So where are the extraterrestrial beings?

Not so very long ago, astronomers were theorising about life on practically every planet in the solar system whereas today the Earth is sometimes envisaged as the only haven for living beings... have the extraterrestrial beings disappeared in-between times?

The planet Earth (photographed here by the crew from Apollo 13, see this portfolio) is for the moment the only celestial body known to shelter living beings. Credit: NASA

Extraterrestrial life, that is living beings on planets other than the Earth, has been fascinating mankind since the dawn of time. We no longer think about the legends concerning creatures from the sky, not forgetting the reasoning of the Ancient Greek thinkers who, theorising about a possibly infinite Cosmos, logically envisaged other inhabited worlds. However, the idea did not have unanimous support and the concept of the Earth being at the centre of the Universe (and therefore the only inhabited celestial body), put forward by Aristotle and later developed by Ptolemy in his famous astronomical treatise “Amalgest” in the 2nd century after Jesus-Christ, became established in western culture.

A solar system full of life!
With the progress made by astronomy, and with the growth of observation using telescopes initiated by Galileo, the idea of the “plurality of inhabited worlds” resurfaced. During the Age of Reason and through to the beginning of the 20th century, astronomers and scientists believed that the solar system beyond the Earth was abounding in life. The existence of extraterrestrial civilisations, possibly more advanced than ours, was envisaged on Venus, Mars and even on the Moon! This interest for the possible “other” was such that in 1835 the daily New York Sun caused a sensation by publishing a series of 6 articles describing the discovery of fantastic creatures (notably winged humanoids!) on our natural satellite, thanks to a revolutionary telescope; a hoax that was to guarantee the paper excellent sales. Very seriously, on the other hand, French astronomer Camille Flammarion gave a good summary of the views of his time when, in his 1884 book entitled “Les Terres du Ciel” (The Worlds in the Sky), he reflected on what the inhabitants of each planet of the solar system might be like and, sure of his reasoning, put forward the idea that “humanities from the sky are no longer a myth”.

An engraving from the 1884 book “Les Terres du Ciel” (The Worlds in the Sky) by French astronomer Camille Flammarion which illustrates “the sun rising over the canals of Mars”. At the time, scientists were not opposed to the fact that practically every planet in the solar system could harbour life. Credit: public domain

Martian disappointment
It was Camille Flammarion, and to be exact, another of his works on Mars, that would convince American Percival Lowell to devote himself to the study of the red planet. This wealthy scholar from an influential Boston family was to create an entire observatory in 1894 on a hill in Flagstaff, Arizona. He equipped it with a 61 cm diameter telescope, the lens of which came from the prestigious workshops of Alvan Clark & Sons, so that he could observe the Martian “canals” noted by Italian Giovanni Schiaparelli in detail. In actual fact, the term “canali” (the plural of channel or arm of the sea) was wrongly translated into English as “canals”.

Percival Lowell: this wealthy scholar built an observatory in order to study the Martian canals that he interpreted as having been built by an advanced civilisation looking to irrigate a dying planet. Credit: DR

Percival Lowell was unaware of this linguistic subtlety and saw it as proof of the presence of an extremely advanced civilisation, capable of gigantic construction works intended to take water collected from polar ice to arid regions. But all this is just an illusion despite the three books that Percival Lowell was to publish on the subject between 1895 and 1908. Moreover, at the beginning of the 20th century, more and more astronomers questioned the very reality of the canals, seeing them, at best, as an error in interpretation, and at worst, as the autosuggestion of observers who wanted to see them no matter what! In the meantime, Martians became the extraterrestrial beings best known to the general public. In the middle of the twentieth century, extraterrestrials actually became cinema stars as Hollywood took up a theme that had become very popular since the media hyped the story told by American businessman and pilot Kenneth Arnold. This man claimed to have seen a disc-shaped object in the sky from his airplane on 24 June 1947. The press then coined the term “flying saucer”. It was a huge success; accounts continued to flood in and were more and more linked to creatures visiting from other worlds... The subject immediately seduced followers of the paranormal but the scientific community did not get involved.

Where are the extraterrestrial beings? In the 1950’s, they became the stars of numerous sci-fi films. A theme that regularly comes back into fashion because “aliens” make it possible to orchestrate the fears and hopes of our species. Credit: DR

If they exist, where are they?
A scientific approach to extraterrestrial life was, however, continued. In 1950, Italian Enrico Fermi (Nobel prize for physics in 1938) expounded the paradox that bears his name: if extraterrestrial beings exist, where are they? He explained that a civilisation from another world could have colonised the galaxy in several million years by contenting themselves to intersideral travel (from star to star) well under the speed of light. This length of time remains short in comparison to the age of our galaxy and we should therefore, if not have been visited by these strange explorers, at least have seen signs of their activity, radio signals for example.

