“Two possibilities exist: either we’re alone in the Universe, or we’re not.
Both are equally terrifying.”
- Arthur C. Clarke.

Artist’s concept of Kepler 62e, the most ‘Earth-like’ exoplanet found to date. Credit: NASA/JPL/Caltech

Views are shifting, but at present the public perception is that astrobiology is the study of little green men from Mars. However, the reality is very different, as anyone working in the field will tell you. I’ve always seen it as an organic extension and interdisciplinary marriage of the natural sciences, with solid scientific foundations firmly laid by the likes of Carl Sagan and Lynn Margulis. Astrobiology carries significant intellectual clout and I am convinced that contributions made by those working in the field will likely produce some of, if not the most, fantastic discoveries of this century.

This is why outspoken minority opinions that come to dominate discussion can be detrimental to both the public perception of the field, and also the direction and coherence of the discipline itself. When the press is seeking an ‘astrobiologist’ to comment on the latest Curiosity announcement or claim of life from outer space, there is a chance that they will go to the person who shouts loudest, regardless of whether that person represents the broad consensus of others in the field.

Don’t get me wrong, every discipline tends to attract their fair share of eccentrics and contrarians, but if that field is relatively young and already struggling to find a foothold amongst mainstream science in the UK, this can prove a bit of problem. To make matters worse, this field, unlike others, lies on the rational border of the fertile pseudoscientific pastures of aliens and UFOs and associated guff.

So, what to do?

Exclude the relevant parties from the forum and proceed as normal? This strategy risks alienation (ahem), and could end up backfiring as the troublemakers shout from the rooftops about systematic silencing by the ‘academy’ and the existence of an overarching conspiracy to keep their fantastical research from the public, thereby further accentuating the stereotype of the paranoid alien hunter to the public and other academics,and providing them with the attention they originally sought from their peers.

I think the answer is more integration, not less. Yes, these individuals may have made fundamental flaws at nearly every stage of their research, which itself was based on significant misapplication of the scientific method, but that is all the more reason to give them access to the ears and opinions of members in the field. This way, their methods can be improved and some of the more unscientific claims can be weeded out prior to steering any potential publication towards a peer-reviewed journal where its merits can objectively assessed by the wider community.

The organisers of the conference had a difficult decision to make, and made the right one I think by including the research in the schedule. It was then up to the attendees to highlight major errors, foster discussion and debate and attempt to reduce the isolation of this group from the community and the higher standards required to publish good work in this field.

It would have been easy to do this in Edinburgh. Firstly, when given the opportunity, challenge their claims! This could be done after their talk, during a poster session, or in the literature. I’ll be the first to admit, I’m a bit reluctant to do this. It might be that I’m an early career scientist, or because of my inherently British fear of confrontation and misguided diplomatic aspirations toward a plurality of opinions, but there should be a limit. When a claim impinges on that limit the immediate repercussions should take the form of an erudite and impassioned, yet polite, rebuttal. Insist on hard evidence, critically scrutinise methodologies and deconstruct their results – this is science in action and it’s how progress is made.

This is where the attendees of ASB5 may have faltered. We all had multiple opportunities to address the relevant parties and their claims, but instead hid behind a passive-aggressive tut and endured comment after comment of rambling through gritted teeth. I understand that any learned society cannot make assertions and give direct answers to difficult questions, but they can take stewardship of the conversation and advance the discussion in a democratic forum, the rest is up to the audience. We owe it to the hard work conducted by researchers in astrobiology to ensure that we adhere to only the highest standards of scientific investigation and scrutiny as a community because the future of this discipline as a viable and respected avenue for research and funding is at stake.

This the second in a series of posts by me at Things We Don’t Know about the many unknowns involved in the study of planets in the orbit of other stars across the galaxy.

The first planet discovered orbiting another star was detected by astronomers at an observatory in France in 1995. The planet is an enormous gas giant, half the mass of Jupiter, orbiting very close to the Sun-like star 51 Pegasi in the constellation Pegasus, 50 light-years from Earth. The existence of other planetary systems had been predicted by astronomers for centuries and the discovery marked a monumental breakthrough in astronomical research. Since then, rapid improvements in technology and observational techniques have resulted in the discovery of 863 confirmed ‘exoplanets’ to date.

