29.1 Should we expect to find evidence of aliens?
NASA has stated that it might be possible for life to exist on Mars,[1] on Jupiter’s moons Europa and Ganymede,[2] and on Saturn’s moons Titan[3] and Enceladus[4], and NASA Space Scientists predict that about half a billion other planets in our galaxy may be capable of containing life.[5]
The Drake equation can be used to estimate the number of extraterrestrial civilizations in our galaxy that we may be able to communicate with. This was devised by Frank Drake at the first SETI (Search for Extra Terrestrial Intelligence) meeting in 1961.[6]
The Drake equation
| N | = | R* × fp × ne × fl × fi × fc × L, where | (29.1) |
| N | = | The number of extraterrestrial civilizations in our galaxy that we may be able to communicate with. | |
| R* | = | The average rate of star formation per year in our galaxy. | |
| fp | = | The fraction of those stars that have planets. | |
| ne | = | The average number of planets that can potentially support life, per star that has planets. | |
| fl | = | The fraction of the above that go on to develop life. | |
| fi | = | The fraction of the above that go on to develop intelligent life. | |
| fc | = | The fraction of civilizations that develop a technology that releases detectable signs of their existence into space. | |
| L | = | The length of time such civilizations release detectable signals into space. |
Although we can only guess the answers to the last four factors in the Drake equation, in 1961 only the average rate of star formation in our galaxy could be calculated with any degree of accuracy.
Another way to determine the probability of alien life is to consider the single example we have. If the Earth is atypical, and it is currently the only planet in the observable universe to contain life, then it’s still probable that life will evolve on other planets in the future.
The Earth is thought to have formed before 92% of all habitable planets that will ever form in the observable universe.[7] Fewer habitable planets form over time, and so most of these planets will form in the very distant future, hundreds of billions of years from now. If only one civilisation were to ever form in the observable universe, then you would expect it to form somewhere in the middle of the distribution. The fact that civilisation developed on one of the first 8% of planets to form means it’s quite likely that another civilisation will form at some point.
If the Earth is a somewhat typical example, and intelligent life typically evolves about 4 billion years after the formation of a habitable planet,[8] then the first intelligent life forms in the Galaxy could have evolved about 4 billion years ago.[9]
It would only take about half a million years for self-replicating spacecraft to travel across the Milky Way if it were travelling at 10% the speed of light. This means that spacefaring intelligent life would only have to have evolved once on any of the hundreds of millions of possibly habitable worlds, during the last 4 billion years, for the Galaxy to be almost entirely inhabited.
Given that alien life seems like a real possibility, scientists are currently searching for evidence of life both within the Solar System and on exoplanets.
Figure 29.1 |
An artist’s impression of newly confirmed habitable exoplanet Kepler 22-b. |
29.2 The search for aliens
So far, there is no compelling evidence of life outside of Earth, however, NASA is currently searching for evidence that life could exist on Mars,[1] and they intend to search for life on Jupiter’s moon Europa in the 2020s. They have also stated that life may exist in oceans on Jupiter’s moon Ganymede[2] and Saturn’s moons Titan [3] and Enceladus[4].
Many agencies and programs are also searching for signs of life on planets outside of our Solar System. Thousands of exoplanets have already been discovered and it’s hoped that future space telescopes like NASA’s James Webb Space Telescope (JWST) may be able to find evidence of life in a planet’s atmosphere. In order to do this, scientists will use spectroscopy (discussed in Book II) to look for molecules that are produced naturally by living things, like molecular oxygen, or molecules that are artificially produced by intelligent life, like CFCs.
People have tried to communicate directly with life outside of the Solar System. Direct messages to intelligent alien life were placed on the Pioneer and Voyager probes (discussed in Chapter 27), although these will not leave the Solar System’s Oort Cloud for tens of thousands of years. Members of the public have also sent direct messages to exoplanets including those in the Gliese 581 system, which contains at least three planets (discussed in Chapter 28). These were translated into binary and encoded into light waves, which means they can travel at the speed of light.
The Gliese 581 system is about 20 light-years away. This means that Earth’s message to the Gliese 581 system should arrive in 2029[10] and if any of the planets in the Gliese 581 system contain intelligent life forms that are able to reply, we can expect a message sometime after 2049.
We can also search space for artificial signals that may have been sent to us. This is what the SETI (Search for Extra Terrestrial Intelligence) program does. SETI mainly searches in the radio spectrum, around the ‘radio waterhole’. The term ‘radio waterhole’ was coined by the Project Cyclops team in 1971. The Project Cyclops team helped design SETI and was headed by Bernard Oliver, an engineer and vice-president of Hewlett Packard.
Oliver and his team suggested that the most obvious part of the spectrum for aliens to communicate would be between 1400 and 1700 MHz, corresponding to wavelengths of about 21 and 18 cm, which are in the radio spectrum. This is the part of the spectrum where spectral lines for hydroxyls - OH – and hydrogen - H - can be found. Together, these make water – H2O. The Project Cyclops team suggested,
different galactic species might meet there just as different terrestrial species have always met at more mundane water holes.[11]
Figure 29.2 |
Different species sharing a waterhole. |
We can also search for artificial broadcasts, artificial megastructures, and other signs of intelligent life using data from the Kepler spacecraft.
29.2.1 Alien megastructures
It’s been suggested that advanced life forms might form megastructures like Dyson spheres or ‘Ringworlds’ around their stars to harness more of the star’s energy and increase the liveable landmass. This idea was developed by the American physicist Freeman Dyson in 1960.[12] A Dyson sphere with a radius of 1 AU (the distance between the Earth and the Sun), for example, would have a surface area of about 550 million times the surface area of the Earth.
