A habitable planet

So much for predictions. Now the press embargo is lifted we can take a look at what's going on around a small, nondescript, red dwarf star 20 light years away in the constellation of Libra. This star, Gliese 581 had been known to harbor 4 planets, including a couple of super-earths (less than 10 Earth masses) lurking at the very edges of the so-called orbital 'habitable zone'. Now the Lick-Carnegie Exoplanet Survey has crunched through 11 years of radial velocity spectroscopy or 'wobble' data on this star, and are announcing 2 additional planets, including a 3-4 Earth mass world smack bang in the middle of the habitable zone - GL 581 g.

This is the first world that clearly ticks off two key boxes in the laundry list for habitable planets - it's very close in mass to the Earth, and sits at a perfect distance from its parent star to stand a chance of having a temperate surface. It's a wonderful and thrilling discovery, and I'll confess to going out and staring in the direction of Libra last night - although sadly Gliese 581 is too faint to see directly with our beady human eyes. The relative ease (relative being the operative word, it took the might of the Keck observatory to pin this down) of finding this world and its sisters is a potent indicator that planets like this are quite common.

With a 37 day orbit (putting it about 0.15 AU from the 1/3rd solar mass star) there's a good chance that GL 581 g is tidally locked - with a permanent day and night side, although it's by no means clear that tidal locking is inevitable. This poses significant questions about any climate on the planetary surface - something astronomers and planetary scientists have been worrying about for a while for this kind of scenario. A thick enough atmosphere and thermal transport could help even out the drastic day/night temperature difference and keep things stable.

It's a long way from being Earth-2.0 though. The star is small, an M-dwarf, about 100 times less luminous than our Sun and strongly skewed to emitting photons in the infrared. This is also an ancient system, somewhere between 7 and 12 billion years old. GL 581 g is an old, old world bathed in red light. Although larger rocky planets than the Earth should have a more vigorous geophysical history - and the attendant chemical cycling that seems so critical for life - they too cool off with age and eventually suffer from stagnation. Whether GL 581 g has a significant water component or not is also something that we'll have to wait patiently to find out - one, or two generations of astronomical instrumentation in the future.

It's an alien place for sure. But to even be able to discuss these issues in the context of an actual, real, planet only 20 light years away is only a hairs breadth away from revolutionary - welcome to the coming age of exoplanetary science!

Jovian attraction

A beautifully bright object has been hanging in the sky the past few nights. Big enough to distinguish itself from the stars by not twinkling, the planet Jupiter is closer to the Earth at the moment than it has been since the 1960's. This past Friday I found myself peering up through the canyons of Manhattan, and there it was, brilliant enough to outshine the up-lit urban canopy, muscling aside the landing lights of jetliners and helicopters, second only to the post-harvest Moon. There was something enthralling about it. This tiny disk was so distinctly alien. In my mind's eye I superimposed the great gas giant, king of worlds, two and half times as massive as all the other planets in our system put together. Its vast jet streams and storms, its incredible family of 63 moons, the potent magnetic field, the great plasma torus as Io burrows through a tight orbit. There it was, a place in the sky, a tiny condensed blob of mass. It struck me just how vivid this perspective was on the yawning gulf of interplanetary terrain. Gravity does an incredible job at packing matter down to a small volume, our solar system is indeed mostly empty void, but for these extraordinary cusps of curved space.

The scientist's curse is that any semblance of poetry often gives way to curiosity about numbers. As far and as small as Jupiter appeared in the sky it was surely doing more than just bouncing photons into my eyes. All that mass, now a mere 592 million kilometers away, shouldn't I be able to feel the Jovian lure?

It turns out not by much. If Jupiter was at the zenith then it pulls at us with a gravitational acceleration a few hundred millionths that of the Earth. At first I was disappointed, it had felt so much more out there on the street. It's all a matter of relating though. The Empire State Building is a 365 thousand ton chunk of rock and steel, that's about 730 million pounds. When Jupiter sweeps close, and rises into the night, it reaches out and makes this iconic tower weigh about 26 pounds less than normal. That's not much, but 26 pounds is something I can envisage - it's a chunk of cornerstone, maybe a small floodlight. Not so much that anyone would notice, but there nonetheless.

Of course, the lunar and solar tides sweep across us all the time and do far more - but they lack the charisma of King Jove, our planetary gravity lord. If some distant race were to be monitoring our Sun, seeking the tell-tale dips and wobbles as it is pulled at by the planets, then they too would most likely first see the presence of this gas giant. In their catalog of exoplanets Sol b would be the entry. If anyone bothered with this nondescript system it might eventually gain a Sol c, another gas giant orbiting a little further out. Would anything compel them to keep looking, to seek those small inner worlds that might, or might not, be there? Looking in from the outside is very different from looking out from the inside at a bright object in the September sky.

