“Once life appears on a planet, regardless of the circumstances under which it appeared, evolution will push toward occupying every conceivable niche, even when the environments in some niches are very different from that in which life first arose.”
– National Research Council
The Limits of Organic Life in Planetary Systems (2007), p. 29.
The relentless pressures of survival have shaped the panoply of life on Earth throughout the ages. Life has endured oppressive heat, extreme volcanism, deep freezes, vicious asteroid impacts, perhaps even gamma ray bursts, and multiple mass extinction events. Yet, as resilient and adaptive as biology has proven, every organism that has ever been, owes its existence to a delicate game of cosmic chance.
The Earth orbits within a slender ‘habitable zone’ – a ‘slice of Eden’ – around the Sun, which, in spite of the aforementioned pressures and catastrophic events, has provided the foundational conditions for life to burst forth and flourish within a relatively narrow cosmic timeframe. As far as we know, Earth is the only place where this has occurred, but evidence is mounting that many extrasolar worlds enjoy similar blessings from their star.
Credit: Francois Fressin, Harvard Smithsonian Center for Astrophysics
The quest for extrasolar planets (‘exoplanets’) is one of the most rapidly developing fields in Astronomy. Hundreds of exoplanets have been confirmed since the mid 1990s and there are thousands of current candidates, with Astronomers now confidently predicting billions more worlds within the Milky Way Galaxy alone (including many within the classic Circumstellar Habitable Zone). Furthermore, perspectives about what exactly may constitute ‘habitability’ – or even life – in various contexts are broadening, whilst new evidence indicates that DNA building blocks can be created in space. The notion that the Earth alone harbours life amongst this vast spectrum of potential, grows ever more unlikely.
The history of planet hunting is littered with false hope, frustration and suspicion. In his groundbreaking book, The Neptune File, Tom Standage explains that, “by the early 1990s planet hunting was seen as a somewhat disreputable field of astronomy. Funding to search for planets around other stars was extremely difficult to come by; astronomers involved in the field were regarded with suspicion. But still they searched.”
Credit: Jan Skowron
Since exoplanets are distant, faint and overwhelmed by the luminosity of their star, they cannot be observed directly – at least not yet – and must instead be deduced by indirect means. Despite numerous painstaking detection attempts and discovery claims, perhaps most famously by Peter van de Kamp in the 1960s, subsequent studies were unable to provide verification. Most recently, in van de Kamp’s case, Choi et al. (2013) used precise doppler measuring to put the matter to rest. “Previous claims of planets around the star by van de Kamp are strongly refuted,” the authors state. In the early 1990s, however, astronomers finally struck ‘exo-gold’.
In 1992, using the Arecibo Observatory in Puerto Rico, Wolszczan & Frail reported planetary bodies orbiting pulsar PSR1257 + 12 with “almost circular orbits with periods of 98.2 and 66.6 days”; and in 1994, Wolszczan once and for all confirmed, “irrefutable evidence that the first planetary system around a star other than the sun has been identified”. Disappointment followed, as this was no environment suitable for life. Supernova remnant pulsars would pump high energy radiation into their surrounding bodies, lashing at anything that constitutes an atmosphere; and/or bathing a barren surface in lethal x-rays. The meticulous search for a system more suitable to habitability as we know it, continued, bolstered by a new confidence in planet detection methodologies and improving technology. Observers did not have to wait long.
In 1995, the loosened floodgates of discovery were flung wide open, following radial-velocity confirmation by Mayor & Queloz of a planet orbiting Sun-like star 51 Pegasi. Subsequent observations by other research teams demonstrated an “excellent” agreement with the work of Mayor & Queloz. Whilst this close-orbiting planet’s estimated 1,000+°C surface temperature would most certainly ruin your day at the beach, its discovery marks the first ever confirmation that Sun-like stars do host planets. This led to a heightened emphasis on what may be considered the ‘holy grail’ of exoplanetology – the search for Earth-like planets around Sun-like stars. In fact, the tagline for NASA’s Planet Quest initiative is, “The Search for Another Earth”.
