May 11 2012

Bugs from Space: Panspermia and The Interplanetary Transfer of Life

Teeming with Life? The Rho Ophiuchus cloud complex located about 500 light-years away. This view spans about five light-years across. The false-color image is taken from the Spitzer Space Telescope.

Historically, the theory of panspermia (from the Greek pas meaning ‘all’ and sperma meaning ‘seed’) - that life exists throughout the Universe, and is distributed by asteroids, meteoroids and planetesimals - arose as an attempt to address fundamental concerns over the evolution of life on our planet, specifically the ability of life to evolve in the harsh conditions postulated to be present on the early Earth. The theory was re-popularised by Francis Crick (the co-discoverer of the structure of the DNA molecule) and Leslie Orgel in a 1973 paper that rather controversially suggested that life was intentionally sent to the Earth by an advanced civilisation on another planet.

Conditions on the early Earth were unlikely to have particularly conducive for life. A particularly unpleasant period of the Earth’s history, known as the Late Heavy Bombardment (LHB), occurred at the Hadean-Archean boundary, roughly 4 Ga ago, and was characterised by extremely high cratering rates on inner Solar System planets, evident from petrological analysis of impact craters on the mostly undisturbed surface of the Moon. The LHB presents a conundrum when considering the evolution of life on Earth: a series of statistically plausible cataclysmic asteroid or meteorite impacts would have in effect sterilised the planet, boiling the oceans and obliterating vast swathes of terra firma. However, life arose rapidly after the LHB, recorded by carbon isotope analysis of sedimentary rocks to be possibly as early as ~3.8 Ga, in direct contention with our understanding of the probabilities of the critical evolutionary steps required for the evolution of life.  Is it possible that the Earth was seeded with life during, or after, the Late Heavy Bombardment?

Approaching this problem methodically, organisms that survive interplanetary transfer would have to endure ejection from an impacted planet, transit in space and eventual re-entry and impact onto another world, thousands or perhaps millions of years later. Is this really feasible?

As it turns out, it is.

Bacillus subtilis: a surprisingly competent astronaut (isciencemag.co.uk)

Studies analysing the factors associated with the ejection process considered the ability of bacteria to endure the associated extreme pressures, temperatures and acceleration likely to be experienced at the beginning of a trip to space.  A study exposing spores of Bacillus subtilis to peak shock pressures of 32 GPa (gigapascals) and post-shock temperatures of 250 °C, similar to values expected to have been experienced by Martian ejecta, reported survival rates of 10-4, indicating that the high shock-pressures and heating associated with planetary escape may not be detrimental to bacterial survival in the long-term, providing that a significant fraction of the ejecta avoids being heated to > 100 °C. Similar research indicated that, even when Bacillus subtilis spores are subjected to acceleration 2.5 – 25 times greater than would be normally experienced by ejected material, survival rates remained between 40 and 100%.

Similarly, interplanetary space may not be as harsh an environment as initially thought, at least for bacteria encased in several metres of rock. However, there are still the issues of vacuum, long periods of thermal inactivation, desiccation, photolysis of volatiles, impacts with micrometeorites and most significantly ionising radiation, in the form of solar ultra-violet, solar particle events and galactic cosmic rays to deal with. Modelling studies suggest that organisms at the centre of objects greater than 100m in diameter receive a sterilising dose of radiation after 10 to 100 million years in space, whilst centimetre and smaller objects are sterilised in less than 10,000 years. For an estimate of transit duration, Monte-Carlo trajectory analysis used to estimate the likely duration of ejecta in space approximates that the vast majority of Martian meteorites reach the Earth within 10,000 to 100 million years; for approximately 0.1% of Martian meteorites the transit period is less than 10,000 years. On top of this, recent studies suggest that the space environment may actually be conducive to microorganism growth, providing adequate radiation defence is in place, due to a currently undiscovered mechanism disrupting the ability of antibiotics to inhibit the proliferation of bacteria.

Seems plausible so far, but what about landing? With no external evidence of lithopanspermic planetary colonisation as of yet this stage of the transfer process is perhaps the least well understood. The shock of a low angle (≤ 30°) impact is predicted to be less than those associated with ejection, so a viable population may be able to survive landing. Even large organisms, such as the worms recovered from the wreckage of the Columbia space shuttle, may be able to weather re-entry. However, environmental conditions such as nutrient availability and appropriate osmolarity, low toxicity and low predation will dictate the ability of surviving organisms to colonise the planet.

