Are we alone? The search for life beyond the Earth
By Ian Morison
On 22 and 23 October 2015 preeminent UK radio astronomer Ian Morison gave lectures on the search for life beyond earth at YBC gymnasium, Viktor Rydbergs Gymnasium and at the International English School. We have received much positive feedback from the schools about the program.
We can search for evidence of past, or even present, life forms within our own solar system, find evidence of simple life on planets around other stars –- a planet where water could be present has recently be found –- or even detect an intelligent signal from an alien civilization. The speaker was a project scientist in the most sensitive search, Project Phoenix, ever undertaken. Sadly, no signals were detected but a new 10 year search using two of the world’s largest radio telescopes is about to begin and, during the next decade, a giant radio telescope, the Square Kilometer Array, will have the sensitivity to detect alien signals from across the galaxy.
The lectures included a summary of how stars form, die and produce the elements necessary for life to arise, as well as the science of how radio-astronomy works.
Formation of the Elements
In the latter part of its life, our Sun creates the elements Carbon and Oxygen in its core. Somewhat more massive stars create Nitrogen. As these stars die in massive explosions, they eject these elements into space. So these elements along with Hydrogen and Helium are the most common in the Galaxy. Carbon has the most interesting chemistry of any element and thus, we suspect, most other life forms in the Galaxy will be based on the same elements and chemistry. One should thus expect to find life where water is a liquid to allow the reactions that created life on Earth to exist.
Life on Mars?
Mars is a rocky planet having about half the diameter of the Earth but only one tenth its mass. The reddish tint is due to oxides of iron on the surface known as hematite or rust that forms a dust with the consistency of talcum powder. Its atmosphere is very thin, about one hundredth that of Earth’s, and is largely composed of carbon dioxide along with nitrogen, argon and traces of water vapour and oxygen, and as a result the surface faces extremes of temperature – not a suitable abode for life at the present!
Canals and advanced civilizations on Mars?
Mars was first seen through a telescope by Galileo in 1609, but his small telescope showed no surface details. When Mars was at its closet to Earth in 1877, an Italian astronomer, Giovanni Schiaparelli, used a 22cm telescope to chart its surface and produce the first detailed maps. They contained linear features which Schiaparelli called “canali”, the Italian for channel. However, this was translated into English as “canal” - which implies a man made water course - and the feeling arose that Mars might be inhabited by an intelligent race. It should be pointed that a waterway could not be detected from Earth, but it was thought that these would be used for irrigation and so would have fields of crops growing adjacent to them which could be seen from Earth.
Influenced by Schiaparelli’s observations, Percival Lowell founded an observatory at Flagstaff in Arizona, USA, (later famous for the discovery of Pluto) where he made detailed observations of Mars that showed an intricate grid of canals. However, as telescopes improved, fewer canali were seen though the surface did show distinct features. Canali appear to have been an optical illusion, but the myth of advanced life on Mars was not finally dispelled until NASA’s Mariner spacecraft reached Mars in the 1960’s. This was perhaps why Orson Well’s 1938 radio program “War of the Worlds”, broadcast in the form of a breaking news story rather than a play, caused such panic in the United States. (An intriguing image of a huge rock formation, called the “Face on Mars”, was obtained by the Viking 1 space craft in 1961 leading some to suspect that this was a giant representation of a past civilisation. However, a detailed photograph taken by Mars Global Surveyor in 2001 showed it to be a “mesa” – a broad flat topped rock outcrop with steep sides.)
Spacecraft probe the Martian surface
Detailed images taken by the Mariner spacecraft showed vast canyons and giant volcanoes. One of these, Olympus Mons, is the largest known volcano in the Solar System with a caldera of 85 km in width surmounting the volcanic cone whose base is 550km in diameter. The caldera is nearly 27 km above the Martian surface, three times higher than Everest! It was realised that when these giant volcanoes were active, some three thousand million yeas ago, they would have given Mars a far thicker atmosphere than now and the effects of greenhouse gasses in the atmosphere would have allowed the surface temperature to be sufficiently high for water to exist on the surface. Other visible surface features gave ample evidence of water flow over the surface leading to speculation that simple life forms might then have existed on Mars. An intriguing thought is that Mars, being smaller, would have cooled more quickly than Earth and so could have allowed life to form there before it could have done so on Earth. If so, that life could have been brought to Earth on a meteorite (as will be discussed below) and, finding suitable conditions here, could have continued evolving. We could all be Martians!
The Viking Missions to Mars and a heroic failure, Beagle II.
With the possibility of life being, or having been, present on Mars, in 1976 two Viking probes landed on its surface to search for possible evidence. They each carried four experiments to test for any signs of life or organic compounds in the Martian surface. Of the four experiments, two were negative but two, gave somewhat enigmatic results, initially giving a positive results but not in the way that would be expected had life been present. The consensual opinion is that this was not the result of any life forms being present but the result of a chemical reaction due to the fact that the Martian soil is more chemically active than that on Earth. This view has been supported by the discovery of perchlorate in the soil by the recent Phoenix lander, but Gil Levin who designed the Labeled Release experiment that gave a “positive” result still maintains that it had detected life. As Mars has a very thin atmosphere (and no ozone layer), far more ultraviolet light reaches the surface than on Earth. This would prevent the existence of life above ground and so, if life had once arisen on Mars, one could now only expect to find evidence for it beneath the surface. It was nearly 30 years before the next probe to specifically search for evidence of life was sent to Mars. This was the UK designed and built Beagle II that was due to land on Christmas day 2003. It had left its mother ship, Mars Express, on a perfect trajectory a few weeks before, but appeared to have crash landed on its arrival at the surface – a sad moment for the lectuerer who had been charged with receiving its first signals from the surface using the Lovell Telescope at Jodrell Bank.