Credit: Geoff Marcy and Paul Butler, San Francisco State University
Our Sun is a G-type main-sequence star, halfway through its 10 billion year main sequence lifetime. It has nourished our planet long enough for life to evolve to its current state. Our planetary neighbours, on the other hand, do not fare so well. Closer to the beast, Mercury fluctuates between a scorching 427°C and a chilling -179°C; whilst Venus maintains temperatures high enough to melt lead. Farther out, the Sun gives a merciless cold shoulder. Mars has a temperature range of -87°C to -5°C and that’s where any semblance of ‘warmth’ ends (although Earth could theoretically remain habitable at Mars orbit, due to its higher gravity, plate tectonics and magnetic field, which Mars lacks). Earth’s atmosphere and position between the two extremes is, at the present time, perfect for life as we know it, yet the balance is tenuous and finite. Natural or human-induced catastrophes, in the short-to-mid term, could threaten habitability (at least, in the human context); whilst in the long term, the Sun will inevitably roast Earth like a marshmallow, as it expands into its red giant phase.
Determining the likely habitable zones of exoplanets is a complex exercise, with a multitude of variables to consider alongside contemporary technological limitations. However, recent research into a particular class of planetary system candidate has vastly increased the likelihood that small rocky worlds, orbiting relatively favorable stars, may be the galactic norm. The planetary systems in question orbit a class of stars known as M dwarfs (or red dwarfs) – faint, cool, long-lived and abundant stars within the Milky Way. NASA astronomers estimate that hundreds of billions of planets orbit M dwarf star systems within our galaxy alone, with tens of billions of these residing in the habitable zone, according to certain European Southern Observatory astronomers.
Since M dwarfs are much fainter and cooler than our Sun, their calculated habitable zones are much closer in comparison to Earth’s habitable zone. This leads to important caveats. For instance, these stars have a tendency towards stellar eruptions and significant magnetic fields, which may bathe their close inner habitable zone planets in X-rays and UV radiation. Furthermore, as Vidotto et al. (2013) propose, M dwarf magnetic fields may reduce the size of planetary magnetospheres, leading to atmospheric erosion by stellar winds; whilst Barnes et al. (2012) postulate that tidal heating may produce runaway greenhouse effects for certain planets within the habitable zone, thus significantly reducing habitability.
Nevertheless, Delfosse et al. (2012) reiterate that “M dwarfs have been found to often have super-Earth planets with short orbital periods,” which makes them “preferential targets in searches for rocky or ocean planets in the solar neighbourhood”. In fact, Tarter et al. (2007) suggest that evolutionary challenges, such as stellar flaring, may not be as prejudicial as generally assumed, thus, they conclude, “it makes sense to include M dwarf stars in programs that seek to find habitable worlds and evidence of life”. It is this potential that makes these M dwarf star systems intriguing, especially when considered within the context of their sheer multitude throughout the galaxy.
Credit: European Southern Observatory
Jon Swift is a postdoctoral researcher at the California Institute of Technology. He works in Professor John Johnson’s ExoLab group, which hunts for, and characterises extrasolar planets. Swift’s current focus is on the formation and evolution of planetary systems. In late 2012, Swift, Johnson and co-authors submitted a paper – Characterizing the Cool KOIs IV: Kepler-32 as a prototype for the formation of compact planetary systems throughout the Galaxy – that garnered significant public attention.
Using the Kepler space telescope, the research team took advantage of the rare edge-on orientation of the Kepler-32 system (in relation to the orbiting observatory) to study its five planets in unprecedented detail, as they take turns at blocking their star’s light. The Kepler-32 M dwarf system is incredibly compact. Its planets orbit within an equivalent range to one third that of Mercury’s around our Sun (Swift et al. propose that the planets formed at wider orbits, migrating inwards over time). Although, apart from its coincidental orientation, the system is not unusual, and this is what makes it extraordinary in its implications.