What is the surface area of a Dyson sphere?
| Surface area of a sphere = 4π × Radius2 |
| Radius of Earth = 6371 km. |
| Surface area of Earth = 4π × 63712 = 510 million km2 |
| Radius of Dyson sphere of 1 AU = 150,000,000 km. |
| Surface area of Dyson sphere of 1 AU | = | 4π × 150,000,0002 |
| = | 283 million billion km2 |
| Surface area of Dyson sphere of 1 AUSurface area of Earth | = | 283 million billion km2510 million km2 |
| = | 554 million |
Figure 29.3 |
A diagram of a Dyson sphere. |
If Dyson spheres are possible, and alien civilisations can build them, then they may be detected using the same method the Kepler spacecraft uses to look for planets. A fully formed Dyson sphere would block all visible light from the star and would only be detected from its heat or through gravitational effects. But a ring system or partially constructed Dyson sphere would cause the light of the star to flicker as it orbits, periodically blocking the light. This would cause a dip in brightness, and the shape of the dip can be used to determine the shape of the object that made it.
It has recently been suggested that some sort of alien megastructure might be responsible for the strange light curves of the star KIC 8462852.[13] KIC 8462852 is an F-type main sequence star, which makes it very similar to the Sun, and is about 1400 light-years away. This means that light from KIC 8462852 takes 1400 years to reach us, and so we are seeing what it was like 1400 years ago.
Evidence of KIC 8462852’s unusual fluctuations in brightness was first discovered by amateur astronomers working as part of the Planet Hunters citizen science project. They then collaborated with professional astronomers, including the American astronomer Tabetha Boyajian.[17] While Dyson spheres are not discussed in the paper, Boyajian has since discussed the possibility with other scientists,[18] and in October 2015, scientists at SETI spent two weeks looking for evidence of artificial radio signals coming from KIC 8462852, although they did not find any.[13] While it’s possible that there’s an alien megastructure around KIC 8462852, it seems much more likely that the unusual fluctuations will be due to a new type of natural phenomenon.
Artificial megastructures like Dyson spheres may be difficult to detect because many natural phenomena, like clouds of comets, may produce the same effect. If they wanted to broadcast their existence, the inhabitants of a planet could change the shape of their light curve to make it appear obviously artificial.
The shape of a planet’s transit curve could be changed mechanically, by putting large artificially shaped objects into orbit, such as triangles.[19] These would be more efficient if they were placed close to the host star, where they would orbit more quickly – this is Kepler’s second law - although this is a far beyond our current ability. However, a similar effect can be achieved using lasers to distort the light curve.[20]
Information could be beamed along the laser, and so planetary transits may provide a universal way for life forms to commutate using optical light, providing an ‘optical waterhole’ analogous to the ‘radio waterhole’.
Figure 29.4 |
‘Construction crew at work on the colony’ by Don Davis. An artist’s impression of a potential future space colony designed by NASA scientists in the 1970s. |
Self-replicating spacecraft
Self-replicating spacecraft are probably the quickest and easiest way to explore the Galaxy. A self-replicating ship would travel to a star system where it uses resources - such as metals extracted from asteroids, or hydrocarbons, like those found on the moons of gas giants - to create replicas of itself. These then head off in different directions, repeating the process.
A self-replicating spacecraft, travelling at about 10% of the speed of light, would take 10 years to travel one light-year, and 1,000,000 years to travel 100,000 light-years, which is the diameter of the Milky Way. Assuming they are roughly in the centre, and send ships in each direction, the time is halved.
If there are at least half a billion habitable planets in the Galaxy, they will come across one every 24,000 years or so.
Number of stars in the Milky WayNumber of stars containing habitable planets = 300,000,000,000500,000,000 = 600
With an average distance of 4 light-years between stars, it will come across a habitable planet every 4 × 600 = 2400 light-years. Travelling 10% the speed of light, this will take 24,000 years.
If self-replicating spacecraft do exist, they are unlikely to be crewed by biological life forms given the long timescales involved - they may even outlive the species that created them - but they could have artificial intelligences on board or could be considered life forms in their own right. Different self-replicating ships may compete with each other for resources and ‘mutate’ when they replicate, with some mutations helping them better fill their evolutionary niche.
Self-replicating spacecraft may be launched for several reasons:
- Von Neumann probes would be designed to map the Galaxy and transmit information about stellar systems back to its creators. These probes are named after the Hungarian-American mathematician John von Neumann, who studied self-replicating machines.[14]
- Bracewell probes would be specifically designed to find, and communicate with, other alien species. The Australian physicist Ronald Bracewell first proposed this idea in 1960.[15] Bracewell probes will presumably contain some kind of artificial intelligence that can provide information about its creators. If these probes find a habitable world that is devoid of intelligent life, they may wait and see if it evolves, perhaps hibernating until they are activated.
- Seeder ships are designed to spread life throughout the Galaxy.[16] They may even contain the genetic information required to replicate their creators.
- The most threatening self-replicating probes would set about wiping out life in the Galaxy. These might be launched by an aggressive or paranoid species. They might be created by accident, or they might be formed from benevolent self-replicating probes that have mutated or made the conscious choice to stop following their original orders. Self-replicating probes that wipe out life are known as Berserkers. This name is taken from the American science-fiction writer Fred Saberhagen’s Berserker series. Saberhagen took the name from the Norse Warriors who were known to kill indiscriminately, giving rise to the English word ‘berserk’.