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.
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.
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.
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.
 Tyler, Robert H. “Strong ocean tidal flow and heating on moons of the outer planets.” Nature 456, 770-772 DOI: doi:10.1038/nature07571
 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)
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?
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: 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’.
Last week, my first research paper was published in the journal Astrobiology. The paper outlines our method for estimating how long ‘habitable’ conditions may exist for on planets that have been discovered in the ‘habitable zone’ – a concept I regularly discuss on this blog and elsewhere. The run-up to its publication has been surprisingly hectic, and it has received a lot of media attention (see the bottom of this post for a full list of coverage). Whilst this is great for getting the science out there, I want to make sure that there is something available on the internet where I discuss the paper in my own words in case there are any misconceptions about our results.
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. I should stress that the planet itself is not moving, but rather the boundaries of the possible habitable zone that extends around the star are changing, and planets may be left in the heat, or brought in from the cold, as the star ages.
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. The time that a planet spends within the habitable zone can be considered its ‘habitable zone lifetime‘ (HZL). The HZL 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.
We coupled a stellar evolution model, with the classic habitable zone and applied it to planets that had already been discovered in the habitable zone by workers at the Planetary Habitability Laboratory. (Bio)geochemical feedback mechanisms operating on individual planet to buffer the climate will affect the boundaries of the habitable zone, but because these processes are complex and likely planet-dependent, we left them out and assumed constant conditions. We made the same assumptions about the planets’ atmospheres that the original authors of the habitable zone model (Kasting et al. 1993) did: a nitrogen rich atmosphere, with about 300ppm carbon dioxide and no clouds or other complex atmospheric physics or chemistry.
Nevertheless, this produced some interesting results. The Earth seems to be habitable for perhaps 6.29 billion years (Gyr), but this is excluding the influence of humans and our pesky habit of pumping extra CO2 into the atmosphere. This obviously can’t account for other random events (asteroids etc.), and it’s important to remember that we’re making no allowances for the natural biogeochemical cycles of the planet to buffer climate – this is a very simplified picture. Luckily for us, these estimates are similar to those produced by other more complex and Earth-centric models, so we were happy to continue to try to apply the simple model on other planets with reasonable confidence.
Other potentially habitable exoplanets do pretty well too. Kepler 22b may be habitable for 4.3 Gyr, Gliese 581g (if it exists!) will be in a habitable position for 11.2 Gyr, whilst its neighbour Gliese 581d might be clement for 42 Gyr! A huge amount of time. This star system is already approximately 8 Gyr old, so both these planets would be very interesting candidates for further study.
Our intention was to supply these figures so that they could be incorporated in habitability metrics in the future to capture the temporal aspect of the planetary habitability. Also, we hope that this framework can be used with other habitable zone formulations (several updated versions already exist) that focus on different aspects of the planetary system. Further, we hope that we can identify interesting planets for further study by future space telescopes or SETI campaigns. These would be planets that have been habitable for a similar or greater amount of time to the Earth, because we think that the evolution of intelligence will require a very, very long time, so pinpointing worlds with long HZLs would make sense.
I’ve noticed from the very many interviews I did that the press machine is a rapid, yet inefficient beast. I worry now that the purpose of the paper (to find habitable exoplanets like the Earth) has been eclipsed by the fact that we tested it on Earth first, and that we are making some definitive statement about how long we can comfortably live here. We are not. Earth is or test, our standard, our control. The press releases I have seen have put all the emphasis on this small part of the project, (and the fact that we should move to Mars!), whilst in reality we only validated the model against other more complex models for the Earth, and came to similar conclusion.
Further to this, it now appears that some people seem to have taken my work to show that human induced climate change will have little effect in the long term and that it undermines climate research. This really was not my intention. If I could have, I would have avoided all discussion of anthropogenic climate change in the first place, because we were investigating a different question (long term, solar-forced exoplanetary habitability) using a different tool. However, my institution (the University of East Anglia) has a strong reputation for climate science, and I fully support the findings of my colleagues at the UEA and elsewhere that illustrate the warming effects of increased levels of atmospheric CO2 over human timescales, and I did what I could to mention this in my interviews. When we state that the Earth will be in the habitable zone for 1.7 billion years longer, we have left anything that humans could do to the atmosphere in the interim out of the equation out of necessity.
