Is extraterrestrial life possible? How do we look for it? This series of articles attempts to scientifically answer these questions in as much depth as possible in an understandable way. This was probably my most satisfying work yet and I hope it leaves you hopeful and well equipped for the new golden age of exoplanet astronomy now upon us.
Is life on Earth just an extraordinarily freakish accident in a vast universe of a septillion stars? That's 1024 stars according to universetoday.com. Surely not, and for science fiction fans like myself, we think it's only a matter of time before humans achieve the technology required to make the exploration of life on other planets possible.
Can We Get to an Extrasolar Planet?
In order to find evidence for extraterrestrial life, we might imagine ourselves flying through a universe that is effectively small enough to explore in person. Our contemporary science fiction offers great portrayals of interstellar travel amongst myriads of connected planets. Wormholes, space folders and faster-than-light-speed ships bring together alien worlds that are just exotic enough to be fascinating, while having human-friendly gravities and atmospheres and lush ecosystems offering food and shelter. It is easy to get lost in these enticing possibilities, and it is easy to hope that some version of this will someday be realized.
In the movie, Interstellar, worm holes, black holes and the discovery of five dimensional space-time give doomed near-future Earthlings the chance to colonize a new world. It's a really nicely done modern space epic.
|Interstellar film poster (fair use);Wikipedia|
The story achieves some bona fide scientific rigour by hiring theoretical physicist Kip Thorne as a scientific consultant for the film. Kip Thorne is perhaps one of the world's leading experts on the implications of general relativity, and in this regard the movie shines because it brings anti-intuitive concepts to an accessible level. But the physics in general and the planetary science don't stand up so well. Falling down the interior of a black hole intact or flying through a stable wormhole are enormous theoretical stretches. Nonetheless, black holes exist and wormholes are permitted solutions to Einstein's general relativity field equations, which means they are theoretically possible. In fact, there are several wormhole theories. However, most traversable wormholes would require the use of hypothetical exotic matter, matter with negative mass, negative energy density and negative pressure to stabilize them. Traversable wormholes exist only in theory at present and require an equally theoretical form of matter, but could one be built someday? For a brief and easy wormhole refresher try "What is a Wormhole?" by Nola Taylor Redd at space.com. A more vigorous and comprehensive discussion is can be found at Wikipedia. A recent article written by Ph.D. student Matthew Wright for chalkdustmagazine.com offers a really good read for those of you interested in exploring the feasibility of wormhole technology.
Popular science fiction seduces us into thinking that wormholes or other technologies will eventually mean that we can literally reach out and touch distant alien planets. Perhaps the universe is ripe for human exploration and perhaps not. Gravity, another one of my favourite movies (below right), makes the case that space isn't so easy.
|Gravity film poster (fair use);Wikipedia|
It adheres quite well, but not perfectly, to scientific reality according to former NASA astronaut Garrett Reisman. Space in it is portrayed as the deadly and unforgiving reality it is.
It is disconcerting to accept that, as far as we currently know, the entire universe, with the exception of our infinitesimally small biosphere, is deadly to humans and to almost all Earth life.
Below left is the famous 1972 image of Earth taken from Apollo 17 from 29,000 km away. An inseparable part of our solar system and our universe as a whole, Earth is also unique as far as we know. Our oceans, land and atmosphere, taken together, is a biosphere that supports a complex global ecosystem.
A few species can handle the brutal vacuum of outer space, such as some very tough bacterial spores and surprisingly resilient multicellular but microscopic water bears, although how and why the water bears are so resilient remains a mystery.
|Schokraie E, Warnken U, Hotz-Wagenblatt A, Grohme MA, Hengherr S, et al. (2012) - Schokraie E, Warnken U, Hotz-Wagenblatt A, Grohme MA, Hengherr S, et al. (2012);Wikipedia.|
Rather adorable looking, a water bear in its desiccated state can survive temperatures from-272°C to 150°C, pressure greater than six times that of the deepest ocean trench, intense radiation and the vacuum of space (a scanning electron micrograph of one is shown below centre). It can also live without food and water for 30 years.
Space is an airless frozen vacuum filled with unpredictable radiation and dust that can be traveling at close to light speed. A few grains of it could devastate even the toughest spaceship's hull. From this perspective one has to wonder how life ever got started anywhere, let alone Earth.