As part of its new attraction, “Extraterrestrials: are you ready for the encounter?”, the Cité de l’Espace makes it possible for the general public to understand the issues of the search for life in the Universe and notably Fermi’s paradox. Credit: Cité de l’espace

Answers to the paradox are numerous, the first categorically states that there is no such civilisation! We can then go almost endlessly through other hypotheses such as the fact that the Earth is in a neglected area, that these beings are so advanced that humans are of no interest to them or that they are following an absolute principle of no intervention in developing civilisations. American astronomer Frank Drake was interested in radio signals and barely more than 50 years ago (8 April 1960) launched the project Ozma, the first attempt at listening for extraterrestrial emissions by the SETI (Search for Extra-Terrestrial Intelligence). The SETI Institute is still continuing with its research but without any convincing results to date. A year after the launch of Ozma, Frank Drake formalised an equation which bears his name and which tries to estimate the number of extraterrestrial civilisations from our galaxy with which we could come into contact. Its multiple factors include the number of stars, the percentage of planets favourable to life, the lifespan of a civilisation, etc. Moreover, as part of the new exhibition at the Cité de l’Espace in Toulouse entitled “Extraterrestrials: are you ready for the encounter?”, it is possible to play around with the parameters and see the results displayed. It is easy to obtain dozens of potential civilisations just by using the most reasonable of values! The question behind Fermi’s paradox (also explained as part of this attraction at the Cité de l’Espace) remains unanswered.

In 1960, astronomer Frank Drake initiated the first attempt at listening for extraterrestrial signals using radio telescopes. He also put forward an equation, the parameters of which can be adjusted at the Cité de l'Espace’s exhibition devoted to the search for life in the Universe. Credit: DR/Cité de l’espace

Solar system: candidates for life
Using Drake’s equation, several decisive factors are determined or are about to be. The number of stars in our galaxy is therefore one of the known figures (between 200 and 400 billion stars). One important parameter is that of the number of planets of a given star system capable of sheltering life (on average, of course). Now, if we base ourselves on our solar system, the answer is 1. Finding life, even in a simple form, on another planet (or moon as we will see below) changes not only the result of Drake’s equation, but also proves that the development of life is more general and that Earth is not necessarily an exception. It must be said that with the space age, robotic probes are contradicting the idea of “plurality of inhabited worlds” favoured by astronomers in the past. Out go Percival Lowell’s Martian canals and advanced civilisation, “shot down” for good by the first successful flyby of the red planet carried out by Mariner 4, an American spacecraft in 1965. Mankind, via the intermediary of robots, is gradually conducting flybys of all the planets in the solar system and noting the lack of life therein. Only Pluto (who was considered to be a proper planet up until 2006, prior to being “downgraded” to a dwarf planet) remains on the sidelines of this systematic exploration but should be visited by NASA’s New Horizons probe in July 2015. However, Pluto has no pretensions as regards being one of the candidates for life in our solar system. Although all ambitions of finding a civilisation have obviously been abandoned, we are now looking for proof of a simple life form (microbial for example) in the past or the presence of biological niches, confined spaces where living beings could have developed.

The ExoMars rover belonging to the European Space Agency is to be equipped with a drill so that it can analyse the subsoil down to a depth of 2 m. And what if life on the red planet was hiding in biological niches underground? Credit: ESA

On Mars, we are looking for both. The red planet once knew a vastly different climate and some planetologists believe that water was present there in liquid form. But was it there for a sufficiently long enough period for the life cycle to have started? The question currently remains unanswered. Others suggest that Martian subsoil harbours the previously mentioned biological niches, and why not in expanses of water underground? The European Space Agency is planning an ExoMars rover for 2018. It is to be equipped with a drill capable of digging to a depth of 2 m so that this attractive hypothesis can be examined.

Jupiter’s moon Europa is covered with a layer of ice which resembles an ice field. Has life been able to develop in a possible subglacial ocean? Credit: NASA

Other than Mars, further candidates in the solar system are the moons. Europa, Jupiter’s natural satellite is often in the exobiologists’ favour. This moon with its diameter of 3,120 km has a frozen surface resembling an ice field. As a result, the hypothesis of a vast subglacial ocean is riding high and it is easy to imagine an aquatic life form similar to that found in the great depths of our planet and which draws its energy from the heat coming from underwater volcanic structures. But the thickness of the ice field (several kilometres even dozens of kilometres) could well form a barrier that will be difficult for future robots, tasked with going to check whether Europe is sheltering micro-organisms or a slightly more advanced life form, to get through. In the meantime, prudence is the order of the day and that is why in 2003 NASA plunged its Galileo probe into Jupiter’s atmosphere. The spacecraft was, in fact, at the end of its mission and there was a risk of it crashing into Europa. This could have contaminated this moon with germs from Earth as Galileo, not being scheduled to land, had not been subjected to in-depth sterilisation procedures).