In a seminal estimation, the authors used this representative M dwarf system to infer that a staggering number of planets populate this most common of star systems throughout the galaxy. Shortly after its publication, Swift, as lead author of the Astrophysical Journal paper, explained that “basically there's one of these planets per star” in the Milky Way. Given that there are a hundred billion stars or more in our galaxy, these numbers are overwhelming, but, as Swift also pointed out, the team’s probability calculations were conservative. If the dataset were expanded, the average would likely be doubled.
Credit: PHL at UPR Arecibo
With countless numbers come countless opportunities. With this in mind, I followed up with Dr. Swift to discuss the possibility of favourable conditions for life on planets orbiting M Dwarf systems; and how his team’s unique calculations may allow for additional speculation on this issue:
“Indeed M dwarfs do have temperate zones around them in which liquid water could potentially exist. These zones are both closer in and span a narrower range in orbital radius than for stars like the Sun. But they do exist, and it is expected that there are many planets throughout the Galaxy that lie in these temperate regions around M dwarfs. M dwarfs are also known to harbor rocky planets, so it is quite feasible that there are rocky planets with liquid water around M dwarfs. But the prevalence of this specific kind of planetary system is not yet known... The high frequency of planets around M dwarfs makes it easier for life to spring up across the Galaxy, simply because there are more places that it would have had a chance to begin. However, without knowing how difficult it is for life to begin from inanimate matter, we are still a long ways away from being able to deduce an accurate likelihood of life elsewhere in the Galaxy”.
He also cited the work of Tarter et al. (2007), noting that, “while there may be some difficulties in creating or sustaining life on planets around M dwarfs (even in the “habitable zone”), they are viable locations for the development and evolution of life; perhaps even intelligent life”. Swift’s clarifications raise some of the most profound questions of all – how did life begin on Earth - and how may it arise, survive and thrive elsewhere?
If there is one thing that the study of M dwarfs and other planetary systems has shown, it is that if life were to arise elsewhere in the Universe, it certainly has a vast and diverse number of orbiting outposts from which to launch its biochemical journey. As previously discussed, much of the focus in planet hunting centres around finding planets as favourable as possible to life as we know it – lots of liquid water; a solid surface; and a favourable orbital zone around a star without a temperament too much like The Incredible Hulk on a rough day. Why?
Credit: Peter Sawyer Smithsonian Institution
Part of the answer is terracentricity. We search for what we know, and what we know shapes what we expect. Since the only examples of life we have to go by are those found on Earth, we naturally gravitate towards as much “Earth-likeness” as we can deduce elsewhere. The most obvious manifestation of this human tendency is the search for ‘classic’ habitable zone exoplanets. Logically, this approach is sound. As we know, we cohabitate a planet utterly teeming with stunning biological diversity. The imagination need not overstretch to consider a similar situation elsewhere, given similar conditions. However, there are good scientific reasons to make conceptual leaps beyond the terracentric frame of reference; and when exoplanetology meets astrobiology along these lines, intriguing biochemical/evolutionary vistas present themselves for consideration.
“The natural tendency toward terracentricity (a particular set of biological and chemical characteristics that are displayed by all life on Earth) requires that we make a conscious effort to broaden our ideas of where life is possible and what forms it might take,” explains the National Research Council (NRC). “The long history of terran chemistry tempts us to become fixated on carbon because terran life is based on carbon. But basic principles of chemistry warn us against terracentricity”.
In 2007, the Council’s Committee on the Limits of Organic Life in Planetary Systems, produced a landmark review of the scientific literature, aimed at broadening perspectives about the full potential of life, and focusing future astrobiological research accordingly. The Committee was candid about the sometimes deep limitations of current knowledge (even in relation to most microorganisms in Earth environments), although what is known and what can be reasonably speculated upon, constantly redefines the boundaries of possibility. The study of the potential origins, distribution and biochemistry of life raise the most profound, fundamental, challenging and exciting questions facing humanity today.
