Planets concentrate the molecules needed for creating life
Life either began here on Earth after the development of a unique DNA, or was seeded by some kind of alien DNA carried in a comet or some other object. Whichever way life began on Earth, creating life from the genetic code in DNA is easier should enough of the raw materials and molecules are concentrated in a particular region of space. While certain types of clays can do this, so too can a rock the size of a planet because of its strong gravitational attraction. If scientists are to easily find and identify life as truly alien, we need to find planets as this is where we will find complex alien life.
Evidence for planets can certainly be found in our solar system, of which Earth definitely does harbour living things on its surface. However, for the most part, scientists often need a little more convincing. Thus the aim for many scientists today is to find evidence of a planet beyond our solar system at the right distance from the parent star and of the right size if they are to give any serious credence to the idea that advanced lifeforms may exist elsewhere in the universe.
Finding the world's first planet outside our solar system
At present, the most powerful optical, infra-red and radio telescopes on Earth cannot directly observe extrasolar planets, even of the size of Jupiter (the largest planet in our solar system). The intense glare of their parent-star and the great distance of the star from the Earth puts an unfortunate dampener on the scientists' party. In other words, the stars are usually too bright and the planets too close and/or small to detect. Astronomers would need a planet several times the size of Jupiter orbiting at a reasonable distance from a dimly glowing and nearby star if it is to be directly observed.
In the days of Giordano Bruno, he already knew how difficult it would be to detect extrasolar planets when he said in his 1584 book On the Infinite Universe and Worlds:
"...we discern only the largest suns, immense bodies. But we do not discern the earths because, being much smaller, they are invisible to us."
Things haven't changed a lot since those days. So clearly the search for extrasolar planets would have to be done in a more indirect manner.
In January 1983, the Infra-Red Astronomical Satellite (IRAS) provided scientists with evidence of what appears to be the presence of clouds of cool material forming a disc around stars. Such discs are thought to be conducive to planetary formation. There is even a strong possibility that some of the debris detected in the clouds might be of planetary size. However it isn't enough to prove conclusively planets exist beyond our solar system.
Then on 9 January 1992, the world's first undisputed evidence for a "solar system beyond our own" came in the most unexpected way. A group of astronomers from the Arecibo radio telescope in Puerto Rico announced in the British journal Nature the discovery of planets around a distant star called PSR1257+12, located at a mere 1400 light years away in the Virgo constellation.
PSR1257+12 is termed a pulsar by astronomers because of the pulses of electromagnetic energy it emits. This discrete emission of energy is due to its rapid gyroscopic motion that was created at some time in its history after undergoing a massive supernova explosion. In the case of PSR1257+12, the star spins on its primary axis at 161 times a second, which causes its accelerating mass around the equator to produce a powerful gravitational field. So powerful is this field that most of the light emanating from the equator is bent back on itself. Only the polar regions of the star, where the mass of the star accelerates the least, is the gravitational field at its weakest and thus light can escape into space like the beams of a lighthouse. The star then rotates about a second axis, which causes the cones of energy emitted around the poles to sweep across space and so giving the pulsar its characteristic on-and-off effect.
Early observations of these cosmic lighthouses have suggested that the timing for each pulse received was more accurate than a Swiss-made pocket watch. However, the astronomers noticed that the pulses of electromagnetic radiation emitted by PSR1257+12 were not exactly precise, but were arriving too early or too late by 1.5 milliseconds over a period of several months.
Dr Alexander Wolszczan, an astronomer from Pennsylvania State University, USA, has spent three years examining the pulsar's puzzling pulse rate. He attributes it to the doppler effect whereby a slowing down of the pulse rate is due to the pulsar moving away from us, and a speeding up of the pulse rate is due to the pulsar approaching us. It is as if something was pulling the star into tiny circular orbits. That something was later calculated to be of planetary size.
Astronomer, Dr George Gatewood of Allegheny Observatory in Pittsburgh, USA, explains it as follows:
"Imagine two people dancing a polka. As they whirl about to the music, each person - pulled by the other - moves in a series of circles across the floor. Now imagine that one of the dancers is invisible; we can still tell he is there by the motion of his partner."
A total of three planets have been detected so far around PSR1257+12. Two of the planets are estimated to have masses of 3.4 and 2.8 times that of Earth, orbiting the pulsar at distances corresponding to about where Mercury is in our solar system. The third one is much smaller. Calculations suggest this third planet is about 0.01 times the mass of the Earth and orbits the pulsar at a distance approximately half that of the other two planets.
