The presence of life in the Universe has titillated scientists for centuries. The explosion of exoplanet discoveries throughout our Galaxy and beyond in the last 15 years has allowed philosophical exploration to turn into real science. Research in astrobiology, “the study of the origins, evolution, distribution, and future of life in the universe” by astrophysicists, geophysicists, biologists and chemists, has flourished, spurred on particularly by targeted spending programmes such as NASA’s Astrobiology Institute. The fledgling field has been a regular grabber of spectacular headlines – but not always for the right reasons.
Last year, US researchers claimed in a Science paper to have discovered bacteria in California’s Mono Lake that were capable of replacing phosphorous in their DNA by arsenic, allowing them to thrive in arsenic-rich environments. NASA announced this potentially groundbreaking finding with much fanfare: the bacteria GFAJ-1 would be the first lifeform discovered to substitute one of its fundamental DNA building blocks with a different element. But Wolfe-Simon et al’s results were called into question by a number of respected scientists, and the resulting storm of commentary brought out the best and the worst of scientists and of the web. If you want to know more about it, Carl Zimmer has written some good reports on the whole affair for Slate.
Astrobiology at SciFoo
At SciFoo last week I was happy to meet Rosie Redfield, who runs a microbiology lab at the University of British Columbia in Vancouver. Rosie was the first scientist to write an extensive critique of the arsenic paper on her research blog. She’s currently repeating the experiments with the GFAJ-1 bacteria in her own lab and documenting her progress on the blog, which is, irrespective of the controversial back story, an interesting experiment in itself. I participated in a session she organised at SciFoo entitled “Astrobiology: Buzzword or Science?”, in which we had a lively discussion on the phenomenon that is astrobiology. I can’t remember all the participants (sorry!), but among them were SETI’s Jill Tarter, Chris Lintott, Sean Carroll, as well as Rosie and myself.
Within the microbiology community, she explained, there’s the perception that astrobiology papers almost invariably get lots of media attention, while the research is not held up to the same standards as “regular” microbiology work. In other words, the papers rarely (never?) live up to the hype. The arsenic Science paper was a perfect (though not unique) example of that. On her blog she asked readers to send her examples of good astrobiology papers, and didn’t receive any satisfactory responses.
My own feeling is that there’s a huge potential for discovery in astrobiology research. Breakthroughs often happen at the intersection of disciplines, and astrobiology ticks that box by bringing together astrophysicists, biologists, geologists and chemists to study all aspects of life in the Universe: what it is, how it originates and evolves, and how we might detect it over large distances. So I came back from SciFoo determined to find some examples of great astrobiology research from the actual scientific literature rather than from flashy press releases.
Annual Reviews of Astronomy & Astrophysics, always an excellent first port of call for the non-expert, published a review on Astrobiology in 2005 by Christopher Chyba from the SETI Institute and Kevin Hand of Stanford. This paper is a great read for the uninitiated as it covers a lot of interesting history and background on early ideas on life in the Universe. Chyba & Hand describe some elementary principles of astrobiology, such as the properties of stars that form with a planetary system, the concept of the Habitable Zone where planets might host life, and possible pathways to the origins of life. But much of the research it cites is of more of philosophical and historical interest than the cutting edge of research (circa 2005). Even if it had done, the landscape of this fast-growing field has changed quite a bit since then.
In any case, let’s accept that the reasons for doing astrobiology are well documented. But where’s the breakthrough science?
As it happens, I have a prominent astrobiologist working just two flights of stairs up from my office. Lisa Kaltenegger, who joined MPIA from the Center for Astrophysics at Harvard last year, works on modelling of exoplanetary atmospheres specifically to investigate their potential habitability. In recent years she’s (co-) authored a number of nice papers in astrobiology and high-impact astrophysics journals that bring together the disciplines to genuinely contribute something new to our study of life in the Universe.
In Kaltenegger et al (2010), ‘Deciphering spectral fingerprints of habitable planets’, the authors look specifically at the chemical species whose signatures, or combinations of signatures, in the spectra of a planetary body are strong signposts of biological activity, and what properties of the planet could be derived from them. With our current observational facilities, we’ve been able to record spectra of exoplanet atmospheres. In those, we’ve picked up signatures of e.g. methane, carbon dioxide and water. So the paper is definitely valuable in helping us place these observational results into context, and identifying what to look out for.
