the shuttles have been used to lift several bits of International Space Station into orbit. So a cow weighs around 500 kg, depending on the type, but let's run with this number because it's round and convenient. That means theoretically, we could squish 500 cows into a space shuttle and lift them into orbit. Well, that's not very helpful because we're ignoring all the food and water they'd need. I also think they wouldn't quite actually fit into the payload bay. Let's say cows need a metre by two metres of space to stand around in. That leaves us with only around 70 cows in our cargo bay. Well, OK, that means we could use the rest of the space for that pesky food and water I keep mentioning...

Let's say a cow eats 50 kg of hay a day... WolframAlpha tells me that hay weighs around 380 kg for a cubic metre (when pressed, because anything else would be silly in this context...) Our 70 cows would eat more than nine cubic metres of pressed hay a day and since getting to anywhere other than the moon takes at least months... we quickly run into problems being able to carry enough, even without worrying about the water.

It's fair, at this point, to mention that the space shuttles were obviously not designed to carry cows anywhere. They were designed to carry bits of ISS and other space-based equipment into orbit and not further. But in terms of lifting power they and the Soyuz rockets are all we've currently got. Also, cows probably aren't the best thing to start off carrying to other planets, I was just trying to make a point.

It takes a lot of energy to launch anything into space, let alone an ecosystem. Unlike the ISS which was built by launching bits up in manageable chunks, it's not really practical to do that with live animals. Flora poses less of a challenge and the difficult part becomes setting it up sensibly on the other end. Also the part where we haven't actually sent people on very long space flights yet.

More on this topic at a later date.

Stars in their Skies

My intention had been to write another gravity post, this time about tides, tidal locking and Saturn's rings, but unfortunately I've been too busy to fully cover the scope I wanted to. Instead of a half-hearted post on the above, I bring you: stars! Stay tuned for tidal forces next week.

It's full of stars!

There are many different types of stars, as you may recall from the HR diagrams I've discussed previously (I've also drawn a rather crude, annotated HR diagram below). Stars come in a whole spectrum of colours, based on their (surface) temperatures, and these are loosely correlated with their masses. In general (for the main sequence), small stars are red and hence cool (only 3700 Kelvin) and large stars are blue and hence very hot (more than 33 000 Kelvin).

These large blue stars are sometimes called blue giants and small red stars are often called red dwarfs. In terms of mass, these stars can range from over 100 times the mass of the sun for blue giants down to about a tenth of the mass of the sun for red dwarfs. The sun, by the way, is on the cooler and smaller end of the middle of the main sequence.

Aside from descriptive words, stars are also separated into lettered classes based on their temperature. The hottest stars are designated O type, then it goes B, A, F, G (the sun is a G-type star), K and the coolest red dwarfs are M type. I'm not terribly fond of the mnemonic I use to remember them, so if you can think of a good one, please leave it in the comments. ;-)

Main sequence stars generate energy by fusing hydrogen and helium in their cores. They need to generate enough energy to over come the pressure of gravity trying to pull them into their central point. This means that bigger stars burn their hydrogen faster because they need to produce more energy to prevent gravitational collapse. Well, technically it's increased gravitational pressure that drives faster fusion in the core. Smaller stars don't have as much gravitational pressure acting on them, so the hydrogen nuclei in their cores are pushed as close together, meaning that their fusion reactions happen much more slowly and it takes longer for them to burn through all their fuel. So big stars burn bright and fast and die young. Small stars burn more conservatively and lead much longer lives. For reference, the lifetime for the hottest blue stars is around a million years whereas red dwarfs can expect to live a lengthy fifty billion years or so (the universe is currently only 13.4 billion years old). Our sun has been around for four and a half billion years and can expect to keep going for another five billion or so.

A very crude representation of where different types of stars fall on the HR diagram.

Stellar life

Talking about how long stars last is all well and good, but where do they come from and where do they go?