Press coverage of this publication has been extensive. Here is a list compiled by the UEA Press Office:
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?
Last week I had the pleasure of attending the 5th UK Astrobiology Society of Britain Conference in the lovely city of Edinburgh. It was a very enjoyable and well organised few days of interdisciplinary science and good whisky, friendly folk and an obligatory bagpipe recital. However, upon reflection sometime in-between the well-lubricated poster session and the céilidh — replete with some fine displays of motility I should add — it seemed to me that astrobiology in the UK has a potentially serious image problem.
Views are shifting, but at present the public perception is that astrobiology is the study of little green men from Mars. However, the reality is very different, as anyone working in the field will tell you. I’ve always seen it as an organic extension and interdisciplinary marriage of the natural sciences, with solid scientific foundations firmly laid by the likes of Carl Sagan and Lynn Margulis. Astrobiology carries significant intellectual clout and I am convinced that contributions made by those working in the field will likely produce some of, if not the most, fantastic discoveries of this century.
This is why outspoken minority opinions that come to dominate discussion can be detrimental to both the public perception of the field, and also the direction and coherence of the discipline itself. When the press is seeking an ‘astrobiologist’ to comment on the latest Curiosity announcement or claim of life from outer space, there is a chance that they will go to the person who shouts loudest, regardless of whether that person represents the broad consensus of others in the field.
Don’t get me wrong, every discipline tends to attract their fair share of eccentrics and contrarians, but if that field is relatively young and already struggling to find a foothold amongst mainstream science in the UK, this can prove a bit of problem. To make matters worse, this field, unlike others, lies on the rational border of the fertile pseudoscientific pastures of aliens and UFOs and associated guff.
So, what to do?
Exclude the relevant parties from the forum and proceed as normal? This strategy risks alienation (ahem), and could end up backfiring as the troublemakers shout from the rooftops about systematic silencing by the ‘academy’ and the existence of an overarching conspiracy to keep their fantastical research from the public, thereby further accentuating the stereotype of the paranoid alien hunter to the public and other academics,and providing them with the attention they originally sought from their peers.
I think the answer is more integration, not less. Yes, these individuals may have made fundamental flaws at nearly every stage of their research, which itself was based on significant misapplication of the scientific method, but that is all the more reason to give them access to the ears and opinions of members in the field. This way, their methods can be improved and some of the more unscientific claims can be weeded out prior to steering any potential publication towards a peer-reviewed journal where its merits can objectively assessed by the wider community.
The organisers of the conference had a difficult decision to make, and made the right one I think by including the research in the schedule. It was then up to the attendees to highlight major errors, foster discussion and debate and attempt to reduce the isolation of this group from the community and the higher standards required to publish good work in this field.
It would have been easy to do this in Edinburgh. Firstly, when given the opportunity, challenge their claims! This could be done after their talk, during a poster session, or in the literature. I’ll be the first to admit, I’m a bit reluctant to do this. It might be that I’m an early career scientist, or because of my inherently British fear of confrontation and misguided diplomatic aspirations toward a plurality of opinions, but there should be a limit. When a claim impinges on that limit the immediate repercussions should take the form of an erudite and impassioned, yet polite, rebuttal. Insist on hard evidence, critically scrutinise methodologies and deconstruct their results – this is science in action and it’s how progress is made.
This is where the attendees of ASB5 may have faltered. We all had multiple opportunities to address the relevant parties and their claims, but instead hid behind a passive-aggressive tut and endured comment after comment of rambling through gritted teeth. I understand that any learned society cannot make assertions and give direct answers to difficult questions, but they can take stewardship of the conversation and advance the discussion in a democratic forum, the rest is up to the audience. We owe it to the hard work conducted by researchers in astrobiology to ensure that we adhere to only the highest standards of scientific investigation and scrutiny as a community because the future of this discipline as a viable and respected avenue for research and funding is at stake.