Credit: MARUM, University of Bremen and NOAA-Pacific Marine Environmental Laboratory
At present, Earth life is the only form of life and biochemistry we know, but we don’t know how it began. It may have begun in deep sea hydrothermal vents, or perhaps was seeded via panspermia (interplanetary transfer of microorganisms). The latter of these possibilities was considered of high importance by the committee, for if Earth was seeded in this manner, a fundamental biochemical resemblance to other lifeforms could be expected elsewhere in the galaxy; and it could be expected to thrive in a wide variety of extreme environments. However, the committee strongly concluded that “life is possible in forms different from those on Earth” and that these may be highly unusual:
“As discussed in the literature, chemical models of non-Earth-centric life reveal much about what the scientific community considers possible, particularly regarding ways in which systems organize matter and energy to generate life. Thus, truly “weird” life might utilize an element other than carbon for its scaffolding. Less weird, but still alien to human biological experience, would be a life form that does not exploit thermodynamic disequilibria that are largely chemical. Weirder would be a life form that does not exploit water as its liquid milieu. Still weirder would be a life form that exists in the solid or gas phase. In a different direction, yet also outside the scope of life that most communities think possible, would be a life form that lacks a history of Darwinian evolution”.
One thing, above all, which life on Earth at least has taught us, is that once it secures a foothold, it is nigh impossible to eradicate. Assuming the forces of evolution are in play, life’s march forward will be relentless and far-reaching, across a diverse range of environments over time.
Even the most original science fiction writers of the past – and perhaps even today – would be hard pressed to have imagined the extreme lifeforms that have been found on Earth to date, let alone those potentially awaiting discovery elsewhere. Consider, for example, the enigmatic Deinococcus radiodurans – cited as the World’s Toughest Bacterium – especially for its ability to withstand a thousand times more radiation than humans. Deinococcus radiodurans defies the ‘Humpy-Dumpty’ analogy; extreme doses of ordinarily lethal radiation shatter its genome, then it puts itself perfectly back together again.
In addition to long-surviving spore-forming bacteria, radiation-resistant microorganisms such as Deinococcus radiodurans are considered so tough, they may even be legitimate candidates for surviving space travel and seeding other worlds, especially if embedded in rocks and ejected. According to Dr. Stephen Kane of San Francisco State University, “there have even been studies performed on Earth-based spores, bacteria and lichens, which show they can survive in both harsh environments on Earth and the extreme conditions of space”.
Credit: Oak Ridge National Laboratory
However, the transference of life itself may not have been necessary to lay the foundations for life to spark on Earth. It is plausible, as noted by Chyba et al. (1990), and Chyba & Sagan (1992), respectively, that, “Earth accreted prebiotic organic molecules important for the origins of life from impacts of carbonaceous asteroids and comets during the period of heavy bombardment 4.5 x 10(9) to 3.8 x 10(9) years ago”, or that “organic synthesis [was] driven by impact shocks; and... other energy sources”. In 2007, the National Research Council’s Task Group on Organic Environments in the Solar System acknowledged that, “a remarkably broad range of organic compounds has been identified in carbonaceous chondrites,” stating categorically, “the presence of such compounds in meteorites indicates that impacts by meteorites, comets, and dust must have delivered potentially biologically useful organic compounds to early Earth and the other terrestrial planets”.
