That Tingling Feeling


There is a word in Japanese, Yūgen (幽玄), derived from the study of Japanese aesthetics with no English equivalent, that perhaps comes closest to describing the profound sense of the enormity of the cosmos: to despair and be humbled by the insignificance of the struggle against the indifference of the universe, whilst also appreciating the sad beauty of human suffering. I often find myself grasping for a word to describe this reaction when discussing astrobiology with people, other scientists or members of the public, who find the entire field incredibly depressing; who, at some level, acknowledge the futility of our search for meaning in the distant reaches of space. Some find the emotional burden too great to bear, triggering a minor existential crisis. “It’s better not to know”, they say, “Not to think about it. Besides, [insert reality TV show name here] is on!”

On one hand, who can blame them? It’s not like we’re expecting answers to many of The Questions that astrobiology and astronomy are trying to solve in our lifetimes. Science is a gradual process after all, and one that will last as long as there are still questions to be answered. The relative insignificance of our personal lives, our careers and relationships, cast against the enormity of the cosmos and separated by orders of magnitudes of space and time, so clearly presented, can prove a bit too much. The Astronomical Perspective can be overwhelming, and astronomy, as Carl put it, is a humbling experience. I’d like to adopt yūgen as a general descriptor of these feelings.1

Yūgen-inducing perspective: Over the Top. Credit: Luc Perrot

Astrobiology is a scientific discipline practised from deep within in the realms of bounded rationality. These bounds stem from a definite, fundamental and detrimental lack of information about the system, as well as a possible cognitive and technological limitation in processing of the limited information available to us. We definitively lack the resources to arrive at an optimally rational conclusion regarding our place in the universe, the existence of suitably habitable environments elsewhere, and the possibility of life on other planets.  And yet, we know we’re close. We suffer a kind of collective Dunning-Kruger effect regarding how little we know, and how little we know about how little we know. We’re approaching that greatest of unknowns, cobbling together a piecemeal scientific narrative as we go, but missing so many parts of the puzzle that it’s not even clear what it is we’re building. Yet, something innate drives us onwards. Some part of us that has always been, as if a distant memory or half-remembered dream, within our genetic luggage and passed on to us from pre-human ancestors.

The size of our brains relative to our body size (also known as the encephalization quotient (EQ)) has, in fact, gotten smaller in recent times, peaking ~30,000 years ago after 2 million years of expansive growth. I’ll leave the anthropologists to argue over why and what this means, but making some crude assumptions about intelligence and EQ we can assume, therefore, that our extremely distant ancestors may have gazed up at the canopy of the night sky and felt that same intangible yearning as we do. At least, there seems to be no cognitive reasons that they couldn’t have done so. Maybe it was even more pronounced by the gulf of knowledge that separates their knowledge of the cosmos from our own? The bright band of the Milky Way stretched out overhead, unobscured by pollution, but hidden by ignorance; an unknowable story waiting for a narrator, one that would not arrive in earnest for thousands of years. In the meantime, complex and anthropomorphic mythologies were borne and woven by the tapestry of human imagination and fuelled by our penchant for storytelling.

Perhaps, that sense of insignificance, that yūgen, was even more heart-wrenching in the very distant past when we were young, when our contemporary achievements in understanding of our place in the greater Story would seem unfathomable, akin to magic. Perhaps, yūgen has been a driving force in our history as long as we have existed? I’m not suggesting an evolutionary driver akin to bipedalism, but perhaps a minor constituent of the human story that contributed an unquantifiable edge to our tale. An ember burning near the edge of the campfire of humanity’s intellectual awakening, smouldering away throughout the ages whilst we built our temples and cities, waged our wars and battles, waiting for the spark of enlightenment to burst into an inferno of curiosity and discovery.

That’s why I’m optimistic about our search. Sure, we may not find any concise answers to the ‘big’ questions in our lifetimes, and we’ll probably always have that sense of yūgen when faced with incomprehensible enormity on galactic and light year-scales, but rather than hiding in the dark, we should embrace the feeling of astronomical despair and turn it into a creative force for discovery! If you don’t like being insignificant, find something that makes you significant. Yūgen will be passed on to the next generation of curious scientists and philosophers, and as it has done in the past, it will drive us on to more profound questions and more mysterious unknowns.


1 If any Japanese speakers are reading this, please let me know if I’m using this word incorrectly – my understanding is that the context is important.

Dead Stars Reveal Mysteries of Planet Formation


This is a guest post by David Wilson, a PhD student in the Astronomy and Astrophysics group at the University of Warwick, where he studies the remains of planetary systems around white dwarfs (see below!). He can be found on Twitter and blogs about various astronomy topics at Stuff About Space.