Considering the time scale of the evolution of the Solar System, the ejecta liberating LHB event and the results of empirical studies on Earth and in space, the possibility of panspermia may not as unfeasible as it first appears…

Apr 12 2012

A Multiplicity of Worlds

This article was originally posted at the European Association of Geochemistry blog (click for link)

Undoubtedly the most exciting exoplanet news of the past week is the discovery of a star system with a total of 9 potential planets, surpassing even our own Solar System in terms of planetary diversity. University of Hertfordshire astronomer Mikko Tuomi discovered the bustling planetopolis around the enigmatic star HD 10180, a Sun-like G-type main sequence star 127 light years distant, using a probabilistic Bayesian analysis technique.

View of the sky around the star HD 10180 (center) Credit: ESO

HD 10180 has been known as a multi-planet system since 2010, but the last analysis of the HARPS data available for the star, carried out by Christopher Lovis last year, seemed to indicate a 6 or 7-planet system was most likely. However, the novel probabilistic methods used by Tuomi are more computationally intense than those previously applied, and confirm the findings of Lovis whilst also adding a further two planets to the planetary inventory of HD 10180.

Tuomi’s Bayesian method, which seeks to evaluate a number of possible scenarios to determine which is most consistent with the observations, finds that an orbital configuration including an eighth and ninth planet, with masses 5.1 and 1.9 times that of the Earth respectively, returns a 99.7% probability.

The planets themselves, denoted HD 10180 b through h, are a diverse bunch, including two Earth-mass terran planets, one superterran, five neptunian and one jovian-sized planet, and all are contained within 3.5 AU – roughly the distance of the asteroid belt between Mars and Jupiter in our Solar System. Despite their proximity, the orbits are predicted to be stable over astronomical time.

Orbital and size visualisation of the HD 10180 system, courtesy of Abel Mendez at the Planetary Habitability Laboratory. The blue-green area denotes the habitable zone. (click for more detail).

The image above, from the Habitable Exoplanets Catalog, provides a visualisation of the orbital system and a comparison of the sizes of the planets. Note that one neptunian, HD 10180 g, is within the habitable zone but is unlikely to be habitable given its large mass, at least not by our definition.

That’s an extraordinary array of sizes and shapes crammed into a comparatively small area, and unseats our Solar System, with a certain 8 planets (excluding trans-neputunian objects, asteroids and dwarf planets – sorry Pluto fans!), from atop the pile of planetary richness, all the while adding to our understanding of the mechanisms of planetary system formation.

Whilst this is certainly an exciting discovery, should we be surprised by the apparent ubiquity of multi-planetary systems? It would be more unusual if this architecture wasn’t the norm, given model predictions. Writing for his Scientific American blog Life, Unbounded, astrobiologist Caleb Scharf notes that the combined masses of the HD 10180 planets would only amount to roughly half that of Jupiter, and given the star’s similarity to our own Sun, its proto-planetary circumstellar disk should have contained a similar amount of material. Therefore, it wouldn’t be surprising if more planets lurked in the HD 10180 system somewhere!

In fact, the same could be said for any of the planetary systems we have detected so far as well as those that we find in the future. Our detection techniques remain biased towards massive, short-period planets that produce readily identifiable signals, particularly when using the radial velocity method, and we suffer from the fact that we have only been collecting data for a few years and so may have missed more orbitally distant, longer period planets.

However, as with most exoplanet discoveries, the detection of this diverse family of worlds serves to put our planet  into some wider perspective – to challenge the notion that Earth and this solar system are particularly unique, at least in an astronomical sense.

Solar systems, it seems, are everywhere.

Mar 13 2012

The Legacy of the Present

Chaos and causality: the course of our history will be defined by the decisions we make now (Space Time Colour by Keith Peters) (artfromcode.com)

How will the future judge us? Will our descendants be proud of our legacy and the achievements we sculpted at this particular juncture in human history, fondly imagining an exciting and revelatory time gone-by? Which of our many mistakes will be remembered? What, or who, will populate the pages dedicated to the present in the historical documents of the future?

By digitally archiving nearly everything, from our words and pictures, to international news and films, to the triviality of daily emails and receipts, we are inadvertently accumulating an enormous and unprecedented time-capsule of cultural and social information, ripe for the data-miners and historians of the future to peruse and analyse. Looking back, our descendants may celebrate the hope and opportunities that we created and exploited in a time of rapid change and uncertainty. This is an age of exploration and discovery unsurpassed by any in the past. We have human beacons in orbit, on the Moon and Mars, the outer planets and two explorers poised to enter the vacuous expanse of interstellar space. We move the Earth at will, harness the power of the atom and circumvent our own biology, and that of the organisms we share this planet with, to our own means.

