It’s Autumn in New York once again, and naturally thoughts turn to…life on other planets! Or at least the thoughts of the audience at Columbia University’s Pupin Hall, where Dr. Joshua Tan gave a lecture on the evening of October 18.
A professor at CUNY-LaGuardia, where he teaches astrobiology, Tan is also a research associate at the AMNH. His recent work includes optical observations of short-period millisecond binary pulsar companions, and neutron star physics more generally.
Dr. Tan began by discussing how to define life. The task is surprisingly difficult, even if, as he quoted Justice Potter Stewart, “I know it when I see it.” There are seven recognized minimum characteristics of life, but as he pointed out, an entity could still satisfy all of them without being alive. For example, our Sun, like other stars, is organized into layers, has a metabolism (nuclear fusion), exhibits homeostasis (which in the case of stars we call hydrostatic equilibrium). Stars, too, undergo growth (from accretion through ignition, main sequence, old age), reproduce (as stars are born from the remnants of prior star populations—after all, other species also die in giving birth), respond to stimuli (a star can bounce back from a shock wave), and evolve (as the metallicity of stars increases from one generation to the next). But no one would think of the Sun or any other star as a living being. So what is missing from the definition?
To these seven characteristics of life, Dr. Tan added a chemical component: life, at least as we know it, uses proteins and nucleic acids (DNA and RNA) to accomplish its tasks. But where do these necessary ingredients come from?
Tan recounted the now-famous Miller-Urey experiment of 1952, in which the two chemists found that by firing electrical sparks (akin to lightning) through a mixture of gases representing the early Earth’s atmosphere, complex organic molecules were synthesized. As he put it, their work offered a recipe for at least the precursors to carbon-based life.
He then took a moment to discuss the role of RNA, which, as opposed to DNA, has the ability to do some of the work of proteins itself, in addition to being capable of reproducing. The RNA World Hypothesis outlines individual steps which could have led from chemical synthesis of the first RNA molecules to the later evolution of DNA and life as we know it today.
Tan then extended the recipe metaphor for the origins of life by discussing the ingredients, pantry, delivery services, grocery store, and producers of these ingredients.
In terms of the ingredients, there are six elements that are essential to life on Earth: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur, or, as he put it, “CHNOPS.”
In terms of a pantry, that would be our planet. But the early Earth lacked an abundance of CHNOPS, and most of the oxygen that was already there was tied up with silicon in rock. How did these elements get to the Earth? Fortunately, when the pantry is bare, there are delivery services to bring ingredients, in the form of asteroid and comet impacts.
Of course the delivery services get their supplies from a grocery store. In the case of life on Earth, the grocery store was simply the interstellar medium, from which the solar system was formed, and where these elements and organics are available.
There are six major producers—think agro-industry—of the elements necessary for life, which include the Big Bang, where nucleosynthesis created the first elements (H, He, and a smattering of Li), dying low-mass stars which cast off their outer atmospheres, exploding massive stars, cosmic-ray induced fission, merging neutron stars, now recognized as the principle source of heavy elements, and exploding white dwarf stars.
Tan then introduced some daunting figures. Of the energy density of the universe, only about 5% is “normal” matter, the rest being dark energy and dark matter. But the chemicals for life that were available to Earth come specifically from cold gas clouds in galaxies, such clouds representing only 1.8% of ordinary matter. So 1.8% of 5%: life is definitely a specialty!
In terms of the kitchens where life is made, Tan showed a diagram of the variety of exoplanetary systems discovered to date, and discussed the importance of habitable zones around stars, while noting that the discovery of underground ocean worlds in our solar system has forced us to enlarge our concept of where in a stellar system life might be found.
Finally, you can’t have a restaurant, at least in NYC, without inspection of the kitchen and food preparation processes. Tan then discussed the various methods which we are using to find exoplanets: the very successful transit and radial velocity methods, but also microlensing, measuring astrometric wobble, and in a very small number of cases to date, direct detection of exoplanets (as is the case for at least one planet orbiting the star Fomalhaut).
Dr. Tan finished by showing a modern diagram displaying forms of life on Earth and their large-scale family relationships. In searching for life elsewhere, we may find lifeforms highly different from us. To illustrate the point, he called our attention to our world’s extremophiles, in particular the Pompeii worm, which thrives in temperatures of 80°C of hydrothermal vents—a temperature which would quickly kill us, and Pyrococcus furiosus, an extremophile whose optimal environmental temperature is 100°C—the boiling point of water!