Is there anybody out there?
When we stop at the gate to enter the playing field, the moment is ripe with anticipation.
The dogs are nothing if not eternal optimists. The dogs’ impending time in the field is full of fantastic possibilities as far as they are concerned:
A discarded lunch bag, perfumed with the residue of a meaty sandwich.
The fresh scent trails left by other visitors.
It’s the latter possibility that seems to create the greatest enthusiasm. As they sit at the gate, the dogs’ quiver with overwhelming exhilaration. They are unable to restrain themselves, voicing the thrill of the moment with eager whines. If they should encounter another dog when loosed in the field, it creates the greatest flourish of activity – rapid bowing, wagging of tails so vigorous that it lifts paws off the ground, and circular runs so fleet that they are over in the blink of an eye. In the moments before they enter the field, it’s really the anticipation of meeting other dogs that seems to wind them up like a tightly coiled spring of expectant tension.
The palpable excitement of discovering others is something we share with dogs. As a species, we share a seemingly boundless anxiousness to see who else might be out there. It is tempting to speculate that this may have been what lead our species out of the confines of the birthplace of humanity, Africa, to colonise all but one continent. It is certain that our explorations were driven by other factors – to outrun pests, disease or climate change; the need or desire for new resources; to escape conflict; or, as part of a more cerebral quest. Nevertheless, irrespective of the motivation, as we turned our attention to new locales, forever was the question “Is there anybody out there?”
Even when we had cast our eyes skyward, prior to the invention of telescopes, we invented stories involving others. We populated our stories about the sky with peoples and animals. These characters played out celestial dramas that provided explanations for the workings of the cosmos. Not surprisingly, the travels of hunters and the hunted, or beings otherwise involved in giving chase, figure prominently amongst these stories.
Stories of people populating the constellations were a common feature of cultures the world over. Small wonder then, when we finally attained the capacity to explore space, part of this exploration involved the search for not just life, but for others like us.
Manned spacecraft were fitted with windows, enabling astronauts to gaze not only back to Earth, but beyond our planet, in the search for others with whom we might share the universe. Importantly, our unmanned space probes were equipped with sensitive instrumentation to sense the existence of life on other planets, including the possibility of residents like ourselves. As reported almost exactly 20 years ago, one of these craft uncovered a planet that showed not only the hallmarks of life, but also potentially intelligent life.
Galileo arrived at Jupiter near the beginning of December in 1995, becoming our first spacecraft to orbit that planet. Over the course of the next 8 years, Galileo would launch probes to examine the details of the Jovian atmosphere, and document features of Jupiter’s moons, including Io’s volcanism, Europa’s icy oceans, and the exospheres surrounding Ganymede and Callisto. Before ending its mission, Galileo also documented a fine ring system encircling Jupiter. On September 21, 2003 Galileo was sent into Jupiter’s atmosphere at a speed of 173000 kilometres per hour, bringing its mission to an end.
While Galileo’s mission in the Jovian system was remarkable, this was not its only contribution to our understanding of the universe. Galileo’s trip to Jupiter was notable for its discoveries as well. For example, in 1991, Galileo undertook the first flyby of an asteroid, 951 Gaspra. Just two years later, Galileo discovered the first asteroid moon – 243 Ida’s Dactyl. As Galileo approached Jupiter in 1994, Galileo watched on as Comet Shoemaker-Levy pounded the Jovian surface.
All of these discoveries would make the Galileo mission notable in and of themselves, but the one discovery that really sets Galileo apart was the discovery of evidence for intelligent life in December of 1990.
Galileo was equipped with instrumentation to enable the characterisation of planetary surface features and atmospheres. A Near-Infrared Mapping Spectrometer (NIMS), Solid State Imager (SSI), and Plasma Wave Spectrometer (PWS) provided data that were relevant to the detection of life – or, at least, as Sagan and colleagues indicate, “life as we know it”. The data in question were evidence of pigments on the planet surface that suggested the capacity to harvest light energy, chemical compositions that were misaligned with expectations, and radio signal emissions that had no natural explanation, except that they were the products of life.
Sagan and colleagues found a planet with an incredible abundance of water – liquid water so abundant that it was distributed widely in large oceans upon the planet’s surface. Water was so abundant as to comprise a significant portion of the planet’s atmosphere as well – condensing there to form clouds.
