The definition of an exoplanet is a planet outside Earth’s Solar System, orbiting another star, or a rogue planet drifting aimlessly in space. These rogue planets are also known as drifting planets, wandering planets, starless planets, and sunless planets – roaming free of any star system deep in the interstellar void of space between the stars. The first exoplanet was detected in 1988 and later confirmed in 2012. As of April 2019, there are 4023 confirmed exoplanets in 3005 different star systems; 656 of these star systems have multiple planets. Exoplanets orbiting stars outside our Solar System are far easier to detect than the rogue planets which don’t belong to any star system.
The Hubble Space Telescope has been a tremendous tool in helping to find previously undiscovered planets beyond the Earth’s Solar System. Having said this, there are a number of ways of detecting Exoplanets such as Transit Photometry(the most successful method), watching for possible planets moving across the body of a star, and Doppler Spectroscopy, where Doppler shifts of the spectrum of the star being observed are measured. Although these two methods find the vast majority of exoplanets, about 85 percent, they tend to be biased towards planets which are orbiting close to the star. Gravitational microlensing, finding very small deviations from the light of a star as a planet orbits around it, taking an image which indicates a planet orbiting around a star, and watching for extremely small perturbations, such as wobble, of a star which might be caused by an orbiting planet are a few other methods.
Planets In The Universe
With our current state of technology we are pretty much limited to searching for planets in our galaxy, especially in the more local region within a few thousand light years. What about planets in further regions of our Milky Way Galaxy? Do other galaxies in the Universe have an abundance of planets?
Let’s take a look at the possibility of exoplanets outside of our galaxy in more detail. Our galaxy, the Milky Way, is part of a cluster of galaxies called the Local Group. This cluster, or group, contains about 54 known galaxies, most of them are dwarf galaxies, but some are also large galaxies such as Triangulum, the Milky Way, and Andromeda(in ascending order of size).
Astronomers now estimate that there are about 50 billion planets in the Milky Way, and roughly 500 million of them are orbiting within their stars habitable zone. That’s a remarkable number of habitable planets in our galaxy alone, and it boggles one’s imagination to think of how many of these 500 million planets might have intelligent civilizations on them. There may be a huge number of planets where intelligent beings of different types could be gazing at their night skies and wondering about the same things that we are.
This is where something like SETI, the search for extraterrestrial intelligence comes in. Astronomers are using giant radio telescopes in search of radio transmissions or signals that could be from intelligent extraterrestrial civilizations. Most frequently they search for radio signals in the range of 1420 to 1720 megahertz – this is because when a hydrogen atom combines with a hydroxyl molecule, it forms a molecule of water, which is the most essential molecule needed for life known. So the frequency range of 1420 to 1720 megahertz is often called the water hole and considered the most likely place where intelligent civilizations might transmit signals in an effort to contact other civilizations in the Universe.
Something that should be noted here is that these radio astronomers could be making assumptions that aren’t really valid. There may well be highly advanced civilizations in our galaxy alone hundreds, thousands, or maybe even eons of years more advanced than we are. Are we to imagine that these super advanced civilizations are still using primitive radio waves for their interstellar transmissions? Radio waves that we’ve only had for a little over a hundred years? Could it be that advanced civilizations discarded radio wave transmissions long ago in favor of more exotic and far more efficient means of communications? Might they be using scalar electromagnetic waves(see Maxwell’s equations), neutrinos, tachyons(a hypothetical particle which always travels faster than light but whose existence has never been proven), or perhaps something far beyond anything which we could imagine?
Stanton Friedman, the famous UFO researcher, has often called SETI ‘silly effort to investigate’ – he believes it unlikely that highly advanced alien civilizations would still be using radio waves. Of course, it could still be possible to receive radio signals from some less advanced civilizations, and we don’t know at this time if more advanced and better means of interstellar communication exist, so this method certainly does have some merit.
We have been talking about the estimated 500 million habitable planets in our galaxy, but what of other galaxies. The Local Group(containing the Milky Way) of 54 galaxies is part of a much larger cluster of galaxies known as the Virgo Supercluster which contains about 1300 galaxies, one of the largest structures in the Universe – it has about 100 galaxy clusters and groups within its diameter of 110 million light years.
But wait, that’s just the beginning! The Virgo Supercluster is now believed to be only one of about 10 million superclusters in the observable Universe(according to the current standard model roughly a sphere with a radius of 16 billion light years). The total number of galaxies in the observable Universe is estimated to be somewhere around 2 trillion. How many galaxies might exist beyond this cosmological event horizon of 16 billion light years is anybody’s guess.
So how many planets are in the Universe? Well, the question is almost pointless since there is no way to know; we can only make an extrapolation based on what limited information we have in the relatively small region of our galaxy. It seems pretty safe to say that there are a lot, an awful lot, of habitable planets out there in the Universe – this is a question that is probably unlikely to ever be fully answered.