However, the passage of time may not reflect kindly on us. Our children, perhaps distant, will study us as remote relatives separated by a gulf of time and knowledge and bear witness to our many and varied failures of foresight, as we our parents’ before us. They will marvel at our fallibility and indifference, reflect on the disasters and injustices narrowly avoided and those sadly and painfully endured. Through the clarity provided by the looking-glass of hindsight, they will picture the beleaguered ark of our young civilisation battered by the waves of ignorance, superstition and intolerance awash in the turbulent melting-pot ocean of this age. Captained as best we can by some modicum of insight and forethought, our leaky ship may just make it to calmer waters yet. The light from the distant shore is weak and easily obscured, yet a beacon of hope and reason guides our course onwards.

The decisions we make now will be our gift, or curse, to them. What tyrants and monsters, manifest of the inhumanity of our time, will carve out their legacies and what atrocities will they commit? These monsters may be people, and often are, but they need not necessarily be so. Rather, they may be fundamental failings of reason and understanding, particularly regarding how we treat each other and this planet; mistakes borne from a myopic lack of perspective that we have all allowed to propagate unchallenged. The crucial, globally relevant decisions we make today, many lacking foresight and made either in haste or with undue hesitation, distorted by corruption and cronyism and sealed by denial and an immature lack of responsibility, will be our legacy. Sensible and objectively necessary modes of action, albeit admittedly depressing and uncomfortable in the short term, are hindered by fickle tribal loyalties over the often short, usually filthy, lifespan of the career politician.

Even now it is easy to recognise those who, in the shadows and back-streets of politics and business, lurk intoxicated and maligned by greed and paranoia and plot the downfall of us all. Those whose machinations, knowingly or otherwise, are determined to dismantle and distort the warnings repeatedly provided by those working to protect our planet from the harm that this uncontrolled, unprecedented ecological and environmental experiment is subjecting upon it. Worryingly, it is these people and their misguidance that will be remembered: those who had the chance to avert a global disaster that may have an untold effect on the future direction of our species and the continued habitability of our planet, and yet did nothing, or even actively fought mitigation attempts at every step. For the psychologists of the future, they will make for rich pickings.

For it is these people, and the organisations and ideologies that they often represent, that epitomise the ugly face of corruption and denialism that may very well go down in the annuls of history as the true monsters of our time. Those who do not heed the repeated warnings that may one day spell the end of our brief stay on this planet, those who put money before reason, denial before rationalism and whose remarkable lack of foresight will condemn us all. They will go down in history books as defenders of the worst facets of human nature. As pitiful, transparent anachronisms and the morally bankrupt pawns of the most destructive, self-centred generation of organisms that this world has ever seen.

There are plenty of reasons to be optimistic though. There is much potential, but sadly little time, to address and mitigate these mistakes. The future, providing we’re in it, will be one where sense has prevailed. By virtue of our continued existence, it has to be. Any conceivable scenario in which the evidence has been ignored, where the bellowing drone of selfish contrarians have deafened the ears of logic and reason, will be a future that we, in our current social, political and cultural form, will not be part of. There may be no one able or willing to document the fall of our civilisation at the hands of our own inherent inability to manage the finite resources of this world. There may be nothing worth remembering. We may be the last chapter in the brief, eventful history of our species, or worse perhaps, the crucial turning point of an irreversible, yet avoidable, slow decline into chaos and decay. Perhaps all intelligent civilisations eventually destroy themselves in this way and perhaps that is why, despite the statistical implausibly of it being so, we seem to be traversing space and time on our own.

But perhaps, we’re different. This may be the defining moment in the history of our species. Can we overcome the pressures we are exerting on the planet, whilst simultaneously fighting those deeply invested in defending the objectively unsustainable means by which we are attempting to secure our future? Undoubtedly, our own shortsightedness may present the biggest challenge humanity has ever faced; overcoming it is our only means to ensure that the history books of tomorrow will be written.

Mar 6 2012

The European Association of Geochemistry Blog

The new blog by the European Association of Geochemistry (EAG), the body that promotes geochemistry and earth sciences in Europe and organises the annual Goldschmidt conference in partnership with its American counterpart, the Geochemical Society, launched last week. I was humbled to be asked to write an article for the launch, and perhaps contribute more regularly too, an offer which I gratefully accepted. I’ll still post regularly on here in the same vein as always, but also ply my wares over at the new blog too.

Have a read of this article for an overview of the new blog. My first post is on the astrobiological potential of Jupiter’s esoteric moon Europa, and can be found here.

AR

Feb 22 2012

Habitable Zone Of Red Dwarfs May Be Larger Than Once Thought

Stretching the spectrum: a hypothetical red dwarf planetary system (Research.gov)

Given that 80% of the stars in the Universe are M-type ‘red dwarfs’, research into the habitability of planets in these stars’ orbits has received relatively little attention in the past as they were generally considered unsuitable for hosting habitable planets due to their low mass and temperatures, as well as the propensity for planets in their orbit to be ‘tidally locked’. However, this trend has shown signs of reversal over the past few years, and habitability assessments have generally returned favourable reviews of M-star planets. The issue of tidal locking, where one hemisphere of a planet constantly faces the star, doesn’t seem to be resolved yet, but more research is being carried out and a definitive assessment may be forthcoming soon.