Orbiters, Rovers and the Phoenix lander
Following a period when many Mars probes seemed doomed to failure, successes in the last decade have greatly increased our understanding of its surface, either when viewed with high resolution cameras orbiting the planet such as those on Mars Reconnaissance Orbiter which began surveying Mars in November 2006, or with the rovers “Spirit” and “Opportunity” which landed in 2004 and are, amazingly, still operational and the “Phoenix” lander which landed in May 2008.
The rovers primary scientific mission was to investigate a wide range of rocks and soils that might hold clues to past water activity on Mars. They were targeted to sites on opposite sides of Mars that appear to have been affected by liquid water in the past: Gusev Crater was a possible former lake in a giant impact crater, and Meridiani Planum, where mineral deposits suggested that Mars had a wet past. They have been highly successful, having together traversed over 20 kilometres on the surface of Mars.
In 2004, scientists showed pictures revealing a stratified pattern and cross bedding in the rocks inside a crater in Meridiani Planum suggesting that water once flowed there, whilst an irregular distribution of chlorine and bromine suggested that it was once the shoreline of a, now evaporated, salty sea. To confirm the “wet past” hypothesis, Opportunity has found hematite, in the form of small spheres nicknamed “blueberries” which could only have been formed inside rock deposits soaked with groundwater.
Also in 2004, NASA announced that Spirit had found hints of evidence of past water in a rock dubbed "Humphrey” which appeared to contain crystallized minerals lodged in small crevices. These minerals been most likely dissolved in water and carried inside the rock before crystallization. When, in December 2007, one of Spirit’s wheels was not turning properly, it scraped off the upper layer of the Martian soil and uncovered a patch of ground similar to areas on Earth where water or steam from hot springs had come into contact with volcanic rocks. Here, such locations are often teeming with bacteria as hot water provides an environment in which microbial life can thrive. Though still operational, since May last year Sprit has been stuck in sand and, as it can no longer orientate its solar panels towards the Sun, may only remain alive for a few more months.
The rovers had never been expected to survive so long as it was thought that dust would soon cover their solar panels to such as extent that they could no longer survive. But scientists had not realized that dust devils – mini tornadoes that can sweep across the Martian surface - could act like a Dyson vacuum cleaner and sweep the panels clean. By March 2005, Spirit’s panels had dropped to 60% of their full capacity, but suddenly this increased to 93%! The following day Spirit was able to film a dust devil as it sped across the Martian surface. Sometimes major dust storms can fill the Martian atmosphere and, when the orbiter Mariner 9 reached Mars in November 1971, the surface was totally shrouded by dust. As the storm gradually subsided, the first feature to be seen was the caldera of Nix Olympia rising high above the surface. Towards the end of June 2007, a series of dust storms blocked 99% of the direct sunlight to the rovers and they were facing the real possibility of system failure due to lack of power. They were both placed into hibernation to wait out the storms and happily survived to face another Martian year.
Further evidence of water locked up beneath the surface in a permafrost came when the Phoenix lander used its scoop to dig out a trough in the soil. This exposed sub-surface ice which, as would be expected due to the thin atmosphere, began to vapourise over the following days. This confirms the observations made by the Mars Odyssey and Mars Reconnaissance Orbiter that there is ice beneath the surface of nearly all the northern half of Mars. But until one can drill down into the surface we will not know the depth, and hence amount, of water ice lying beneath the surface.
Over the last few years streaks have been found on the sides of canyon walls. There is now compelling evidence that they are the result of the occasional flow of salt water (which can be a liquid below zero Celcius) when the surface of Mars is at its warmest. It is just possible that an organism might be able to evolve to survive in such conditions but it is perhaps not that likely.
A possible abode for life on Jupiter’s Moon Europa
Jupiter’s two innermost Moons, Io and Europa are of great interest. Io is the fourth largest moon in the Solar System with a diameter of 3642 km. When high resolution images of Io were received on Earth from the Voyager spacecraft in 1979, astronomers were amazed to find that Io was pockmarked with over 400 volcanoes. It was soon realised that giant tidal forces due to the close proximity of Jupiter would pummel the interior, generating heat and so give Io a molten interior. As a result, in contrast with most of the other moons in the outer Solar System which have an icy surface, Io has a rocky silicate crust overlying a molten iron or iron sulphide core. A large part of Io's surface is formed of planes covered by red and orange sulphur compounds and brilliant white sulphur dioxide frost. Above the planes, are seen over 100 mountains, some higher than Mt Everest − a strange world indeed.
In contrast, Europa, the sixth largest moon in the Solar System with a diameter of just over 3000 km, has an icy crust above an interior of silicate rock overlying a probable iron core. The icy surface is one of the smoothest in the Solar System. Close up images show breaks in the ice as though parts of the surface are breaking apart and then being filled with fresh ice. This implies that the crust is floating above a liquid ocean, warmed by the tidal heating from its proximity with Jupiter. This could thus conceivably be an abode for life and some ambitious proposals have been made for a space craft to land and burrow beneath the ice to investigate whether any life forms are present!
