Weird Worlds: Gleise 581

A quick note before I jump into the meat of this week's post. From next week, I'm going to be on holiday for about two weeks. I'm going to try to queue up a couple of blog posts to self-update while I'm gone, but if something goes wrong I may not be able to do anything about it until I'm back in normal-internet land.

So this week, I thought I'd do another entry in my Weird Worlds series. This time about a planetary system which has gained quite a bit of media attention: Gliese 581. (Pronounced something like "glee-zeh", if you're curious.)

You may have heard of Gliese 581 discussed in the media with phrases like "second Earth" or "super Earth in the habitable zone" being thrown around. So how like Earth are we talking? Which part of the habitable zone? What sort of star is Gliese 581 anyway? Read on!

The Star

First the basics: Gliese 581 (which I will now refer to as Gliefeo because I am lazy and typing numbers is annoying it rolls off the tongue better) has four confirmed planets and two unconfirmed planets. For the sake of not getting too carried away I will focus on the four confirmed planets, although it was one of the unconfirmed ones which drew a large slice of media attention.

Gliefeo itself is a red dwarf star only about a third of the mass of our sun and a bit less than a third of the size. This means that to be within the habitable zone, its planets need to be significantly close to it than Earth is to Sol. Furthermore, the stellar environment these planets will be living with is very different to ours. Here is a nice Space.com article about it.

All of Gliefeo's planets were detected using the radial velocity method.


The Planets

As I mentioned above, Gliefeo's four confirmed planets are all quite close to their sun. In fact, as the image below (taken from my favourite exoplanet app) shows, all four are well within the orbit of Mercury (the grey circle). And the second image below (also taken from my favourite exoplanet app) shows two blue circles indicating radial distances of 0.1 AU and 0.3 AU (an AU is the distance between Earth and sun).

Gliese 581 and its four confirmed planets designated, from innermost to outer: e, b, c, d. The grey outline shows where the orbit of Mercury would be if Mercury's orbit were picked up and plonked around Gliese 581. The green annulus shows the habitable zone for Gliese 581 and both Gliese c and d are within the habitable zone for at least part of their orbits. Image from Exoplanet iOS app by Hanno Rein.

A representation of the Gliese 581 system. The inner blue outline denotes an orbit of radius 0.1 AU and the fainter outer circle (click to enlarge) shows an orbit of radius 0.3 AU. The four planets from inner to outer are designated e, b, c and d. Image from Exoplanet iOS app by Hanno Rein.

Some technical details on each of the planets (all taken from the Extrasolar Planets Encyclopaedia) :

• Gliese 581 e:
• Furthest distance from Gliefeo: 0.03 AU
• Length of year: 3.15 Earth days
• Mass: 1.94 Earth masses
• Gliese 581 b:
• Furthest distance from Gliefeo: 0.041 AU
• Length of year: 5.37 Earth days
• Mass: 15.64 Earth masses = 0.91 Neptune masses = 1.08 Uranus masses
•  Gliese 581 c:
• Furthest distance from Gliefeo: 0.07 AU
• Length of year: 12.9 Earth days
• Mass: 5.36 Earth masses
• Gliese 581 d:
• Furthest distance from Gliefeo: 0.22 AU
• Length of year: 66.8 Earth days
• Mass: 7.09 Earth masses = 0.41 Neptune masses = 0.49 Uranus masses

The last two planets, c and d, are the most interesting because of their location in the habitable zone.

Habitability

So are the two outermost planets of Gliefeo habitable? Well, probably not. In fact, chances are, none of these planets are able to support Earthlike life. Ignoring the issues with the star itself, as mentioned in the Space.com article I linked above, there are problems with all four planets.

Gliese 581 e is the closest to Earth in mass which, depending on its size (and composition) could mean that we could safely walk upon its surface from a gravitational point of view (well, safely is relative, but we probably wouldn't die instantly). However, it's so incredibly close to the star that it would definitely be to hot to support life.