Where water was absent, there appeared to be continents. The continents were coated in places with a unique pigment. This pigment was able to absorb light in the red wavelengths, but not very effectively in the green. In fact, the pigment would be seen as green under visible wavelengths of light.
In addition to water, the atmosphere was unique in that it had unusually high concentrations of oxygen and methane. These concentrations are unusual inasmuch as thermodynamic equilibria are concerned. That is, if normal chemical processes were at work, there is no way that the oxygen and methane concentrations could be so high. Normally these molecules would be turned over, dissipated, by normal chemical mechanisms, as well as the action of light. Somehow, these oxygen and methane were being maintained at concentrations that defied thermodynamic equilibrium.
Finally, radio emissions emanated from the planet. Several frequencies could be detected in the amplitude modulation range of radio emissions. We are familiar with amplitude modulation emissions, better known as AM radio. The AM emissions increased in intensity as Galileo approached the planet, and remained stable for many hours. This indicates that the signals were derived from the planet surface, and that they defied the properties of other radio signals that tend to be fleeting, or unstable in transmission. Crucially, the AM radio emissions seemed to encode usable information, but Galileo was unable to decode it.
As Sagan and his colleagues note, “Life is a hypothesis of last resort”. That is, one has to rule out all other hypotheses before life can be invoked to explain the observations. Moreover, all lines of evidence have to be consistent with the hypothesis. In the case of the data collected by Galileo, “life as we know it” is the only reasonable hypothesis. It is the only hypothesis that can explain the phenomenal chemical disequilibria, the existence of the light-harvesting pigments, and the transmission of the radio signals.
In 1990, Galileo swung by our planet, taking advantage of our gravity to slingshot itself out into space. As it did, Galileo collected data to test the hypothesis that there is intelligent life on Earth. Using technology that is now almost a quarter of a century old, it was able to discover intelligent life on our planet. Galileo detected the water that is essential to life on this planet. It identified the chlorophyll pigment that is the basis for the photosynthesis – harvesting the energy from sunlight to make the sugars that fuel plants and animals. Galileo sensed the imbalance of oxygen, produced during photosynthesis, and methane, produced by microbes. Finally, Galileo picked up on our AM radio transmissions. While talk radio may undermine the notion of intelligence in information content, AM radio does reflect a profound inventive intelligence found in our species.
The Galileo mission provided a good proof-of-principle – a positive control – that suggests that our space probes would be able to detect life as they ventured out into the universe. That is, of course, “life as we know it”. Galileo provides hope that we should be able to sense the kind of life that has evolved under the special circumstances of Earth’s history in our solar system. Assuming that we encounter planets that exist under similar circumstance in other solar systems, we should have the capacity to determine if a similar type of life has emerged there. Of course, it may be that there are other ways for life to evolve that might challenge our capacity to detect it. These would be false negatives in our search for extraterrestrial life – a problem that may require something beyond, literally, blue-sky thinking to resolve.
The Galileo test for life on Earth was designed to help us identify life elsewhere. In the past 50 years, spacecraft and Earth-bound detectors have revealed almost 1000 exoplanets. Thousands more will likely come to light in the coming decades. We can make use of the lessons learned from Galileo to design tests to determine if any of these harbour life. It’s an exciting possibility that the universe holds other planets that share a life-yielding history with our Earth.
Like the dogs, we stand at a gate of discovery in excited anticipation of there being others like us out there. There may come a day when, like the dogs in the playing field, our anticipation is realised, and we encounter others like our kind in the vast playing field of the universe.
This said, in the interim, it’s worthwhile remembering that Galileo already discovered life in our universe. We know for a certainty that life is right here, right now. It is amazing in its diversity and complexity. What’s more, there is a universe of things that we don’t know about the life with which we share this planet.
Galileo’s discovery of intelligent life on Earth is a wonderful reminder to value the life that we know is there already. We may not be alone, but until we determine if that is the case, let’s appreciate the life that is already around us. It may be that the only “life as we know it” is right here.
Sagan C, Thompson WR, Carlson R, Gurnett D, & Hord, C (1993) A search for life on Earth from the Galileo spacecraft. Nature 365: 715-721