 

The Martian Methane Chronicles

Since the firm detection of methane in Mars' atmosphere announced in 2009 (confirming earlier, more ambiguous results) it's been tough waiting for the next steps, impatience abounds. Methane is one of those compounds that has a predominantly biological origin on the Earth. Both here and on Mars, simple models of atmospheric chemistry suggest that methane molecules shouldn't last for very long - so if you see them then something is actively putting them into the air. Originally the models indicated that on Mars methane might last for as much as a couple hundred Earth years. The big shocker was that it was getting wiped out in less than one Earth year - showing large seasonal dependency.

We still don't understand either this result, or what's producing the methane (future measurements of isotopic compositions - heavy vs. light carbon and hydrogen - may indicate if biology is involved, since life  usually prefers light nuclei). Some beautiful new results by Fonti & Marzo, using Mars Global Surveyor data, add much needed detail, but also add further layers to the mystery.

Their extraordinary map (click on the image) shows the atmospheric distribution of methane during the Martian fall, three Martian (6 Earth) years ago. Where does the methane hover over? It lurks around both regions sculpted by past volcanism (Tharsis and Elysium) and - rather provocatively - around a region where we know there are large amounts of subsurface water ice (Arabia). This is wonderful news - there's definitely a connection with the most recently active geophysical sites, the same sort of places that could produce the kind of warm, chemically rich, subsurface environments loved by the types of microbial life we are familiar with. Whether the methane itself is geophysically produced, or comes from extinct or extant life, we now know that there are places on Mars where conditions have been pretty juicy.

The hitch is that by the end of Martian winter most of this methane vanishes. Something is very efficiently scrubbing the atmosphere on Mars. Prime candidates are wind driven particulates and tough oxidizers like perchlorates (which are good for making fireworks, no, honestly) that may sweep through the skies. But come the relative warmth of spring and summer, the methane reappears, either released from frozen deposits or, just possibly, maybe, tantalizingly, the result of ongoing biological activity - the blooms of Mars. Are we witnessing the first signs of a very alien ecological system? If biology is responsible then what does this cleaning system imply for the lifestyle of organisms? In effect the methane excreta is tidied up before it can pollute, like some efficient recycling program. Is there anything equivalent here on Earth?

Pushing up daisies

Life is opportunistic. About a 150 million years ago flowering plants did not exist on the Earth, today we are positively tripping over the things, so what happened? While there are many factors involved, a particularly interesting one has come up for recent discussion - and relates to a previous post on these pages.

A couple of weeks ago Bond & Scott published a paper in the rather wonderfully named scientific journal 'New Phytologist' that discusses how flowering planets, or angiosperms, spread during the Cretaceous some 65 to 145 million years ago. The novel aspect to this work is the suggestion that critically during this period, because of an elevated atmospheric oxygen level compared to today - perhaps to 25% rather than our paltry 21% by volume, surface fires were much more pervasive. I talked about this general phenomenon a while back.

So, the picture goes like this. Wildfires (well, I guess every fire was 'wild' during the Cretaceous) would have been significantly more frequent with higher atmospheric oxygen. This would have posed a significant challenge to surface plant life. Long-lived and slow growing species, like larger conifer trees - which have an ancient lineage - would have a hard time regenerating their populations fast enough. Imagine a cosy little spot, a fire rips through, everything burnt to a crisp. Seeds arrive, new plants grow, but sure enough another fire comes tearing across the land. Only those plants that had grown fast enough to mature and dump out the next round of seeds (carried off by wind and newly minted mammals and birds) would stand a chance at producing another generation. It's a vicious cycle, the fast growing angiosperms (one presumes helped along by insect and animal pollination) not only outrun the fire cycle, but they quickly produce the next round of fuel.

The upshot is that flowering plants don't get as much competition for resources from the previously dominant types of vegetation - in essence the weedy daisies win the day. The evidence for all this combustible carnage lurks in the remarkable charcoal deposits, and charcoal fossils from this geological period.

It's another example of the incredibly intertwined nature of life on a planet, and another great example of the constant 'what ifs' of evolution. Would an Earth that had always kept a low oxygen level have ended up with flowering plants - and the particular effect this implies on continental albedo and biosignatures?

Impending discovery

You really have to hand it to some people. As if it weren't bold enough to go around looking for planets around other stars, and trying to figure out things like the putative climates on as-of-yet-undiscovered terrestrial type worlds (guilty), someone has to go and tell us when the first genuinely Earth-like planet will be discovered. Doing anything in early May 2011? Block out your diary.