Twenty seven years ago astronomers noticed something strange about the white dwarf star GD29-38.

White dwarfs are dead stars, the burnt out carbon cores of stars like our Sun which have exhausted their hydrogen fuel; incredibly dense, incredibly hot balls of matter roughly the size of the Earth. Because of their high temperature, tens of thousands of degrees, all white dwarfs glow blue.

But the light from GD 29-38 wasn’t just blue. When it was split into a spectrum, separated into a rainbow of separate colours, there seemed to be something else there. Something shining with an infrared light, beyond the range of our eyesight.

Initially the discovers were excited, as the red light could have come from an orbiting brown dwarf, a mysterious object several times bigger than a planet but much smaller than a star. But both the white dwarf and the infrared source were pulsating slightly, periodically getting brighter and dimmer. If the red light was from a separate object, then it shouldn’t have pulsed in time with the white dwarf.

An asteroid plummets to its doom around the white dwarf GD 29-38. Studying the debris left from these asteroids can reveal the chemical composition of exoplanets. Image Credit: NASA

The spectrum also revealed metals in the white dwarf’s atmosphere, heavy elements like calcium, magnesium and iron. These were also out of place, as white dwarfs have such a strong gravity that anything heavier than hydrogen or helium should have sunk down into their cores long ago. The metals must be falling onto the white dwarf from the space around it- but how did they get there?

It took until 2003 for the origin of the mysterious infrared glow to be found, during which time many more white dwarfs with similar red spectra and metal polluted atmospheres were found. The explanation was that the infrared light is coming from a disc of dusty debris surrounding the white dwarf.

This debris was formed from the wreckage of an asteroid, leftover from when GD29-38 was a Sun-like star with its own system of planets. The dust in the disc rains down onto the white dwarf, explaining the metals we see in the atmosphere.

The spectrum of GD 29-38. Along the bottom is its wavelength, or colour, going from blue on the left to invisible infrared on the right. The vertical axis shows how bright the white dwarf is at each wavelength. The difference between the blue white dwarf and red dust cloud can be clearly seen. Image Credit: NASA

The spectrum of GD 29-38. Along the bottom is its wavelength, or colour, going from blue on the left to invisible infrared on the right. The vertical axis shows how bright the white dwarf is at each wavelength. The difference between the blue white dwarf and red dust cloud can be clearly seen. Image Credit: NASA

The story of how the debris disc got there is a result of the turbulent formation of the white dwarf. As it runs out of fuel a star swells up to a huge red giant, then blows away roughly half of its mass in an immense stellar wind, leaving the tiny white dwarf core.

With the gravitational force at its heart cut in two, the system of planets around the dying star is thrown into chaos. Planets begin to migrate outwards, trying to reach orbits twice as far away from the central star as before. As they do this, they risk coming into close contact with each other.

Some of the planets survive these encounters and carry on as they are. Others, especially when a big Jupiter sized planet is involved, are thrown out of the system into the depths of interstellar space. And some are scattered into the centre of the system towards the white dwarf.

These unlucky asteroids and dwarf planets fall in towards the white dwarf until they reach a point known as the tidal disruption radius. There the tidal force, the difference in gravitational pull between the parts of the asteroid nearest the white dwarf and the areas further away, becomes so great that the asteroid is ripped apart, forming the dusty debris disc that we see as an infrared glow.

The discovery of this process lead to an important conclusion. As the dust rains down onto the white dwarf it becomes visible to our telescopes. If we can measure what metals there are, and how much of each there is, then we can reveal the chemical composition of the asteroid or planet that formed the disc. We can ask, and answer, the question: “What are planets made of?”

Two decades ago we only knew about the eight planets in our solar system (Pluto was never a planet, it was just mislabelled). Now we know of over a thousand planets, new worlds orbiting hundreds of stars. Through our telescopes we can measure the size of these planets, what their masses are, and even in some cases get a glimpse into their atmospheres.

But we can’t find out what they’re made of, what the geology of these newly discovered planets is like. This means that we don’t know for sure if the way that the rocky planets are built in our solar system, the particular mix of iron, oxygen, magnesium, silicon and other chemicals that make up the Earth and its neighbours, is the way all planets are built.

The metal polluted white dwarfs form a perfect laboratory, presenting us with rocky objects that have broken apart into their chemical components. By observing as many as we can, we can begin to explore the chemical diversity of planets and planetary systems. We can see if the way our planets are built is the normal way to construct a planet, or whether Earth is even more unique than we thought.