Gliese 581 b is the largest of the four at around the size of Neptune. This almost certainly makes it a gas giant with a very dense atmosphere that would crush us if we tried to find a surface. It would also be very hot, not just because of it's proximity to the star, but because of the insulating effect of that thick atmosphere.

Gliese 581 c skirts the inner edge of the habitable zone, probably making it too warm for comfortable life. It's heavier mass also suggests a thicker atmosphere than Earth's (although this is purely speculation) and it could be more similar to Venus in terms of climate. That is to say: very inhospitable. It's surface gravity would probably also be a bit too strong for us, though this would depend a bit on composition.

Gliese 581 d falls into the class of planets somewhere between Earthlike and (mini) gas giant. We're not completely sure at what mass point planets stop being rocky and turn into mini gas giants.

(Side note: the mini and the giant should really cancel out, shouldn't they? Maybe we should call them gas balls to distinguish from larger gassy planets like the gas giants Jupiter and Saturn. But then, where would you draw the line? I suspect it would end up depending on the composition of the gas at lesat partially.)

So while Gliese 581 d spends roughly half its time in the habitable zone and half beyond it, it's probably not inhabitable itself. However, if it had any rocky moons like the solar system gas giants we know and love, those moons have a reasonable chance of being habitable. With the added advantage that they're going to be (probably) tidally locked to the planet, not to the sun, allowing the sun to more evenly warm its surface.

It's interesting, really, that of all the exoplanets discovered so far—not just around Gliefeo—the area most likely to be habitable is a moon of a gas giant. Really that says more about our detection techniques than anything else, but it does suggest some interesting possible world building.

Requests?

As I mentioned in the intro, I'm going away and hope to have some blog posts prepared in advance. However, my brain is all used up organising travel-related things without much room left for thinking about stories or reading new articles (my primary sources of blog post inspiration). So if any of you lovely readers have any requests for future posts, drop me a line in the comments!

Gravity inside solid (and hollow) bodies

I have in the past talked about gravity on the surface of a planet and ways of creating artificial gravity on things like space stations. Today, I thought I'd talk about what happens when you start to move into a planet, particularly deep beneath the surface (perhaps more applicable to asteroids, small moons and other bodies without plate tectonics, although I can envisage a story taking place inside a gas giant that might need to take this sort of thing into account). Alternatively, this also applies to giant hollow spheres, y'know, if that's your Big Dumb Object of choice.

Alternate titles for today's post could be "Why Jules Verne's Journey to the Centre of the Earth was inaccurate from a physical point of view (as well as biologically), even taking the physics of the day into account" or "The Shell Theorem". I thought I'd stick with something both short and explanatory.

Hollow Shell

OK, this is the simplest case so I'm going to start here. Consider a large hollow sphere. Make it massive enough that it has an appreciable gravitational attraction from the outside (Hmm... no one really talks about the gravitational field of the Death Star, probably because Star Wars is full of unexplained artificial gravity, but that sort of structure would be ideal to keep in mind).

From the outside, it would behave the same way as a non-hollow sphere of the same mass. Basically, with anything roundish you can just pretend that all the mass is at the centre and do your gravitational force equations from there. That's what we do for planets. And it works fine if you're floating around outside or standing on the surface, but things are a little different once we move inside. Below, I've put a little doodle of a shell with some labels to make it clearer what I'm talking about.

The grey circle is our spherical shell. R is the radius of the shell, r is the distance from the centre to the location of interest. I used snowmen with jaunty hats as stand-ins for stick figures because I found them in the list of symbols in Helvetica. You can pretend they're space-suited people with comms devices on their heads (or brain slugs) if you prefer.

So outside the sphere, at positions A and B, a regular calculation of the gravitational field based only on the mass, M, of the sphere and the distance, r, of the snowman from the centre of the sphere. The acceleration due to gravity is then:

g is acceleration due to gravity, G is a constant, M is the mass of the sphere and r is the distance from the centre. The equation applies for objects outside of the sphere (ie positions A and B) in the image above.