Arbesman and Laughlin posted a few days back a rather provocative paper that includes the word 'Scientometric' in the title. [I had to look this up, since I first assumed this was lifted from an episode of the Simpsons. Apparently it's the science of measuring and analyzing science, well, there you are. Much is to do with the impact particular scientific works have on a field]. The paper, to be fair, employs some reasonable statistical methods to try to see what the trend line is for the rate of exoplanet discovery versus planetary properties. In this case the authors devise a metric for planetary habitability based - albeit in a fairly sophisticated way - on planet mass and distance from parent star, and then employ a favorite statistical trick called 'boot-strapping'. Bootstrapping is one of those methods that you resort to when you really have a limited idea of what it is that your sample is actually sampling - and when there is noisy data.

The results of their analysis suggest that early next year (the month of May) we might expect Doppler searches for exoplanets to turn up something in the Earth-mass range and in the liquid-water orbital zone around a star. Transit searches don't fare so well, with no convergence on a date, tough luck Kepler - although the reason is that there just aren't enough transiting planets yet discovered in the right orbital range to allow for a meaningful prediction. It's an admirably immodest prediction. They even refer to the so-called Hawthorne Effect - whereby their publicizing this result may in fact influence the race to achieve what they predict - although they suggest that astronomers will be too busy struggling with the challenges of the task to care much.

What I think is most interesting is not this result itself (if they're right they get to brag, if they're wrong no-one will really care), but that these - genuinely smart - guys went to the effort. That alone I think suggests that the scientific community is sensing convergence on the quarry. The caveat to all this is, at the risk of sounding pessimistic, that planets meeting this particular habitability metric could still be dry, barren, atmosphere-less rocks. Finding such worlds would indeed suggest that several boxes can be checked in the search for Earth-2, but it's not the end of the story.

The smell of disaster

Here's the beginning of a radical thought. We expend a lot of scientific muscle on figuring out what can make conditions on small rocky planets as 'Earth-like' as possible: the long running theme of 'habitability' as defined by liquid surface water, temperate climate, organic chemistry, stability. It's fair enough, we have to start somewhere in our search for other worlds with life, we need to stack the deck towards something that we stand a chance of recognizing. However, this is only part of the template, the bit that is convenient and tidy.

If there is one inevitable thing about life it is that particular variants, species, modes of existence, are all prone to extinction. A new work by Drake & Griffen in Nature this week makes this point rather succinctly. They show, by subjecting water flea populations to a series of unfortunate events, how the population dynamics of a species can fundamentally shift due to environmental changes. Fluctuations in population numbers occur even in stable environments, but the character and size of these fluctuations changes in degrading environments, and beyond a certain point there is no recovery. Long before it all goes down the tube there are clear statistical indicators that things are not well - population sizes drop as the tree begins to fall.

Extinction is a fact of life. When we consider what the signatures are of life on exoplanets - chemical fingerprints in an atmosphere, selective photon absorption by bio-molecular structures (red edges), seasonal variations in albedo - we typically assume things are cozy on the ground. Given that (big) extinction events seem to happen pretty fast, one would think that the odds favor us observing a planet in the calm between these periods. However, we need to be careful about convenient truths.

We are most likely to be able to sniff out the signs of life on a terrestrial-type planet when it's in full swing. Suppose a world is having a particularly fertile episode, chock-a-block with organisms, but not a stable situation. It's prime for collapse. Relative populations will swing high and swing low. At the high point for some, a planet may show the greatest bio-signatures, and make itself far more tasty for our prying telescopic eyes. Without running the numbers it's impossible to give a precise answer, but it would seem that the odds will be shifted. We may be most likely to find not the signs of normality, but the signs of a system approaching some kind of biological collapse - just like the stock market, it's all about the fluctuations.

Another way to think about this is that success breeds vulnerability. If one measure of evolutionary success is the ability to occupy as many niches as possible, with as big a population as possible, then there are many terrestrial examples. This success comes with a price though. That level of dominance may alter the global environment itself (either chemically, or in terms of restraint on other species). If something degrades, or intra-species population dynamics get out of hand, the whole system will head south.

So, the radical thought. It may be that if/when we begin to see signs of biospheres on distant worlds around other suns, these will be places on the verge of collapse, overrun by certain types of life, populations fluctuating to unsustainable heights - albeit perhaps over timescales of hundreds or thousands of years. Our limited detection sensitivity will catch only the outliers.

It's not all doom and gloom though. If a long term goal of exoplanetary science and astrobiology is to place the Earth in proper context, and to potentially help ourselves, then watching disaster unfold elsewhere may be most helpful.