In 2012, a research team from Goddard’s Astrobiology Analytical Laboratory reiterated, “Meteorites may have served as a molecular kit providing essential ingredients for the origin of life on Earth and possibly elsewhere”. “For the first time,” explained Dr. Michael Callahan, lead author of the 2011 study that examined Carbonaceous meteorites, “we have three lines of evidence that together give us confidence these DNA building blocks actually were created in space”. Researchers were previously much less certain whether these DNA components were actually created in space, but Callahan et al.’s findings strongly support an extraterrestrial origin. Glavin et. al. (2012) examined nonterrestrial amino acid excesses in the Tagish Lake meteorite and concluded, “significant enantiomeric enrichments for some amino acids could form by abiotic processes prior to the emergence of life”; and whilst Pizzarello & Shock (2010) acknowledge the high degree of uncertainty surrounding the possibility of biogenesis after exogenous delivery, they maintain, “the selective abundance of biomolecule precursors evident in some cosmic environments and the unique L-asymmetry of some meteoritic amino acids are suggestive of their possible contribution to terrestrial molecular evolution”.
Credit: David Hardy
The growing awareness of the extent of this type of transference of material – when considered alongside increasing evidence of the poly-extremophilic potentialities of certain organisms, and the possibility of alternative biochemistries and geneses – inevitably leads to the continuing reassessment of ‘habitability’. In turn, this evolving broad spectrum understanding helps to refine the parameters and technologies of future missions. If life has been found to survive in environments previously considered unimaginable, on Earth, it is plausible that such life may have – or may still – exist on worlds outside the classic Circumstellar Habitable Zone. Whilst the possibilities for extrasolar planetary systems in this context may be almost endless, the logical and only practical first place to search for such evidence, is in our own Solar System. For this reason, the 2011 Decadal Survey, Vision and Voyages for Planetary Science in the Decade 2013-2022, lists moons like Europa, Enceladus, and Titan as top research targets:
“What were the primordial sources of organic matter, and where does organic synthesis continue today? The surfaces and interiors of the icy satellites display a rich variety of organic molecules—some believed to be primordial, some likely being generated even today; Titan presents perhaps the richest planetary laboratory for studying organic synthesis ongoing on a global scale. Europa, Enceladus, and Titan are central to another key question in this theme: Beyond Earth, are there modern habitats elsewhere in the solar system with necessary conditions, organic matter, water, energy, and nutrients to sustain life, and do organisms live there now?”
Whilst viable missions to answer these questions are currently of a robotic nature, longer term goals include human missions to Mars, the Moon and beyond. As human missions carry a greater risk of contamination, it has been said that, “nothing would be more tragic in the... exploration of space than to encounter alien life and fail to recognize it either because of the consequences of contamination or because of the lack of proper tools and scientific preparation”. For this reason, future research will also focus on better understandings of life and habitability as demonstrated by Earth life, and will continue to consider possible biological alternatives beyond the terracentric frame of reference.
Credit: European Space Agency
Dr. Isaac Asimov once likened the joys of scientific inquiry, as it moves through history, to a man rising at dawn to fish, waiting happily all day for the occasional twitch of his line. Wouldn’t it be more practical for him to order all the fish he wanted from the fish market, via telephone?, Asimov asks. Perhaps, but the true joy is in the process of awaiting those elusive twitches. Within the context of the current discussion, merely knowing how much we don’t know about life’s origins, structures, and potential distribution throughout the Cosmos, makes the anticipation of what we may yet discover – the vast potential of what could be out there – all the more enjoyable.
In summary, we know there are potentially hundreds of billions of extrasolar planets, just in our Galaxy alone; and that many of these may fit the current precepts of classic habitability. We know there are countless interplanetary objects that have spread the organic building blocks of life in all directions through space; and perhaps even life itself. We know that the type of life to which we have become accustomed ranges from extreme fragility to extreme resilience; and that virtually nowhere on Earth does life not exist. We also know that life has survived at least five mass extinction events throughout our planet’s history. We don’t know exactly how life began on Earth, how common it may be throughout the Galaxy, what form it may take, or how it may spawn from world to world. In the future, we may not always know where or how to search for life elsewhere, or how long we may take to find it – or even if we ever will – but the tantalising prospect of those occasional twitches at the end of the line, will make for an enjoyable day at the lake.