The Null Hypothesis: When Do We Declare a Barren World?

This is a guest post by Euan Monaghan, a post-doctoral researcher in the Department of Physical Sciences at The Open University, where he studies the habitability of the subsurface of Mars. You can find him on Twitter

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Astrobiology is the search for life elsewhere in the universe. When this search is focussed on a specific world, there’s a chance—quite a good chance it would seem—that this search will turn out to be fruitless; that there will be no life to be found except the terrestrial life we bring along with us in the process. But can we ever say for sure?

This piece is focussed on Mars, but the idea applies to all worlds targeted for astrobiological exploration. The particular habitats on Europa, Titan or Kepler-62e might be different to those found on Mars, but the question is the same everywhere: does this world host life?

Scientific progress has made the martians of our imagination progressively smaller and more insignificant. No longer the grand canal builders of old—no longer even considered to be multi-cellular—the optimistic amongst us imagine microbes in briny pockets kilometres beneath a hostile surface; their presence deep underground given away by a subtle disequilibrium in the gases of Mars’ tenuous atmosphere. If the martians are there, they’re in hiding.

As we gain a greater understanding of the geologic and climatic history of Mars, a subterranean biosphere doesn’t seem so unreasonable. While Mars was likely warm and wet long before the Earth was, it is also smaller and so cooled faster. It couldn’t hold onto a thick, warming atmosphere for long and so its surface water was gradually lost, both out into space and down into the planet’s interior, to be fixed within the structure of minerals, frozen as permafrost or trapped in groundwater aquifers beneath layers of ice. And as Mars cooled and the water descended, so did the planet’s habitable zone, until it was hidden from view.

Recurring slope lineae in Coprates Chasma may be due to active seeps of water; a clue to a possible subsurface biosphere? (Credit: NASA/JPL/University of Arizona, HiRise)

The habitability of any extra-terrestrial environment is estimated through the study of life adapted to extreme conditions on the Earth. This ‘envelope of life’, with its upper and lower boundaries of temperature, pressure, salt tolerance and so on, is expanding all the time. The relatively recent discovery of our own deep subsurface biosphere, as well as its remarkable diversity and extent, has broadened our concept of what we consider to be a habitable environment. It is with this ever-more subtle knowledge of our own world that we turn back to the planets in our search for life.

The next logical step in that search, for Mars at least, is a detailed study of its atmosphere. In early 2016 the European Space Agency will launch a mission to do just that: the ExoMars Trace Gas Orbiter (TGO) will perform a more comprehensive inventory of the martian atmosphere and the respective abundances of its gases than ever before. It is hoped that the results of this study will provide an insight into active processes occurring deep underground. But then again there is the very real possibility that the TGO will arrive in orbit and find no signs of life, however tentative. The null hypothesis—Mars is a barren world—would still stand. Should we then give up on our search, or do we commit time and resources to a strategy of ever more sophisticated astrobiological exploration, all the while striving to prevent accidental contamination by terrestrial life?

The inevitable moments when we decide to re-focus our search for life beyond the Earth should not be considered moments of pessimism. The universe has too much potential.

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. 

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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.

Are Exoplanets Habitable?

thingswedontknow

This the fourth and final article 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. It started off their coverage for World Space Week 2013.

As the catalogue of planets orbiting other stars (called exoplanets) known to us continues to grow, increasing discoveries of potentially ‘habitable’ planets are likely to follow. The ‘habitable zone’ (HZ) concept, which was introduced in a previous post, is becoming increasingly important to our interpretation of these announcements. However, when used unilaterally as it often is, the HZ metric may be misleading – and should rather be considered as a good initial indicator of possible habitable conditions, interpreted relative to other available planetary characteristics.

To the best of our knowledge, Hungry Space Bears aren’t really the leading cause of failure on interplanetary missions. Image copyright ©Luke Surl, used with permission.

The habitable zone describes the theoretical distance (with both upper and lower limits) at which a given planet must orbit a star to support the basic fundamental requirements for the existence of life based on our understanding of the evolution of the biosphere on Earth. It is often referred to as “the Goldilocks Zone“, since it looks for the region “not too hot, and not too cold”. The concept is based on terrestrial (rocky, as opposed to gaseous or icy) planets that exhibit dynamic tectonic activity (volcanism and/or possible plate tectonics) and that have active magnetic fields to protect their atmospheres from high energy stellar particles that could strip it away. The composition of atmosphere is assumed to consist of water vapour, carbon dioxide and nitrogen with liquid water available at the surface, as on the Earth. Liquid water is the key; the giver of life and the fundamental factor in defining the habitable zone in any planetary system.

