“Two possibilities exist: either we’re alone in the Universe, or we’re not.
Both are equally terrifying.”
- Arthur C. Clarke.
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?Given that a multitude of habitable worlds exist, many covered in a primordial cocktail of complex, biologically useful compounds, it seems that the Milky Way should be teeming with life. So, where is everyone? This question has proved tricky, paradoxical even. Accordingly, it’s known as the Fermi Paradox after the Italian astronomer who first posited the riddle to the wider scientific community, where it was met with unexpected consternation. Over 50 years on and it remains a question without an answer. SETI pioneer Frank Drake devised an equation to address the problem, called the Drake Equation, which attempts to provide an estimate of the likely number of other civilisations in the Milky Way. However, the huge uncertainties involved in each stage of the calculation limits its predictive powers to more of interesting thought exercise than a robust scientific methodology.
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.
The Blue Marble Space Institute for Science is a not-for-profit research organisation that is using PetriDish.org to fund a modelling project that seeks to identify the signs of industrial activity in the atmospheres of extra-solar planets. Find out more about the project, including more about the authors, their methods, the possible outcomes of the project and a breakdown of the costs, here:
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.
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?
* – 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.
It’s been a busy couple of weeks for exoplanetary discoveries, but also for me, which explains why I’ve taken so long getting round to writing about them.
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.
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.
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:
It’s certainly an exciting time to be working in this field; nearly each new week brings another interesting discovery. Keep looking up!
-Carl Sagan. Cosmos
In my last post I discussed how it was possible to make tentative estimates about the total amount of time that a planet spends in the habitable zone, also known as its habitable period, and why this is important. In this post, I’d like to put numbers to those estimates.
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
-William Shakespeare, Sonnet LX
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.
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.
As you may know if you frequent this blog often, I spend a fair amount of time writing about planets that astronomers spend a lot more time discovering. My main interest in these worlds lies with their ‘habitability’, a rather esoteric and loosely defined term that is primarily concerned with describing how broadly livable these planets are, in a very Earthcentric way. Planetary habitability is an extremely complex recipe that turns climatic, planetary and geological ingredients, added in just the right quantities, into a warm, salty, non-toxic broth. Perhaps life on other planets, if it exists, has completely different requirements, but without a good sample of inhabited planets teeming with life we can’t really be sure and have to make this assumption for now.
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 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.
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.
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.
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.