Despite what might be extreme odds, scientists are hopeful people, and as you will read, life itself is hopeful too. The search starts closest to home by investigating ourselves and other planets and moons in our own solar system. Mars, Saturn's moons, Titan and Enceladus and Jupiter's moons Europa, Ganymede and Callisto, have been and continue to be explored for signs of life. Life could exist on any of these worlds, perhaps under Europa's thick icy exterior shell within a liquid ocean kept warm by the moon's internal tidal forces, for example. Looking further outward, we hope that scientific missions like the Kepler spacecraft, launched in 2009, will find new worlds in alien solar systems where life could also be present.
|An artist's rendition of the Kepler spacecraft in flight by NASA|
Data extrapolated from Kepler so far tells us there could be as many as 40 billion Earth-sized planets orbiting in what scientists call habitable zones. Those zones could be around Sun-like stars (11 billion planets) or, more commonly, red dwarf stars (around 30 billion planets) and that is just within our Milky Way galaxy alone.
|NASA image of a hypothetical planet and two moons orbiting the habitable zone of a red dwarf star, created using data from the Kepler Space Telescope|
NASA's ever-expanding online exoplanet archive, as I write this, stands at 3374 confirmed exoplanets. Surely Earth can't be the only planet where life arose?
Getting There Would Be a Very Long Journey
Unfortunately, the vastness of the universe works very much against investigating an exoplanet in person. Planets orbiting other stars, or exoplanets, are further away than we can imagine. Our closest star system, consisting of three stars, is Alpha Centauri, 4.4 light years away. This system has been intensively observed for over 15 years and no planets had been confirmed orbiting any of these stars, but planets have always been possible. Recent computer simulations of the Alpha Centauri system suggested that a habitable planet could form there, and in fact, just last week (August 15, 2016), rumours began to swirl that an Earth-like exoplanet had been discovered orbiting Proxima Centauri (one of the three stars).
|In the night sky, Alpha Centauri (left bright dot) and Beta Centauri (right) are visible. Fainter Proxima Centauri is highlighted in the tiny red circle below and between the two brighter stars. Image is by Skatbiker;Wikipedia.|
As I write this today (August 23, 2016), a rocky planet has indeed been confirmed orbiting Proxima Centauri! The planet is called Proxima b. Two ground-based telescopes in Chile run by the European Southern Observatory just confirmed its existence indirectly by monitoring how its star, Proxima Centauri, wobbles under the planet's gravitational influence. They have released tantalizing artist's conceptions of what the planet might look like.
|An artist's rendering of Proxima b orbiting the red dwarf star Proxima Centauri. The double star, Alpha Centauri AB appears as a singular small white dot to the right of Proxima Centauri. Credit to ESO/.Kommesser|
|An artist's rendition of what the surface of Proxima b might look like. It is a potentially Earth-like world. Credit to ESO/.Kommesser|
This exoplanet is about 1.3 times the size of Earth. Even though its star appears distant and is relatively small and dim in the image above, Proxima b orbits just 7.5 million km away from Proxima Centauri. Compare this distance to Earth's orbital radius of 150 million km, 20 times further. This means that Proxima b's year is just 11.2 Earth days long and it is likely to be tidally locked under its star's gravitational pull (like our moon is). Because it is so close to its star, this means that the same side always faces its star. This close orbit puts Proxima b in the middle of the Proxima Centauri 's habitable zone, where surface water, if it is present, could exist as a liquid. While such a discovery is very exciting, there are reasons why life might not have a chance there. Conditions on this planet would likely be harsh. Although red dwarf stars live much longer than our type of star (trillions of years compared to our Sun's 10 billion year lifespan), their radiation output is unstable. Large sunspots develop often, drastically reducing its infrared (heat) output. These periods could rapidly cool Proxima b's face side to below freezing. At other times, devastatingly powerful flares develop over just hours, showering the surface with X-rays that would be deadly to most of Earth's surface life. Proxima b might have a protective atmosphere that shields the surface from radiation just as Earth does, but star flares would be expected to erode away any atmosphere over time, and it is unknown if the planet has a magnetosphere, which would envelop and protect its atmosphere. If the planet has liquid surface water, it is possible that life could develop underwater, protected there from unpredictable radiation. It's also possible that life on a planet orbiting a red dwarf might evolve strategies such as radiation-proof armour or avoidance behaviours like burrowing down or moving under water. The fact that Proxima b is tidally locked also does not preclude the possibility of life there. Recent computer analyses suggest that surface winds could distribute heat, reducing wide discrepancies in surface temperature, which would allow for a potentially habitable band between the planet's hot baked inner face and frozen outer face.
Until this discovery, the closest confirmed rocky exoplanet was Gliese 674 b, which orbits the star Gliese 674, a red dwarf star located 14.8 light years away.