To date we’ve discovered around a dozen white dwarfs with enough chemicals to compare their systems in detail with our own. So far, they look fairly similar to the Earth, a hopeful sign. But we need many more to truly explore this area, and over the next few years myself and others will be scouring the sky, using the Hubble Space Telescope above us and an array of telescopes on the ground. We will find more metal polluted white dwarfs, measure the chemicals of the planetary debris around them, and begin to explore in detail what things you need to build a planet.

Lost in Space: Finding a Sense of Place in the Cosmos

This is a guest post by Sean McMahona PhD student in the School of Geosciences at the University of Aberdeen. Sean’s research applies geological perspectives and techniques to astrobiological problems ranging from the origin and distribution of life in the universe to the origin of methane in the Martian atmosphere. Visit his excellent blog, Fourth Planetfor more on his research, his impressive space art and photography, and writings.


“Though a planetary perspective is a magnificent and enriching thing, places, not planets, are the core of human experience. It is from places that we build our world.”

—    Mapping Mars, Oliver Morton (2002)

“He stood thereby, though ‘in the centre of Immensities, in the conflux of Eternities,’ yet manlike towards God and man; the vague shoreless Universe had become for him a firm city, and dwelling which he knew.”

—    The French Revolution: A History, Thomas Carlyle (1837)

Last year, in a car park in Aberdeen, I saw Jupiter through a telescope for the first time. What I saw was not the familiar red-spotted giant from the Nasa photographs, that great bronze bauble marbled with cream like artisan coffee—no. What I saw, through a gap in the Scottish clouds, was a pale round smudge with three white specks for moons. It was not dramatic but it was a strange and lovely moment. It reminded me that Jupiter, the other planets, and even the distant stars and galaxies, are no less real, no less here—albeit further away—than Scotland, clouds, car parks, and me. They are on the same map, sharing our geography, our humdrum commonplace reality.

In our eagerness to be inspired by astronomical imagery, we are often tempted to forget this fundamental sameness. Documentaries about the cosmos besiege us with spectacular graphics, rousing orchestral music and rapturous, lyrical narration. In the tradition of Carl Sagan, we are urged to adopt a “cosmic perspective”, in which the Earth dwindles to an insignificant1 “mote of dust suspended in a sunbeam”. Meanwhile, digital space art is reliving the Romanticism of 19th Century painting: balance, proportion and subtlety are abandoned in favour of vertiginous perspectives, extremes of colour and contrast, and sublime, mystical lighting: silhouetted planets disintegrate into vast purple nebulae bristling with crepuscular rays. Thus, it seems that an ecstatic, almost mythical vision of outer space, emphasizing above all its spiritual and aesthetic grandeur, has taken root in popular culture.

McMahon juvenilia. This is what I thought space looked like when I was 17. I have since changed my mind.

McMahon juvenilia. This is what I thought space looked like when I was 17. I have since changed my mind.

Maybe that vision has some role to play in attracting public interest to the space sciences. But paradoxically, it can make the “wonders of the universe” seem less accessible than ever; profound, ethereal, miraculous, even unreal. It bolsters the popularity of astrology by reinforcing the illusion that planets and stars are unfathomable, heavenly beings: much more plausible aids to divination than ordinary material things. Most worryingly, it can give the impression that space exploration is an esoteric spiritual quest, unrelated to ordinary human problems and unfit for serious attention from media, government or young, career-minded scientists.

Perhaps the “numinous” view of space reflects a deeper failure to grasp the implications of the Copernican Revolution. Somehow, I suggest, we still make some kind of basic ontological distinction between the heavens and the Earth2. Consequently, we are unable to feel truly embedded in our extraterrestrial environment, which remains a transcendent, detached and coldly beautiful space rather than a homely, material, lived-in place. The Apollo programme helped to bridge that gap for a generation, transforming the moon from an icon of celestial indifference into a humanly intelligible landscape—rather like a golf course, in fact, replete with bunkers, buggies, flags and footprints3. Revealingly, many people today find it easier to believe that the whole thing was a hoax.

A Summer 2012 photograph by NASA's Curiosity rover inside Gale Crater on Mars.

A Summer 2012 photograph by NASA’s Curiosity rover inside Gale Crater on Mars.

The sharp, vivid photographs taken by NASA’s Curiosity Rover can have a similar effect, reminding us that the martian surface is a real place, not so different in appearance from the rocky deserts of Libya or the High Arctic. Despite our unsophisticated cultural relationship with outer space—a mixture of mythology, indifference and reverence—a crewed mission to Mars in the next thirty years now seems very likely. I hope that mission will allow the next generation to feel more at home in the universe, more fully at ease with the fact that even Milton Keynes4 is part of the Milky Way. What we stand to gain is not an exalted “cosmic perspective” but simply a richer, more expansive sense of place, of where it is that we live our lives.


