Biosignatures in exoplanet atmospheres – Giovanna Tinetti (UCL,UK)
Life on Earth is found in the most inhospitable environments. And as we find ever more exoplanets (now approximately 3700 and counting) in great diversity, it seems quite plausible to search for life on these alien worlds. But how do we do this? Astronomer Giovanna Tinetti (University College London, UK) explained how the atmosphere of exoplanets have been studied.
Tinetti started her talk by briefly discussing the variation in size, composition and orbit of the current sample of exoplanets. Statistics are limited, and the types of exoplanet we have found so far is strongly determined by the method used to detect them.
It is now possible to take spectra from the atmosphere of exoplanets when they transit their star (as viewed from Earth). But this technique is limited to planets in an orbit close to their parent star which transit at short intervals, as this allows for multiple measurements. Planets in wider orbits could be studied by direct observation, masking the parent star.
Readings from exoplanet atmospheres have in some cases shown water vapour and carbon-bearing molecules. But what kind of signature would point to life on such a planet? Life will impact the composition of the atmosphere, explained Tinetti, and it will trigger a chemical disequilibrium. But our knowledge of biospheres is limited to Earth. Our planet shows near- infrared reflection from photosynthetic pigments. But on other planets, those pigments might be different – depending on the light emitted by the parent star.
‘The definition of a biosignature has not changed since the 1970ies’, Tinetti said, and is mainly based on our knowledge of life on Earth. ‘So far, the definition of habitability is based on a very simple concept: you need liquid water.’ In the next decade, she would like to do a census of the atmospheric chemistry of non-habitable planets, with a variety of temperatures, sizes and mother stars. Only then we should be prepared to tackle planets in the habitable zone, to get more statistics on which to base a biosignature of life on other planets.
Prospects for life on Mars – Inge Loes ten Kate (U Utrecht, NL)
In our own solar system, we can actually go and look for life on other planets (or moons). Astrobiologist Inge Loes ten Kate (University of Utrecht, the Netherlands) discussed the possibility of life on Mars. Ten Kate has worked on the Curiosity Rover program. Based on what we know of the conditions under which life evolved on Earth, she listed the requirements for life: a solvent, the right physicochemical conditions (e.g. temperature, acidity), presence of major elements and trace elements needed for life, organic compounds and available energy.
All the conditions she listed should be met in one place (location?) for life to emerge. On Earth, hydrothermal vents are interesting candidates, as are shallow pools. Hydrothermal vents are studied as one-pot-synthesis scenario: all conditions in the same location and all the necessary molecules synthesized locally. Shallow pools rely on getting part of the necessary molecules through external delivery by for example comets or meteorites. Additionally, the latter would be flooded with UV radiation from the young Sun. Next, Ten Kate compared her list with studies performed in three Martian locations by different Rovers: Endeavour crater and Meridiani Planum (both visited by Opportunity) and Gale Crater (visited by Curiosity).
Out of these three candidates, only Gale Crater ticks all the boxes. In this crater, formed some 3.7 billion years ago, a shallow pond or an impact crater with some hydrothermal activity might have spawned life. But, how likely is this scenario? ‘It’s not looking too bad’, Ten Kate concluded. She is currently running simulations within the context of the Origins Center, to investigate the role of extraterrestrially delivered organic compounds using her PALLAS Athena simulation facilities. She will also test the ‘one pot origin of life’ hypothesis using the Origins Simulator to be developed by one of the Origins Center fellows.
What makes planets habitable? – Charley Lineweaver (ANU, Australia)
One thing we do know about Mars is that it is not teeming with life. But that might be a normal situation, explained astrobiologist Charles Lineweaver (Australian National University) – it could very well be the case that once life gets started, abiotic planetary feedback systems will wipe it out (the Gaian bottleneck).
Even though life is present in the most inhospitable corners of the Earth, our planet still harbors lots of deserts, where there is too much heat, not enough nitrogen, carbon or other vital elements. And of course, life is limited to a very thin layer of some 10 kilometers. Based on our knowledge of Earth, we have defined ‘habitability’ for other planets. Lineweaver explained that it is not just the temperature which defines whether life can exist, but also the history. The now famous TRAPPIST planetary system has no less than seven planets in the habitable zone. ‘But these planets have been scorched by their star for a billion years.’
Furthermore, apart from a planetary orbit in the ‘habitable zone’, the entire planetary system may have to be in a habitable ‘Goldilocks’ zone of the Galaxy, where stars have high enough metallicity and the number of supernovae is small enough for emerging life to escape roasting. And the habitable conditions shouldn’t disappear.
Disappearance is probably likely, Lineweaver argued. Ice formation increases the albedo of a planet, which increases the reflection of sunlight and thus decreases the temperature. This could lead to runaway ice-albedo. On the other hand, a runway greenhouse effect like the one that occurred on Venus is also likely. It is possible that some life can change the atmospheric conditions, preventing the abiotic positive feedback and thus keep the planet habitable. Lineweaver: ‘So to remain habitable, a planet may need to be inhabited.’ And to remain inhabited, mutations are needed that jointly prevent runaway processes towards ‘too cold’, ‘too hot’ or other inhospitable circumstances. ‘All this could be very rare.’
Thus, if this Gaian Bottleneck scenario is correct, the origin of life could be common, but on most planets life would quickly disappear before advanced life could evolve. This could be a solution to the Fermi Paradox (the question why no advanced life form has yet contacted Earth). If there is a Gaian Bottleneck, we should not expect lots of planets with advanced life.
Beyond prebiotic chemistry – Lee Cronin (U Glasgow, UK)
After talking nearly two days about the origin of life, chemist Lee Cronin (University of Glasgow, UK) stated that we might be better off to abandon all definitions of life. We just don’t have enough data for a good definition, he argued. He favors the approach to focus on what life does: ‘Evolution creates objects or artifacts that are so complex, they wouldn’t exist without life.’ For example it is possible to imagine molecules that are simply too complicated to originate from abiotic processes in any abundance.
Cronin introduced the concept of ‘pathway complexity’, the number of steps needed to create an object, as a tool to threshold which artifacts are created by living processes. It would make the search for life on other planets simpler: ‘This way, you don’t need to find a life form, just a complex artifact.’ Preliminary studies show a correlation between measured physical properties correlate with pathway complexity. In this way it is possible to target which objects are alien artifacts and the product of alien life.