How many planets are there? As astronomers hunt for planets orbiting other stars, we are starting to form a picture of how many planets there are in the galaxy. Image credit: Luke Surl

Unlike the direct observation of stars, the detection of planetary bodies requires astronomers to use a number of indirect methods to infer their existence. Due to the immense distances involved, the distance between any planet and their host star when viewed from Earth is tiny, and the brightness of the star itself effectively blinds instruments and obscures any planets in their orbit, which are much less bright by comparison. Therefore, astronomers have devised a number of ingenious methods to tease out planet data from their observations, but they require a great deal of skill, a generous helping of statistical analysis and a pinch of luck.

The most successful means of planet detection to date, yielding roughly 58% of all discoveries, is called the radial velocity method. This technique exploits the fact that the host star and its planets orbit a common centre of mass, and the planets exert a tiny ‘tug’ on the star that results in a very slight wobble – a signature that can be detected and used to infer the existence of one or more planets. Another successful indirect method of detection, responsible for a third of exoplanet discoveries, is called the transit method. When viewed from the Earth, a planet orbiting a star periodically passes in front of the star (‘transits’) and obscures a very small amount of its light, resulting in a tiny but consistent reduction in the amount of light received by Earth-based instruments. The amount of light that is blocked out provides some information about the size of planet, as larger planets will obscure relatively more light, and the frequency and duration of the transit can be used to infer the distance from the star that the planet orbits. NASA’s Kepler space telescope, launched in 2009, uses this method and it has proved extremely fruitful, resulting in the discovery of 105 confirmed exoplanets to date. Additionally, there are a further 2,740 potential planets (called ‘planet candidates’) detected by Kepler awaiting confirmation.

Kepler’s search area extends 3000 light years from Earth along the Orion Spur of the Milky Way. Image copyright © Jon Lomberg, used with permission.

However, the science of exoplanet detection is by no means certain; many teams use different statistical methods to isolate exoplanet signals, and the lack of consistency means that many discoveries are initially met with scepticism. With little means of directly imaging these planets, debate continues about the existence of a number of exoplanet candidates, and the finer details of many confirmed planetary systems. Also, the methods mentioned above tend to favour large planets as their effect on their star (either by increased ‘wobble’ or by concealing more light during transit) is proportionally greater.

We find ourselves at an exciting, but also frustrating, juncture at the birth of exoplanet detection. Our 862 planet sample is impressive and the effort and skill of the astronomers responsible for their detection should be applauded. However, we have only begun to scratch the surface of planet discovery. Kepler can survey an impressive 100,000 stars, but that is only one millionth of the total stars in the Milky Way Galaxy. Many, many more stars and planets remain out of reach of our telescopes, at least for the foreseeable future.

First direct picture of an alien planet orbiting a Sun-like star. Image credit: Gemini Observatory

Admittedly, to say that no planet has been directly imaged would not be quite accurate. Some extremely large planets, in most cases 5 or 10 times the mass of Jupiter, orbiting at great distances from their stars have been directly imaged. These first pictures represent great steps forward for exoplanet research, but technological constraints impose limits on the size and orbital distance of planets able to be imaged in this way, and the direct imaging of small, Earth-like planets orbiting relatively near to their host stars is not yet possible.

In my next post, I hope to take a more detailed tour through the current exoplanet catalogue to highlight some of the interesting and exotic planets that inhabit our galactic neighbourhood, and illustrate what the diversity of these planets can tell us about the Earth and our Solar System.

This the first in a series of posts by me at Things We Don’t Know about the many unknowns involved in the study of planets in the orbit of other stars across the galaxy. 

Since the middle of the last century, against the backdrop of greatly expanding space technology and understanding, scientists have wondered about our place in the vast universe and whether we are alone or not. When it comes down to it, why would we be? There is no reason, be it physical or chemical, life couldn’t exist elsewhere. At first glance it seems that we live on a relatively normal planet, our parent star is of a fairly common variety and our corner of the galaxy isn’t all that extraordinary. Water and other ‘building block’ organic compounds, thought crucial for life in any imaginable form, are relatively abundant throughout the galaxy.

There are at least 100 billion (that’s a 1 followed by eleven zeroes) stars in the Milky Way galaxy alone; many we now know come complete with a family of planets in their orbit. On top of that, several of these newly-discovered ‘exoplanets’ are not that different from the Earth in mass or orbital distance from their parent stars. In fact, a recent study calculated that a staggering 17 billion Earth-like planets are likely to exist in the Milky Way alone! Surely, more than one of those worlds would have life of some kind or the other clinging to its surface? And if there was life, even if it was almost vanishingly rare, could another species with a similar level of intelligence to humans exist on another one of those billions of planets out there in the reaches of space?