1     This strain of rhetoric characteristically fails to observe that human beings adjudicate the significance of the universe, not the other way around.

2      Douglas Adams exploited this confusion to humorous effect, juxtaposing ordinary things with cosmic phenomena: the “restaurant at the end of the universe,” the “whelk in a supernova” and so on; “you may think it’s a long way down the road to the chemist but that’s just peanuts compared to [the size of] space”.

3      Some readers will know that the American astronaut Alan Shephard did in fact play golf on the moon; two golf balls remain there.

4       Milton Keynes is an architecturally unprepossessing English town and home to the Open University, where much British space research has been conducted.

The hunt for an Exo-Earth: How close are we?

This is a guest post by Hugh Osborn, a PhD student in the Astronomy and Astrophysics group at the University of Warwick. Hugh’s research involves using transit surveys to discover exoplanets. Visit his excellent blog, Lost in Transitsfor more on exoplanets, their detection and his research.


In the 1890s Percival Lovell pointed the huge, 24-inch Alvan Clark telescope in Flagstaff, Arizona towards the planet Mars. Ever the romantic, he longed to find some sign of life on the Red Planet: to hold a mirror up to the empty sky above and find a planet that looked a little bit like home. Of course, in Lovell’s case, it was the telescope itself that gave the impression of life, imposing faint lines onto the image that he mistook for canals. But, with Mars long since relegated to the status of a dusty, hostile world, that ideal of finding such a planet still lingers. In the great loneliness of space, our species yearns to find a world like our own, maybe even a world that some other lineage of life might call home.

51 Pegasi: Home to the first exoplanet discovered by humans (Copyright: Royal Observatory Edinburgh, Anglo-Australian Observatory, and AURA)

A hundred years after Lovell’s wayward romanticism, the real search for Earth-like planets began. A team of astronomers at the University of Geneva used precise spectroscopy to discover a Jupiter-sized world around the star 55-Peg. This was followed by a series of similar worlds; all distinctly alien with huge gas giants orbiting perishingly close to their stars. However, as techniques improved and more time & money was invested on exoplanet astronomy, that initial trickle of new worlds soon turned into a flood. By 2008 more than 300 planets had been discovered including many multi-planet systems and a handful of potentially rocky planets around low-mass stars. However, the ultimate goal of finding Earth-like planets still seemed an impossible dream.

In 2009 the phenomenally sensitive Kepler mission launched. Here was a mission that might finally discover Earth-sized planets around Sun-like stars, detecting the faint dip in light as they passed between their star and us. Four years, 3500 planetary candidates and 200 confirmed planets later, the mission was universally declared a success. Its remarkable achievements include a handful of new terrestrial worlds, such as Kepler-61b and 62e, orbiting safely within their star’s habitable zones. However, despite lots of column inches and speculation, are these planets really the Earth 2.0s we were sold?

While such worlds may well have surfaces with beautifully Earth-like temperatures, there are a number of problems with calling such worlds definitive Earth twins. For a start the majority of these potentially habitable planets (such as Kepler-62e) orbit low-mass M-type stars. These are dimmer and redder than our Sun and, due to the relative distance of the habitable zone, such planets are likely to be tidally locked. The nature of such stars also makes them significantly more active, producing more atmosphere-stripping UV radiation. This means, despite appearances, ‘habitable’ planets around M-dwarfs are almost certainly less conducive to life than more sun-like stars.

Even more damning is the size of these planets. Rather than being truly Earth-like, the crop of currently known ‘Habitable planets’ are all super-Earths. In the case of Kepler’s goldilocks worlds, this means they have radii between 1.6 and 2.3 times that of Earth. That may not sound too bad, but the mass of each planet scales with the volume. That means, when compression due to gravity is taken into account, for such planets to be rocky they would need masses between 8 and 30 times that of Earth. With 10ME often used as the likely limit of terrestrial planets, can we really call such planets Earth-like. In fact, a recent study of super-Earths put the maximum theoretical radius for a rocky planet as between 1.5 and 1.8RE, with most worlds above this size likely being more like Mini-Neptunes.

So it appears our crop of habitable super-Earths may not be as life-friendly as previously thought. But it is true that deep in Kepler’s 3500 candidates a true Earth-like planet may lurk. However the majority of Kepler’s candidates orbit distant, dim stars. This means the hope of confirming these worlds by other techniques, especially tiny exo-Earths, is increasingly unlikely. And with Kepler’s primary mission now ended by a technical fault, an obvious question arises: just when and how will we find a true Earth analogue?