In December 2013, NASA reported that images taken by the Hubble Space Telescope indicate the presence of hydrogen and oxygen above the moon’s southern hemisphere. The observations are consistent with 200-km-high plumes of water vapour. If this can be proved, then it might be possible to detect organic molecules or even evidence of life without having to drill down through the ice. The ESA “Juice” mission, due to be launched in 2022, will make two close flybys in the 2030s and might even be able to fly though any plumes that might exist near the moon’s equator. NASA has made some preliminary plans for an extended mission to Europa called “Europa Clipper” which would spend a year or more in the vicinity of the enigmatic moon.
The Search for Other Worlds
This is one of the most exciting areas of research being undertaken at the present time with the discovery of new planets being announced on a monthly basis. This chapter will describe the techniques that are being used to discover them and then discuss their properties. Perhaps a word of warning might be in order. An obvious quest is to find planetary systems like our own which could, perhaps, contain planets that might harbour life. So far, to many astronomers' surprise, the vast majority of solar systems found have been very unlike our own which might lead one to the conclusion that solar systems like ours are very rare. I have even heard this point of view put forward by an eminent astro-biologist. But I do not believe one should draw this conclusion. For reasons that will become apparent, the techniques largely used to date would have found it very difficult to detect the planets of our own solar system so it should not be surprising that we have so far failed to find many other similar solar systems. As new techniques are used, this situation will improve, but it will be some time before we have any real idea how often solar systems like our own have arisen in the galaxy. The story of the discovery of the first planet to orbit a sun-like star is very interesting in its own right, but, in order to appreciate its nuances, we need first to understand how this, along with the great majority of planets so far detected, has been discovered.
The visual detection of planets orbiting normal stars.
It has been long thought that the detection of planets by direct imaging was not feasible due to the fact that the light reflected from the planet would be lost in the glare of the light from the star.
In the infrared, stars are less bright than in the visible and the brightness difference is reduced, so making detection easier. In fact, a planetary sized body had been detected in orbit around a brown dwarf. This had been achieved using one of the 8m telescopes of the VLT in Chile with the use of adaptive optics to correct for atmospheric turbulence. This is very effective in the infrared and so allows telescopes to achieve higher resolutions and thus allow planets at small angular distances from their star to be seen.
Planets observed in the infrared
In November 2008, a team of astronomers using the 10m Keck and 8m Gemini-North telescopes announced the discovery of three planets orbiting the star HR 8799, 129 light year distant. Again, the observations were made in the infrared and, in addition, an occulting device was used to remove much of the light from the star.
An infrared image showing three planets in orbit around the star HR8799.
HR8799 is quite close to the position in the sky of 51 Pegasai, the first sun-like star to have a planet discovered in orbit around it. The three planets are several times the mass of Jupiter and even the closest has an orbital radius equal to the Sun-Neptune distance of ~30 AU.
The Radial Velocity (Doppler wobble) method of Planetary Detection.
Our own solar system gives us a good insight into this method and its strengths and weaknesses. Astronomers often use, as in this book, the phrase “the planets orbit the Sun”. This is not quite true. Imagine a scale model of the solar system with Sun and planets having appropriate masses and positions in their orbits from the Sun. All the objects are mounted on a flat, weightless, sheet of supporting material. By trial and error, one could find a point where the model could be balanced on just one pin. This point is the centre of gravity of the solar system model. The centre of gravity of the solar system is called its “barycentre”, and both the Sun and planets rotate about this position in space.
As Jupiter is more massive than all the other planets combined, its mass and position have a major effect on the position of the barycentre, which will thus lie a distance from the centre of the Sun in the approximate direction of Jupiter. In fact, if Jupiter were the only planet orbiting the Sun, the barycentre of the solar system would actually lie outside the Sun. When all of the major planets lie on one side of the Sun, as happened in the 1980’s − allowing the Voyager spacecraft missions to the outer planets − the barycentre is further from the Sun’s centre and when Jupiter is on the opposite side to the other planets it is nearer the Sun’s centre. On average, the barycentre is at a distance of ~1.25 solar radii from the Sun’s centre, varying between extremes of ~0.3 and 2 solar radii.
Suppose that we observed the Solar System from a point at a great distance in the plane of the solar system. We could not see the planets − their reflected light would be swamped by the light from the Sun − but, at least in principle, we could detect their presence. Due to the Sun’s motion around the barycentre of the Solar System, it would at times be moving towards us and at other times moving away from us. If we could precisely measure the position of the spectral lines in the solar spectrum we could measure the changing Doppler shift and convert that into a velocity of approach or recession. The Solar System as a whole might, of course, be moving either away or towards us so we would see a cyclical change in velocity about a mean value.
The change in wavelength of a spectral line as the star orbits the barycentre of its solar system.
Again, for the sake of simplicity, let us assume that our Solar System has only one planet: Jupiter. The Sun would be seen to rotate around the barycentre once every 11.86 years, the period of Jupiter’s orbit. Given our calculation of the position of the barycentre, we can thus calculate the speed of the Sun in its orbit about the barycentre which turns out to be 13 m/sec so that the difference between the maximum and minimum velocities would be 26 m/s.
The current precision in Doppler measurements is of order of 2-3 m/s, but the hope is that, in time, this might improve to ~0.5 m/s. Very high resolution spectrometers are used to observe the light from the star whose light is first passed through a cell of gas to provide reference spectral lines to allow the Doppler shift to be measured.