Planets around pulsars
In 2006, scientists succeeded in discovering another phenomenon associated with pulsars. The accretion disc surrounding a pulsar can form new planets. As Deepto Chakrabarty of the Massachusetts Institute of Technology said:
"What's remarkable here is this process of planet formation — which we associate with the birth of stars — seems to also be able to occur at the end of the stellar lifetime, sort of a renaissance of the system, in some sense.
"It says that this planet formation process seems to be a much more robust process than we initially realised."
He supports this with observations of another pulsar in the Milky Way containing planets located 13,000 light years from Earth in the contellation Cassiopeia using NASA's Spitzer Space Telescope capable of tracking infrared light.
But don't expect life to suddenly arise on a planet around a pulsar. Deadly levels of high frequency x-ray and gamma radiation and the constant collisions on the surface of a planet from matter in the accretion disc would make it virtually impossible for life to take hold let alone reach any form of intelligence.
While all these observations of extra-solar planets might give greater confidence to the scientists that ETs probably exist, we have still to detect the existence of Earth-sized planets.
In 2004, a new technique showed it might be possible to detect extrasolar planets approaching the size of the Earth. Known as gravitational microlensing, this new method involves a phenomenon predicted by Albert Einstein's General Theory of Relativity whereby the gravitational field of one object in a solar system can bend and focus the light from another object lying directly behind it. When light is focused, there is a brightening of the object, detectable by Earth-based telescopes. This technique works best when the ecliptic plane of the extrasolar system in question is almost edge on to an observer on Earth and the star showing a brightening effect is small relative to the size of the other object.
As Ian Bond of the Institute for Astronomy in Edinburgh, Scotland, explained: "The real strength of micro-lensing is its ability to detect low-mass planets."
As a result of this technique, a planet was detected around a star lying 17,000 light years in the constellation Sagittarius. The planet is larger than Jupiter and lies three times farther from its star than the Earth is from the Sun. More details about this discovery and technique is available in the 10th May 2004 edition of Astrophysical Journal Letters.
Still not quite Earth-sized?
Well, on 26 January 2006, scientists officially announced the discovery of a planet approximately 5.5 times the mass of the Earth (but still considered much less massive than all the Jupiter-sized objects detected so far around other stars). This cool, rocky planet called OGLE-2005-BLG-390Lb (scientists must be running out of memorable names to give to so many astronomical objects) is orbiting a small red dwarf approximately one-fifth the mass of our Sun, taking 10 years to complete a revolution around the star. The planet lies at a distance equivalent to between Mars and Jupiter in our solar system. Scientists made the discovery in the constellation Sagittarius, not far from the central bulge of our galaxy. It is approximately 22,000 light years away.
What about a planet at the right orbit?
No doubt the scientists have listened and are working hard to serve the evidence on a platter.
A water-world Earth-sized planet?
On 25 May 2007, a team of European researchers led by Stephane Udry from the Geneva Observatory in Switzerland announced in the journal Astronomy and Astrophysics they had discovered using the HARPS instrument on the European Southern Observatory 3.6 metre telescope in La Silla, Chile, on 24 April 2007 the lightest exoplanet detected so far.
Orbiting the seemingly unremarkable red dwarf located 20.3 light years away in the constellation Libra, scientists initially claimed Gliese 581C is more Earth-like than any other extrasolar planet detected to date partly because of its mass (being the lightest of all exoplanets) but also it was thought the planet was situated just far enough away from its parent star to allow the right Earth-like temperatures (estimated in the range between 0 and 40°C) to permit water to exist in the liquid phase and hence most likely to support life.
More refined calculations in April 2009, however, have now made Gliese 581D, another exoplanet lying slightly further out from the red dwarf, to be more Earth-like in temperature.
A total of four exoplanets have been detected.
Gliese 581C is roughly 5 times as massive as Earth with about 1.5 times Earth's diameter. It orbits the red dwarf once every 13 days. Computer models created by Werner von Bloh of the Institute for Climate Impact Research in Germany and his team suggest this planet would be too hot to support liquid water (an important requirement for life to exist). While not as hot as Venus in our solar system, the planet is likely to have a runaway greenhouse effect exceeding 100°C forcing water to boil away and leaving behind a choking atmosphere of carbon dioxide and methane.