It should be relatively easy to spot a number of limitations of the habitable zone concept already; we are still unsure of the atmospheric composition of many of the planets we have already discovered, which would significantly affect any habitability analysis.

Also, we assume that any potential exobiology (the biology of life on other worlds) would have the same requirements as Earth-based life, which may not necessarily be so. The wide variety of extremophile organisms (those able to tolerate extremes of temperature, pressure, salinity, radiation etc.) on Earth might mean we should extend the parameters of the habitable zone beyond those originally considered. All in all, the idea of a habitable zone is a great thought experiment, but it may not necessarily translate into a warm, clement planet in reality. Planetary processes, such as tectonics and atmospheric greenhouse effects, warp the boundaries of the habitable zone. Furthermore, astrobiologists are now considering the very real possibility of salty liquid waterexisting in massive sub-surface oceans of Jupiter’s icy moon Europa, a body well outside of the defined habitable zone of our solar system1.

Another good example of the limitations of using the habitable zone concept in isolation was the furore that resulted from the discovery of the first planet definitively found to be within the habitable zone of its star, Kepler 22b, in late 20112. The popular science media and news outlets were awash with articles and posts describing Kepler 22b as “Earth’s twin” and “Earth 2.0” based solely on the fact that it has been discovered to be orbiting within the habitable zone of the Sun-like star Kepler 22. The media circus surrounding this announcement was an unusual situation, and one that had not been afforded to many other exoplanet announcements before or since. It’s clear that the possibility that this distant world may be suitable for life had spurred the imagination of scientists and the public alike. However, what was usually skipped over, or not mentioned at all, is that Kepler 22b has a radius 2.1 times that of the Earth, and estimates of its mass range from 10 to 34 times that of our planet. The large uncertainty in these figures are due to the method used in its detection, more about which can be found in this previous post in my TWDK series. The unknowns inherent in the discovery of Kepler 22b meant that it could be either a warm, ocean covered rocky planet with a greenhouse atmosphere similar in composition to that of the Earth, or a gaseous planet with crushing gravity and surface temperatures closer to a lead-melting 460 °C, depending on its mass and composition. These are attributes we cannot yet determine effectively.

compisite artist's impressions of Kepler 22B as a watery world and as a gas giant

Kepler 22b: Oceanic paradise, or hellish furnace?
Images courtesy of the Planetary Habitability Laboratory at UPR Arecibo

More data and better detection technology will provide the answer in time, but until then it remains important not to over-hype planets that are only borderline habitable in the very best case scenario as this will most likely be damaging to the public perception of this exciting field in the long term.

Recently, there has been revitalised interest in the habitable zone concept itself, which was first proposed in 1953, with updated estimates based on new climate models published in the scientific literature, as well as increased use of integrated habitability metrics which take other planetary factors into account. However, our understanding of the factors that control the habitability of extrasolar planets is at a very early stage, as is our grasp on the limits that life can endure, and it remains too early to say with much confidence that we have discovered another world suitable for life.

References:

[1] Tyler, Robert H. “Strong ocean tidal flow and heating on moons of the outer planets.” Nature 456, 770-772 DOI: doi:10.1038/nature07571

[2] Borucki, William J et al. “Kepler-22b: a 2.4 Earth-radius planet in the habitable zone of a Sun-like star.” The Astrophysical Journal 745.2 (2012): 120. (PDF)

The Search for another Earth

thingswedontknow

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’.

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.

Enough Time for Life: Part II

We are like butterflies who flutter for a day and think it’s forever.

 -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 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

Like as the waves make towards the pebbl’d shore,
So do our minutes hasten to their end

 -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.

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.

Enough Time for Life: Part I

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 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.

HZZAMS

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.

A Multiplicity of Worlds

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

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

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

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

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

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

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

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

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

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

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

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

Solar systems, it seems, are everywhere.

Habitable Zone Of Red Dwarfs May Be Larger Than Once Thought

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

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

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

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

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

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

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

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

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Click here for the Astrobiology article (requires subscription).

ResearchBlogging.org

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