The measurement accuracy of this method would thus be sufficient to detect the presence of Jupiter in orbit around the Sun. However, in order to be reasonably sure about any periodicity in the Sun’s motion one would need to observe for at least half a period and preferably one full period. So observations have to be made on a time scale of many years in order to detect planets far from their sun. This is the major reason why few planets in large orbits have yet been detected − the observations have simply not been in progress for a sufficiently long time. But there is one other limitation that you might have realised: should we observe a distant solar system from directly above or directly below, then we would see no Doppler wobble and hence could not detect any of its planets. Unless we have additional information that can tell us the orientation of the orbital plane of a distant solar system, we can only measure the minimum mass of a planet, not its actual mass. If, for example, we later observed such a planet transit across the face of its sun then we would know that the plane of its solar system included the Earth so that the derived mass is the actual mass of the planet, rather than a lower limit.
A single planet in a circular orbit will give rise to a Doppler curve which is a simple sine wave. If the orbit of the planet is elliptical, a more complex, but regularly repeating Doppler curve results. In the case of a family of planets, the Doppler curve is complex and will not repeat except on very long time scales. It can, however, still be analysed to identify the individual planets in the system.
In a manner similar to the way in which we calculated the orbital motion of the Sun due to Jupiter, one could calculate Sun's orbital velocity due to the Earth. This is 0.1 m/s, well below the current and predicted future sensitivity of the radial velocity method so other methods are required for the detection of Earth-like planets. As other techniques − discussed below − come to fruition and longer periods of observation are analysed by the radial velocity method, solar systems like our own are beginning to be found − but, as yet, we cannot say how common they are.
Two American astronomers, Paul Butler and Geoffrey Marcy, were the first to make a serious hunt for extra-solar planets. They began observations in 1987 but, assuming that other planetary systems were similar to our own, did not expect that any planets could be extracted from the data for several years so their data was archived for later analysis. They would have thus been somewhat shocked when the discovery of a planet orbiting a star called 51 Pegasai was announced by Michael Mayor and Didier Queloz on October 6, 1995. The star 51 Pegasi, or 51 Peg for short, lies just to the right of the square of Pegasus and is a type G5 star, a little cooler than our Sun, with a mass of 1.06 solar masses. Meyer and Queloz were studying the pulsations of stars, which also cause a Doppler shift in the spectral lines as the star “breathes” in and out. With a sensitivity of only 15 m/s they had not really expected to discover planets but, greatly to their surprise, they found a periodicity in the motion of the star 51 Peg having a period of 4.23 days and a velocity amplitude of 57 m/s. The plot is very close to a sinusoid showing that the orbit is very nearly circular.
From the velocity of the star and the period of the orbit we can first calculate the circumference and hence the radius of the star’s motion. 4.23 days is 365,472 seconds, so the circumference is 57 x 365,472 m or 20,831,904 m, giving a radius of 3,315,500 km. This is thus the distance from the centre of the star to the barycentre of the system. You may remember that one can to calculate the mass of the Sun given the orbital period and the distance of the Earth from the Sun given the given the universal constant of gravitation, G (which has been found by experiment). In just the same way the mass of the star 51 Peg is found to be 1.06 x 2 x1030kg. One can then find the distance of the planet from the star that came to 0.052 AU. This is well within the distance of 0.39 AU at which Mercury orbits our Sun and only about 10 times the radius of the star.
As the number of known close orbiting gas-giants increased, there became a reasonable chance that the plane of some of their orbits will include the Earth and so, once each orbit, the planet might occult the star, giving a measurable drop in its brightness.
Let us estimate the brightness drop if a Jupiter sized planet occulted our Sun as seen from a great distance. The Sun has a diameter which is ~10 times that of Jupiter, so that its cross sectional area will be ~100 times that of Jupiter. When Jupiter occulted the Sun, the effective area will drop from 100 to 99 − a ratio of 0.99 − and give a drop in brightness of 1%.
With care, such accuracy in measurement is achievable and on November 5th 1999 two teams detected the transit of a planet, previously discovered by the radial velocity method, in orbit around the star HD 209469. During the transit, the brightness of the star dropped by 1.7%.
In 2002, a planet OGLT-TR-56b was discovered by the transit method and later confirmed using the radial velocity method. Then, in 2006, the Hubble Space Telescope made a survey of 180,000 stars up to 26,000 light-years away towards central bulge of our galaxy. The survey discovered 16 candidate extra-solar planets of which three have since been confirmed. Such confirmation is required as the technique has a high rate of false detections. If all 16 were confirmed, it would imply that there would be of order six thousand million Jupiter sized planets in the galaxy. Five of the newly discovered planets were found to orbit their sun with periods of less than one day. The candidate with the shortest period − just 10 hours − is only 1.2 million km from its relatively small, red dwarf sun and has an estimated surface temperature of 1400 K. It must be at least 1.6 times the mass of Jupiter in order to prevent the tidal forces from the star splitting the planet apart.
The Kepler Space Observatory
Kepler is a space observatory in solar orbit launched by NASA with the aim of discovering Earth-like planets orbiting other stars using the method of planetary transits. The spacecraft, which was launched on March 7, 2009, is named after the Renaissance astronomer Johannes Kepler. It was designed to survey a region of the milky way in the constellation Cygnus and continuously monitor over 145,000 main sequence stars using a 95 Mpixel CCD array analysing the light collected by its 1.4-metre primary mirror. Three hundred, 6 second exposures, are summed to give an effective exposure of 30 minutes. These images are then pre-analysed in the spacecraft to reduce the data that has to be downloaded to Earth.