Detection of planets the size of Earth is very difficult. The reduction in the star's luminosity as a miniscule planet transits across it is almost immeasurable (via a process called transit photometry). Smaller planets like Earth also have very little gravitational impact on their star's rotation, so orbital wobble is also very difficult to detect (this process is called radial velocity). This is why the confirmation of Proxima b took some time. There are several established and proposed exoplanet detection methods, most of which are indirect, and all of which are ingenious. Check them out with the link. At present, an exoplanet cannot be observed directly even through our most powerful space telescopes because it is too tiny compared to its far brighter host star. This might be about to change, as you will see.
As you have noticed, we are talking about distances of light-years. Astronomers use this scale because it fits best with distances between stars and galaxies. A light-year is the distance that light (traveling at 300,000 km/s) travels in one year. It is approximately 10,000,000,000,000 km, basically just a bunch of meaningless zeros. To help put this distance in perspective, consider Voyager 1.
|Artist's impression of Voyager 1 in flight supplied by NASA|
The most distant human-made object, Voyager 1 recently exited our solar system and entered interstellar space. Its average velocity is about 61,000 km/h. To compare, the fastest recorded Earth vehicle is the SR-71 Blackbird, which officially clocked in at about 3500 km/h. Traveling at 61,000 km/h for almost 39 years, Voyager 1 has traveled over 27,000,000,000 km but it is still only 18.1 light hours away. That's about 1/486th of a light year! What all this means is what seems very fast to us is very slow on the cosmic scale. Even if we could travel at light speed, which is forbidden according to special relativity*, it would take us 4.2 years to reach Proxima b.
*Star Trek spaceships have no problem achieving faster-then-light warp drive but in reality Albert Einstein's special relativity gets in our way. As succinctly and perhaps enigmatically put by Wikipedia, "according to special relativity, the energy of an object with rest mass m and speed v is given by γmc2, where γ is the Lorentz factor." The Lorentz factor simply tells us how time, length and mass change when an object is moving relative to an observer. An observer (you) doesn't notice this factor when an object is moving much slower than light speed (say, a bus passing by) or when it is stationary. When v is zero, γ is equal to one, which gives rise to the famous E = mc2 formula for the mass–energy equivalence of any stationary object with mass. The catch comes when the γ factor approaches infinity as velocity approaches light speed. This means it would take an infinite amount of energy to accelerate an object with any mass to the speed of light. Photons of light don't count because they have no mass. Einstein's theory of special relativity is thoroughly confirmed by physical evidence (consider all the data from particle accelerators, where the behaviours of particles with mass traveling close to light speed are accurately predicted by this theory). This theory is accurate for describing objects traveling through space-time when the gravitational field is not extremely powerful, but when you are describing objects in motion near black holes or when you are dealing with massive galaxies you must switch to general relativity. The two theories are different but consistent: general relativity becomes special relativity in a weak gravitational field.
If we could travel at the same velocity as Voyager 1, it would take us almost 2600 years to reach Proxima b. And Voyager 1 is a relatively light (825 kg) unmanned spacecraft. A much heavier manned craft with life support systems and a few astronauts would be much more difficult to accelerate. For a manned flight analogy we can look at the Apollo missions. If we traveled as fast as the Apollo 10 as it swung around the moon (reaching a peak velocity of about 40,000 km/h), it would take us 1.5 times longer still to reach Proxima b, almost 4000 years! These technologies are decades old of course. It is now theoretically possible using fission or fusion pulse propulsion technology to attain velocities of up to 100,000,000 km/h. The first conceptual spacecraft using this technology was the Teller Ulman thermonuclear unit powered Orion starship, ironically based on work done as long ago as the 1960's.
|Conceptual drawing of a fusion pulse propulsion spacecraft under the Project Orion mission by NASA|
This velocity is about 1/10 the speed of light, which means it would take about 42 years to reach Proxima b, much better but still half of a human lifetime unless some kind of stasis system could be developed as well. Not surprisingly, NASA is working on that too.
In April, 2016, scientists and engineers announced the start of the Breakthrough Starshot Project, a research and engineering project aimed at developing a postage stamp-sized sail-equipped nano-craft probe (called StarChip) that could accelerate to 20% the speed of light using powerful lasers. This would be the fastest vehicle yet but it won't be easy. The laser will have to have a power output on the same scale as a large nuclear plant to achieve just a few Newtons of thrust (and this is why it will have to weigh only a few grams). Sub-gram scale cameras and processors are already available technologies but a small enough (probably plutonium-238) battery needs to be developed and a long list of other challenges described here will have to be overcome first.