A paper published in Astrobiology this month has bolstered the habitability assessment of red dwarf systems even further. Manoj Joshi, now at the University of East Anglia, and Robert Haberle at the NASA Ames Research Center, have considered the effect that the longer wavelength spectra of M-stars may have on the ice-albedo feedback operating on planets within their habitable zones. Albedo describes the fractional reflectivity of a given surface, from 0 (nothing reflected, a hypothetical ‘black-body’ ) to 1 (all light reflected). On Earth, the albedo of ice is ~0.5 (50% of light reflected), whilst snow has an albedo of ~0.8.

The ice-albedo feedback is a fundamentally important abiotic feedback mechanism that has a powerful control over the planetary climate: it describes the ability of ice and snow to reflect light away from the surface, thereby cooling it further and causing more ice/snow to form, which continues to exacerbate the effect in what is termed a ‘positive’ or destabilising feedback loop. More ice, more light reflected away, cooler temperatures, more ice and so on.

The ice-albedo feedback is thought to have been at least partially responsible for the ‘Snowball’ or ‘Slushball’ Earth events that occurred in the late Proterozoic eon, approximately 600 million years ago, which saw the Earth frozen from pole to pole, with possible refugia at the equator. This interpretation is still rather contentious within the geosciences, but most researchers agree that the Earth experienced a period of extreme glaciation around this time, but its full extent, and how the Earth emerged from this deep-freeze, is still not fully understood.

The amount of incident light, as well as atmospheric greenhouse effects, exhibit a strong control on the ability of the ice-albedo feedback to enter a ‘runaway’ state by preventing temperatures from falling below a critical level of ice cover. Accordingly, this mechanism is often considered a controlling factor on the outer boundary of the habitable zone because of its very powerful ability to destabilise the planetary environment into an irreversible state of complete glaciation.

Joshi and Haberle constructed a simple model to test how the the ice-albedo feedback would operate on planets within the habitable zones of M-stars when considering the longer wavelength, lower energy emissions of these stars. Red dwarfs, as their name suggests, emit much of their radiation in the red and near-infrared portion of the electromagnetic spectrum. Observations from the red dwarfs Gliese 436 and GJ 1214 mentioned by the authors show that they emit much of their radiation at wavelengths greater than 0.7 μm, and significantly more in the 3 to 10 μm region than would be expected from a ‘black-body’ hypothesised M-type of a similar temperature. The albedo of ice and snow begins to decrease at wavelengths greater than 1 μm, and therefore the albedo of snow and ice covered surfaces on planets in the orbit of red dwarfs would be proportionally lower than that of the same surface on Earth (or any other planet in orbit around a G- or K-type star), meaning they reflect less radiation away from the surface, and that the ice-albedo feedback mechanism is weakened. For example, the authors show that snow or ice covered surfaces on planets orbiting GJ1214 may have albedos of 0.43 and 0.23 respectively, representing a significant decrease in the amount of incident light reflected from the surface and a dampening of the ice-albedo feedback mechanism.

Because of the diminished effect of the ice-albedo feedback mechanism around red dwarfs, the authors propose that their habitable zone may be 10-30% further from the star than was previously considered. This finding has a significant impact on the search for habitable exoplanets and for astrobiology, and, as is often the case with good science, has been drawn from a relatively simple experiment – in this case, by analysing the reflectivity of frozen or snowy surfaces under the observed radiative regime of red dwarfs. It seems that the tide really is turning in terms of our understanding of the habitability of planets in the orbits of red dwarfs, and that these numerous and ubiquitous stars should receive renewed research and observational attention.

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Click here for the Astrobiology article (requires subscription).

ResearchBlogging.org

Manoj M. Joshi and Robert M. Haberle (2012). Suppression of the water ice and snow albedo feedback on planets orbiting red dwarf stars and the subsequent widening of the habitable zone Astrobiology, 12 (1) DOI: http://arxiv.org/abs/1110.4525

Feb 7 2012

Gliese 667Cc: A new ‘Super-Earth’ basking in the light of three Suns

Astronomers announce with excitement the latest exoplanet found to be orbiting within the habitable zone of its star. In addition, the newly discovered Gliese 667Cc is a member of a very unique orbital system. Its parent star, the red dwarf Gliese 667C itself orbits a binary system of two K-type stars, Gliese 667A & B at an enormous distance roughly equivalent to 6 times that between the Sun and the dwarf planet Pluto. Accordingly, the distant binary system, whilst bound gravitationally, has no affect over the planetary environment of Gliese 667Cc, nicknamed ‘Vulcan’ by astronomers after the triple-star system home to Star Trek‘s Spock. I’m not much of a sci-fi fan, despite my interest in all things exoplanet, so I’ll stick to an shortened ‘Cc‘ for brevity.