On the other hand, Gliese 581D is 8 times as massive as the Earth. So it should have no trouble retaining adequate quantities of water and other materials. Furthermore the water can potentially exist in liquid form assuming a mild greenhouse effect is present in the atmosphere, making it currently the best candidate for finding alien life albeit very primitive, possibly the size of bacteria (or very flat, multi-legged people with brains looking like pancakes). As Manfred Cuntz, an astronomer at the University of Texas at Arlington, USA, and a member of von Bloh's team said:
""This planet is actually outside the habitable zone. It appears at first sight too cold. However, based on the greenhouse effect, physical processes can occur which are heating up the planet to a temperature that allows for fluid water."
To further fuel the fire of potentially finding alien life on this planet, Jaymie Matthews, an astronomer at the University of British Columbia in Canada, has studied the red dwarf using the Canadian space telescope. He has realised how stable the red dwarf is. There were very few solar flares during the time he observed it over a period of 6 weeks and even in those instances the extra energy emissions from the star would quickly be dampened down by a thick atmosphere and deep ocean around Gliese 581D.
If that's not exciting enough, scientists have noticed the star is at least as old as our Sun. As Matthews said:
"We know it took about three and a half billion years for life on Earth to reach the level of complexity that we call human, so it's more encouraging for the prospects of complex life on any planet around Gliese 581 if it's been around for at least as long."
While scientists are not expecting life around Gliese 581D to be terribly advanced, the search is definitely on for a planet that looks exactly like the Earth at the right distance around a more Sun-like star. If the trend in the evidence so far gathered is anything to go by, somehow it seems likely the arguments of some ancient Greek philosophers for an Earth-like world will eventually come to fruition.
The Kepler Mission - Searching for Earth-sized planets
The search for Earth-like planets around single Sun-like stars has reached fever pitch.
The increasing confidence among a number of scientists in finding Earth-size planets like our own around other stars has pushed our technology to the limits.
Now NASA has come up with a new technology to enable the detection of such planets to be made a little easier. Sent into space on 9 March 2009, the Kepler space telescope is designed to detect planets as small as 0.5 times the mass of the Earth around other stars (although with a bit of imagination from the NASA scientists we hope the stars will be more Sun-like for the sake of determining the likelihood of finding advanced alie life in our universe). If exoplanets between 0.5 and 2 times the mass of the Earth exists in what scientists call the habitable zone, it will mean the chances of finding ETs will be bumped up by a few orders of magnitude thank you very much.
The Kepler probe carries a 0.95-metre diameter Schmidt telescope with an array of highly sensitive charge coupled devices (CCDs) for measuring the light from the stars called a photometer or light meter. It is designed to take in up to 100,000 stars in one area of the sky along our Milky Way arm where we are situated (in the constellations Cygnus and Lyra).
And it needs to be able to gather this many stars. Apart from collecting enough information for statistical purposes, it should be remembered that the technique does require enough stars in the field of view of the telescope's eye to help detect enough planets around stars where the solar systems are edge-on to our field of view (these distant planets have to pass in front of the stars for the probe to do its job). If the exoplanetary solar systems are not aligned edge-on, the technique will not work.
Also the technique works best when the telescope is orbiting in space because we know on Earth how stars can change their brightness very quickly, especially near the Earth's horizon when we look up at the night sky. This flickering effect is caused by the Earth's atmosphere as the air flows and there is a variation in temperature at different altitudes. But in space, there is no air. Stars will shine constantly in space unless they undergo a supernova explosion or some other reason for varying the brightness (e.g., a planet moving in front of a star). As NASA said:
"The photometer must be space-based to obtain the photometric precision needed to reliably see an Earth-like transit and to avoid interruptions caused by day-night cycles, seasonal cycles and atmospheric perturbations..." (1)
With the telescope placed in an Earth-trailing heliocentric orbit, it will maintain its view of a large number of stars throughout the mission, observing any variation in the brightness of stars. Should there be a consistent cyclic reduction in brightness over time before returning to normal, it is likely an exoplanet has moved across the face of the star. Once this information is obtained, it is possible for scientists to use Kepler's Third Law of planetary motion to calculate the size of the planet's orbit (i.e. the time it takes to go around the star) as well as the mass of the star itself. Finally the size of the planet is determined simply by how much of a reduction in the brightness of the star has occurred with respect to the size of the star. Then scientists will know whether the orbit of the planet is within the habitable zone and is of the right size to potentially harbour life.