Its aim was to discover dozens of Earth-size extrasolar planets in or near their habitable zones and estimate how many of the billions of stars in our galaxy have such planets. Its initial planned lifetime was 3.5 years, and it can easily be seen why three to four years would be needed to detect a planet, like our Earth, orbiting its sun in one year. It would need at least two and, to be sure, three transits of the planet. One might be lucky and the first transit could occur on the first day of observation so only a year and one day would be needed to observe the second transit one year later with two years and one day needed to observe a third, confirmatory observation. However, the first transit might only occur at the end of the first year and so three years would be needed to observe three transits. Even only requiring two transits, only half of those with orbital periods of 2 years could be found with an observing period of 3.5 years.
When the first observations were analysed, it was found that the noise levels in the system were higher than expected − partly due to a greater intrinsic variability in the stars themselves that had been expected. This meant that to fulfil all the mission goals more transits would be needed to reliably detect a planet and it was planned to extend the mission to 7.5 years, taking observations up to 2016.
The detection of planets depends on seeing very small changes in brightness (Kepler could detect changes of 80 parts per million) so that variable stars would not be worthwhile candidates. Within a few months, about 7,500 stars from the initial target list were found to be variable and so dropped from the target list to be replaced by new candidates.
As in the case of the Hubble Space Telescope (HST), the telescope pointing was controlled by a set of reaction wheels (a form of gyroscope). You may remember that those in the HST were replaced during the service missions and so it was able to be kept operational for many years. Those in Kepler could not be replaced and, on May 11th 2013 the second of four reaction wheels failed which, sadly, brought the initial collection of science data to an end.
Kepler had detected well over 1,000 planets one of which, Kepler 22b, is a rocky planet that lies within the habitable zone of its sun-like star. Though ~2.5 times the diameter of the Earth, its calculated surface temperature would allow water to exist on its surface. It is thus the first planet discovered where life could possible exist.
Evidence for Life?
Let us put ourselves in the place of an advanced civilisation not too far distant in the galaxy. Could we tell if life existed on Earth? The answer is, in fact, yes: we could, and the detection would be based on the taking of an infra-red spectrum of the atmosphere of our planet. If they took spectra of Mars or Venus they would find a flat spectrum with a single deep absorption band due to the presence of carbon dioxide in their atmospheres. But that of our Earth would look very different. The presence of water vapour in our atmosphere would lower the outlying parts of the spectrum and there would be three absorption bands, not one. Along with that due to carbon dioxide, they would find a band due to methane which would be a marker either for life (think cows) or volcanic activity. But, more significantly, they would find a band due to ozone. Ozone can only exist in an atmosphere if there is free oxygen and, as oxygen is highly reactive, unless it is being replenished by some means any will soon disappear. The means by which oxygen is being replenished in our atmosphere is by the action of photosynthesis – a feature of plant life on Earth.
In the same way, should we find evidence of water vapour and ozone in the atmosphere of an exo-planet we could be pretty sure that some form of life might exist there.
I am often asked why we tend to restrict our search for other life forms to locations that are similar to the conditions on Earth. Why should we impose the facts on our own existence on other life forms? My justification for this is twofold. You will have seen that in the stars, somewhat more massive than or Sun, nitrogen is created as they fuse hydrogen into helium and, in the latter stages in the life of stars like our Sun first carbon and then oxygen are created. Thus the life forms on our planet are very largely composed of what are the most common elements in the Universe. Further, it is generally recognised that carbon has the most complex chemistry of any element and has a complete subject, organic chemistry, devoted to it. So, our life forms are based on the most common elements linked by the chemistry of carbon. Is it not likely that the vast majority of other life forms will use a similar chemistry?
SETI, the Search for Extra-terrestrial Intelligence, has now been actively pursued for close on 50 years without success. However this does not imply that we are alone in the Milky Way galaxy for, although most astronomers now agree that intelligent civilisations are far less common than once thought, we cannot say that there are none. But it does mean that they are likely to be at greater distances from us and, as yet, we have only seriously searched a tiny region of our galaxy. It will not be until the mid 2020’s that an instrument, now on the drawing board, will give us the capability to detect radio signals of realistic power from across the whole galaxy. It is also possible that light, rather than radio, might be the communication carrier chosen by an alien race, but optical-SETI searches seeking out pulsed laser signals have only just begun.
The Story So Far
The subject may well have been inspired by the building of the 76-metre Mk1 radio telescope at Jodrell Bank in 1957. In 1959 two American astronomers, Giuseppe Guccione and Philip Morrison, submitted a paper to the journal Nature in which they pointed out that, given two radio telescopes of comparable size to the Mk 1, it would be possible to communicate across interstellar distances by radio. They suggested a number of possible nearby, sun-like, stars that could be observed to see if any signals might be detected. This list included Tau Ceti and Epsilon Eridani, both about 10-12 light years distant. They also pointed out that the radio spectral lines of H and OH, whose frequencies would be known to all civilisations capable of communicating with us, lie in a very quiet part the radio spectrum and could act as markers at either end of a band of frequencies that might be used for interstellar communication. This band of frequencies has become known as the “water hole” (as H+OH = H20).
The following year Frank Drake, the father of SETI, using a 25m telescope at Green Bank, West Virginia, spent 6 hours every day for 2 months observing Tau Ceti and Epsilon Eridani in what was called Project Ozma − after L. Frank Baum's imaginary land of Oz. They did detect two brief signals in what should be a protected band for radio astronomy but it is thought that these were transmitted by the, then, top secret, U2 spy-plane!