Habitability

The Gliese 667C system revolves around a M1.5V red dwarf, a small star only 31% as massive as the Sun and much less luminous, located 22 light-years away from Earth in the constellation Scorpius. The habitable zone extends from 0.11 AU out to 0.23 AU, well within the orbit of Mercury if superimposed onto the Solar System.  Cc has a minimum mass equivalent to roughly 4.5 Earths and orbits at 0.12 AU, straddling the inner edge of the habitable zone. Accompanying Cc in orbit is Gliese 667Cb, a large (5.7 Earth masses) planet nestled at 0.05 AU, and possibly another planet of equal mass, dubbed Gliese 667Cd, at 0.24 AU.

Gliese 667 Cc performed very well in a habitability assessment undertaken by the Habitable Exoplanet Catalog (HEC), ranking as the planet with the greatest habitability potential of all discovered exoplanets to date:

Habitability assessment of Gliese 667Cc by the Habitable Exoplanet Catalog (information and graphics by HEC, 2012)

Figures in red are subject to large uncertainty, and will only be refined with more detailed observation. A quick refresher of the HEC metrics in the context of Cc: ESI is the ‘Earth Similarity Index’  and consists of several planetary characteristics, namely radius, density, escape velocity, and surface temperature that are used to determine the relative similarity of the planet to Earth on a scale from 0 (completely dissimilar) to 1 (identical). An ESI 0f 0.82 represents an ‘Earth-like’ world, but the large mass (5.2 as the mean expected mass) of Cc has negatively affected this value.

SPH is the Standard Primary Habitability, a measure (from 0 to 1), calculated from surface temperature and humidity, of the ability of the planet to support terrestrial primary producers. In the case of SPH, Cc outranks even the Earth! Its position half-way between the very centre of habitable zone and its inner edge, represented here by the metric HZD, means that it is extremely favourable to supporting a ecosystem of primary producers similar to those on Earth. However, as a red dwarf, Gliese 667C emits much of its radiation in the red, near-infrared (NIR) and infrared (IR) portion of the electromagnetic spectrum. Red dwarfs like Gliese 667C are also known to be more variable and prone to flaring.  The affect of this shift in wavelength would have very negative repercussions for Earth-based photosynthetic mechanisms which utilise visible light, but the possibility of photosystems evolved to exploit lower-energy NIR/IR radiation is hypothetically possible.

Other values to note are the comfortable planetary temperature of 29 °C, large mass and somewhat more suppressive gravity. A year on Cc lasts 28 days. Unfortunately, it isn’t possible to determine whether Cc is a rocky, watery or gas planet without an accurate measurement of its size, a parameter still unavailable at this stage. The effects of a possible atmosphere cannot be accounted for just yet but a thick greenhouse of water vapour, carbon dioxide or methane would elevate the planetary temperature beyond that considered habitable.

Lack of public interest

So it seems that Cc  is the new champion of the habitable planet competition being held by scientists on Earth, and the evidence seems to back up their claims. Why then the lack of public interest? Outside of popular science websites and publications, news of this new planetary utopia is hard to find. Contrast the scarcity of coverage with the hype surrounding Kepler 22b two months ago, and I fear the predictions I made in these posts may have come to fruition. The wider public is bored; they’ve heard it all before and become desensitised our disinterested. Kepler 22b is habitable, so is Gliese 581d and now so is Gliese 667Cc. It’s disappointing, but inevitable, that the furore of excitement surround these planet discoveries wasn’t sustainable. The thing is, we still haven’t stumbled across the perfect Earth analogue, a replica of our watery, rocky globe. Yet. We will do, and when this day comes and the discovery is announced, I fear the room may be empty save for a few dedicated science correspondents that realise the very real implication of finding a planet like this.

Update (08/02)

It seems that in my haste to bemoan the lack of mainstream press coverage of Cc, I neglected to detect the underlying politics of the announcement. The main reason that Kepler 22b attracted so much more attention is that Cc was not announced by NASA. The NASA PR machine is an effective beast. Also, the discovery of Gliese 667Cc was first announced last November by a European team of astronomers led by Xavier Bonfils from Université Joseph Fourier in Grenoble, France. However, it’s confirmation came yesterday from an international team lead by two American astronomers, Guillem Anglada-Escudé and Paul Butler from the Carnegie Institute for Science. Cc‘s discoverer is therefore under debate.