Artist’s impression of Gliese 667Cc, a possible Earth-like exoplanet 22 light years distant, in the constellation Scorpius.
Credit: ESO/L. Calçada

What does this apparent silence say about us and our planet? Are we the product of an extremely fortunate evolutionary accident resulting from the interplay between our astronomical and planetary environment? On some distinguishable level, the search for other intelligent species is a thinly veiled search for our own place, both physically and philosophically and convincing proof of a co-existent alien civilisation would most likely have significant scientific, social, political and religious ramifications.

Today, researchers in the burgeoning scientific field of astrobiology attempt to tackle these kinds of open questions, as well as many others in disciplines spanning chemistry and geology, astronomy, biology and even economics and the social sciences. In my completely biased opinion, studying exoplanets is one of the most exciting areas of science to be working in right now, and the rate of new advances and discoveries are progressing at breakneck speed (for science, anyway). However, even despite these recent findings, our understanding of the processes operating on these planets remains regrettably threadbare. Given the immense distances involved and sensitivity required, only limited data is available for a given planet and some large uncertainties remain even when information has been collected. We have yet to image an exoplanet directly, and it may be decades before the technology is available to do so.

Over the course of several posts, I’ll do my best to illuminate the cunning techniques that are being used to tease exoplanet data out of the noise, and explain how the limitations of contemporary technology are driving the development of new methods of remote planetary investigation. Despite the difficulties involved, a picture of our planetary neighbours is beginning to emerge and the results have been surprising and exciting in equal measure.

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When viewed from space, the Earth glows like a blue marble under the light of the distant Sun, bobbing gently in an unimaginably vast sea of darkness. Oceans of azure water lap against the winding, jagged coastlines and pure-white clouds swirl gracefully across its face, temporarily obscuring from view the extensive brown-green landmasses below. At first, there is little to suggest that beneath the clouds, scuttling around the coasts, intelligent* bipedal apes are busying themselves with their daily activities; most utterly absorbed by their own inflated sense of self-importance and certain of their centrality to all the workings of the cosmos. However, with the exception of a couple hundred satellites, a permanently occupied human outpost and sea of debris in low Earth orbit, we have remarkably little effect on the environment of space outside the Earth. We assume that not much of our global civilisation can be detected from astronomical distances, excluding the banality of 1960s television that is currently washing across star systems 50 light years from here, carried outwards from the Earth by radio waves.

‘Earthrise’ – Taken by Apollo 8 crew-member Bill Anders on December 24, 1968 while in orbit around the Moon (NASA)

If however, somewhere out there in the menagerie of stars that is the Milky Way, an alien astronomer was perched at his (or her) telescope one night staring out into dark, and our Solar System happened to come into view, what would they see? The blinding glare of the Sun would obscure our family of planets from direct view**,  but perhaps some information could be gleamed via other methods nonetheless. Using radial velocity measurements or transit timings for example, a whole host of planets seem to be present around this particular G-type star:  four gas or ice giants and possibly four smaller bodies. If our exo-astronomer ran their evening’s observations through their superior spectrometer however, chances are they may be surprised by the results returned from one tiny planet in the orbit of this humdrum star.

Spectrometers measure the properties of light, at first emitted by stars (in the this case, the Sun) but then altered by the constituent gases of the planetary atmospheres through which the beam passes on the way to the receiving instrument. Different gases absorb light at different wavelengths to produce characteristic spectra, and the composition of the atmosphere can be teased out of the noise with sufficient skill and instrument capabilities. The high levels of oxygen, methane and other gases associated with biological or industrial activity detected in the atmosphere of this planet should result in the alien equivalent of a raised eyebrow or two. Methane and other reducing gases are usually rapidly oxidised in the presence of oxygen, meaning that detecting an appreciable amount in the atmosphere of an otherwise relatively oxidised planet may suggest that a biological mechanism is responsible for its continual replenishment. This kind of atmospheric disequilibrium is termed by astrobiologists a ‘biosignature’ for this very reason.

