Is there anybody out there?
The Fermi paradox is the discrepancy between the strong likelihood of alien intelligent life emerging (under a wide variety of assumptions) and the absence of any visible evidence for such emergence. We use this intriguing unlikeness to derive an upper limit on the fraction of living intelligent species that develop communication technology . indicates average over all the multiple manners civilizations can arise, grow, and develop such technology, starting at any time since the formation of our Galaxy in any location inside it. Following Drake, we factorize as the product of the fractions in which: (i) life arises, (ii) intelligence develops, and (iii) communication technology is developed. This averaging procedure must be regarded as a crude approximation because the characteristics of the initial conditions in a planet and its surroundings may affect the three phenomena with high complexity. In this approximation, the number of communicating intelligent civilizations that exist in the Galaxy at any given time is found to be , where is the average production rate of potentially habitable rocky planets with a long-lasting () ecoshell and is the length of time that a typical civilization communicates. We estimate the production rate of exoplanets in the habitable zone and using recent determinations of the rate of gamma-ray bursts (GRBs) and their luminosity function, we calculate the probability that a life-threatening (lethal) GRB could make a planet inhospitable to life, yielding . Our current measurement of then implies at the 95%C.L., where we have taken such that propagation distances of Galactic scales (), ensuring that any advanced civilization living in the Milky Way would be able to communicate with us.
Is there anybody out there?
\FullConference35th International Cosmic Ray Conference - ICRC217 -
10-20 July, 2017
Bexco, Busan, Korea
On May 25, 1961 President Kennedy’s announcement to put a man on the moon and bring him back safely before the end of the decade set the advent of human exploration of space for NASA, culminating to the landing on the Moon on July 16, 1969. It is difficult to believe that this is the only time such an event has ever happened in the history of the universe. On the other hand, if there is alien life capable of pulling off such a feat we must ask, as Fermi did, where is everybody? .
By adopting the starting point of a first approximation of the answer, we can write the number of intelligent civilizations in our galaxy at any given time capable of releasing detectable signals of their existence into space using a quite simple functional form,
where is the average rate of star formation, is the fraction of stars with planetary systems, is the number of planets (per solar system) with a long-lasting () ecoshell, is the fraction of suitable planets on which life actually appears, is the fraction of living species that develop intelligence, is the fraction of intelligent species with communications technology, and is the length of time such civilizations release detectable signals into space (i.e. the lifetime of the communicative phase) .
where represents the production rate of habitable planets with long-lasting ecoshell (determined through astrophysics) and represents the product of all chemical, biological and technological factors leading to the development of a technological civilization. indicates average over all the multiple manners civilizations can arise, grow, and develop such technology, starting at any time since the formation of our Galaxy in any location inside it. This averaging procedure must be regarded as a crude approximation because the characteristics of the initial conditions in a planet and its surroundings may affect , , and with high complexity. In this work we estimate the production rate of exoplanets in the habitable zone and the rate of planetary catastrophes which could threaten the evolution of life on the surface of these worlds. Armed with these estimates we use our current measurement of to set an upper limit on .
The star formation rate in the Galaxy is estimated to be [4, 5]. This estimate has been derived assuming the Kroupa initial mass function (IMF) [6, 7]. The shape of this IMF is lognormal-like and exhibits a peak around , suggesting there are roughly 2 stars per . Altogether, this yields . Now, only 10% of these stars are appropriate for harboring habitable planets. This is because the mass of the star to be sufficiently long-lived (with main sequence lifetimes larger than 4.5 Gyr) and to possess circumstellar habitable zones outside the tidally locked region .
Next, in line with our stated plan, we estimate a rough probability that a habitable planet will survive and remain in a habitable zone to present day. Gamma-ray bursts (GRBs) are short-lived, luminous explosions, thought to originate from relativistic plasma launched at the deaths of massive stars. The widely accepted interpretation of GRB phenomenology is that the observable effects are due to the dissipation of the kinetic energy of a relativistically expanding fireball . The physical conditions in the dissipation region produce a heavy flux of photons with energies above about 100 keV. It has been suggested that a nearby (Galactic) GRB may destroy the ozone layer, possibly making it damaging to life on Earth.
GRBs are generally divided in two groups according to their duration: long ) and short (). Short GRBs are weaker and hence their life threatening effect is negligible . Throughout we consider only long GRBs. We want to estimate the expected number of GRBs that can terminate life in a planet that is situated at a Galactic radius . To this end we should take into account the following considerations:
- The luminosity-rate function
, which measures the number of GRBs with a luminosity in a small range around a given value occurring per unit time and volume, is given by
where , , , , and . Here, is the volumetric rate of GRBs at . We consider a fiducial value of . To accommodate the metallicity bias determined in  we follow  and take correction of a factor . It has been noted in  that such a low metallicity correction factor yields an upper limit on the volumetric rate of long GRBs . To derive our upper bound on we will adopt , since the larger the number of GRBs, the smaller the planets with long-lasting ecoshell, and therefore the larger the value of . In general, any GRB regardless of its luminosity could terminate life if it is close enough to a planet. This is taken into account in our next point.