The only difference, really, between A and B is that at B the distance from the centre is equal to the radius of the sphere (r = R) so you can just substitute R for r.

Let's look at C now. In the centre of the sphere, the gravitational pull from each bit of shell is exactly balanced by the gravitational pull of the bit of shell directly opposite. Easy. All the gravitational forces from all the bits of the shell cancel out and overall there is no gravitational pull in any direction so the centre snowman would feel weightless. (Technically not free-fall as we could use in orbit since there is no falling involved. Just saying.)

OK, what about snowman D then? This is the really cool part. As with C, every bit of the shell pulls on him in different directions. You might think that D would then fall towards the nearest edge of the shell, but this isn't the case. Although D is closer to some edges than others, the forces still all cancel out. How nifty is that? The actual mathematical derivation is a bit complicated but you can read about it on wiki if you are interested. Instead, I present a graphic that qualitatively explains it in a very hand-wavey way:

I highlighted two bits of shell above which would cancel each other out. Remember that as well as mass, the gravitational pull also depends on proximity and in fact drops off with the inverse square of proximity (inverse square means 1/r²). There is less mass in the closer bit of shell but the larger blob of shell is larger in such a way that the further distance cancels out the increased mass exactly. Also my drawing is fairly rough but hopefully you get the idea.

Wow, that turned out even more vague an explanation than I'd hoped. Oh well.

One more thing I offer without any explanation (other than Gauss's Law mentioned and linked-to in brackets) is this: The shell theorem is not confined to only spherical shells. Within any shape of enclosure, there will be no gravitational force due to the enclosure so long as no part of the enclosure extends inside itself. What I mean is, it has to be hollow. Also, it has to be closed, meaning it has no holes in it (but I think that holes with a symmetric counterpart on the opposite side would be OK within reason).

Inside solid bodies

Now for a solid sphere. Hopefully, it is obvious by now that outside the solid sphere gravity behaves normally (the same as points A and B on the first diagram). The next diagram only includes points C and D.

Again, R is the radius of the now solid sphere and r is the distance from the centre to the snowman/brain slug of interest.

For the sake of plausibility, let's just assume our snowmen/brain slugs are in a little spherical cavity inside the planet (and not buried under a tonne of rock). For C at the centre, we can just pretend he's inside a particularly thick shell. As with the thin shell considered earlier, this gives us a weightless snowslug at the centre.

D is a little bit more complicated and we need to consider it in two parts. First the section of the sphere that's outside of D's distance from the centre. That is, the shell of thickness R - r, if D is r from the centre, can be treated just like a shell from the first example and hence cancels out. That leaves us with the bit of sphere closer to the centre than D is. This bit we can treat the same way as if it were a regular sphere we were standing on top of:

As above, but here M(r) is the enclosed mass; the mass within r, the distance from the centre.

Easy! Even better, if we assume that the sphere is of uniform density (lots of things—Earth, gas giants, stars—aren't but asteroids probably are), then we can simplify this even further since the enclosed mass then becomes a simple function of the volume and, skipping the maths, we end up with a linear relationship: a straight line that runs from the surface gravity at r = R to zero at r = 0. Nifty!

Remember, R is the radius of the sphere (which shouldn't change!) and r is the distance from the centre.

So now, if you have a sizeable asteroid and you want to bury a base half way to the centre, you can work out what acceleration due to gravity your colonists/soldiers/evil genii will experience. Huzzah!

What was with that Jules Verne reference at the start?
Oxygen does not get denser closer to the centre of the Earth. Really, I shouldn't complain too much since he tried (outside of, y'know, the dinosaurs and the ability to walk the long distance to the centre of the Earth) but in some ways it's the trying not hard enough that sometimes makes it worse. But props for using the speed of sound to work out how far apart they were. -1 mark for not also taking air density into account.

A correction: You can't fall off Pan.