Future exoplanet missions may well be numerous, but are they cut out to discover a true Earth-like planet? The recently launched Gaia spacecraft, for example, will discover hundreds of Gas Giants orbiting Sun-like stars using the astrometry technique, but it would need to be around a hundred times more sensitive to discover Earths. New ground-based transit surveys such as NGTS are set to be an order of magnitude better than previous such surveys, but still these will only be able to find super-Earth or Neptune-sized worlds.

The Transiting Exoplanet Survey Satellite (TESS) (

Similarly, Kepler’s successor, the Transiting Exoplanet Survey Satellite which is due to be launched in 2017, will only be able to find short-period planets with radii more than 50% larger than Earth. HARPS, the most prolific exoplanet-hunting instrument to date, is also due for an upgrade by 2017. Its protégée is a spectrometer named ESPRESSO that will be able to measure the change in velocity of a star down to a mere 10cms-1. Even this ridiculous level of accuracy is still not sufficient to detect the 8cms-1 effect Earth’s mass has on the Sun.

So despite billions spent on the next generation of planet-finders, they all fall short of finding that elusive second Earth. What, precisely, will it take to find this particular Holy Grail? There is some hope that the E-ELT (European-Extremely Large Telescope), with its 35m of collecting area and world-beating instruments will be able to detect exo-earths. Not only will its radial velocity measurements likely be sensitive enough to find such planets, it may also be able to directly image earth-analogues around the nearest stars. However, with observing time likely to be at a premium, the long-duration observations required to find and study exo-earths could prove difficult.

Alternatively, large space telescopes could be the answer. JWST will be able to do innovative exoplanet research including taking direct images of long-period planets and accurate atmospheric spectra of transiting super-Earths and giants. Even more remarkably, it may manage to take spectra of habitable zone super-Earths such as GJ 581d. But direct detection of true Earth-analogues remains out of reach. An even more ambitious project may be required, such as TPF or Darwin. These were a pair of proposals that could have directly imaged nearby stars to discover Earth-like planets. However, with both projects long since shelved by their respective space agencies, the future doesn’t look so bright for Earth-hunting telescopes.

After the unabashed confidence of the Kepler era, the idea that no Earth-like planet discovery is on the horizon may come as a surprisingly pessimistic conclusion. However not all hope is lost. The pace of technological advancement is quickening. Instruments such as TESS, Espresso, E-ELT and JWST are already being built. These missions may not be perfectly designed to the technical challenge of discovering truly Earth-like planets, but they will get us closer than ever before. As a civilisation we have waited hundreds of years for such a discovery; I’m sure we can hold out for a few more.

Rarely-Done Planets

This is a guest post by David Waltham, Reader in Mathematical Geology at Royal Holloway, University of London. David’s new book, Lucky Planet, is out in April 2014. Visit his ‘Strange Worlds Catalogue‘ for more exoplanet oddities. 


One of the unlucky planets?

The issue of manmade global-warming seems far removed from questions of exoplanet habitability but there is a close link.  A planet whose climate is highly sensitive to greenhouse-gas changes is also a planet that responds strongly to increasing heat from its aging star; and it’s hard for such a world to remain habitable for long. The Earth seems to be one such world (that’s why global warming is such a threat) but it has never-the-less remained habitable for billions of years.  How it managed pull off this trick is an intriguing, but not particularly new, mystery.

In 1972 Carl Sagan and George Mullen recognized that, since our Sun produced 30% less heat when she was young, surface temperatures on the early Earth should have been far below freezing. However, geological evidence showed running water when our world was just a few hundred million years old.   Sagan and Mullen called this the faint young Sun paradox and, forty years later, there is still no consensus on how to resolve it.  However the concept of climate sensitivity, an idea refined over the last thirty years by climate scientists interested in anthropogenic global-warming, now gives us a clear framework for discussing the issues.

Climate sensitivity tells us how much warmer a planet becomes for a given increase in the heat it receives.  It’s a bit like going from gas-mark 5 to gas-mark 6; how much hotter does this make an oven?  At gas-mark 6 more gas is being burnt and temperature rises but, in a badly insulated oven for example, the increase would be less than expected.  Similarly, different planets warm up by different amounts for a given increase in heating and this difference in climate sensitivity depends upon the relative strengths of positive and negative feedbacks in the climate system.  As I’ll show below, the faint young Sun paradox occurs because Earth’s high climate sensitivity is incompatible with the flowing of liquid water on her surface when she was young.