Earth Like Planets
Many stars have been shown to have more than one planet orbiting it, much as the planets in our Solar System orbiting around our Sun. Of the various Sun-like stars which have been discovered, it is estimated that about one in five has at least one Earth-like planet orbiting in a habitable zone around the star – this simply means that these type of planets could conceivably support some sort of alien life, anywhere from the more simple to much more complex, perhaps even intelligent creatures. There are estimated to be about 200 billion stars in our Milky Way Galaxy – making an extrapolation from what has been discovered so far we can hypothesize that there could be as many as 40 billion Earth-like planets orbiting in habitable zones of stars, especially if we include non-Sun-like stars such as red dwarfs.
Earth Like Planets
We have been able to make some extrapolations from the increasing number of exoplanets that have been discovered in recent years that our Milky Way Galaxy should be full of billions of Earth-like planets – only a very very small number has been discovered thus far; far more will no doubt be discovered in future years. But what of the Earth-like planets that have been discovered so far? Well, here are just a few of the most Earth-like planets that we know about outside our Solar System so far.
Gliese 667Cc is a prime example of an Earth-like planet – it orbits the third star, Gliese 667C, in a triple star system called Gliese 667 in the constellation Scorpius which is about 23.62 light years from Earth. Gliese 667C is a red dwarf star with the planet Gliese 667Cc orbiting in the middle of a habitable zone – this planet is considered a super Earth-like planet with about 3.7 times the mass of the Earth and an equilibrium temperature of about 39.6 degrees Fahrenheit.
Kepler 22b is an exoplanet orbiting a Sun-like star, Kepler 22, in the constellation Cygnus about 638 light years from Earth. Its mass is roughly twice that of the Earth and it has an equilibrium temperature of about 12.2 degrees Fahrenheit. The composition of Kepler 22b is not believed to be like that of the Earth, although what it precisely is remains unknown.
Kepler 69c is another super Earth-like planet orbiting a Sun-like star called Kepler 69, visible in the constellation Cygnus and located about 2430 light-years from Earth. This Earth-like planet is believed to have a rocky surface with a mass of about 6 times that of the Earth and an equilibrium temperature of about 527 degrees Fahrenheit. In the case of this planet, Earth-like would likely refer to its size and composition compared to the Earth – it would seem quite apparent that the surface temperature would be far too much for life to evolve, but even this is not completely certain since this is a mean temperature which implies cooler areas on the planet.
Kepler 62f is a super Earth-like planet orbiting a star called Kepler 62, smaller and cooler than the Sun, which is about 1200 light years from Earth and located in the Constellation Lyra. This planet has a mass of about 2.8 times that of the Earth with an equilibrium temperature of about -85 degrees Fahrenheit. Despite the somewhat cool mean temperature, it turns out that Kepler 62f is one of the more promising Earth-like planets in terms of habitability, in large part because it is thought to be covered by a large ocean, which greatly enhances the chance of the evolution of alien life forms. Not only that, but Kepler 62 is thought to be a very stable star with a very long lifespan up to about 30 billion years which would help in the possible evolution of life.
Kepler 186f is another exoplanet which more closely resembles the Earth in size, with a mass roughly 1.4 times that of Earth. It orbits the red dwarf star Kepler 186 in a habitable zone about 582 light years from Earth in the constellation Cygnus. With an equilibrium temperature of -121 degrees Fahrenheit, although this is somewhat cool, it is still within a range where life could possibly
Another Earth-like planet close in size to the Earth is called Kepler 442b and orbits the star known as Kepler 442 in the constellation of Lyra about 1206 light years from Earth. It orbits Kepler 442 in the habitable zone of the star – it has a mass about 2.3 times that of the Earth with an equilibrium temperature of -40 degrees Fahrenheit. Because of its mass relative to volume, it is likely to have a rocky solid surface. More investigation needs to be done to determine its suitability for alien life forms.
There is another planet with Earth-like characteristics, Kepler 452b, that has been called Earths cousin, or Earth 2.0. It orbits in a habitable zone around a Sun-like star called Kepler 452 which is located in the constellation Cygnus about 1402 light years from the Earth. Its mass is approximately five times that of Earth but since its radius is only about 50 percent greater than Earths, it probably has a rocky solid surface. Something very interesting about this planet, and probably why it has been called Earth 2.0, is that it has a temperature equilibrium of about 17 degrees Fahrenheit which is a little warmer than the Earths, and it orbits around the parent star Kepler 452 about every 385 Earth days – very similar to the 365 days for the Earth to complete an orbit around our Sun.
It is not known whether or not this planet has a large amount of water on its surface which would be conducive to the evolution of alien life, but from what is already known it should certainly be examined more intensely in years to come.
