Exoplanets are planets that are found outside our solar system and thus may be called extrasolar planets. Thousands have been discovered and millions more will be detected in the future owing to our vast universe and our constantly improving detection technology and techniques.
Due to the minimal technological advancements of earlier centuries little confirmation of the existence of exoplanets was made compared to today; exoplanet detection in current times is now on an exciting exponential curve.
The discovery of these heavenly objects started in the 16th century at around 1584 when an Italian Catholic monk, Giordano Bruno supported Corpernicus’ theory that other planets and Earth exist in orbit around the sun. Isaac Newton also quoted in the General Scholium that if fixed stars are centers of similar systems, they will be constructed according to a similar design and subject to the dominion of one.
In the early history of these discoveries there were obviously many false claims from the 19th century; for example Captain W. Jacob of the East India Company’s Madras Observatory had some claims that the Binary 70 Ophiuchi has a planetary body within its system and Thomas J.J. Seo of the University of Chicago supported Captain Jacob’s findings saying that there was evidence of a dark body in the system with a 36-year old orbital period. Despite all this, Forest Ray Moulton discredited these findings publishing that the orbital parameters theorised for that system would make it unstable. There were other false claims; in the 1960s, Peter Van de Kamp claimed to detect two exoplanets around Barnard’s star but failed to verify it; in 1980, Dr. Bradford A. Smith of University of Arizona and Dr. Richards Terrile made infrared Sightings of a disk of dust around Beta Pictoris Star but failed to prove it was an exoplanet.
In April 22nd 1992, radio astronomer Aleksander Wolszczan and Dale Frail detected two extrasolar planets orbiting the pulsar 1257+12 (an unusual supernova remnant) and was considered the very first credible discovery but in actuality, the very first was in 1988, by the Canadians Bruce Campbell, Stephenson Young and G.A.H. Walker from the University of Victoria and The University of British Colombia. Due to the use of primitive methods such as radial velocity, they remained skeptical of their findings. During the ‘90s reports came out to support and also to discredit these findings but in 2003 the exoplanets were accepted as orbiting the Gamma Cephei Star.
Other discoveries include the one by the Swiss team Michel Mayor and Didier Queloz of an exoplanet orbiting a main-sequence G-type Star (51 Pegasi) on 6th October 1995. They were supported by Geoffrey W. Marcy’s team as well as Paul Butler who confirmed the exoplanet’s existence. In 1999, Upsilon Andromedae discovered the first main-sequence star with multiple planets.
From radial velocity to advancements such as high-resolution spectroscopy and measuring gravity influence on the motion of an exoplanet’s parent star, has revolutionized extrasolar planet discovery. With France's CoRoT mission in 2006 and NASA’s Kepler in 2009 the rate of exoplanet detection has risen and by the 25th November 2013, 1048 exoplanets had been discovered in 794 planetary systems.
How many Exoplanets have been discovered?
Currently, there are 1048 extrasolar planets in 794 planetary systems which have been verified. Massive ground has been covered by the Kepler mission by NASA: as of the 4th of November 2013 they had 3568 exoplanet candidates still to be proved and verified, but owing to the technological capabilities of the Kepler project most of them are sure to be exoplanets. Most of the extrasolar planets are giant planets and size up to our very own Jupiter and Neptune.
In October 2013, 990 exoplanets were confirmed by size as follows; 148 of these exoplanets are roughly the size of Neptune, 7 are the size of Mars, 711 of them are Jupiter sized (meaning most exoplanets tend to have a large radius), 3 of them are Mercury sized, 11 of them are the size of Earth and 110 of them are super-earth sized. The term super-earth refers to a planetary body where the mass of the exoplanet is slightly higher than that of the earth but still lower than that of the gas giants such as Uranus.
There have been so many discovered exoplanets now and each is given its own unique name. Some of the exoplanets include 2M 0746+20 b with a radius of 0.97, BD-061339b with a radius of 1.3, CoRoT-25b with a radius of 1.08, HATS-3 b with a radius 1.381 and Kepler-53b with a radius of 18.6489525. The radii referred to here is a comparison with the radius of Jupiter and as you can see, the extrasolar planets are also named in accordance with the laboratories or discoverers who detected them; NASA's Kepler mission has named their findings Kepler, then a number e.g. the Kepler-53b.
One of the first exoplanets to be discovered was the one around Gamma Cephei AB which was discovered by radial velocity variation. Another was the HP 114762 b which was first thought to be a brown dwarf but later proven to be an exoplanet with an 89 day orbit span. In 1995, 51 Pegasi b was discovered as the first exoplanet orbiting a main –sequence star with a 4.2 day orbit. Gliese 876b is also another 90s exoplanet which was the first exoplanet that was orbiting around a red dwarf star.
Coming into the 21st century, extremely rapid discoveries were taking place due to the technological improvements in the field. Some examples of these 21st century exoplanets include the oldest exoplanet yet. It was discovered using the Hubble Space telescope by Steinn Sigurosson and his team.
NASA’s Kepler mission has also been at the forefront of these extra solar planets with quite a few discoveries such as Kepler 11 on 3rd February of 2011 and the most recent Kepler-69c on 7th January of 2013 using the Kepler mission space observatory. There are still thousands upon thousands, upon thousands of exoplanets out there in the universe just waiting to be discovered, so it won’t be long until the current statistics and numbers change.