The coverage of Gliese 667Cc also seems to suffer from a somewhat of a geographical disconnect. Daniel Fischer, who runs the excellent ‘The Cosmic Mirror‘ site, notes that the coverage of Cc has been extensive in his native Germany because of Anglada-Escudé’s link with the University of Göttingen. Parodies and further analysis can be found here and here, respectively (in German – thanks Google Translate!).

It seems that the story of Gliese 667Cc is far from over.

Jan 23 2012

We, the pioneers.

As Voyager 1 cradles the edge of our Solar System, poised to enter the vacuous expanse of deep space, we are approaching a milestone that many on this planet are not aware of. As this magnificent example of human engineering leaves the confines of the warm embrace of our Sun, at ~120 AU a now faint and distant beacon in the enveloping darkness, we will become an interstellar species. The gravitas of this monumental achievement should not be overlooked.

Whilst it remains theoretically feasible that our universe may be teeming with life, intelligence of space-faring calibre may be exceedingly rare. We, the product of a knife-edge balancing-act between biological, geochemical and astronomical implausibility, are lucky to be  here at all.  The inordinate complexity, the innumerable coincidences and the eventual culmination of 3 billion years of evolution – we stand on the peak of the impossible, gazing out into the void, with Voyager as our first envoy to the stars.

It is unlikely, but not impossible, that any interstellar civilisation has come before us. We’ve been listening for our galactic neighbours, via the enormous ear of SETI, for over 50 years to no avail. No radio chatter, no xenoarchaeology nor ambassadorial spacecraft. Given the ubiquity of planet forming material, and what we consider the relative normality of our watery home, the emptiness – the silence, is paradoxical.

Voyager 1 is preparing to leave the heliosphere and enter the interstellar medium (IOP.com, 2011)

The galaxy is ~13.2 billion years old and our 4.5 billion year-old Solar System has orbited its centre ~25 times. This planet has been habitable for around 4 billion years, and based on our best estimates, we have another half a billion or so to go before the evolution of the Sun renders the planet uninhabitable. We’ve been hitching a ride through space for one hundredth of one percent (5 million years) of the age of our planet, and have had space technology for one thousandth of that  time (50 years).  Assuming this is the case for most habitable planets, and knowing as we do that exponential colonial growth is impossible, it seems likely that if intelligent civilisations had arisen at any point in the history of our galaxy, and at some coordinate closer to the galactic core, there has been little evidence to suggest that they ever made it out this far. Given that colonisation infers a survival value, the fact that nearby planets give no indication of being inhabited leads to the conclusion that there are likely to be no other colonisers out there.

What conclusions can we draw from the silence? Well, conjecture abounds. Perhaps the galaxy is teaming with civilisations who have consciously hidden themselves from us until we overcome some technological or societal hurdle that would usher our entry into the ‘galactic club’ – perhaps superluminal travel or the formation of a world government? Who knows. In the immediate future, and without too much speculation, we can possibly infer that we may be the only intelligent civilisation ever to have arisen, in this neighbourhood anyway. If so, that places quite a burden on us, whether we realise it or not, to protect our planet and each other until such time that we can make our own way through the stars. We, or most likely our distant descendants, may be the sole custodians of the true meaning of existence, nature and the universe; the formulators and keepers of the ‘theory of everything’. Their success, and ours in the meantime, depends on the decisions we make now.

We are the pioneers, but we are also most certainly endangered by our own machinations. Up to this point, some of those decisions have been rather poor and have possibly compromised the very habitability of the planet we draw life from. Others, like Voyager et al. have been great. This humble, unassuming vessel represents the first step of an infant civilisation adopting a truly universalist, extrospective outlook. With 10 – 15 years of power left, Voyager will continue to take measurements and beam information back to Earth on the transition through the heliopause and the composition of the interstellar medium. After its batteries have died and its instruments have gone silent Voyager will continue to obediently sail through the depths of space on a mission lasting an eternity; a mission with no end and no more formal objectives. The spacecraft will not decay in the vacuum of space and its form and technology will be preserved indefinitely as a timecapsule to the stars. Long after the Earth has ceased to exist, Voyager will remain.

What a truly magnificent thought! It is humbling to be part of the first generation of interstellar human beings and an honour to have Voyager as our flagship.

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My science friend Luke drew a comic about the Voyager probes, which can be found here.

Jan 12 2012

New Planets for the New Year

2011 was an exciting year for exoplanets, mainly thanks to the Kepler mission and its abundance of results, and 2012 looks set to be even better. Even though we’re not quite half-way through January, a number of exoplanet discoveries have already been announced.

