Is Anybody Out There? Julia Bandura, Michael Chong, Ross Edwards ISCI 3A12 - LUE March 23rd, 2017 -graphic of scientists believe that with our rapid technological advancement, there may be ways to “We stand on a great communicate with them. This article will explore attempts to transmit threshold in the human messages to potential intelligent life forms, history of space attempts to search for incoming alien signals, and analyze explanations for the silence that we have exploration” (Sara Seager, 2014) insofar encountered. However, before considering any form of communication, whether receiving or radiation levels (since high UV radiation can be transmitting, we must first consider where to look. damaging to replication molecules like DNA), and Where is Life in the Universe? liquid water. Water is especially important: all known life on Earth requires liquid water to Logically, a scientist that hopes to communicate survive. As such, within a solar system, the with extraterrestrial beings must assume that: 1) traditional ‘habitable zone’ is defined as the extraterrestrial intelligent life exists in the universe, imaginary disc around the host star where water 2) it exists in high enough abundance that radio will remain in liquid form. A significant number of communication is possible, 3) a transmitted radio exoplanets have been discovered within this zone signal from Earth will be picked up by a receiver, around their host star, but only a fraction of them and 4) the message will be translated successfully. are considered ‘Earth-like’, meaning that their Each assumption comes with challenges that make surface conditions and sizes are similar to Earth’s the process of creating a radio message appropriate (Rekola, 2009). for transmission extremely complicated. Space telescopes such as Kepler (launched by Before we begin the search for intelligent life, it is NASA in 2009) have discovered just over 1000 important to recognize where life of any sort may exoplanets (as of 2015), a relatively small exist in the universe. In general, scientists look for proportion of which can be seen in the green belt known precursors for life, like heavy elements in Figure 1 (the image is biased as it excludes (such as carbon, oxygen, or nitrogen), low FIGURE 1: EXOPLANETS NEAR HABITABLE ZONES. This image shows the number of exoplanets found in habitable zone (green), and their relative sizes. Only the smaller, labelled planets have radii less than 2.5 times that of Earth (Méndez, 2017). exoplanets that are not near the habitable zone). atmosphere. Even climate cycles are also thought But being inside the habitable zone does not mean to impact habitability, which can be highly that we should immediately categorize them as a sensitive depending on the orbit of the exoplanet candidate for a planet with life. Pat Brennan, a (Gómez-Leal et al., 2016). Another method for science writer for NASA explains that if a planet is determining the habitability of exoplanets involves “too big, too uncertain, or circling the wrong kind spectral analysis in order to determine chemical of star…”, even within the traditional habitable composition, which finds ‘fingerprints’ of carbon, zone, we can usually ignore them as candidates for oxygen, ozone, nitrogen, and other known organic life in the catalogue of exoplanets (Brennan, 2016). molecules: prerequisites for life as a we know it So what do scientists look for? Usually planets that (Kaltenegger & Selsis, 2009). Unfortunately, the look like the only planet we know to have life so fact remains that despite all of these efforts, we far: Earth. Scientists determine if a planet is similar have yet to find any evidence of life existing to Earth by using the Earth Similarity Index (ESI). anywhere but Earth in the universe. This quantifies the Choosing Targets similarities between the With prospects like this, Earth and the planet of the likelihood that interest for certain intelligent life exists seems parameters such as stellar even more bleak, but a flux, radius, density, escape famous equation created by velocity, and surface Frank D. Drake in 1961 temperature. The output appears to demonstrate number for a planet’s ESI that perhaps our search is (from 0 to 1, where 1 not as futile as it seems. indicates a planet identical The Drake Equation, as it to Earth) can vary is known today, depending on how FIGURE 2: MESSIER 13. Messier 13 is a large star cluster estimates the number of many parameters are in the Hercules constellation (KuriousGeorge, 2016). extraterrestrial considered in the ESI civilizations with the equation (Schulze-Makuch et al., 2011). means to communicate with other intelligent life. Furthermore, the ESI at which planets are In the Milky Way, this number has been estimated considered Earth-like is somewhat subjective. to be anywhere from 10 civilizations to 10 million. ‘Earth-like’ on the ESI can mean from 0.6 This