Time will be a crucial factor in this experiment as any exoplanet detected could involve more than one planet, or the star could show evidence of serious sunspots moving over its surface. However, given enough time, the larger exoplanets lying further out from the star will have moved out of the way and the sunspots would disappear leaving behind the smaller rocky planets nearer the star. Then we will know what we have got. The first results won't come for at least a year and probably up to 5 years for the results to be reliable and consistent. (2)
And even if we do find an Earth-sized planet of the right size and distance as the Earth is from our Sun, what then? Can we learn anything more about the planets?
First Earth-like exo-planet in a habitable zone found
Yay! It has taken a little while, but at last scientists have confirmed the existence of the first Earth-sized planet within the habitable zone of a star. And no, it isn't our own planet (a welcome change). The Kepler space telescope has detected a planet of the right size as our own lying at a distance of 490 light years away. And more amazing, it actually sits at the right distance from its parent star known as Kepler-186 (it is the fifth and outermost planet in this system) to allow water to exist in liquid form for a very long time. Elisa V. Quintana of the SETI Institute at NASA Ames Research Center in Mountain View, California, USA, gave her backing to this claim when she said:
"This is the first definitive Earth-sized planet found in the 'habitable zone' around another star. Finding such planets is a primary goal of the Kepler space telescope. The star is a main-sequence M-dwarf, a very common type. More than 70 percent of the hundreds of billions of stars in our galaxy are M-dwarfs."
Not exactly a sun-like star like our own. But then as they say, beggars can't be choosers. At any rate, there is still a high probability that primitive alien life does exist on the surface of this distant world even if the star is a typical red dwarf commonly found throughout the Milky Way. This is especially true since the distance from the star is enough to prevent a "tidal locking" situation where the planet stops rotating and has one side constantly facing the star leaving one side very warm or hot and the other size very cold (unless liquid oceans can even out the temperature variations). Another problem is that red dwarfs tend to be of the flare-up variety and there is a risk this star might just be one of those types. At the moment there is no evidence to suggest this, making it one of the more stable red dwarfs around.
Of course, all this assumes the planet is not depleted of important greenhouse gases in the atmosphere that might lead to an irreversible Ice Age. Oops, we shouldn't let that one get out of the bag. Otherwise we will need yet another piece of technology to observe the atmosphere of this planet and use spectroscopic analysis to determine the atmospheric composition to see if it contains the right chemical elements.
Probably another NASA project to come in the next 25 years.
Still, if you need something to keep you awake, you can read the latest results from the Science journal, Volume 344, Number 6181, pp.277-280, published on 18 April 2014, under the title, "An Earth-Sized Planet in the Habitable Zone of a Cool Star". An online version can be found here.
In yet another effort, the successor to the Hubble Space Telescope known as the James Webb Space Telescope is expected to have enough viewing power to directly observe Earth-like planets in nearby stars. It will be so powerful that scientists believe they can make an analysis of the atmospheres of these distant planets to help determine the likelihood of finding life beyond our solar system. Unfortunately, it won't have the resolving power to observe a planet like Kepler-186f discussed above a distance of 490 light years away is just a bit too much. A star within 12 light years? Now that's a more realistic achievement.
Not to be outdone in this field, we now have NASA considering the idea of combining a suite of the latest and most sensitive space telescopes with revolutionary new imaging technologies to allow NASA to study all aspects of planetary formation outside our solar system. The system will be so sensitive, NASA claims it will be able to photograph planets as small as the Earth in the habitable zones of distant solar systems. And, using spectroscopic analysis, scientists should be able to analyse the atmospheres of exoplanets by measuring the amount of gases like carbon dioxide, water vapour, ozone and methane. Then atmospheric chemists can give a probability of just how likely extraterrestrial life exists. The system is known as the Terrestrial Planet Finder (TPF).
Until the technology is built, we now know enough planets exist beyond our solar system. Between 1995 and 2005 over 170 exoplanets have been discovered, as of 2009 the figure has reached 300, and by April 2010, the figure has shot up to 443 orbiting around 350 or so stars. And not all are the massive Jupiter-sized bodies scientists had first come to observe. It is looking pretty clear at this stage that extrasolar planets of various sizes are common throughout the Universe, including Earth-sized ones. And already the first has been found in the habitable zone. It won't be long before these sorts of planets will become routinely detected around more and more stars in our Milky Way and soon we will be counting them like they were confetti.