Planetary atmospheres are something we are all intimately familiar with; the Earth’s is the medium in which all of our lives play themselves out. Ours is filled with life-giving oxygen, greenhouse gases essential (in the right balance) to maintaining planetary climate and ozone that shields us from the Sun’s harmful rays. If humans are to ever colonise Mars, atmospheric engineering on a global scale would be essential to provide a clement climate. Without the thin envelope of gases that clings to the surface of our planet, life as we know it would be unlikely to exist, and the advanced civilisations of intelligent species like humans would be impossible. However, we probably take for granted the atmosphere’s ability to act as a mirror of our industrial and technological activities detectable at light-year distances, able to preserve the unique signatures of the gases associated with these processes and hold them there for those with the correct instruments to see.

Under the watchful eye(s) of our distant alien astronomer’s stern but fair supervisor, and following a long and arduous proposal to the relevant funding bodies of their world during which detractors on the committee would openly balk at the possibility of advanced life outside of their star-system, more observing time would be begrudgingly allocated to collecting data about this strange planet in obvious thermodynamic disequilibrium. A soup of exotic chemicals are now detected: high and increasing amounts of CO₂ and constantly replenished methane along with a suite of more harmful and industrially produced compounds like chlorofluorocarbons (CFCs). There is no known biological mechanism for producing CFCs, so their detection in the atmosphere of this planet is a strong indication of the activities of industry, termed a ‘technosignature’ in line with the naming conventions of the field. The exo-astronomer has struck gold (or the equivalently rare element on their planet); they have detected strong evidence of a technologically advanced species at work, despite having never seen the surface of their planet itself. In doing so, they have forever altered the way their civilisation views itself – one of perhaps many in a vast, galactic family. Whilst they are given a passing mention in the local paper, statues of the members of the funding committee are erected in a square of their nation’s capital, for the whole project was their idea from the outset.

Ignoring the thinly-veiled allegorical critique of science funding on Earth, this is the theory that lies behind the most recent proposal out of the aptly-named not-for-profit Blue Marble Space Institute of Science (BMSIS). Their project, currently seeking funding at PetriDish.org, aims to use computer modelling techniques to simulate the hypothetical spectra of planets that have elevated levels of CFCs in their atmospheres. Whilst out of our reach at present, the hope is that instruments of the future will be able to examine the atmospheres of these planets to search for signs of life, and these hypothetical signatures would be readily available for comparison against data received from the planet of interest. They will form a standard by which to determine whether the received spectra are the result of accidental or intentional alteration by another global civilisation light years distant.Acquiring science funding from kickstarteresque sites like PetriDish.org is ideal for this kind of small project; perhaps too close to the politically-charged line that NASA is willing to tread when it comes to funding SETI projects, but with sufficient outside interest to attract funding and a mandate directly from the public. The four BMSIS investigators are looking for $24,000  to cover their costs, with a minimum donation of $1. Addressing a resolvable problem within the field with admirable foresight, optimism and cost-effectiveness and detached from the bureaucracy of tax-payer funded science institutions, surely this is the kind of research that should be at the forefront of astrobiological research?

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* – whether humans are truly ‘intelligent’ or not is open for debate, as this video of a ‘haunted toaster’ illustrates all too well.

** – assuming a similar level of observational technology to that of the astronomers of contemporary Earth, which remains statistically unlikely.

On the 28th of August, the Kepler mission announced the discovery of a unique binary star two planet system. The Kepler 47 family consists of a binary pair, a G-type star – about 84% as massive as the Sun, and a smaller M-type red dwarf roughly 36% of the Sun’s mass, but only 1.4% as luminous. Two planets have been observed to be orbiting the pair. The closest is of these is Kepler 47 (AB) b, estimated (from mass-radius relationships) to be between 7 and 10 Earth masses, but the error on this figure remains large. The outermost planet, Kepler 47 (AB) c, is Neptune-sized (16 – 23 Earth masses) and is orbiting within the habitable zone, although due to its large mass it is unlikely to fulfil the traditional requirements for planetary habitability. The configuration of the Kepler 47 system illustrates the fact that stable multi-planetary orbits can exist around binary stars, and brings the total of circumbinary planets to six.

Artist’s impression of the Kepler 47 system. (NASA/JPL-Caltech/T. Pyle)

On the 29th of August, a new planet was added to the Habitable Exoplanets Catalog (HEC) bringing the total to six (including: Gliese 581d and g, Gliese 677Cc HD 85512b, Kepler 22b). Super-Earth Gliese 163c was established to be orbiting within the habitable zone of its 0.40 Solar mass star by an  international team working at the European HARPS project. It completes an orbit in 26 days and has a mass no less than 6.9 times that of the Earth. The custodians of the HEC database have given Gliese 163c an Earth Similarity Index (ESI) rating of 0.73, establishing it as the 5th ‘most habitable’ exoplanet discovered to date, despite exhibiting possible surface temperatures of 60 °C or above.