Frank Drake with the 25m telescope at Green Bank where he carried Project Ozmay
The "WOW!" Signal
Since then there have been nearly 100 serious SETI searches. In 1977 a telescope called “Big Ear” operated by Ohio State University, which had been carrying out an all-sky SETI survey since 1974, picked up a signal that appeared to have all the right characteristics. It is called the “Wow” signal as the astronomer analysing the data wrote the word in the margin of the computer printout. Sadly, in follow-up observations, no signal has ever been picked up from the same region of sky.
To make a radio message as easy as possible to detect over interstellar distances it would almost certainly be in the form of a very slow “morse-code” type signal with a band width of a Hz or less − in contrast to an audio transmission requiring a bandwidth of several KHz. To detect such signals requires highly specialised receivers with millions of channels covering the band of frequencies being searched. Paul Horowitz at Harvard, a leader in this field, developed receivers to simultaneously analyze 80 million channels each with a bandwidth of 0.5 Hz. These were used to search the whole of the “water hole” using the 25m Harvard–Smithsonian telescope at Oak Ridge in projects META and BETA.
Projects SERENDIP and Phoenix
Two significant searches have used the 305m Arecibo Telescope in Puerto Rico. The first of these, Project SERENDIP, still continues whilst the second, Project Phoenix, terminated in 2003. SERENDIP, under the auspices of the University of California, Berkeley, is using the Arecibo dish in “piggy-back” mode with a dedicated feed system observing the sky close to wherever other astronomers are pointing the telescope. Though the SETI observers have no control over what part of the sky is being observed, over a few years most of the sky accessible to the telescope will be observed, much of it several times over. SERENDIP is thus looking for signals that are seen on more than one occasion from the same location in the sky. A small part of this data, relating to a narrow band of radio frequencies close the 1400 MHz Hydrogen Line, has been analysed by home computers across the world in what is known as SETI@home. After a few years a number of signals with appropriate characteristics had been detected several times and a special observing session was set up to observe these in detail. However no signals appeared in the data to confirm a real detection.
This does highlight a real problem; a signal from ET might be transitory and one really needs to make an immediate confirmation that any signal has an extra-terrestrial origin. This was the premise of Project Phoenix that arose out of the NASA SETI project when the American Congress cut funding. This had been managed for NASA by the SETI Institute who then raised private funds to continue the targeted search part of the NASA programme and observe around 800 nearby sun-like stars. In project Phoenix, two telescopes were used to make simultaneous observations so that any signals originating within our solar system could be eliminated and there would be an immediate confirmation of any extra-terrestrial signal. Initially pairs of telescopes in Australia and the USA were used, but NASA had helped pay for a major upgrade to worlds largest radio-telescope at Arecibo in Puerto Rico and had ~30 weeks of observing time allocated to use it to carry out SETI observations in Project Phoenix. By chance, at a conference on large radio-telescopes in 1996, I happened to sitting next to their project scientist who told me about the proposed use of Arecibo and that they would need a very large radio telescope to operate in tandem with it. I immediately suggested that they use the 76 Lovell telescope at my own observatory, Jodrell Bank − still then the forth largest radio telescope in the world. This came to pass and the receiver system was installed on the telescope in the summer of 1998 with observations beginning that autumn.
The Arecibo Telescope used for the SETI@home observations and Project Phoenix
Due to their separation across the Atlantic any local interference at either telescope could be immediately discounted. In addition, as a result of the rotation of the Earth and the change in received frequency introduced by the Doppler effect, a signal from beyond our solar system would be received at Jodrell Bank at a precisely calculable frequency which is approximately 2 KHz lower in frequency than that received at Arecibo. Thus, when Arecibo detected a possible alien signal, the receiver at Jodrell, offset in frequency by the required amount, attempted to confirm the signal. This enabled the elimination of any signals received from Earth itself or satellites orbiting nearby in the Solar System. The system was proven each day by observing the very weak signal from the Pioneer 10 spacecraft, then more than 10 million km from Earth and far beyond Pluto. It hardly need saying that no positive signals were detected.
The Drake Equation
The lack of success prompts one to ask what the likelihood is that other advanced civilisations exist in the galaxy who would be attempting to contact us. If we do not expect there to be any other civilisations then there would not be a lot of point in searching. This problem was first addressed by an eminent group of scientists at a meeting organised by Frank Drake at Green Bank in 1961. As an agenda for the meeting he came up with an equation which attempts to estimate the number of civilisations within our galaxy who might be attempting to communicate with us. Known as the “Drake Equation”, it has two parts. The first part attempts to calculate how often intelligent civilisations arise in the galaxy and the second is simply the period of time that such a civilisation might attempt to communicate with us once it has arisen.
A plaque at Green Bank Observatory commemorating the Drake Equation
Some of the factors in the equation are reasonably well known; such as the number of stars born each year in the galaxy, the percentage of these stars (like our Sun) that are hot enough, but also live long enough, to allow intelligent life to arise and the percentage of these that have solar systems. But others are far harder to estimate. For example, given a planet with a suitable environment it seems likely that simple life will arise − it happened here on Earth virtually as soon as the Earth could sustain life. But it then took several billion years for multi-cellular life to arise and finally evolve into an intelligent species. So it appears that a planet must retain an equable climate for a very long period of time. The conditions that allow this to happen on a planet may not occur very often. Our Earth has a large Moon which stabilises its rotation axis, its surface is recycled due to plate tectonics and this releases Carbon Dioxide, bound up into carbonates, back into the atmosphere. This recycling has helped keep the Earth warm enough for liquid water to remain on the surface and hence allow life to flourish. Jupiter’s presence in our Solar system has reduced the number of comets hitting the Earth; such impacts have given the Earth much of its water but too high an impact rate might well impede the evolution of an intelligent species. It could well be, as some have written, a “Rare Earth”. How many might there be amongst the stars?