Yesterday the three smallest exoplanet candidates to date were unveiled at a NASA press conference. The three planets orbit an M-type star, denoted ‘Kepler Object of Interest’ (KOI) 961, which has about 60% the mass of the Sun but only one sixth its diameter. The three planets, KOI 961.01, KOI 961.02 and KOI 961.03 follow a diminutive orbit analogous to the moon system of a gas giant: the most proximate planet has a semi-major axis of roughly 1.5 million kilometres, or 0.010 AU, which is slightly more distant than the orbit of Titan around Saturn. All three planets’ orbits could be roughly contained within the orbit of Iapetus, Saturn’s third largest moon, which has a semi-major axis of 3.5 million kilometres.

The planetary system of the star 'KOI 961' consists of the three small planets in a very tight orbit. The size of the 'planet' is representative of it's radius (x-axis is on a logarithmic scale). Also included is the habitable zone (inner boundary in yellow, outer in blue), Gliese 581d (a larger planet that also orbits an M-type star) and Earth and Mars for scale.

The planets themselves are well within the ‘hot zone’ of the star, outside of the inner boundary of the habitable zone, with equilibrium temperatures (excluding greenhouse effects) of between 960 (KOI 961.01) and 1150 °K (KOI 961.02).  Their radii are reported as 0.57, 0.78 and 0.73 Earth radii respectively, the smallest of which is comparable to Mars. Kepler continues to surprise; these small planets in their miniature orbits are a testament to the sensitivity of the instrument, as well as an indication of the variety of  planetary systems and possible orbital configurations across the galaxy.

Circumbinary Planets

Artists rendition of the circumbinary planet Kepler 35b orbiting the binary Kepler 35(AB)

Also announced yesterday were the planets Kepler 34b and 35b, unique in the fact that they are both ‘circumbinary’ planets. This means that, along with Kepler 16b first announced in December 2011, they orbit two stars  in an orbital configuration now thought to be common throughout the galaxy. Weighing in at ~70 Earth masses Kepler 34b orbits two Sun-like (1.04 and 1.02 Solar masses) stars over the course of a 284 day orbit. The stars themselves orbit one another in 28 days and are separated by 0.22 AU, or ~32 million kilometres. At ~40 Earth masses, Kepler 35b is slightly smaller and orbits the binary pair Kepler 35A and B, with 0.80 and 0.88 solar masses respectively, at a distance of 0.60 AU. The stars are separated by 0.17 AU, or 25 million kilometres. Both planets appear to be within the HZ distance of their respective stars, but their complex orbits are likely to complicate this metric and any conclusions regarding their habitability will have to await further study. However, considering their masses and densities (0.61 and 0.41 g cm-3 respectively) they are likely to be low-density gas giants with little potential for habitability despite their residence within the habitable zone. Nevertheless, the possibility of orbiting habitable moons cannot be excluded.

Principal investigator Dr. William Welsh of San Diego State University says that the unique orbital configurations of  circumbinary planets would make for novel and interesting climate dynamics due to large multi-periodic variations in insolation over the course of a single orbit. Speaking at the American Astronomical Society meeting on behalf of the Kepler Science Team he said, “It would be like cycling through all four seasons many times per year, with huge temperature changes. The effects of these climate swings on the atmospheric dynamics, and ultimately on the evolution of life on habitable circumbinary planets, is a fascinating topic that we are just beginning to explore.”

Dec 21 2011

Kepler 20

So, more exciting news from the Kepler team: Kepler 20, a planetary system of five planets orbiting an Earth-like star 946 light years away in the constellation Lyra was announced at a NASA press conference yesterday. This time, none of the planets, from Kepler 20b through -f, were estimated to be orbiting within the habitable zone of Kepler 20 but instead had very short orbital periods of between 3 and 77 days, putting them all within the orbit of Mercury if superimposed onto our Solar System.  The importance of the Kepler 20 system is that two of the planets (-e and -f) are Earth-sized, or possibly even smaller, making them the smallest extrasolar planets yet discovered. The remaining three planets -b,-c and -d are sub-Neptune sized. I’ve done my best to illustrate their orbital configurations here (click for a better view):

 

Diagram of the Kepler 20 planetary system relative to the habitable zone distance, Earth and Jupiter. The mass estimates for Kepler 20e and -f is the mean of the upper and lower estimates provided by Fressin et al. (2011).

 

In terms of habitability, there is not much we can garner from the Kepler 20 planets. They are all in the ‘hot zone’ in very close proximity to the star. Their equilibrium temperatures are likely to range from 1014 °K (Kepler 20b) to 369 °K (Kepler 20d) – much too hot to host terrestrial biology within our current experience and understanding. It is possible, within ‘theoretical considerations‘ (i.e. planet formation model output), that Kepler 20e and -f are rocky, and that Kepler 20d may have a thick water-vapour atmosphere.