Gliese 163 c infographic: Warm Superterran Exoplanet in the Constellation Dorado (PHL @ Arecibo/HEC)

Speaking to online science network io9, HEC lead scientist Professor Abel Méndez in the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo said, “Gliese 163c ranks fifth in our current list of six potentially habitable exoplanets because it is nearly twice the size of Earth and its temperature is also higher, but it’s still an object of interest for the search of biosignatures by future observatories.” The HEC has yet to assess Kepler 43 (AB) c, but it is not likely to fare well in habitability assessments due to its large mass.

Bringing my own (as-of-yet-unpublished, but in preparation) research into planetary habitable periods to the table, Kepler 43 (AB) c has a residence time within the habitable zone of approximately 3.9 billion years, whilst Gliese 163c can be expected to within the habitable zone for at least 22.6 billion years. The habitable zone is now populated by 8 planets (including the Earth), and looks a bit like this:

The habitable zone and confirmed habitable zone exoplanets. The dashed lines indicate differing models of cloud cover. Data points are not to scale. (Author’s own research)

It’s certainly an exciting time to be working in this field; nearly each new week brings another interesting discovery. Keep looking up!

The source of the Perseids is dust and debris contained in a relatively dense ‘cloud’ impacting the upper atmosphere of the planet and burning up due to rapid deceleration due to increased aerodynamic drag. The shower has been observed for millennia,  the first recorded sighting was in 69 BC, and most of the dust and debris responsible for the shower was pulled off a comet a thousand years ago. The particles that produce this astronomical light-show are generally tiny, on the order of centimetres, and pose little threat to the Earth below.  However,  the same cannot be said for their parent, comet Swift-Tuttle.

Composite Image of The Perseid Meteor Shower from Mount Hood (Gary Randall, 2012)

Comet Swift Tuttle (designation: 109P/Swift–Tuttle) is a typical Halley-like long period comet. It tears through the inner solar system when nearing the closest approach of its 133 year orbit around the Sun; an orbit that takes it out 12 AU past Pluto to 51 AU, and all the way back again.  Its last close encounter with Earth was in 1992, and it won’t return until 2126.

For a while following its rediscovery in 1992, almost 10 years away from its expected position, the orbital evolution of the comet was not well constrained and there was considerable cause for alarm when it was estimated to be on a collision course with Earth in 2126. Concern was justified:  its nucleus is 26km in diameter, considerably larger than the 10 km impactor that is thought to have caused the Cretaceous-Paleogene (K-T) mass extinction event 65 million years ago. However, reanalysis of ancient records of observations and improved calculations that included the effects of nucleus evaporation confirmed that the comet is on a very stable orbit and poses little threat to Earth for the next 2000 years.

That said, in a 1997 book by South African/American radio astronomer Gerrit Verschuur, comet Swift-Tuttle was described as the most dangerous object known to man for it’s ability to cause catastrophic damage if it was to impact the Earth. An exceptionally close encounter is expected in 4479, bringing Swift Tuttle to within 0.03 AU (approximately 4 million km) of the Earth – roughly 10 times the mean Earth-Moon distance. Travelling at a relative velocity of 60 km per second, Swift-Tuttle would unleash the equivalent of a devastating 3.2×10¹⁵ tons of TNT upon impact – 27 times the energy of the K-T impactor. For comparison, the largest nuclear weapon ever detonated was a ‘mere’ 50 megatons (10⁶). It would very likely cause huge loss of life across the planet and result in a mass extinction unlike any known previously, whilst placing unbridled pressure on the capacity for human civilisation to recover. If the initial impact was survived, tsunamis, wildfires, earthquakes, years of darkness and a toxic atmosphere would follow. Harvard astrophysicist John Chambers estimates the chance of collision in 4479 to be 1 in 1,000,000. Best of luck to our descendants 2467 years from now!

It is worth bearing this in mind when you gaze up over the next few nights to witness the magnificent sight of the ancient dust of this comet burning up in our atmosphere, for one day their parent may put on a somewhat more spectacular, if devastating, show.

Imagination will often carry us to worlds that never were. But without it we go nowhere.

