The 'big' coming event will be the launch of the James Webb Telescope in 2018, successor to the Hubble and Spitzer Space Telescopes, with unprecedented resolution and sensitivity from long-wavelength visible to the mid-infrared. The discoveries are just going to keep coming but none of them are going to change the fundamental problem that, as biological creatures of this planet, the anti-biological conditions of space and the vast distances involved are going to see physical exploration beyond our solar system as something for the very far distant future.
So why be interested in exo-planets other than scientific curiosity for its own sake. First, because the very far distant future is still possibly an era when humans or more differently evolved humans may have mastered both conditions and distance, even if it is only to send non-biological surrogates or machinery capable of seeding planets with our biology. Second, though we can tell very little about exoplanets now, it is more than intriguing and would have enormous effects on our culture if we found one that was sending out signals, whether deliberately or not, that showed that something conscious like ourselves had evolved as we have evolved (or differently).
We would be faced with some interesting challenges that would precede our decision to invest in further exploration - are these less or more developed, can we communicate, would they be friends or enemies, opportunities or threats? Anything beyond philosophical thought experiments (which should be handled by philosophers and not scientists) though is speculation, 'hard' science fiction. It is useful to explore philosophically the many various possible scenarios for the future: this is not a waste of time but it is, in practical terms, futile in the twenty first century. The real issues to consider arise from our own nature and perceptual and conceptual abilities to deal with radical discoveries. By the time that we think we know that there is something out there with which we are going to have contact, we will probably have evolved ourselves or, at least, have advanced our powers in the area of thought if only because of the practical use of artificial intelligence. So, let's stick to the facts here and not try to be a second division Arthur C. Clarke.
At the moment, exoplanets can be indexed according to their similarity to earth. These are known as Earth Analogs. You might equally see terms like Twin Earth, Earth Twin, Second Earth, Alien Earth, Earth 2 or Earth-like Planet. Whether these planets are more or less likely is still a philosophical debate about what deduction from science can tell us. As of about two years ago, the majority opinion of astronomers was something along the lines of their being as many as 40 billion Earth-sized planets orbiting in the habitable zones of sun-like stars (11 billion of these, depending on the source) and red dwarf stars within the Milky Way Galaxy. This was a calculation based on Kepler space mission data. The nearest is around 12 light-years away which sets us the standard of hitting the speed of light safely for well over a decade of travelling before we can even observe one at close quarters with the human eye.
This, however, is not a calculation from observation but a 'could' based on the so-called 'mediocrity principle' which assumes that if we are as we are within a giant system then we are probably pretty average or mediocre and not so very special, so we should expect to find other things like us around. Philosophers can be highly critical of the assumptions behind the mediocrity principle (which we won't go into here) so it is probably best to say that the 'jury is out' and the case is, as in Scottish law 'not proven' but that it is a decent working assumption on which to base continued investigation until the data changes.
It is equally reasonable to suppose that we are accidental or exceptional and there are no planets like us that could bear life, let alone develop evolved consciousness with culture. This latter is the Rare Earth Hypothesis which starts by stating just how improbable it was that conditions would be right for multicellular life, let alone evolved consciousness. The debate can be studied from the Rare Earth Hypothesis entry in Wikipedia and from there you can check out the extensive references to Extraterrestrial Life The bottom line is that it is largely hypothesis and speculation. Nobody knows very much. It is just theory.
In the same realm of hypothesis and theory is the debate about terraforming in which engineers join scientists in positing that planets that do not currently bear life could be transformed by planetary scale projects into habitable homes for humans. This, of course, is more immediately interesting if it can be applied to a near neighbour like Mars but we are far from having the resources or knowledge to consider a project that would take aeons to complete. Even more theoretical work would posit alternative earths in multiverses or parallel universes. None of this is currently of any functional value although it is all very entertaining and stimulating. At a certain point, science fiction may become a distraction more than it becomes an aid to creativity.
However, there is practical work to be done - other than exploring space for data that might confirm the many earths or the rare earths model as more likely (or something inbetween). First, there is the search not only for habitable planets but also for signals that might come from habitable (or non-habitable) planets or deep space. Second, there is the science of astrobiology which is essentially about the conditions that may be possible or necessary for any form of life whatsoever to exist outside the Earth and where we might expect to find it. Third, there is the science of planetary habitability itself which is about comparing what has happened on earth with conditions on planets and hypothesising the relationships between planets and the creation of life forms.
What has emerged is an Earth Similarity Index, developed by NASA and SETI, which scales exo-planets as similar to Earth (the understandable model for habitability in an anthropocentric mind-set) on a range from zero to one where the Earth is one. The details are in the Wikipedia link but an ESI of 0.8 to 1.0 would cover any rocky terrestrial planet. The index is not to be taken as implying habitability at all - it is simply what it says on the tin, the similarity of a body (including large satellites) to the Earth in terms of mass, radius and temperature. Currently, the closest confirmed planet to the Earth is Gliese 667 Cc only 22.7 light years away. This rather 'cool' artist's impression of the planet should, of course, be taken with a pinch of salt in terms of detail but it gets across something of what such a planet may look like, the closest yet to us of the sort of planet which might be targeted for colonisation that we know of at this level of detail ...