The announcement of Proxima b could help move such a mission forward but it could still take decades of development before a probe can be sent to the exoplanet. The trip itself would then take about 20 years, and it will take another 4.2 years after that to start to receive data back to Earth. I probably won't be around for that historic breakthrough but younger readers certainly could be.
Unfortunately once the problem of going fast enough is solved another one awaits. The velocity itself becomes a significant issue. Micro-meteors and even cosmic dust (which is mostly free hydrogen and helium atoms and small molecules) can become, in effect, hard radiation striking the spacecraft with enough kinetic energy to mechanically damage or embrittle the hull over time. Magnetic or electrical shielding wouldn't work because most of the space dust is electrically neutral. Even drag from these particles would become an issue. This will be a significant technical hurdle even for the tiny StarChip.
Cosmic radiation is an additional concern. Voyager 1 had the benefit of being enveloped in the protective solar radiation bubble of the Sun's heliosphere for most of its life.
|This NASA image of Voyager 1 and Voyager 2 is from 2005 while Voyager 1 was still in the Sun's heliosheath (grey bubble) where interstellar gas and solar wind mix. Voyager 1 has now crossed outside the heliosphere into interstellar space.|
When Voyager 1 recently exited into interstellar space, it measured a 40 times increase in plasma (ionized gas) density) and a large increase in galactic cosmic radiation. Cosmic radiation or cosmic rays are high-energy protons and other small nuclei streaming through space from supernovae and other highly energetic sources. Flying through space very close to the speed of light, they pose perhaps the most significant barrier to interstellar space travel. They might mean that manned interstellar flight is unfeasible. A single proton at near light speed, for example, can have as much kinetic energy as a baseball traveling at 90 km/h. Imagine millions of them striking a spacecraft or an astronaut on a spacewalk. Not only would they present a deadly radiation threat to humans, but they could also damage the electronics onboard. Even an unmanned interstellar probe, including one that does not approach relativistic velocity, would have to stand up to all the cosmic radiation of interstellar space. Cosmic rays (positively charged nuclei) could be deflected by strong magnetic shielding, a potential silver lining. The StarChip's electronics could be shielded but it will have to be a very effective and very lightweight solution.
Perhaps the only practical choice is to send an unmanned probe such as Starchip. It's not the dream as I think most humans have an innate need to experience new things in the flesh. However, travel time in terms of passengers would no longer be critical. Still, this option comes with a time-related challenge. To visit exoplanets, it will need a power source that will work for decades, centuries or even millennia. Voyager 1 is powered by three radioisotope thermoelectric generators. The radioactive decay of plutonium-238 generated 470 watts of electrical power at launch time but the fuel output has been declining ever since because plutonium-238 has a half-life of just under 89 years. The thermocouples are also wearing down over time. Voyager 1 is expected to continue to be able to power its operations until about 2025. If a radioactive power source were to be used for an interstellar probe on a mission much longer than Proxima b, it will have to have a much longer half-life than plutonium-238.
Another possible complication of interstellar travel, and communication as well, also has to do with time. A relatively close exoplanet such as Proxima b is 4.2 light years away so any electromagnetic (EM) signal we received back from a probe that reaches it, perhaps a radio signal, would not arrive at Earth for 4.2 years. One communication cycle would take over 8 years. Therefore, even travelling to our nearest rocky planet will require a long-term vision and immense patience
While a probe mission to Proxiima b seems plausible, unmanned missions to other exoplanets much further away could take generations of human lifetimes to complete. It will be critical to make good planet choices first, and that is the most technologically feasible aspect of exoplanet exploration. Scientists have an ever-increasingly sophisticated arsenal of indirect detection technologies at their disposal. Wikipedia lists and describes them well. An exoplanet's orbit, mass, shape, atmosphere, density, circumference, surface composition, surface temperature and magnetic field can potentially be inferred by using some of these technologies. For example, more than 50 exoplanet atmospheres have been inferred so far. Scientists can in theory deduce information such as molecular gases present, temperature, pressure, density, day/night temperature gradients, and vertical atmospheric structure by observing the planet using various types of spectroscopy and polarimetry, by transit imaging and by observing the variation of brightness with the planet's phase. Clouds (as fairly clear reflective signatures) have even been detected in the atmosphere of gas giant Kepler-7b, which is about 1000 light-years from Earth.
|An artist's rendition of Kepler-7b is shown on the right compared with Jupiter (left). Aldaron, a.k.a. Aldaron;Wikipedia|
Next we will explore the possibility that intelligent extraterrestrial life might be trying to signal or communicate with us, and we might be alerting them to our presence as well.