This the second in a series of posts by me at Things We Don’t Know about the many unknowns involved in the study of planets in the orbit of other stars across the galaxy.
The first planet discovered orbiting another star was detected by astronomers at an observatory in France in 1995. The planet is an enormous gas giant, half the mass of Jupiter, orbiting very close to the Sun-like star 51 Pegasi in the constellation Pegasus, 50 light-years from Earth. The existence of other planetary systems had been predicted by astronomers for centuries and the discovery marked a monumental breakthrough in astronomical research. Since then, rapid improvements in technology and observational techniques have resulted in the discovery of 863 confirmed ‘exoplanets’ to date.
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.
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.
Admittedly, to say that no planet has been directly imaged would not be quite accurate. Some extremely large planets, in most cases 5 or 10 times the mass of Jupiter, orbiting at great distances from their stars have been directly imaged. These first pictures represent great steps forward for exoplanet research, but technological constraints impose limits on the size and orbital distance of planets able to be imaged in this way, and the direct imaging of small, Earth-like planets orbiting relatively near to their host stars is not yet possible.
In my next post, I hope to take a more detailed tour through the current exoplanet catalogue to highlight some of the interesting and exotic planets that inhabit our galactic neighbourhood, and illustrate what the diversity of these planets can tell us about the Earth and our Solar System.
This the first in a series of posts by me at Things We Don’t Know about the many unknowns involved in the study of planets in the orbit of other stars across the galaxy.
Since the middle of the last century, against the backdrop of greatly expanding space technology and understanding, scientists have wondered about our place in the vast universe and whether we are alone or not. When it comes down to it, why would we be? There is no reason, be it physical or chemical, life couldn’t exist elsewhere. At first glance it seems that we live on a relatively normal planet, our parent star is of a fairly common variety and our corner of the galaxy isn’t all that extraordinary. Water and other ‘building block’ organic compounds, thought crucial for life in any imaginable form, are relatively abundant throughout the galaxy.
There are at least 100 billion (that’s a 1 followed by eleven zeroes) stars in the Milky Way galaxy alone; many we now know come complete with a family of planets in their orbit. On top of that, several of these newly-discovered ‘exoplanets’ are not that different from the Earth in mass or orbital distance from their parent stars. In fact, a recent study calculated that a staggering 17 billion Earth-like planets are likely to exist in the Milky Way alone! Surely, more than one of those worlds would have life of some kind or the other clinging to its surface? And if there was life, even if it was almost vanishingly rare, could another species with a similar level of intelligence to humans exist on another one of those billions of planets out there in the reaches of space?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.
It’s been a busy couple of weeks for exoplanetary discoveries, but also for me, which explains why I’ve taken so long getting round to writing about them.
On the 28th of August, the Kepler mission announced the discovery of a unique binary star two planet system. The Kepler 47 family consists of a binary pair, a G-type star – about 84% as massive as the Sun, and a smaller M-type red dwarf roughly 36% of the Sun’s mass, but only 1.4% as luminous. Two planets have been observed to be orbiting the pair. The closest is of these is Kepler 47 (AB) b, estimated (from mass-radius relationships) to be between 7 and 10 Earth masses, but the error on this figure remains large. The outermost planet, Kepler 47 (AB) c, is Neptune-sized (16 – 23 Earth masses) and is orbiting within the habitable zone, although due to its large mass it is unlikely to fulfil the traditional requirements for planetary habitability. The configuration of the Kepler 47 system illustrates the fact that stable multi-planetary orbits can exist around binary stars, and brings the total of circumbinary planets to six.
On the 29th of August, a new planet was added to the Habitable Exoplanets Catalog (HEC) bringing the total to six (including: Gliese 581d and g, Gliese 677Cc HD 85512b, Kepler 22b). Super-Earth Gliese 163c was established to be orbiting within the habitable zone of its 0.40 Solar mass star by an international team working at the European HARPS project. It completes an orbit in 26 days and has a mass no less than 6.9 times that of the Earth. The custodians of the HEC database have given Gliese 163c an Earth Similarity Index (ESI) rating of 0.73, establishing it as the 5th ‘most habitable’ exoplanet discovered to date, despite exhibiting possible surface temperatures of 60 °C or above.