Until then, all we need is some bright scientist to figure out how to venture to the stars in a technology that would allow humans to participate in the journey. Surely it can't be that hard to do?
What will alien solar systems look like?
With the prospects of finding extrasolar planets looking good with each passing year, including direct observations of protoplanetary systems in the Orion Nebula with the help of the Hubble Space Telescope, there are two other conditions that must be met if we are to have an excellent chance of finding sufficiently complex and intelligent life on another planet. The first condition is that the planet must have a cool solid rocky surface not only to protect life from the heating effects of hot molten iron inside the planet but also for intelligent life to emerge from the oceans and eventually develop a technology. And the second condition is that there is an optimum planetary size to ensure essential raw materials to support life are retained on the surface.
Are there planets in the universe satisfying these two conditions? Already scientists have found a rocky planet about 5 times the mass of the Earth in January 2006. That's an excellent start. And now Steven Dole thinks they should be common throughout the universe.
In an attempt to simulate solar system formulation, Dole fed all the available astronomical theories on planetary formation into his computer and let the machine create models of solar systems that could exist in the universe. The result was this: All the planetary systems looked remarkably similar to our own (shown below). Furthermore, there will be rocky planets located close to a star.
Below is typical of the results obtained by Dole:
Will alien Earth-like planets have similar mineral contents to our own planet?
While the chances of finding ETs are looking mightily better the further scientists look into this issue, not every scientist in the world is completely convinced. Maybe all the observations and computer simulations are just lucky results or a pure coincidence in the minds of some of these hard-headed rational people? Okay. So what about having the right mineral content to Earth? Are there Earth-like planets out there that could look very similar to Earth with the right minerals, and how common could they be? The importance of the right minerals and ratio of those elements cannot be underestimated in the search for ETs. As astronomer Professor Brad Gibson of the University of Hull in the U.K. realised:
"The ratio of elements on Earth has led to the chemical conditions 'just right' for life. Too much magnesium or too little silicon and your planet ends up having the wrong balance between minerals to form the type of rocks that make up the Earth's crust. [Moreover,] too much carbon, and your rocky planet might turn out to be more like the graphite in your pencil than the surface of a planet like the Earth."
Can science provide the answer?
Well, ask and you shall receive!
Apparently a new computer simulation with more detailed and up to date data on planetary formation and the typical composition of nebulas has surprised astronomers involved in the study after observing the results on the computer screen. It would appear that nearly all rocky planets, including Earth-like planets with similar planetary sizes as our own planet, will have remarkably similar mineral content and ratios. Earlier calculations had previously indicated that only one in three Earth-like planets will be a close match. Now the prospects of Earth-like planets looking very similar to ours has dramatically increased in favour of finding ETs.
The astronomers were not expecting the results to be what they were. As Gibson, a member of the study group who conducted the simulation at the E.A. Milne Centre for Astrophysics located in the University of Hull, said:
"At first, I thought we'd got the model wrong! As an overall representation of the Milky Way, everything was pretty much perfect. Everything was in the right place… But when we looked at planetary formation, every solar system we looked at had the same elemental building blocks as Earth, and not just one in three. We couldn't find a fault with the model, so we went back and checked the observations. There we found some uncertainties that were causing the one-in-three result. Removing these, observations agreed with our predictions that the same elemental building blocks are found in every exoplanet system, wherever it is in the galaxy."
However, as Gibson added, "even with the right chemical building blocks, not every planet will be just like Earth."
Gibson clarified what he meant by this when he said:
"Conditions allowing for liquid water to exist on the surface are needed for habitability," he said. "We only need to look to Mars and Venus to see how differently terrestrial planets can evolve. However, if the building blocks are there, then it's more likely that you will get Earth-like planets and three times more likely than we'd previously thought."
The fascinating findings were presented on 8 July 2015 at the U.K.'s National Astronomy Meeting in Llandudno, Wales.
If, after all of this, scientists are still not totally convinced that ETs probably exist in the universe, you can be sure more computers simulations and observations of the universe will keep them busy over the coming years.
Or maybe the question we should be asking is, is the evidence already here to prove ETs existence?