It is now known that there is a very wide range of exoplanets. For example, some exoplanets are so close to their parent star that they complete an orbit in only a few hours, while others are so far away that it can take thousands of years to complete an orbit – some are so far out from the parent star that it is not definitively known if they are gravitationally bound to the star(in effect, part of the star system). The most massive exoplanet discovered so far is known as HR 2562b; this planet is in orbit in a circular debris field around HR 2562 and is believed to be about 30 times as massive as Jupiter, although its exact mass is unknown. In fact, it is so massive that it was originally thought to be a brown dwarf companion star to HR 2562, although now it is being classified as a planet. The smallest exoplanet in terms of mass discovered so far is named Draugr which is about twice the mass of the Moon. It is the innermost planet orbiting a pulsar star(a very rapidly rotating neutron star emitting a beam of electromagnetic radiation; the only time the star is visible is when this beam of radiation points toward the Earth) about 2300 light years from Earth in the constellation of Virgo.
The nearest known exoplanet is Proxima Centauri b, about 4.24 light years from Earth, orbiting Proxima Centauri B about every 11.2 days – it is about 1.3 times as massive of the Earth and is in a habitable zone around the star. All of the exoplanets definitively discovered so far are in our Milky Way Galaxy, although it is now thought that many more billions of exoplanets, many of them Earth-like, must exist in galaxies beyond our own galaxy. Since it is known now that there are many billions of other galaxies in the observable Universe alone, the total number of Earth-like worlds could conceivably be in such huge numbers that it would be almost incomprehensible – how many of these worlds could have intelligent life is anybody’s guess and really beyond speculation at the present time.
Rogue planets are planets that are free floating in interstellar space, not belonging to any star system. These wandering, sunless worlds actually directly orbit the center of the Milky Way Galaxy(and of course, other galactic centers as well), not bound by gravity to any other stars. It is thought that in our Milky Way galaxy alone there could be billions of rogue planets drifting around the galactic center, taking many millions of years to complete one galactic orbit. Needless to say, the very nature of these dark starless worlds drifting in the black void of interstellar space make them exceptionally difficult to detect.
As we have seen by the discoveries of various Earth-like planets in recent years, there is certainly a very significant possibility of some form of extraterrestrial life. If this life does exist, there is an open question of what form this alien life would take. Well, there isn’t any real answer to this question; we only have the life that has been studied here on Earth to be our guide. If extraterrestrial life does exist, which seems quite likely now in light of the increasing exoplanet discoveries, it could follow similar patterns to those of Earth, or be completely different altogether.
Exoplanets continue to be discovered at an accelerating rate; as we have better and more advanced instruments, such as space telescopes, an increasing number of planets outside our Solar System are being found. We already have a pretty good idea that with the billions and billions of Earth-like planets in the Milky Way Galaxy alone, not to mention billions of other galaxies in the Universe, there surely is a very high probability of extraterrestrial life forms.
But what of the existence of intelligent extraterrestrial, or alien, life? What are the probabilities of that being a reality? This is a much narrower question than that of extraterrestrial life in general. After all, intelligent alien life would require a much higher level of evolution and a lot more things would have to go right in that process for this to occur. The parent star would need to have a fairly high degree of stability, otherwise evolving life on a planet could be wiped out before reaching a high level. Other disasters could occur which could end life on such a planet – one example could be an asteroid impact – this is what many believed wiped out the dinosaurs on Earth. And if life on an exoplanet did evolve enough to reach an intelligent level, would these aliens acquire wisdom quickly enough to avoid destroying themselves someway – with nuclear weapons, nanotechnology, bioweapons, artificial intelligence, etc.?
It may well be that there have been many intelligent civilizations in our galaxy alone that have come and gone, to exist no more. There may have been highly advanced civilizations, perhaps much more advanced than our own, that existed many millions of years ago in the past, but which are extinct in the present time. We keep searching and searching the stars with our powerful radio telescopes, but the success of definitively finding an intelligent civilization among the stars has eluded us so far. But then again, maybe truly intelligent alien civilizations stopped using radio waves for communication long ago, it might be that they have developed a far superior method – neutrinos, tachyons(hypothesized – we don’t know if they exist or not), types of scalar electromagnetic waves, or even something else that we can not even imagine today. Surely our current state of physics is not the final one; we’ve only had automobiles, planes, and electricity for around a hundred years or a little better. It seems likely that the physics of several hundred years, a thousand years, or even more will be quite different from what we know today. Maybe faster than light communication and travel as well will be easily possible in the distant future – who knows?
Whatever the case, I really think it is very likely that intelligent life in the Universe, and our galaxy in particular, does in fact exist. Why don’t they contact us? Well, first of all, they might not have any more in common with us than we do with rats. Or it could be, as some believe, that they already have – a small percentage of UFO sightings continue to elude any conventional explanation. We don’t have the answers to these questions at the present time, and it may be a VERY long time before we do, but we’ll keep trying.