Speaking to online science network io9, HEC lead scientist Professor Abel Méndez in the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo said, “Gliese 163c ranks fifth in our current list of six potentially habitable exoplanets because it is nearly twice the size of Earth and its temperature is also higher, but it’s still an object of interest for the search of biosignatures by future observatories.” The HEC has yet to assess Kepler 43 (AB) c, but it is not likely to fare well in habitability assessments due to its large mass.
Bringing my own (as-of-yet-unpublished, but in preparation) research into planetary habitable periods to the table, Kepler 43 (AB) c has a residence time within the habitable zone of approximately 3.9 billion years, whilst Gliese 163c can be expected to within the habitable zone for at least 22.6 billion years. The habitable zone is now populated by 8 planets (including the Earth), and looks a bit like this:
It’s certainly an exciting time to be working in this field; nearly each new week brings another interesting discovery. Keep looking up!
Around this time every year, the Earth, on her year long trundle around the Sun, passes through the Perseid cloud of cometary debris. The resulting month long encounter produces arguably the most prolific and spectacular meteor shower for northern observers – the Perseids. As many as 100 “shooting stars” an hour may be visible at its peak in mid-August and the shower is eagerly awaited by sky-gazers for it’s dazzling and reliable display of colourful meteors and fireballs.
The source of the Perseids is dust and debris contained in a relatively dense ‘cloud’ impacting the upper atmosphere of the planet and burning up due to rapid deceleration due to increased aerodynamic drag. The shower has been observed for millennia, the first recorded sighting was in 69 BC, and most of the dust and debris responsible for the shower was pulled off a comet a thousand years ago. The particles that produce this astronomical light-show are generally tiny, on the order of centimetres, and pose little threat to the Earth below. However, the same cannot be said for their parent, comet Swift-Tuttle.
Comet Swift Tuttle (designation: 109P/Swift–Tuttle) is a typical Halley-like long period comet. It tears through the inner solar system when nearing the closest approach of its 133 year orbit around the Sun; an orbit that takes it out 12 AU past Pluto to 51 AU, and all the way back again. Its last close encounter with Earth was in 1992, and it won’t return until 2126.
For a while following its rediscovery in 1992, almost 10 years away from its expected position, the orbital evolution of the comet was not well constrained and there was considerable cause for alarm when it was estimated to be on a collision course with Earth in 2126. Concern was justified: its nucleus is 26km in diameter, considerably larger than the 10 km impactor that is thought to have caused the Cretaceous-Paleogene (K-T) mass extinction event 65 million years ago. However, reanalysis of ancient records of observations and improved calculations that included the effects of nucleus evaporation confirmed that the comet is on a very stable orbit and poses little threat to Earth for the next 2000 years.
That said, in a 1997 book by South African/American radio astronomer Gerrit Verschuur, comet Swift-Tuttle was described as the most dangerous object known to man for it’s ability to cause catastrophic damage if it was to impact the Earth. An exceptionally close encounter is expected in 4479, bringing Swift Tuttle to within 0.03 AU (approximately 4 million km) of the Earth – roughly 10 times the mean Earth-Moon distance. Travelling at a relative velocity of 60 km per second, Swift-Tuttle would unleash the equivalent of a devastating 3.2×1015 tons of TNT upon impact – 27 times the energy of the K-T impactor. For comparison, the largest nuclear weapon ever detonated was a ‘mere’ 50 megatons (106). It would very likely cause huge loss of life across the planet and result in a mass extinction unlike any known previously, whilst placing unbridled pressure on the capacity for human civilisation to recover. If the initial impact was survived, tsunamis, wildfires, earthquakes, years of darkness and a toxic atmosphere would follow. Harvard astrophysicist John Chambers estimates the chance of collision in 4479 to be 1 in 1,000,000. Best of luck to our descendants 2467 years from now!
It is worth bearing this in mind when you gaze up over the next few nights to witness the magnificent sight of the ancient dust of this comet burning up in our atmosphere, for one day their parent may put on a somewhat more spectacular, if devastating, show.