On 14 January 2005, the European probe Huygens made the first landing on Titan, Saturn’s biggest moon (illustration, photograph of the surface on the right). Credit: ESA

Two other moons are of interest to hunters looking for some form of life in the solar system and they both orbit Saturn: Titan and Enceladus. The only moon endowed with an atmosphere, and moreover, one with a composition close to that of the primordial Earth, Titan (5,150 km in diameter) has long fostered high hopes. A little more than 5 years ago, on 14 January 2005, the European probe Huygens landed on its surface: a first. This frozen world knows methane rain which forms vast lakes in its polar regions as the Cassini probe that carried Huygens confirmed. Despite this moon’s complex chemistry based on organic molecules (carbon-based), any life form for the moment is conspicuous by its absence and the environment does not appear particularly favourable (intense cold at -180°C, as the atmosphere expels a large part of the low solar radiation available, and the lack of CO2). Some theoretic models are based on the presence of an underground liquid ammonia ocean with a layer of liquid water at a depth of 200 km. These are extreme conditions that exist there, but they could possibly harbour a biological niche.

Enceladus: with its jets of water, this small moon of Saturn’s has become a serious candidate in the search for life in our solar system. Credit: NASA

Still in orbit around Saturn, the much more modest Enceladus (513 km in diameter) is a great surprise. Cassini’s detailed analysis of the jets that spew out from faults situated near to its south pole changed the status of this moon to that of a candidate for a simple life form in our solar system. As far as the planetologists are concerned, these jets prove that pockets of liquid water exist under Enceladus’ frozen surface. And where there is a pocket of liquid water, there is also potential for a biological niche. True, it is not a question of advanced beings, but why not micro-organisms adapted to these extreme conditions such as are to be found on Earth? The answer to this question will require more missions to Saturn and perhaps one day the landing of a robot on Enceladus, which will constitute a tremendous technological challenge!

Rare Earth or Earth twins?
As interesting as the candidates for life in our solar system might be, they are only candidates and not havens of life. To date, therefore, the Earth stands out as the only celestial body known to harbour living beings. Consequently, although the absence of any evidence does not mean evidence of an absence, scientists have put forward the “Rare Earth” hypothesis, notably defended by Donald Brownlee, astronomer at Washington University and in charge of NASA’s Stardust mission. They believe that the examination of the conditions that led to the appearance of life on our planet and its development into an advanced civilisation shows that it is highly improbable that such a scenario could have occurred elsewhere. “Rare Earth” therefore answers Fermi’s paradox by asserting that extraterrestrial beings are not coming forward as the sequence of circumstances favourable to their existence is virtually impossible. Some astronomers deem this conclusion rash due to the constant progress made in the search for exoplanets, those planets that orbit suns other than our own.

An artist’s illustration showing an exoplanet, a far-off world that orbits a star other than ours. Perhaps extraterrestrial life is hiding beyond our solar system. Credit: ESO

Today, more than 400 worlds have been inventoried and none of them resemble our blue planet. Is the “Rare Earth” hypothesis to be confirmed? No, because current techniques favour the detection of giant planets close to their star. The Corot space telescope belonging to the CNES, the French Space Agency, has, however, the capacity to find “super-Earths”, that is worlds with a mass twice that of the Earth. NASA also has another more recent space telescope dubbed Kepler that is able to spot Earth twins (same size, same mass) up to 1,500 light years away. The results of these two missions (the gathering and analysis of the data are in hand and could take several years) will provide a first pertinent statistical sample relating to the percentage of star systems sheltering one or more planets likely to harbour life (which is one of the parameters of Drake’s equation).

Corot (left) and Kepler (right): two space telescopes respectively tasked with finding super-Earths and Earth twins. Credit: CNES/NASA

In the longer term, we envisage sending telescopes that will fly in formation into orbit, thereby simulating a giant mirror by combining their light via interferometry. It will then be possible to accurately analyse the light spectrum of these other worlds so that signs of life such as the presence of oxygen, CO2 and chlorophyll molecules can be sought and perhaps an image, several pixels wide, of an Earth twin obtained. Such progress will not necessarily provide a final answer to the question “So where are the extraterrestrial beings?”, but it will show that although the chances of “winning the lottery of life” are slight, the Universe, being a compulsive player (our galaxy alone contains between 200 and 400 billion stars), has perhaps won more than once.

A project to fly space telescopes in formation and combine their observations via optical interferometry. Such a mission would give us the possibility of examining the atmosphere of exoplanets via spectroscopy. Credit: ESA

Published on 15 April 2010