This shows a sunset. The brightest star is the red dwarf Gliese 667 C, part of a triple star system. Two more distant stars, Gliese 667 A and B appear in the sky also to the right. There may be tens of billions of rocky worlds like this orbiting faint red dwarf stars in the Milky Way.
The Habitable Exoplanets Catalog is held at University of Puerto Rico at Arecibo. Gliese 667Cc has an ESI of 0.84 but there are two other confirmed exoplanets with higher ESIs - Kepler 438b (0.88) and KOI-1686.01 (0.89). Kepler-438b is 470 light-years from Earth. The Kepler reference refers to NASA’s Kepler telescope which was launched in March 2009, costing $600m and with a mirror about 60% the size of Hubble's, precisely to search for habitable zone Earth-sized planets in the Milky Way using just one instrument, a photometer which continuously records the brightness of stars, monitoring 1,500 stars simultaneously in a targeted block of the sky. When a planet crosses a star, its blockage of light permits it to be identified but the measurements are miniscule. Kepler merely identifies candidates. Ground-based observers and scientists then take over to confirm with about 10% of sightings proving to be false alarms. Kepler 438b received most media attention at the beginning of the year as the 'most earth-like planet'- only 12% larger and 40% more illuminated although its star is smaller than ours.
However, this information is probably already out of date. Exoplanets frequently have their data revised as new information comes in or is analysed. Some 'habitable planets' turn out not to be so habitable at all. It is hard for any non-specialist to get reliable information. The truth is that while many habitable (not the same as earth-sized planets so that reduces the Milky Way number from 11bn to 8.8bn) exoplanets are posited, only 1,000 confirmed exoplanets have actually been found (although this is a remarkable scientific achievement with over another 2,000 under investigation through Kepler, backed up by confirmatory ground observation). Of this, perhaps just over a dozen are confirmed as within the so-called habitable zone albeit with around 54 candidates to be confirmed. A habitable zone is a definable region around a star where a planet with sufficient atmospheric pressure can maintain liquid water on its surface, hence the water in the artist's impression of Glise 667Cc.
Ben Solomon has suggested that life sustaining planets be named zoetons on the principle that a spacefaring civilisation ought to start defining its terms in advance along the lines of what is going to be useful for that new emergent culture. The idea strikes me as premature. We are way off being spacefarers to the extent of requiring a new cultural paradigm. While the effort to think in these ways may be interesting, they ultimately fall into the category of speculative science. Nevertheless, science fiction may find the following paragraph from Solomon's blog posting in Lifeboat News useful in regulating its universes ...
Taking a different turn, to complete the space faring vocabulary, one can redefine transportation by their order of magnitudes. Atmospheric transportation, whether for combustion intake or winged flight can be termed, “atmosmax” from “atmosphere”, and Greek “amaxi” for car or vehicle. Any vehicle that is bound by the distances of the solar system but does not require an atmosphere would be a “solarmax”. Any vehicle that is capable of interstellar travel would be a “starship”. And one capable of intergalactic travel would be a “galactica”.Speculation almost has to be rife in this area because travelling into space and finding new homes is the stuff of the dreams that turned many youngsters into scientists. There is more grounded speculation that there may be planets out there (super-earths) that are 'even more habitable' than Earth. This speculation suggests that we are, not unreasonably, privileging Earth as the most habitable simply because we grew up there and that there may be planets that are more amenable to life than ours. This leads, in turn, to the call by a minority for some redirection of the search to include planets of some types outside the classic habitable zones around sun-like stars and red dwarves.
The debate is useful because it acts as check and balance on automatic assumptions about what, in the search for life (as opposed to just habitability for humans), we should be looking for - for example, underground oceans on planets well outside the 'zone' may be as likely to be where life is to be found as a rocky planet inside the zone. However, according to Ravi Kopparapu, a Penn State University physicist (according to the National Geographic article cited in the paragraph above):
"there is a very good reason why the binary habitable zone concept is important and relevant" ... Currently, when astronomers discover a planet, all they can learn about it is its mass and radius, how much light it receives from its star, and occasionally the composition of its upper atmosphere. Until scientists develop the techniques to study a planet's surface features, tectonic activity, and geological composition, the habitable zone concept remains the best guess of its habitability, says Kopparapu."The James Webb Space Telescope should radically extend the range of our knowledge about these and related factors. It will be a step up but this is not a vehicle for fly-bys and close observation. The planet-finding programme will be extended significantly in the coming years with new space telescopes. NASA is launching Tess (Transiting Exoplanet Survey Satellite) and the ESA will launch Cheops (CHaracterising ExOPlanet Satellite) in 2017. ESA will follow up with a larger planet finder, Plato, by 2024. The specific mission (despite the critique of those who think the search is too limited in scope) is to find Earth Analogs within the nearest (to us) habitable zones of sun-like star systems in as many locations as possible. The European E-ELT telescope which being built in Chile, is being designed to analyse the atmospheric composition of these planets and a better judgment made of habitability and even whether life is probably there already.
So, it is not impossible that the world will be stunned to find that a planet with all the atmospheric characteristics of life is proven probable rather than possible within a decade or two. How to get there, if we want to get there, is another matter entirely.
Note: There are a large number of entries on exoplanetology available from the relevant Wikipedia Template There is also an amusing if outdated Popular Science infographic of all the known exoplanets at the beginning of 2014.