aa r X i v : . [ phy s i c s . pop - ph ] M a r The search for life and a new logic
Douglas Scott ∗ and Ali Frolop † Dept. of Physics & Astronomy, University of British Columbia, Vancouver, Canada (Dated: 1st April 2020)Exploring the Universe is one of the great unifying themes of humanity. Part of this endeavour isthe search for extraterrestrial life. But how likely is it that we will find life, or that if we do it will besimilar to ourselves? And therefore how do we know where and how to look? We give examples ofthe sort of reasoning that has been used to narrow and focus this search and we argue that obviousextensions to that logical framework will result in greater success.
I. INTRODUCTION
Motivations given for Solar System exploration mis-sions, as well as for studies of exoplanets, often havethe search for life at the very top of the list. Pickingsome examples, the stated science goals for the whole ofNASA’s Mars Exploration Program are to “study Marsas a planetary system in order to understand the forma-tion and early evolution of Mars as a planet, the historyof geological processes that have shaped Mars throughtime, the potential for Mars to have hosted life , and thefuture exploration of Mars by humans”,[1] while in Eu-rope “The goals of ExoMars are to search for signs ofpast life on Mars ”.[2] Elsewhere in the Solar System, theaims of the Dragonfly mission to Titan are “to searchfor chemical signatures that could indicate water-basedand/or hydrocarbon-based life ”[3] and the Europa Clip-per will “investigate whether the icy moon could har-bor conditions suitable for life ”.[4] Moving further afield,“The Origins Space Telescope will trace the history ofour origins from the time dust and heavy elements per-manently altered the cosmic landscape to present-day life . . .
How common are life-bearing worlds? ”[5] and “TheHabitable Exoplanet Observatory is a concept for a mis-sion to . . . search for signatures of habitability such aswater , and be sensitive to gases in the atmosphere pos-sibly indicative of biological activity , such as oxygen orozone”.[6]Beyond these few examples, there are countless others.In general, astronomers cannot talk about planetary ex-ploration or exoplanetary observational studies for morethan a couple of sentences without mentioning the searchfor life.Is this reasonable? Is there no motivation for study-ing a planet other than to search for life? While somecynical people might suggest that the reasons for thissingle-minded focus are sociological or political [7], weare merely scientists, and so in this paper we will con-centrate only on what rational thinking can say aboutthis question. Let us turn to the most basic aspect ofthe scientific process, namely logic. There is a famous ∗ Electronic address: [email protected] † Electronic address: [email protected] syllogism that illustrates how logical reasoning works: • A All elephants are grey. • B Mice are grey. • C Therefore mice are elephants. [8]The search for life elsewhere in the Universe follows asimilar form of dialectical thinking: • A The Earth has life. • B Some other places are like the Earth. • C Therefore these other places have life.We will suggest that this is not only logically sound,but that extending such reasoning gives us a way to selectspecific places where life is much more likely to be found,as we will discuss in Section V.
II. HISTORICAL DIGRESSION
First, let us go back to the time of ancient Greece[9], when several philosophers, notably Leucippus [10],Democritus and Epicurus, argued that the Universe waslarge and contained a multitude of life-bearing worlds.This idea of “Cosmic pluralism” was continued by Mid-dle Eastern scholars and was promoted in Europe byGiordano Bruno, among others. It was formalised in the1686 book “Entretiens sur la pluralit´e des mondes” [11]by Bernard Le Bovier de Fontenelle. Deeply intertwinedwith religious thinking [12], the basic concept was thatthe Creator would surely not have made all these worldswithout purpose, and hence each world must have beenmade for its inhabitants.As Sir David Brewster [13] put it, when “we tracethroughout all the heavenly bodies the same uniformityof plan, is it possible to resist the influence that thereis likewise an uniformity of purpose; so that if we finda number of spheres linked together by the same bond,and governed by the same laws of matter, we are enti-tled to conclude that the end for which one of these wasconstituted, must be the great general end of all, – to be-come a home of rational and God-glorifying creatures”.To rephrase this argument: • A The Earth was made for humans. • B Other planets exist. • C Therefore there are beings on all other planets.This “plurality of worlds” and cosmic-abundance-of-life concept was popular in the 17th, 18th and 19th cen-turies. It was promoted by such luminaries as Adams,Herschel, Huygens, Locke, and Newton. Camille Flam-marion’s 1862 book specifically devoted to the topic, “LaPluralit´e des mondes habit´es” [14], went through 33 edi-tions in 21 years and includes statements such as “we whoinhabit this world are only a few out of all the worlds”.In 1837, popular astronomy author Reverend ThomasDick [15] went through a series of five arguments for lifeon other worlds in the Solar System, leading to an esti-mate that there were 21,894,974,404,480 inhabitants intotal [16]; he did not include the Sun in his calculation,although he acknowledged that its surface area would al-low for a larger number of inhabitants than all of theplanetary bodies. However, William Herschel had al-ready stated that “we need not hesitate to admit thatthe sun is richly stored with inhabitants”.