- Carl Sagan (Cosmos, 1980)

Since the dawn of civilisation, humans have gazed up at the stars and planets overhead. Even now, separated from our forebears by an expansive gulf of time, technology and knowledge, the stars remain distant, esoteric but evocative targets. Our curiosity and thirst for understanding drives us on, pushing the limits of human endurance, engineering and science to the point where 528 humans from 38 nations have flown beyond the tenuous envelope of gases clinging to the surface of the Earth into wilderness of space. A first, unsteady and cautious step into the vast unknown that surrounds our tiny globe. Of these, only 12 have stepped foot on the surface of the Moon. At over 385,000 km away, reaching the desolate face of our lunar companion remains the pinnacle of manned spaceflight capability, yet it is a mere stone’s throw from Earth in astronomical terms. We peer out from the relative safety of our home, edge into the abyss that surrounds us and tentatively contemplate its content.

The delicate squishiness of the human form is not conducive to the hostile environment of space. Fleshy bags of meat and fluids don’t travel well in a vacuum, the near absolute-zero temperatures dessicate skin and lung and our fragile bones snap and break easily under undue strain. Bombarded by radiation, and far from the protective effect of the ozone layer, our cells mutate and die.  Ingenuity and engineering have surmounted these problems in the short term by wrapping our bodies in spacecraft and suits, but the frailties of our terrestrial form remain.

As with many aspects of our lives, we have increasingly outsourced the monumental task of space exploration to robotic envoys. Obedient, unfaltering and better able to withstand the hardships of space travel, these metallic pioneers are our eyes and ears in the depths of space, straddling the boundary of the known and unknown to help us elucidate the mysteries of our near and distant planetary neighbours. Beacons in the fog, they light the way out into space.

Moreover, these scientific emissaries are more than merely (very expensive) collections of navigational equipment, cameras, sensors and propulsion. They are more than laboratories, more than the experiments they conduct, or the raw data they return. More too than the images they record, most never seen by the eyes of a human. These magnificent machines, representative of the peak of human exploratory technology are much greater than the sum of their parts. Often the result of years of international collaboration, teamwork, anguish and joy, these are the ambassadors of our knowledge, the manifestations of the spirit of human curiosity and the first steps of a lonely species wandering out into the darkness. Whilst they wander space in isolation, they have the dreams and imagination of many people behind them.

This is why, when a launch fails or an unmanned probe goes missing, the loss is felt by us all. The cost can be counted in dollars or euros, but the real price is the setback to the campaign for understanding that our failed or lost probe was spearheading. A scout lost to the enemy. I’ve heard stories of folks who cried at the loss of Beagle II (the British-built Mars lander lost to the Martian atmosphere in 2003/4), and who amongst us are not moved by xkcd‘s wonderful homage to the late (but very successful) MER Spirit rover?

On the eve of the landing of MSL Curiosity, the most complex rover ever designed, it is worth bearing in mind the hard work and dedication that it took for the latest generation of scientists and engineers to push the limits of our understanding and put a car-sized robot on Mars. I wish all those involved in the construction and operation of this wonderful machine the best of luck. Earth is rooting for you!

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Follow Curiosity’s landing live at JPL’s site here

]]> http://andrewrushby.com/2012/08/02/men-and-machines/feed/ 3 andrewrushby Spirit (by xkcd) http://andrewrushby.com/2012/06/05/enough-time-for-life-part-ii/ http://andrewrushby.com/2012/06/05/enough-time-for-life-part-ii/#comments Tue, 05 Jun 2012 20:24:19 +0000 Andrew Rushby http://andrewrushby.com/?p=759 Continue reading →]]> We are like butterflies who flutter for a day and think it’s forever.

 -Carl Sagan. Cosmos

This figure plots the results as a function of star mass, running along the horizontal axis. The vertical axis is in units of billions of years, and is on a logarithmic scale. The dashed line running through the middle (‘mean habitable period’) represents the habitable period that would be expected if a planet was located right in the centre of the habitable zone at the beginning of the star’s lifetime. I’ve included it to highlight the fact that lower mass stars have longer habitable periods. I’ve also included the Earth and Mars, as well as the four habitable exoplanet candidates mentioned in the preceding post.