Nebulae (singular: nebula) are giant clouds of dust and gas in space. Even though the particles in a nebula are fairly spread out, over enough time gravity pulls them together into clumps. When these clumps get big enough that the gravitational pressure holding them together is great enough for fusion to start in the core, they "turn on" and switch from warm balls of gas into blazing young stars. The rest of the gas and dust that didn't make it into the protostar before it turned into a star proper either get blasted away by the new stellar wind unless they already clumped together enough to form planets.

After that, the star lands on the main sequence, based on its mass. What happens when it uses up all its fuel and reaches the end of its main sequence life varies depending on its mass. Smaller stars, like our sun, will throw off their outer layers and swell up into a red giant. The star will eventually eject all its outer layers and all that will be left is the star's exposed core; a white dwarf. A white dwarf no longer fuses hydrogen or helium or anything else. Instead it just slowly radiates away all of its heat until it eventually (over trillions of years) cools. White dwarfs are basically just spheres of carbon or oxygen or a mix of the two, depending on the initial star. Those news stories you might have seen about the "largest diamond in the universe"? Those are talking about carbon white dwarfs.

If we start with a larger star—big enough that after the star has ejected all its outer layers the core that's left behind is more than 1.4 times the mass of the sun—then the core will be too massive to remain merely a white dwarf. Instead it will collapse in a giant explosion known as a supernova. In the immense pressures exerted in the supernova, the protons in the old stellar core combine with the electrons to form neutrons (atoms are usually made of protons, neutrons and electrons). This type of star, composed entirely of neutrons and not found on an HR diagram, is called a neutron star. A really massive star can, after a supernova, collapse into a black hole, an even denser object (previously mentioned here).

Life elsewhere

Since the sun is a G type star and has a life-bearing planet, of course G type stars elsewhere could have life-bearing planets too. There is also a reasonable chance that F and K type stars, which are fairly similar to our sun, could also harbour life. The biggest problem is when we look at very massive stars. When we're talking about stellar lifetimes in the millions of years, then there probably isn't enough time for life to form. It took four or so billion years for life on Earth to get to the stage it is now. If the sun had only lasted for ten million years before exploding, then we wouldn't be here.

What about red dwarf stars then? They live for a very, very long time, so there's certainly ample time for life to develop. However, because the habitable zone is so close in to the star (because the star is so dim and cool), there are two issues:

1. The planet will probably be tidally locked, meaning that one side is always facing its sun while the other never gets any direct heat or light. It's likely in this case that both the day side and the night side will be permanently too hot or cold. The ring of the terminator (the boundary between the day side and the night side) would probably be the best bet for life developing, temperature-wise.
2. Small stars seem to have more flares than larger stars. Flares aren't terribly conducive to life, particularly at such close proximity. This is still an active area of research, however.

But, theoretically, in optimal conditions life could develop on a planet orbiting a red dwarf. Also, both of those issues are things I will probably blog about in the future.

As I mentioned last week, just because life can't develop there, doesn't mean humans can't try to colonise planets around different and interesting stars. Of course, colonies with a five or so million year time-limit might be a bit sort sighted, but if there are planets in suitable places, it could be good for a laugh.

The humanly habitable zone

If you want your little book people (otherwise known as characters) living on the surface of a planet that isn't Earth and without constant artificial support, you probably want the planet to be habitable. This is where the habitable zone (and hence this blog post) comes in.

Why is it a zone?

First let's talk about what we need from a planet. Earth is great; it gives us lots of handy life-sustaining conditions; air, water, the right amount of gravity, sunlight and radiation... Of course, the reason the Earth is so well-tuned to keeping us alive is that we evolved on Earth. It's not tuned to us, we're tuned to it. There is no reason that aliens couldn't evolve in very different conditions to those found on Earth. Even on Earth there are many forms of life such as extremophiles which live in conditions we humans couldn't survive in. There's also a good chance that life is possible on Europa or Titan (moons of Jupiter and Saturn respectively), but on the former it would have to be in a subsurface ocean and the latter has a very different atmosphere and composition to Earth, meaning that life couldn't be water-based.

Aliens are all well and good, but if we're interested in what conditions humans can live unsupported on a planet's surface, we need to be much more specific.