Climate sensitivity is usually expressed by how much warmer the Earth becomes if carbon dioxide concentrations are doubled.  Doubling of CO2 is expected by the end of the current century and so this is a very concrete way of expressing the expected impact.  The best guess is that climate sensitivity is in the range 1.5-4.5 °C .  This range is largely based upon computer models of the present-day climate system but it is backed up by simulations of Earth’s past climate which only match observations when similar climate sensitivities are used .  If anything, these geological studies suggest that the computer estimates are too low but let’s be conservative and stick with the computer models.  What does a climate sensitivity of 3 °C predict concerning temperature changes over the life time of our planet?

To calculate this we need to re-express climate sensitivity in a slightly different way.  Doubling CO2 increases heating at the Earth’s surface by 3.7 Wm-2 but, to produce an equivalent amount of heating at ground level, solar radiation must go up by 5.3 Wm-2 because some is reflected back into space.  Thus, temperatures go up 3 °C if solar heating increases by 5.3 Wm-2.  Earth’s climate sensitivity is therefore 0.6 °C per Wm-2.  Heat from the Sun has actually gone up 90 Wm-2 over the last 4 billion years and so temperatures should have risen more than 50 °C.  This implies a young Earth that endured average temperatures near -40 °C and that is inconsistent with liquid water anywhere on our planet’s surface.

An obvious objection to this analysis is that the ancient climate system was very different to that of the modern Earth and so the present-day climate sensitivity may not be relevant.  That’s a fair point but we can get around it by concentrating instead on the Phanerozoic Eon (i.e. the last 542 million years) when there is no reason to think that climate sensitivity would have been massively different to today.  Solar heating has increased 15 Wm-2 over this time and so temperatures should have risen by about 10 °C but there is no evidence whatsoever for such a rise.  Analysis of oxygen isotopes in ancient marine organisms suggest that Phanerozoic temperatures have fluctuated around a steady mean or perhaps even dropped a little.  Thus, whether we look at the whole of Earth’s history or just the last half-billion years, there is no evidence for the expected overall warming despite the steadily increasing luminosity of our Sun.  What’s going on?

Tropical Sea Surface Temperatures over the Phanerozoic ()

Tropical Sea Surface Temperatures over the Phanerozoic (after Vizier et al., 1999)

The missing part of the puzzle is that Earth itself has evolved, both geologically and biologically, during its long history.  For example, the slow growth of the continents and the biological evolution of more effective rock-fragmenters (e.g. lichens and trees) has steadily increased the efficiency with which CO2 is removed from the atmosphere by the chemical reaction of acid-rain on volcanic rock.  Another greenhouse gas, methane, has also greatly declined through time as oxygen levels have grown following the evolution of photosynthesis.  Furthermore, land, especially plant-covered land, is more reflective than sea and so, as the continents grew and as they became colonized by life, more of the Sun’s heat has been reflected into space.  These processes, and perhaps others, cooled our planet as the Sun tried to warm it.

Two opposing forces therefore fought for dominance of climate trends and, coincidentally, roughly cancelled out.  But what produced this coincidence?  Some would ascribe it to the Gaia hypothesis that a sufficiently complex bio-geochemical system will inherently produce environmental stability.  However there’s no credible mechanism for this and, in any case, Gaia may have confused cause and effect: Earth’s complex biosphere didn’t produce a stable climate; rather a stable climate was a necessary precondition for a complex biosphere.  If this is right, then biospheres whose complexity and beauty rival that of the Earth will be rare in the Universe.  On the majority of those few worlds where life arises, it will all-too-soon be frozen by bio-geochemistry or roasted by its sun.  However a few worlds will, purely by chance, walk the fine line between these fates long enough for intelligent life to arise.  We live on one of those rare, lucky planets.

A Multiplicity of Worlds [RSS]

Earth-like planet (Image credit: Sean McMahon)

I wrote an article for the October edition of the Royal Statistical Society’s Significance magazine about statistics and exoplanets. You can download a .pdf copy here.

The Search for another Earth


This the third 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.

 In my last post I broadly covered the techniques for finding planets around other stars in the galaxy, as well as the role this technology plays in defining the current limits on our knowledge. We have discovered 885 other planets to date, but how many of them are like the Earth and why is this important?

As we live on a rather lovely watery planet ourselves, we seem to have a natural inclination to seek out others just like it because we consider them to be the most likely for hosting life. Why? Well, because our current sample of ‘inhabited planets’ stands at just one, we have a very limited understanding of where the boundaries for life lie as well as the important factors that affect habitability when considering the broad characteristics of life-bearing worlds. If other inhabited planets exist, is the Earth typical within the sample or an outlier? Are the furnaces of close-in gas giants the cradle of most flavours of life in the universe, or maybe the frigid surfaces of icy worlds in the far-flung outer regions of their star system?