large margin of error occurs because many of (tolerable for extremophile organisms) to 0.8 the probabilities used to calculate this number are (acceptable for plants or animals as we know impossible to determine, such as the percentage of them). For reference, Mars has an ESI of 0.7 and planets with life on which intelligent life has the prospect of finding life there now is fairly evolved, or the percentage of planets on which the unlikely. Kepler has discovered several planets that intelligent life is able to communicate with other surpass the 0.8 threshold, notably Kepler-438b intelligent life. Regardless, the Drake Equation is (0.88), and Kepler-296e (0.85) and exomoons that an interesting thought experiment that shows how have reached an ESI of up to 0.96 (Denza & common intelligence could potentially be, even Denza, 2016). within our own Galaxy (Maccone, 2011). However, the ESI often overlooks one of the most To choose a target destination for radio important aspects of determining an exoplanet’s transmission with the hope of reaching intelligent habitability: its atmosphere. Even small variations life, a good strategy is to increase the chances of it in a planet’s orbital path can have huge impacts on reaching a large number of stars, by simply terrestrial atmosphere, which shields the planet choosing a part of the sky where more stars are from solar radiation. A so called ‘habitable’ planet visible. The first radio transmission, known as the with an atmosphere that is too thick or non- Arecibo message, was sent to a globular cluster of existence is thought to have a much lower chance stars known as Messier 13 (M13) (Figure 2) in the of sustaining life than a planet with an Earth-like Hercules constellation in 1974, which contains at and cultural reasons for choosing the North Star as least 300,000 stars. The closest stars are a destination (Evans et al. 2008). approximately 25,000 light years away from Earth, There exist also many illegitimate messages sent but M13 itself is about 145 light years across in out for humorous or celebratory reasons. volume, meaning the the signal would continue to travel through the cluster well after having reached For example, a video advertisement for Doritos its destination. During the transit time, M13 is tortilla chips was transmitted to the Ursa major expected to move relative to our solar system, but constellation in 2008. While it’s interesting to only by about 25 light years (Fairbairn, 2008). ponder what aliens would think of our flat Nevertheless the sheer distance between ‘us’ and triangular snack foods, the actual possibility of this ‘them’ is so astronomical that by the time the message making meaningful contact is slim to message arrives at M13 around 24,957 years in the none. What criteria then must a transmission future, our species may very well have forgotten satisfy to be considered a “serious” or about it. “scientifically relevant” effort to make contact? Since the Arecibo message, targets for radio Alexander Zaitsev, a prominent author in the field transmissions have become closer and more of interstellar transmission, argues there are three specific. “A Message From Earth” was sent to a basic criteria that filter out most of what he calls red dwarf system called Gliese 158 in 2008, which ‘pseudo-messages’, such as the Doritos is orbited by three ‘super-Earths’ (planets with advertisement. These criteria are: (1) the choice of mass between 1 and 10 times Earth’s mass), one target star; (2) the energy required per bit of (Gliese 158g) located within the habitable zone of information; and (3) availability of a decoding ‘key’ the star (Levenson 2011). The system exists within inside the message. The first criterion considers the constellation of Libra and due to its relatively whether we are transmitting to a plausible close location, the outgoing message is expected to candidate for life, which is obviously necessary if arrive in February of 2029 (Zaitsev 2012). If Gliese there is to be any two-way communication. The 158g has an atmosphere, it could be considered second criterion is concerned with whether the habitable, but assuming intelligent beings might message will reach the target. When sending radio roam its surface may be a long shot. Either way, messages through interstellar space, noise is added we will not know for certain until after 2050, being to the signal by the cosmic microwave the minimum amount of time it would take for us background. This is the fundamental limitation on to receive a response, assuming the message is interstellar communication (Messerschmitt, 2015). received and translated almost immediately, and The Shannon Limit, named after the famous assuming that the intelligent beings respond to our communications scientist Claude Shannon, tells us message (Zaitsev 2012). the limit for reliable information transfer rate depends on the signal-to-noise ratio.