The larger Kepler 20 b, -c and -d were detected using a combination of photometry and astrometry, but due to their relatively small photometric and/or gravitational effect, the smaller Kepler 20 planets were uncovered using a statistical method that excluded the possibility that the detected signal was caused by a binary star eclipsing Kepler 20. The Kepler team have now provided proof that they are able to detect Earth-sized planets, even in very short period orbits. Coupled with the discovery of Kepler 22b in the habitable zone a few weeks ago, their brief of discovering Earth-like planets with the habitable zone of distant stars is partially complete – all we need now is to find an Earth-sized planet, like Kepler 20e, in the habitable zone, like Kepler 22b. It really is only a matter of time now.

Dec 8 2011

Uninhabitable planets in the Habitable Zone.

Following the excitement surrounding the discovery of potentially Earth-like exoplanet Kepler 22-b (K22-b) earlier this week, the ‘habitable zone’ (HZ) concept is becoming increasingly important to our interpretation of these announcements. Used unilaterally, the HZ metric may be misleading and it should be considered as a rough guide, interpreted relative to other available planetary characteristics.

The HZ describes the theoretical distance (with both upper and lower limits) at which a given planet must orbit a star to support the basic fundamental requirements for the emergence of life based on our understanding of the evolution of the biosphere on Earth. The HZ theory is based on terrestrial (rocky, as opposed to gaseous or icy) planets that exhibit dynamic tectonic activity, that have active magnetospheres and atmospheres of water, carbon dioxide and nitrogen with liquid water available at the surface. Liquid water is the key; the giver of life and the fundamental factor in defining the HZ in any planetary system.

It should be relatively easy to spot a number of limitations of the HZ concept already; we are still unsure of the composition of K22-b which would significantly affect any habitability analysis.  Also, we assume that exobiology would have the same requisites for life which may not necessarily be so; this alludes to our inability to detach ourselves from our anthropic bias. The ubiquity of extremophilic organisms (those able to tolerate extremes of temperature, pressure, salinity, radiation etc.) on Earth has also lent some credence to the extension of the parameters of the habitable zone beyond those originally included. All in all, the idea of a habitable zone is a great thought experiment, but it may not necessarily translate into reality. Planetary processes, such as tectonics and atmospheric greenhouse effects, warp the boundaries of the HZ. Astronomers are now considering the very real possibility of liquid water existing in sub-surface oceans of Jupiter’s moon Europa, a body well outside of the habitable zone of our solar system.

I think that the hype surrounding the discovery of K22-b has highlighted the limitations of using the habitable zone concept in isolation when considering the potential for life on other planets. It certainly provides an excellent starting point to narrow down the enormous Kepler catalogue, and it will play an important role in identifying habitable planet candidates in the future. Clearly however, other factors need to be taken into consideration. I’ve seen numerous articles and posts describing Kepler 22-b as “Earth’s twin” and “Earth 2.0″ based solely on the fact that it has been discovered to be orbiting within the habitable zone of Kepler 22. What is usually skipped over, or not mentioned at all, is that K22-b has a radius 2.4 times that of the Earth, and estimates of its mass range from 10 to 34 times that of our planet. With a surface gravity possibly nearing 6 g, an ‘Earth-twin’ this is not. My favourite resource for planetary habitability, the Habitable Exoplanet Catalog run by UPR Arecibo has released an updated best and worst case scenario analysis for K22-b:

I covered the worst case scenario in my last post, but here it is again (updated):

Kepler-22 b habitability assessment from the HEC

A gaseous planet with huge mass and crushing gravity, and still a very hot 42 °C for those that can stand being squished to a pulp. With a greenhouse atmosphere similar in composition to that of the Earth, the temperature would be roughly -15 °C; with Venus’ super-greenhouse, the surface would be closer to a lead-melting 460 °C.

This estimation is based on an ocean planet with a rocky core, no continents and thick cloud cover.  It’s mass has been downgraded slightly, but its still an impressive ~10 times that of Earth. Its surface gravity isn’t quite as oppressive as first thought, but it would still be very uncomfortable especially in the heat. The conclusion is that it would be habitable, but only at a push, and you certainly wouldn’t choose to spend your summer holidays on Kepler 22-b.

I can only reiterate the conclusions of my last post; it’s important not to over-hype planets that are only borderline habitable in the very best case scenario. It’s unscientific and will be damaging to the public perception of exoplantology in the long run. There are only so many times that the exoplanet community can cry wolf and expect the public to be interested in these kind of Earth analogue stories. When an excellent candidate comes along, which I firmly believe that it will, there is a risk that this monumental announcement will be made to an empty room.