Waterbear, taken by scanning electron micrograph

Some lifeforms live in extremely tough environments, and have even survived space vacuum conditions – like this water bear. Image credit Bob Goldstein and Vicky Madden (Creative Commons)

It might be fun to speculate about all the various forms and shapes that other life might take, but this lies outside the remit of science. It seems obvious to us that only on a planet able to support life would organisms (like intelligent Homo Sapiens) eventually evolve, but this instils in us a fundamental bias towards planets like Earth: it remains beyond our perspective to consider the possibility that can life operate outside of the physical and biological boundaries that we are familiar with. It therefore seems unsurprising that the limits of life lie so perfectly within those experienced on Earth, and why we seek out other Earth-like planets as possible oases of biology. This bias is known as the anthropic principle and is an important philosophical consideration to bear in mind when considering the search for ‘habitable’ planets.

Nevertheless, many of the projects that exist to catalogue exoplanets are looking for ‘Earth-like’ planets: about the same size as Earth and at a similar distance from their star where the amount of incoming light produces temperatures that allow water to exist on the surface of these planets as a liquid. Liquid water plays a central role in the search for other Earth-like worlds because it is considered to be an essential requirement for life that is used as a solvent for biochemical reactions and is crucial to the operation of cells; no life exists on Earth that can survive without water. This water-centric distance is known as the habitable zone, or ‘Goldilocks zone’, because the temperature is ‘just right’! Different star-types have habitable zones that extend to different distances: the habitable zones of large, bright and young stars are further away than those of small, dim and cool stars.

Being within the habitable zone is important, but there are many, many other factors to consider before a planet can be labelled as ‘Earth-like’ or ‘habitable’. Planet size, age, density, orbital characteristics, atmospheric pressure and composition, the existence of an active geological cycle with volcanism and plate tectonics and the properties of the other members of the star system, to name a few. The habitability of planets is a complex and multifaceted property that we are only beginning to investigate, but it seems that a single measure (like residence the habitable zone) is insufficient to capture the true nature of the planet itself. This is why the growing catalogue of exoplanets has prompted the development of integrated ‘habitability indices’ that incorporate a number of factors into a single measure to determine how similar an exoplanet is to the Earth. One such measure, called the Earth Similarity Index (ESI) has been developed by researchers at the Arecibo observatory and attempts to rank planets discovered in the habitable zone on a scale from 0 (completely dissimilar to the Earth) to 1 (identical to the Earth) across a range of factors including size, density, atmospheric properties and temperature. According to this measure, the ‘Top 10’ most habitable planets we’ve discovered so far fall into a range between 0.50 and 0.82. For reference, our cold and dry neighbour Mars has a rating of 0.64, so it seems that none of these planets represent a suitable replacement for the Earth just yet.

The planet ranked most highly in this measure is called Kepler 62e and was discovered recently by the Kepler space telescope: the latest in a series of remarkable finds from this workhorse of planetary detection. This planet is orbiting within the habitable zone of an orange star slightly smaller and less bright than our own 1200 light years distant, but the planet itself is somewhat larger than the Earth and may be covered by a global ocean. At present, this distant world represents the pinnacle of exoplanetary habitability, yet it is far from being another Earth.

Kepler 62e
Kepler 62e: An artist’s concept of the most ‘Earth-like’ planet found to date
Image Credit: NASA/Ames/JPL-Caltech

Our occupation with the search for an ‘Earth analog’ masks the fact that there is still plenty about this planet we don’t know. For example, exoplanet researchers consider an active geological cycle to be essential for long-term habitability because the geochemical coupling between the oceans, atmosphere and planet interior is essential for ‘recycling’ nutrients through the Earth’s system. However, there are many unanswered questions about how this process operates on the Earth, and how it would function on planets that are different sizes. Modelling studies from different teams return seemingly contradictory results: some suggest that a similar mechanism to plate tectonics is inevitable, while others propose the opposite and infer a very different ‘lid’ type mode. These scenarios result in very different outcomes in terms of surface morphology and overall habitability, yet without direct observations it seems unlikely that this problem will be resolved soon.

We are also very limited by the detection limits of our instruments in this area: Kepler can only tell us the size of the planet – because it is proportional to the amount of light from the star that it blocks out to produce a detectable signal – but not the mass because we don’t know what it is the planet is made of. It is therefore very difficult to accurately model or estimate many of the surface or subsurface processes that may be occurring on these planets as mass is a very important factor in many aspects of planetary dynamics. Further to this, we are most likely decades away from being able to investigate the atmospheres of small, Earth-like planets in any detail.