[17] Moreover,astronomer Johann Bode [18], describing the inhabitantsof the Sun, stated: “Who would doubt their existence?The most wise author of the world assigns an insect lodg-ing on a grain of sand and will certainly not permit . . . thegreat ball of the sun to be empty of creatures and stillless of rational inhabitants who are ready gratefully topraise the author of life”.Herschel also talked about the Moon, stating in 1780that there was a “great probability, not to say almostabsolute certainty, of her being inhabited”[19] and in1795 he added that “the analogies that have been men-tioned are fully sufficient to establish the high probabil-ity of the moon’s being inhabited like the Earth”.[20] Ithad already been known since the time of Galileo, thatthe Moon possessed seas and volcanic craters. However,further evidence of life appeared in a series of articlespublished in The Sun newspaper in New York in 1835[21], based on new observations by William Herschel’sson John. These articles discuss how forests, fields andbeaches could be seen on the lunar surface, and with alittle more scrutiny, bisons and sheep, as well as bipedalbeavers, blue goats, unicorns and man-bats.[22]Hence we see that, during the 19th century, the SolarSystem was understood to be teeming with a great va-riety of living creatures, and presumably the rest of theUniverse also. Following the usual logic, William Her-schel’s final conclusion was that “if stars are suns, andsuns are inhabitable, we see at once what an extensivefield for animation opens itself to our view”.[23]
III. MARS
Proponents of the study of the biota of Mars are ingood company, since they are following the same lines ofreasoning as the champions of “cosmic plurality”, namely • A Earth has life. • B Mars is similar to Earth. • C Therefore Mars has (or did have) life. Since the 17th century, we have known that the rota-tional period of Mars is approximately the same as theEarth’s. Over time, improvements in the measurementsgrew along with the ideas of “cosmic plurality”. Hence,as it became clearer that a Mars day is very similar to anEarth day, there was growing obsession with the question:is there Life on Mars?[24] This quest was also encouragedby apparent evidence for water on the planet, includingthe famous canals [25] seen by Percival Lowell.[26] Thusfollowed decades of Martians appearing in books, motionpictures and radio broadcasts.[27]More recently, many missions to Mars have focused onthe search for evidence of biological activity. Althoughthere are continuing claims that such evidence has beenfound, the general consensus is that Mars might be bar-ren today. But since Mars is so similar to Earth, andthe logic is so unassailable, then if Mars has no life now,it must be that it had life at some other time. Henceattention has focused on looking for evidence of water onancient Mars.
IV. WATER
Our home planet is about 70 % covered with waterand swarming with living organisms (if not necessarilyintelligent living organisms [28]). By the now-familiarlogic, it is obvious that liquid water is necessary for thedevelopment and sustainment of life. In other words: • A The Earth has water. • B The Earth has life. • C Therefore, where there is water there must be life.“Habitability” then equates to the presence of H O,not as ice or steam, but in its liquid form.[29] A planetin a habitable region is also referred to as being in the“Goldilocks Zone”.[30]But how do we know we are looking in the right placesfor life? We simply defer to the so-called “streetlight ef-fect”, which states that usually the light has been placedin just the right place for you to be able to see the thingyou are looking for. This follows the same exacting rulesof deduction that we have described above.
V. THE NEW LOGIC
To further extend this line of reasoning, might we notexpect that bodies sharing further attributes with theEarth will have a higher chance of harbouring life?Our planet has several characteristics that make it spe-cial. For example, Earth’s orbital inclination is very closeto zero [31] – hence we should look for life on planets withalmost no orbital inclination. Perhaps we should alwaysfocus our attention on the third planet from the star inany exoplanet system, or on the fifth largest planet?Earth is also the greenest place in the Solar System,suggesting that we should search for life on planets withthe same colour as the Earth.[32] Additionally we onlyhave a single, large moon, which may be beneficial forlife, [33] and hence we can ignore planets with too few ortoo many moons.A particularly fruitful search may be in any planetarysystems we find that initially look like they have nineplanets but turn out to have only eight.However, we have only been considering the obviousreasons that the Earth is special. Following the thinkingdescribed earlier in this paper, it is clear that any char-acteristic similar to Earth’s should make life compulsory,according to pure logic. Hence other bodies whose namesalso start with the letter “E” should be good bets. Infact this has already been confirmed in the Solar System,where Europa and Enceladus have been highlighted forfuture searches for life.Another popular place to look is Titan, and, while it does not start with an “E”, it has the same number of let-ters as “Earth”, making it another obvious target. More-over, it starts with the same letter as “Terra”, the Latinname for Earth.[34]Maybe we should concentrate on places with lots ofNa Cl [35], while avoiding those with almost none? [36]As a last suggestion, perhaps planets whose names mean“dirt” in one of their native languages are likely to hostlife? [37]We hope that some of these ideas [38] will be pur-sued by future targeted exoplanet observations, as wellas SETI searches. Following the same rigorous logic thathas been applied by centuries of researchers of extrater-restrial life, we hope that readers of this paper will comeup with visionary ideas of their own.[39] [1] mars.nasa.gov [2] [3] dragonfly.jhuapl.edu [4] [5] asd.gsfc.nasa.gov/firs/ [6]