The things I mentioned earlier (air, water, heat, etc) depend on two things:

1. Planetary properties such as size and composition, and
2. Planetary location.

The first of those is most easily summarised as having to be similar to Earth to sustain unaided human life. The latter is the subject of this blog post.

You see, although the planet needs to be similar to Earth, its sun doesn't have to be that similar to our sun. Different stars output different amounts of light and energy (previously discussed in terms of how bright they are). If we were to pick Earth up and put it in orbit around a larger, hotter star then, if we didn't want to be burnt to a crisp while all our water boiled, we would have to put Earth in a further out orbit to compensate for the extra energy coming off the star. Similarly, if we put Earth around a smaller, cooler star, we'd need to put it in a closer orbit to stop it turning into a snowball.

The optimal region in which to put a habitable planet is called the habitable zone. It's a zone because there's no reason for life to have not evolved on Earth if it had been slightly closer or further away from the sun. I'll get back to life evolving on other planets at the end of the post, though.

There are a few different ways of defining what constitutes a habitable zone. Some definitions involve the region in which liquid water is possible, temperature-wise (since our biochemistry depends heavily on liquid water), other definitions go a bit further and include things like carbon cycles and the greenhouse effect (pdf link, sorry). For the purposes of the habitable zone calculations which follow, I'm going to use the definitions given in Selsis et al. (2007).

Before that, though, let's work out how warm a planet is based on how far from its sun it is. I am using a formula adapted from a Melbourne Uni 3rd year physics lab manual because it's simpler than the one given in Selsis et al. (2007), although they both give the same results. The approximate temperature of a planet, given a stellar temperature and radius and ignoring complex atmospheric effects (but assuming that an atmosphere exists) is:

T is the approximate temperature of the planet in Kelvin, T_* is the temperature of the star in Kelvin, A is the albedo of the planet (for reference, Earth's is 0.306, according to wiki), R_* is the radius of the star and d is the distance of the planet from the star.

The only tricky thing here is that both R_* and d have to be in the same units, so either both kilometres, or both AU etc.

According to Selsis et al. (2007), life is possible in the range where the temperature is between 277 K and 394 K. Rearranging the above equation then with these values and substituting R_* in annoying units for solar radii (the HR diagram I linked to a few weeks ago can help you estimate this much more easily than in km or AU), we find that the habitable zone for any main sequence star in AU is given by:

Quantities as above. R_☉ is solar radii (multiples of the radius of the sun) and d now must be in AU.

So all you have to do now is work out what kind of star you want, find out it's general properties (easily googleable if you're using a real star) and throw in the numbers. Unless you have plans to make your planet unusual, it's probably best to keep A as the Earth's albedo. That said, habitable distance will change with albedo so the only reason not to change it is because albedo reflects the planetary composition (and we probably want to keep a similar composition to Earth...).

For reference, Selsis et al. (2007) find the sun's habitable zone to be between 0.95 and 2.4. (Note that they use a different albedo for the Earth.) Wiki lists some other numbers which seem to vary mostly in the outer edge prediction.

I should also point out that all of this assumes that there aren't any other stars near our planet. Things get a bit more complicated with multiple stars around, something I will definitely address in a later post.

Evolving elsewhere

As well as the habitable zone, there is something known as the continuously habitable zone (or CHZ). This is the region around a star which remains habitable throughout the star's (main sequence) lifetime. (Main sequence lifetime because different stars end their main sequence lives in different ways, mist of which make the continued survival of planets complicated at best. More on this in a future post, I think.) The reason we need to worry about different times in a star's life is that stars tend to increase their rate of energy output as they age and use up fuel (because as more fuel is used up, hotter core temperatures are required to sustain fusion which leads to an increase in luminosity). The CHZ is narrower than the habitable zone calculated at any given time during the star's lifetime and it is possible that as the star's luminosity changes, the habitable zone could shift so that the planet moves into or out of it over (astronomical) time.

Of course, this doesn't matter so much if we just want to find a planet to throw humans at. Continuous habitability becomes more relevant when we're talking about life evolving. Evolution from scratch takes a long time, but human civilisation to date has lasted an insignificant amount of time, on an astronomical scale.