It was widely assumed that once multi-cellular life had formed, evolution would drive life towards intelligence, but this tenant had been challenged in recent years − a very well adapted, but not intelligent, species could perhaps remain dominant for considerable periods of time preventing the emergence of an intelligent species.
The final factor in this part of the equation is the percentage of those civilisations capable of communicating with us who would actually choose to do so. Our civilisation could, but currently does not, attempt to communicate. Indeed there are some who think that it would be unwise to make others aware that here on Earth we have a nice piece of interstellar real estate! Any attempts at communication are very long term with the round travel time for a two way conversations stretching into hundreds or thousands of years. It would be hard at present to obtain funding for such a programme. Estimates of 10% to 20% are often cited for this factor. This may well be optimistic.
The topic of “leakage” radiation from, for example, radars and TV transmitters is often mentioned as a way of detecting advanced civilisations that do not choose to communicate with us. But this is, in my view, unlikely. Any signals that could be unintentionally detected over interstellar distances are, by definition, wasteful of energy. Already, on Earth, high power TV transmitters are being replaced with low power digital transmissions, satellites transmissions are very low power and fibre networks do not radiate at all. The “leakage” phase is probably a very short time in the life of a civilisation and one that we would be unlikely to catch. It could be that airport radars and even very high power radars for monitoring (their) “near-Earth” asteroids might exist long term and give us some chance of detecting their presence but we should not count on it.
When all these factors are evaluated and combined the average time between the emergence of advanced civilisations in our galaxy is derived. If we find it hard to estimate to estimate how often intelligent civilisations arise it is equally hard to estimate the length of time, on average, such civilisations might attempt to communicate with us. In principle, given a stable population and power from nuclear fusion, an advanced civilisation could survive for a time measured in millions of years. Often a period of 1000 years is chosen for want of anything better. This length of time is critical in trying to estimate how many other civilisations might be currently present in our galaxy. If, for example, a civilisation arose once every 100,000 years − a not unreasonable estimate − but typically, civilisations only attempt to communicate for 1000 years it is unlikely that more than one will be present at any given time. If, however, on average, they remain in a communicating phase for 1 million years then we might expect that 9 other civilisations would be present in our galaxy now.
When the Drake Equation was first evaluated, the estimates of other civilisations were quite high; numbers in the 100,000’s or even 1 million were quoted. Nowadays astronomers who try to evaluate the Drake Equation are far less optimistic. Many estimates are in the ten’s to hundreds and there are a minority of astronomers who suspect that, at this moment in time, we might be the only advanced civilisation in our galaxy.
The truth is we just do not know. It was once said with great insight that “the Drake Equation is a wonderful way of encapsulating a lot of ignorance in a small space”. Absolutely true, but an obvious consequence is that we cannot say that we are alone in the galaxy. SETI is our only hope of finding out.
Stars are not strong emitters of radio waves, it is thus not difficult to generate a signal that can be detected at great distance in the presence of the radio noise produced by a star − specifically that at the centre of the solar system from which a signal was being transmitted. (This would also be in the beam of the radio telescope that was attempting to detect the signal.) When SETI was first mooted it was believed that, in the visible part of the spectrum, it would be impossible to outshine a star, but Charles Townes, having invented the laser, immediately realised that a laser might be able to generate very high intensity pulses that could, for brief instants of time, easily outshine a star. Thus the basis of optical SETI was laid. Laser systems close to what are required are now being developed for “Star-wars” type weapons and nuclear fusion power plants so we can easily envisage that advanced civilisation would have them.
Dan Werthimer and Geoff Marcy at the University of California, Berkeley and Paul Horowitz at Harvard University have been pioneers in O-SETI. The Berkeley pulsed laser search is directed by Dan Werthimer and plans to observe 2,500 nearby, largely sun-like, stars, looking for very short bright pulses that might last a billionth of a second or so, transmitted by a powerful pulsed laser operated by a distant civilization. A second Berkeley search is for laser signals that are on for a large fraction of the time. This search, directed by Geoff Marcy, is a 1,000 star program to search for ultra narrow band signals in the visible part of the spectrum (analogous to radio-SETI). They plan to search though thousands of extremely high resolution spectra for very sharp lines. Much of the data has already been taken in an ongoing (and highly successful) planet search.
Horowitz’s first detector system piggy-backed onto the Harvard University’s 61 inch telescope whilst it was carrying out a survey of 2500 nearby sun-like stars. First light was in 0ctober 1998. The very sensitive detectors used to look for nanosecond time scale pulses are prone to false triggering so the light beam is split into two and both detectors have to detect an event for it to be significant. During the first 27 months of observations of the Harvard system two detections were made on average every three nights. The detection events appeared to be uncorrelated with stellar magnitude and did not exhibit any periodicity. In fact, there is was no clear evidence that they originated from light entering the telescope from the direction of the targeted star.
The Harvard group then combined their targeted search efforts with a group at Princeton to make simultaneous observations at two separated sites. By November 2003, 16,000 observations totalling 2,400 hours had been made but no pulses were detected simultaneously at the two sites. (Having first taken into account an appropriate time delay due to their separation.)