NASA’s Kepler satellite has delivered a bumper crop of new exoplanet discoveries: earlier this year, the mission team released data on over 1,200 possible new planets from just a tiny fraction of sky. More than 50 of these were suspected to orbit in their parent stars’ habitable zone, i.e. with conditions potentially favourable to life. Kaltenegger & Sasselov (2011) examine the full sample of Kepler candidates, and with more detailed calculations and careful consideration of uncertainties, come up with their own estimates of the number of planets whose orbital parameters and physical characteristics may render them friendly to life. Their calculations strongly reduce the number of candidate rocky planets within their host stars’ habitable zones.
These papers are just a few examples of work that I think addresses the core goals of astrobiology while remaining thorough and balanced. As they’re extensively referenced, they’re an excellent starting point for those who want to explore the subject.
Where’s the “Astro-”?
A limitation of this type of research is that “habitability” is defined purely from our experience here on Earth. It assumes that a significant amount of liquid water must be present on the planet’s surface to act as a solvent and that biology is carbon-based – basically that the spectral signatures of life on other planets would look similar to those we see in the Earth’s atmosphere, when we look down from space. These are of course quite restrictive assumptions. There’s no reason for life not to develop via other chemical pathways; we just don’t know about it, and therefore have no clue what effect such life would have on a planet’s atmosphere. For now, that’s all we can observe.
In that respect, the discovery of weird organisms living in scorchingly hot volcanic vents, dark seabeds, and hostile chemical environments is extremely interesting, in that it helps expand our definition of life here on Earth.
But it seems to me that some research funded in astrobiology, such as (but not exclusively!) the infamous arsenic paper, fails to make any connection to space or planets or astronomy. In other words, it’s biology. So I’m curious as to why such work is funded in astro-biology programs, particularly by NASA, which is after all a space agency.
You could argue that it doesn’t really matter who funds what, as long as great research gets done. But as a result of this wide disparity of topics covered under the astrobiology umbrella, some of which contain no astronomy or no biology, the field seems to be a jumble of vaguely related topics rather than a joining of expertise to answer specific problems. In our discussion, Chris drew the comparison with the astrochemistry field, where the input of chemists has been hugely successful in helping us understand processes in complex astrophysical environments such as star forming regions. There are dedicated labs for astrochemistry, where chemists and astronomers perform experiments that specifically aim emulate conditions in space.
That kind of targeted collaboration doesn’t seem to be widespread (yet) in astrobiology.
Maybe it’s just a matter of time before more focus emerges from new collaborations. Every week seems to bring new discoveries in (exo)planetary science at the moment, and it’s exciting to see this field coming into maturity. Has astrobiology caused a revolution in science? Not yet – and that’s ok. There’s altogether too much hype, but there’s solid science behind it. I hope that astrobiology’s recent PR troubles don’t stand in the way of this progress.
A Few Asides
(1) I’m not an astrobiologist and my work is not paid for by an astrobiology programme. However, as you probably know by now, I do work on MIRI, the mid-infrared instrument for JWST, and the part of the instrument I work on, the low resolution spectrograph, has as a primary goal to characterise the atmospheres of exoplanets. JWST will be ideally suited to this kind of observations, which are extremely challenging with ground-based telescopes. So if you care deeply about this kind of research, you should support the continued funding of JWST.
(2) I also realise I’ve limited myself to the search for life on exoplanets in this post – of course we’re also searching for life closer to home, in our own Solar System. The study of exotic lifeforms on Earth becomes more relevant when we think about exploration of our planetary siblings and their moons.
(3) While reading up on this topic, I came across a number of decent life- and exoplanet-related media articles, all published in the last week or so. Science has a piece by Yudhijit Bhattacharjee on exoplanet research, its history and future prospects. In the Guardian, Ian Sample writes about a recently published paper that tries to judge how an alien encounter might pan out for mankind. Welcome to exo-bio-sociopolitics! On the same topic the Guardian has a blog post by astronomer Alan Penny, who makes a case for searching for extra-terrestrial intelligent life.
Chyba, C., & Hand, K. (2005). ASTROBIOLOGY: The Study of the Living Universe Annual Review of Astronomy and Astrophysics, 43 (1), 31-74 DOI: 10.1146/annurev.astro.43.051804.102202
Kaltenegger L, Selsis F, Fridlund M, Lammer H, Beichman C, Danchi W, Eiroa C, Henning T, Herbst T, Léger A, Liseau R, Lunine J, Paresce F, Penny A, Quirrenbach A, Röttgering H, Schneider J, Stam D, Tinetti G, & White GJ (2010). Deciphering spectral fingerprints of habitable exoplanets. Astrobiology, 10 (1), 89-102 PMID: 20307185
L. Kaltenegger, & D. Sasselov (2011). Exploring the Habitable Zone for Kepler planetary candidates ApJ, 736 arXiv: 1105.0861v2