This simple model, the results of which are outlined in the image above, estimates the Earth’s total habitable period to be approximately 4.91 billion years, meaning that it will end about 370 million years  from now. That sounds like a long time, and in the context of human time-scales, it certainly is. Even geologically, the world of  370 million years ago was a very different place. It was the height of the Late Devonian period, and a full 172 million years after the Cambrian explosion saw the rapid diversification and speciation of some the earliest complex eukaryote life. The first forests were in the process of transforming the landscape of the supercontinent Gondwana, unconstrained by the lack of large herbivorous animals, and the first tetrapods were appearing in the fossil record. Who knows what transformations the world and life will undergo during the next 370 million years?

I should note that the error bars for these numbers are high, and I’m making no concrete predictions here for the inhabitants of the world 369 million years from now to call me out on. The habitable zone as a theory itself is fraught with assumptions that are, at this stage of understanding, regrettably necessary and regularly challenged and amended.

The Clock is Ticking

It remains intrinsically unsettling to consider the fact that at some point our lovely blue-green home planet will eventually lose its ability to support life. It is certain that, whether after 4.91 billion years or not, the edge of the gradually advancing theoretical boundary of habitability will near planet Earth; now an apocalyptic world of blistering heat and desolation, unrecognisable from today’s lush, watery paradise. As Sol’s mass, radiative output and surface temperature steadily increase,  the Earth’s climate will eventually become scorching. The fundamental biogeochemical mechanisms that help to regulate the Earth’s climate will break down, buckling under the strain of the ever encroaching Sun, and a ‘runaway greenhouse‘ crisis will result. Caused by the evaporation of the oceans and the initiation of a irreversible water vapour/temperature feedback mechanism, the runaway greenhouse is thought to be responsible for the of climate of Venus today. High temperatures result in more water vapour in the air and higher humidity, which in turns boosts the temperature further causing more evaporation and more humidity. Eventually the Earth will become enveloped in thick, impenetrable cloud, insulating the surface and acting like an planet-wide pressure cooker, undoubtedly heralding the end of life on the Earth as we know it.

As the Sun grows larger and hotter, high energy particles from the solar wind will eventually strip away this thick atmosphere which will be forever lost to space. The parched, molten husk of the Earth, former home to countless organisms and every human ever to exist, as well as the stage to every single event, from the minuscule to the revolutionary that took place for nearly 5 billion years, will probably be devoured by the Sun long after it has become inhospitable for life, an incomprehensibly distant 7 billion years from now.

What Earth may look like 5-7 billion years from now – after the Sun swells and becomes a Red Giant. (Wikipedia)

The Earth, my friends, is lost. But fear not, perhaps we could move out to Mars? Our dusty neighbour will move into the habitable zone approximately 1.7 billion years from now, and stay there for the remainder of the Sun’s main sequence lifetime. The Sun in it’s death throes will make for an incredible sight in the Martian sky. However, Mars has a very chaotic orbit, making it difficult to determine exactly where it will be in the distant future. On top of all this, it’s hard to predict what conditions will be like around the ageing Sun.

Well, so much for the Earth and Mars. Let’s hope that in the preceding 370 million years our descendants make it to a better world.

The Lives of Planets

The Super-Earth Gliese 581d (top left of plot) has an approximate habitable period of over 50 billion years. I don’t know about you, but I have real difficultly grasping the truly unfathomable immensity of that amount of time. Research suggests that its star, red dwarf Gliese 581, is approximately 8 billion years old, and therefore the habitable zone has been home to Gliese 581d for 1.4 times as long as the Earth has existed for, yet it is only 13% of the way through its total habitable period.  Still, this isn’t to say that it’s ‘habitable’; there are plenty of other factors (its large mass for example) that suggests that it’s not a place where life would thrive. Although, given 50 billion years who knows what evolution could throw up?

Gliese 667Cc, also orbiting a red dwarf star, will be in the habitable zone for 1.8 billion years because it formed straddling the inner edge – it won’t be (relatively) long until the heat of its star overwhelms its ability to maintain a habitable environment, if it has one at all.  It’s a similar story for the Super-Earth HD 85512 b. Despite it’s location in the habitable zone, it’s still too close to be habitable for any considerable length of time – a mere 603 million years which, if we draw on Earth’s evolutionary history for comparison, is barely enough time for the denizens of the Cambrian to make themselves comfortable, if we extrapolate backwards (and ignore the ~3.5 billion years that it took to get to this stage in the first place).

Kepler 22b is another excellent candidate for a habitable planet, orbiting well within the habitable zone and remaining there for 3.4 billion years. On Earth, 3.4 billion years ago, it is thought that the first primitive organisms had emerged and were building reefs (stromatolites) and going about their daily business of dividing and multiplying – the kind of stuff that modern bacteria tend to fill their lives with. From these humble beginnings we emerged eons later; perhaps the same can be true on Kepler 22b?