In 2003, the telescope at Harvard was decommissioned, and so the targeted search ceased. Since then, the group have planned and commissioned an all-sky survey using a custom-built 72 inch (1.8m) telescope that came into operation in April 2006. The telescope is equipped with an array of 1024 light sensors that observes an area of sky 0.2 degrees wide by 1.6 degrees high. The telescope is a transit instrument observing a given declination on the celestial sphere as it passes due south. During one (sidereal) day of observations a 360 degree round strip of sky, 1.6 degrees high, will thus be observed. The next night the altitude of the telescope will be adjusted to observe an adjacent strip so that after ~200 clear nights the whole of the sky visible at Princeton will have been observed and the survey repeated.
The 1.8-m Harvard "Optical SETI" Survey telescope
In 2015, a new telescope, the 2.4-m Automated Planet Finder, also joined in the Optical SETI search.
The future of radio-SETI
It has been a long term dream of SETI astronomers to have a large dedicated telescope of their own. This dream is has been realised, in part, with the partial construction of the Allen Telescope Array (ATA) at Hat Creek in California. The ATA was conceived as a combined project of the SETI Institute and the Radio Astronomy Laboratory at the University of California, Berkeley to construct a Radio Telescope that will search for extra-terrestrial intelligence and simultaneously carry out astronomical research. (The Berkley collaboration ended in April, 2012, and the project is now managed by SRI International, an independent, non-profit research institute.) This is, as one might suspect, not a telescope “as we know it” and is exploiting the great advances in computing technology to build a highly flexible instrument where, as Jill Tarter of the SETI Institute points out, “steel is being replaced by silicon”.
The cost of building a large single dish antenna tends to rise as the cube of the diameter. The equivalent area could, in principle, be made up of an array of smaller antennas, in which case the cost only rises as the square of the diameter. However the task of combining the signals from the individual elements must also be taken into account. With the reducing cost of electronics; from the receivers on each antenna, their fibre-optic links to the central processing system and the correlators that combine the data − this smallD/largeN approach has become both feasible and cost effective. But, in addition, there is a far more fundamental reason why this approach is particularly appropriate for SETI purposes. A large single antenna is only sensitive to signals received from a very small area of the sky defined by its “beamwidth”. For a 120-metre antenna observing in the region of the “water-hole” this would be of order 7-8 arc minutes. (The use of multi-beam receiver system can increase this by a small factor.) Let is suppose that, as in the ATA, the same effective area is made up by combining the signals from 350, 7x6 m antennas. These small antennas will have a beam width of ~120/7 times greater (beamwidth scales directly with diameter) giving ~146x125 arc minutes − over 2 degrees! At the heart of the array, the signals from all antennas are combined together to form a beam of comparable size to the single 120m antenna and having the same sensitivity. So nothing is lost. But there is much to gain. If, in additional electronics, the signals from each antenna are combined in a slightly different way then a second narrow beam can be formed anywhere within the overall beam of the small antennas as shown in the figure below. But if one can form a second beam, then with further electronics one can form a third, a fourth and so on. So the ATA will have multiple beams and could thus observe many stars simultaneously whilst the Berkley group could be observing pulsars or other astronomical objects in the same area of sky.
The multiple beams formed with the Allen Telescope Array
The first phase comprising 42 antennas was commissioned in the autumn of 2007 and began its SETI observations with a survey of the galactic centre. However, in April 2011 due to funding shortfalls, the ATA was placed in operational hibernation but then, having found some short-term funding, operation of the ATA was resumed in December that year.
Antennas of the Allen Telescope Array
In 2015, a Russian Oligarch pledged $100 million over ten years for a new SETI search initially using the 105 m Greenbank Telescope in the USA and the 64 m Parkes Telescope in Australia.
In many respects the ATA is a technology demonstrator for what would be the ultimate radio-SETI instrument − at least for many decades. A major, 38M euros, European design study is now underway to finalise the technology for what is called the Square Kilometre Array (SKA). As its name implies, it aims to have a total collecting area of one square kilometre − a million square metres and the equivalent of ~90 x 120m single dishes in size. It would thus be nearly two orders of magnitude more sensitive than the ATA. Like the ATA it will also use the largeN/smallD approach, though the individual antennas might be somewhat larger, perhaps 16m in size. There will be a central core of antennas with outlying “stations” located on a number of “spiral arms” radiating from the centre of the array, their separation increasing away from the array centre. The overall size of the whole array will be more than 3000 km so it needed to be located in a large, sparsely populated, area. The Northern Cape in South Africa has been chosen as the location of the central array.
The SKA will be a 17-country collaboration and require funding of ~1.5 Billion euros, so it is a truly major project. Construction of the SKA is scheduled to begin in 2016 to allow initial observations by 2019 and full operation by 2024. The headquarters of the project are at the Jodrell Bank Observatory in the UK. Like the ATA it will have the ability to form multiple beams so whilst, for example, a search might being made for pulsars, beams could also be in use searching for any alien signals.
Should we be disheartened that no signals have as yet been detected? Not really, for as Peter Backus of the SETI Institute has made clear, of the ~100 searches that have taken place since 1960, only SERENDIP and Phoenix (because of their use of the giant Arecibo Telescope) have had the sensitivity to detect signals from beyond our immediate locality in space. The use of the SKA will extend the search further out across and so have a realistic chance of detection if, as many astronomers now believe, intelligent life is thinly spread around the galaxy.
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