- The fluence
measures the amount of energy per unit area arriving to a planet from a distant GRB. If the distance between the GRB and the planet is , an isotropic emission and conservation of energy implies
where is the total energy released by the GRB.
The effects that a copious flux of gamma rays may cause on the Earth atmosphere have been studied in [19, 20]. A fluence of could cause a depletion of roughly 68% of the ozone layer on a time scale of a month, whereas fluences of and could cause depletions of about 91% and 98%, respectively. This implies that a fluence of could cause some damage to life, while will wipe out nearly the whole atmosphere causing a catastrophic life extinction event. Note, however, that the complete removal of all life on an Earth-like planet is a very unlikely event . The critical fluence gives the limit on acceptable fluence on a planet. Following , in our calculations we take as our fiducial life threatening critical fluence.
- The hazard distance
of a GRB, characterized by its total energy , measures the length for which the fluence is higher than the critical fluence . Any planet within this distance will have life terminated. For a fixed GRB energy and critical fluence, we use (4) to obtain the hazard distance
where in the second rendition we have assumed that the total GRB energy is roughly the average ( half) of the peak flux . A good but rough estimate of follows from the assumption that the typical duration of long GRBs is .
- The fraction of hazardous galaxy
measures the fraction of the total galactic mass that is within the hazard distance , for any point at radius from the Galactic center. The fraction of galactic mass that is contained within a surface element at radius is given by,
Defining the unit vectors and perpendicular to , we can write and . Since ,
Defining , we have , where runs from to , so that
- The rate of life-threatening GRBs
Taking into account all of these considerations we now turn to estimate the expected number of GRBs that can terminate life in a planet that is situated at a Galactic radius . As it turns out is . This means that for time scales we expect to have (on average) a small number of GRB events. This kind of estimate is then well suited to Poisson statistics. The Poisson distribution is a discrete probability distribution for the counts of events that occur randomly in a given interval of time (or space). Of particular interest here, the probability of having GRBs when the expected average is is given by . The probability of having or more GRBs is . The average number of GRBs during a time is , and therefore
In Fig. 1 we show probability contours of at least one GRB having occurred in the past time with enough flux to produce significant life extinction on Earth. Since the probabilities get too high in most of the parameter space of interest, the legend does not show the probability (which would be too close to ), but instead the parameter defined by
(12) gives the usual correspondence between probabilities and the standard deviation for a normal distribution, which leads to . Some value correspondences between and the probabilities in are , , and . The circles indicate selected values of and . Our estimates are in good agreement with those given in Table II of . These findings seem to indicate that a nearby GRB may have caused one of the five greatest mass extinctions on Earth. However, it is important to remind the reader that the estimates shown in Fig. 1 rely on an upper limit of the volumetric rate of long GRBs; namely, .
Coming back to our calculation, we can use (11) to compute the probability of having at least one lethal GRB, for a critical fluency and a fixed lookback time , as a function of the distance . Life has been evolving on Earth for close to 4 Gyr [30, 31], but complex life is well under 1 Gyr old, and intelligent life is only a Myr old at most. In what follows we adopt and 4 Gyr as critical time intervals for life evolution . In Fig. 2 we show the probability of having one or more lethal GRBs for a critical fluency and and 4, as a function of distance to the Galactic center.
All we need to do now is add the components together to arrive at the production rate of habitable planets with a long-lasting ecoshell,
It is of interest to study how depends on the different parameters involved. According to (3), (10) and (11), the dependence of in and can be grouped in the same functional dependence. Introducing , one can rewrite , where and . We can then rewrite (13) as , where
Finally, to determine the upper bound on we must decide on the possible minimum . Herein we consider such that propagation distances of Galactic scales (). This would provide enough time to receive electromagnetic (and/or high-energy neutrino ) signals from any advanced civilization living in the Milky Way which is trying to communicate with us.
As of today, the non-observation of evidence of advanced civilizations implies that models of predicting are excluded at the 95% C.L. .
In summary, in this paper we have derived an upper bound on the average fraction of living intelligent species that develop communication technology: at the 95% C.L. Future observations
could help to tighten this bound. In particular, a new arsenal of data will certainly provide an ideal testing ground to improve our understanding about: (i) the occurrence of exoplanets in the habitable zone, (ii) the early star formation rate models, and (iii) the GRB phenomenology. The past few years have witnessed the discovery of more and more rocky planets that are larger and heftier than Earth. Finding the Earth-twins is a higher order challenge, because these smaller planets produce fainter signals and hence only a few have been discovered. Technology to detect and image Earth-like planets has been developed for use of the next generation space telescopes. The Transiting Exoplanet Survey Satellite (TESS) is NASA’s next step in the search for planets outside of our solar system, including those that could support life. The NASA roadmap will subsequently continue with the launch of the James Webb Space Telescope (JWST)  and perhaps the proposed Wide Field Infrared Survey Telescope - Astrophysics Focused Telescope Assets (WFIRST-AFTA) early in the next decade . The ability to detect alien life may still be years or more away, but the quest is underway.
We thank Michael Unger for discussion and Pink Floyd for inspiration. This research was supported by the U.S. NSF (Grant PHY-1620661) and NASA (Grant NNX13AH52G).
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