In the End…

I realise this has been quite a long article, and I appreciate you sticking it out to the end. I hope that you found it as interesting to read as I did to write. The concept of habitability through time hasn’t been explored in great detail, and I hope to refine these numbers and tweak the model and its assumptions to improve the accuracy of the estimates in the future. Nevertheless, I found it an interesting, and rather humbling, thought experiment if nothing else.

Perspective is important, and yet always in short supply. We’re currently 92% of the way through our planet’s habitable period, enjoying the twilight years of its habitable lifetime. We have to remember that the Earth isn’t going to be able to shelter us indefinitely and that all planets’ lives come to an end at some point. It’s worth bearing that mind when considering that despite our delusions of grandeur, our brief residence on this planet has been a fleeting blip in its long and tumultuous history. Our future may well be too.

A reasonably good place to start looking for planets hosting these conditions is the ‘habitable zone‘ of stars, a concept that I’ve discussed before. The habitable zone describes an area around a star where a planet, if it was discovered to be orbiting within this area, could have liquid water on its surface. Stars of different masses and classifications have different habitable zone distances, and not all planets in the habitable zone are habitable: some may be too massive, others too small, many wouldn’t have the correct mix of atmospheric constituents, others may have no atmosphere at all. In fact, there are more reasons to think that planets, whether inside or outside the habitable zone, are more likely to be completely unsuitable for (Earth-like) life than there are to consider the opposite.

However, whilst habitability is variable in space, it is almost certainly variable in time as well. The habitable zone isn’t a fixed distance: its boundaries move outwards as the star undergoes main-sequence evolution, growing larger and hotter over time. More massive stars (classifications F, G and K) have the shortest main sequence lifetimes and therefore the habitable zone boundaries around these stars migrate outwards at a proportionally more rapid rate. Low mass stars, M-stars for example, have extensive lifetimes on the order of tens or hundreds of billions of (Earth) years, and therefore their habitable zones are relatively more static in time.

The Habitable Period: A Measure of Habitability Through Time

The habitable zone for stars of different masses at the point of entry on to the ‘main sequence’. The horizontal axis shows the distance from the star in astronomical units (AU) on a logarithmic scale. The dashed boundaries illustrate the uncertainty of the HZ when cloud cover is taken into account.

The habitable zone for stars of differing masses at the end of their main sequence evolution.

The time that a planet spends within the habitable zone can be considered its ‘habitable period‘. The habitable period of a planet is an important factor when considering the possibility of life on these worlds. A planet with a long habitable period is perhaps more likely to host complex organisms that require more time to evolve, if we make the assumption that evolution by natural selection is a universal constant, operating in a similar way in potential exobiological systems as it does on Earth. An alternative means of speciation has not been discovered on Earth, and natural selection has withstood 200 years of intense scientific scrutiny and analysis relatively unscathed. As before, with a sample of one assumptions have to be made.

Building on this idea, if it is possible to determine the extent of the habitable zone at the beginning and end of the star’s main sequence lifetime using modelling techniques, and estimate the approximate age of the star, then a rate of outward migration of the boundaries of the habitable zone can be derived and quantifying the habitable periods of these planets becomes a possibility.

The figures above go some what to illustrating this point: the image on the left shows the extent of the habitable zone of different stars at the stage at which the star enters the ‘main sequence‘ – the beginning of its hydrogen-burning life. I’ve included the Earth, Mars and the confirmed habitable zone exoplanets from the Habitable Exoplanet Catalog and plotted them at their semi-major axes. Note that the Earth and Kepler 22b are comfortably within the warming embrace of their respective suns’ habitable zone at this stage, whilst the other planets remain fairly peripheral. The figure on the right shows the same planets in the same relative orbital locations, but at the end of their star’s lives. Earth, Kepler 22b and most of the other planets, with the welcome exception of Mars (not likely to be at this location in the future anyway because of its chaotic orbit), have all been relegated to the dangerous and inhospitable ‘hot zone’ nearest the star as the boundaries of the habitable zone migrated past their positions at some point during stellar evolution. The rate at which the imaginary boundaries move outwards is proportional to the mass of the star, as discussed above.

I used a very simple model to estimate exactly how long these planets will spend in the habitable zone and I’ll post the results in the coming days.