While writing a story which happens to be set on Pan, one of Saturn's moons, I realised that I had made an erroneous statement in a past blog post. Of course, I had to correct it.

In that post I made a passing comment that on Pan, the gravitational force of Saturn is greater than that of the moon itself. That statement was true. However, I went on to say that on the Saturn-side of Pan, you'd fall off because the gravity of Pan wasn't strong enough to overcome Saturn's gravity. This last part isn't quite true.

If you were on Pan, despite its weak gravity, you would be hurtling around Saturn at the same speed that Pan does, which means that you would automatically be going fast enough to stay in orbit (your centripetal acceleration would be balancing Saturn's acceleration due to gravity), no matter which side of Pan you were on. Admittedly, the low escape velocity (about 25 km/h) and the slight difference in gravitational pull from Saturn between the near and far ends would make it easy to fall off the planet and slowly spiral in towards Saturn, but you certainly wouldn't be falling upwards.

The more accurate statement I should've made was that if you were floating around in the vicinity of Pan's orbit and Pan came past you, its gravity would not be strong enough to pull you in over Saturn's gravity. No matter how close to it you were (even if you could touch the surface), if you were not already moving along with it, then Saturn's gravity would win out and you would fall towards Saturn, rather than towards Pan.

The original post has been amended to reflect the above correction.

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!

What really takes place at science conferences...

This is a quick world-of-science demystification post.

I get the feeling that what non-scientists think science conferences are like and what they're actually like don't quite overlap as much as they probably should. The TV show Big Bang Theory doesn't help. Brawls don't tend to break out and people are rarely drunk on stage, even students.

Of course, I'm writing this from an astrophysics conference and I've never been to a biological conference, for example (though I have been to a general physics one), so it's entirely possible that biologists do brawl... But I doubt it.

Edit: a biology friend on FaceBook informs me that they don't brawl either. She also reminded me that I should mention the after-hours drinking. No one's drunk on stage, but in the evenings, especially after the conference dinner, drinking late into the night is not unusual.

The basic format is a series of talks in a lecture theatre. There will usually be some longer invited talks and then the majority of talks that people have to apply for. You put in for a talk by submitting an abstract (online) and, if the organisers think your talk will be interesting and if it fits into the program (and if you have some modicum of credibility rather than being a walk-in off the street), then you will get a talk.

If it's a large conference, there might be streams where talks on different topics take place simultaneously in different rooms.

Usually, there will also be a bunch of people who present posters. These people will often (but not exclusively) be students or other researchers who don't have quite enough new material to fill out an entire talk. People who don't get to do a talk, but want to, will usually make a poster instead. Posters will be displayed in some room or corridor and other attendees will wander around during tea and lunch breaks, viewing said posters and talking to their presenters. Sometimes there might be something called a sparkler session (or at least, that's what we call it at Australian astro conferences) where poster presenters get 30 to tell the audience why they should bother looking at their posters.

Overall, the most exciting things that happen at scientific conferences are heated discussions about science, technological malfunctions and bad puns. No brawling, d'you hear that, Big Bang Theory?

The Evolution of a Science

As you may have gathered from the intro of my previous post, I've been at a conference, preceded by a winter school, this week. I had planned to have a post prepared before I left, but it was not to be. Instead you get a post inspired by said winter school. (And incidentally, this was written entirely on an iPad soft keyboard. It wasn't a bad experience, if you're wondering.)

One of the talks I attended spoke about where we are today in understanding galaxies. As part of the talk the presenter also went over the history of the scientific study of galaxies, which got me thinking about how a scientific field evolves in time and what we need to consider when we're inventing a fictional scientific field. Then, obviously, I decided this would make a good blog post and here we are.

This isn't really going to take many societal effects into account, which could be very important in some fields, especially when religion disagrees with scientific discoveries. What I am instead going to talk about is how the scientific progress gets made using examples from galaxy astronomy/astrophysics and a few other fields.