We find ourselves poised at the very beginning of the search for another Earth, but the few results that we have at the moment are nevertheless very inspiring. The diversity of exoplanets discovered in the last decade is astounding, and small, rocky planets do not seem to be rare. My bold prediction is that Kepler will soon find a world that is seemingly like our own in size, temperature and orbital characteristics, but even so there are still very many unknowns that need to be addressed before any planet could be labelled as ‘another Earth’.

The Atmospheric Mirror

‘Earthrise’ : A blue marble, floating in a sea of blackness.


When viewed from space, the Earth glows like a blue marble under the light of the distant Sun. Azure oceans lap against the jagged coastlines and pale clouds swirl gracefully across its face, temporarily obscuring from view the brown-green landmasses beneath. From this vantage point, there is little to suggest that intelligent bipedal apes are scuttling around the coasts; confident of their centrality to all the workings of the cosmos, yet mostly unaware of the intricate complexities of its operation.

With the exception of five hundred  operational satellites amidst a sea of orbital debris, one permanently occupied space station in low Earth orbit and two intrepid robotic explorers on the planet next door (Opportunity and Curiosity), humans have little visible presence outside of the Earth. In spite of our delusions of grandeur, we assume that no evidence of our global civilisation could be detected from light-year distances.

However, if we imagine that somewhere in the menagerie of stars that make up our local neighbourhood in the Milky Way, on a planet not too dissimilar from ours, an alien astronomer was perched at his (or her) telescope one night staring out into the dark when our Solar System happened into view. What would they see? Just another star on their survey, if relatively young and brighter than most, but perhaps one of many observed that evening. Initially, the blinding glare of the Sun would obscure our family of planets from direct view. Luckily, there are a number of ways to circumvent this problem. Using indirect planet detection techniques familiar to us such as radial velocity measurements or transit timings, the planetary companions of this curious yellow dwarf star are revealed:  four gas giants and four smaller worlds. If the exo-astronomer ran their observations through their superior spectrometer however, chances are they may be intrigued by the results from one tiny blue planet in the orbit of this humdrum star.

Spectrometers measure the properties of light, first emitted by stars 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 mirrored in the light can be teased out of the noise with sufficient skill. The high levels of water vapour, oxygen, methane and other gases associated with biological activity discovered in the atmosphere of this planet should result in the alien equivalent of a raised eyebrow. Methane is a ‘reduced’ gas and is usually rapidly destroyed in the presence of oxygen, meaning that detecting an appreciable amount of both may suggest that a biological mechanism is responsible for their continual replenishment. This mismatch is identified as a ‘biosignature‘ – a sign that this planet may harbour life.

Planetary atmospheres are something we are all intimately familiar with. The Earth’s is flush with life-giving oxygen, greenhouse gases essential (in the right balance) to maintaining a clement climate and an ozone layer that shields us from the Sun’s harmful rays. Most of us will never leave its gaseous embrace, and without it life would be extremely difficult. However, we take for granted the atmosphere’s ability to act as a mirror of our activities detectable from astronomical distances, able to reflect the unique signatures of the gases injected into it and hold them there for those with the correct instruments to see.

Further studies by the inquisitive alien astronomer would reveal a soup of exotic chemicals in the atmosphere of this distant little planet: increasing levels of carbon dioxide along with a suite of destructive, industrially produced compounds like chlorofluorocarbons (CFCs). There is no known biological pathway for producing CFCs, so their detection in the atmosphere of this planet is a strong indication of the activities of industry. They have struck gold (or the equivalently rare element on their planet) by discovering compelling evidence for the existence of another technologically advanced species. In doing so, they may have forever altered the way their civilisation views itself – one of perhaps many in a vast, galactic family.

Cloaked in an imaginative example, this is the theory that lies behind using spectroscopy as a method of detecting life, and perhaps even advanced civilisations, across the depths of space. Two promising space telescopes, TPF (NASA) and Darwin (ESA), were cancelled due to budgetary constraints, so for now at least interstellar planetary spectroscopy remains out of our grasp. However, the hope is that instruments of the near-future will be able to examine the atmospheres of exoplanets to search for these signs of life. Until they can, it might be worth remembering that we might not be the only ones able to gaze into the Earth’s atmospheric mirror.

Perhaps we should try to keep it clean?

Detection and Discovery of Exoplanets

thingswedontknowThis 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, for TWDK

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.

Where is Everyone? The Fermi Paradox, Astrobiology and Exoplanets


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

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.