Breaking it down

The way I see it, the development of a scientific field can be broken down into four stages:

1. Discovery
The field first has to be discovered. This is a pretty basic requirement. In the case of galaxies, it was thought a hundred or so years ago that the Milky Way was the entire universe and contained everything we could see in the night sky. Then other galaxies outside of our own were discovered or, more accurately, it was realised that that "spiral nebula" in Andromeda was not actually within the Milk Way) and a field was born.

2. Classification
Once a bunch of galaxies had been discovered, Hubble and others started classifying them based on obvious characteristics of appearance. We actually still use a classification system based on Hubble's. However useful it is to be able to say, "Well, that galaxy there is elliptical, that one is a late-type* spiral," it wasn't quite giving us more information just yet.

3. Analysis
This is the part where instead of just collecting things, we start analysing them in different ways. While Hubble was looking at galaxies with optical telescopes, he also took their spectra. It was at approximately this point when he noticed that all the far away galaxies were moving away from us (looping back to the discovery point) and the field of modern cosmology was born.

Since Hubble, of course, many other people have studied galaxies. As new data became available, thanks to the progression of technology, we discovered dark matter (from studying the dynamic properties of galaxies), we learnt that galaxies can interact and merge and we have been able to observe them at all sorts of different wavelengths leading to the discoveries of a variety of properties of galaxies and other things. We have started mapping the universe (which, if you hadn't guessed, is significantly larger than just the Milky Way), inventing models like hierarchical assembly (sorry, I couldn't find a sufficiently lay link for this one) to fit our data and we are now much better equipped to study the evolution of galaxies.

4. Understanding
The last few points I made in the previous section are tied in with starting to really understand galaxies. This is the stage when we start to understand what's going on and become able to make predictions. As technology develops further, we can test our predictions more and more precisely and, sometimes this leads to discoveries of discrepancies and, again, we loop back to analysing and trying to explain these.

As hinted above, these aren't distinct stages. There is almost always going to be some overlap and a considerable amount of looping as the field progresses. And during the development of the field of galaxies, a whole lot of other (sub-) fields were born such black hole physics (well, more specifically, AGN), dark matter and dark energy (which are actually completely unrelated to each other).


*Don't get me started on why Hubble's ideas of "late-type" and "early-type" galaxies irritate me greatly.


Technology-driven advances

I mentioned above that some of the new discoveries were made when new technology made new data available. In the absence of new data what sometimes happens is that more and more elaborate theories are invented to explain bits of observations that we just don't have enough information to address.

An obvious example that springs to mind is the celestial spheres rotating in the sky which were once used to explain orbital mechanics. The idea was that the stars were embedded in a sphere made of ether or quintessence (or insert fifth element of choice here), surrounding the Earth which rotated around the Earth, accounting for the motions of the stars across the night sky. Then more spheres, with each of the planets, sun and moon embedded in one each, were added to explain the motions of the nearer celestial objects. As observations and measurements improved, more spheres were added to account for things like the precession of the equinoxes/solstices. Even Copernicus, when he came along, kept the celestial spheres and just changed them so that, other than the moon, they rotated around the sun rather than the Earth.

It wasn't until Kepler came along and developed his laws of planetary motion that we moved from celestial spheres to orbits and then, shortly after, Newton came up with gravity and proved Kepler's laws. Kepler was able to do this thanks to the more precise measurements of planetary motions made by Tycho Brahe. New observations made it possible to move forward and, indirectly, contributed to a new field (Newtonian gravity) to be born.

Moving forward

Now we have absolute proof of a lot of things in astronomy and astrophysics (and many other areas of science). Basically, we know stuff now. But we don't know everything, not by a long shot. Remember, just over a hundred years ago, scientists thought that we knew almost everything and only a few small details were left to be filled in. Then quantum mechanics was discovered.

I like to think of the pool of human knowledge as fractal; the more we know, the greater the area of the fractal and the more branches of knowledge we develop, the larger and more visible the infinite perimeter between knowledge and known unknowns becomes.