The vast majority of known exoplanets were confirmed in October 2013; illustrating the exponential rate of discovery which is characterizing the field today. One can only imagine the wealth of worlds still to be discovered, and perhaps even travelled to, in the not too distant future.
Earth-like Exoplanets
When an exoplanet is considered to be Earth-like, it can be seen from two angles or perspectives. Earth-like can refer to both the size and the topography of the planet, or it can also refer to its general habitability properties for Earth based life. On the 4th November 2013 the Kepler mission astronomers reported that there could possibly be 40 billion earth-sized planets orbiting both sun-like stars and red dwarfs within their habitable zones. The habitable zones are regions around a particular star where a planet with a sufficient amount of atmospheric pressure is able to maintain liquid water on its surface and thus it is a rough Earth analog and it can thus potentially favor life. This is also known as the CHZ (Circumstellar Habitable Zone)
Erik Petigura, a Berkeley grad student, decided to write an algorithm to apply to data that he downloaded from the Kepler mission log on the same topic: combing the data for signs of Earth-like extrasolar planets. They pulled out over 16000 transit star events and after through each of these visually, Petigura and his team determined that there were around 603 earth-like extrasolar planets in the Kepler data alone.
Some of the extrasolar planets that are known to resemble earth’s topography making them terrestrial include the Alpha Centauri Bb which has a surface temperature of 1200k. Another is Tau Ceti e, a thermo planet which means it orbits within the inner edge of the habitable zone of its star. However Tau Ceti f, another earth-like extra solar planet, is a psychroplanet, which means that it orbits on the outer part or edge of the habitable zone of its star. Another example of these psychroplanets includes the Gliese 581 d.
Other examples of Earth-like planets include Gliese 876d, 82 G Eridani b, HD 40307 c and HD40307 d, but since they orbit too close to their respective stars, they have a very high atmospheric temperature. Additionally, there is HD 85512 b which is a thermo planet and until recently it was considered to be the best candidate for a habitable extrasolar planet in comparison to Earth, until the discovery of Gliese 667c. A mesoplanet which was identified to be even more Earth-like than HD 85512 b. Many of these mesoplanets have mostly been discovered over the past six years as very good candidates of prime Earth-like extrasolar planets that can support life. Some of these mesoplanets include K01-1686 01, the Kepler-62 e, Gliese 667 Cc, the Kepler-61 b and the Kepler-22 b. All of these extrasolar planets are described as being a warm superterran in comparison to earth, which is described as a warm terran planet.
All of these discoveries and analyses, the majority of which are a result of NASA’s Kepler mission, are a bid to find an analog to our own planet Earth in our galactic neighbourhood. It is likely that humanity will become a two planet (or more) civilisation in the future, perhaps due to the immense degradation that is occurring on our home world.
Gliese 581 D
This is one of the many Earth-like exoplanets or extrasolar planets that humanity has been able to discover. It is like Earth in so many ways. Though it is yet to be proven to be a terrestrial extrasolar planet (being rocky and having a topography similar to Earth), it has still shown signs of being so. It is also within the Circumstellar Habitable Zone (CHZ) of the star that it orbits, Gliese 581. This means that it has the ability to maintain water in a liquid state on its surface, given that it has the sufficient atmospheric pressure to maintain liquid H20. It is thus among the dozen or so exoplanets that have been confirmed from the 54 candidates by the various institutions such as CoRoT in France and NASA’s Kepler mission project.
Gliese 581 d and the rest of its planetary system is located roughly 20 light years away from earth, in the well known Constellation of Libra. It was considered a super earth (an extrasolar planet having a higher mass than earth but a significantly lower mass than that of the solar system’s smaller gas giants which are Neptune and Uranus) by its discoverers, who were made of a team from the Geneva Observatory which was headed by Stephane Udry from Switzerland. This took place in April of 2007. At first the team had concluded that Gliese 581 d lied just outside of the habitable zone thus making it too cold to maintain liquid water, but two years later on April 2009, they reassessed their findings and with more observations determined that it lied just at the edge of the habitable zone, with an orbital period of 66.87 days rather than the original analysis. Stephane also detected that there could be at least one or more large oceans on this exoplanet, making it one of the first discovered super Earths within a habitable zone that also has a large water mass.
Gliese 581 d was also discovered with its sister extrasolar planet, Gliese 581 c, using the HARPS instrument which is in the European Southern Observatory. Stephane and her crew took the 3.6 meter telescope to La Silla, Chile, to discover this extrasolar planet using radial velocity. Its system has two working models of either four planets, where Gliese 581 d would be the third planet, or six planets, where Gliese 581 d would be the fifth planet.
This exoplanet’s star has a magnitude of around 10.5 mv, a mass of 0.31 M, which is the solar mass, a radius of around 0.29 R, which is the solar radius, and a temperature of approximately 3480 K, which is the temperature in Kelvin. The star is relatively old with an approximate age of seven to eleven gigayears. A gigayear is a unit that refers to one billion Earth years. Gliese 581 d orbits its star at distance of 0.21847 AU. Gliese 581 d is an extraordinary extrasolar planet and many of its characteristics are yet to be discovered in more detail.
Exoplanet HD 58812 B
This is one of the many discovered exoplanets; and not only that, it is Earth-like (meaning that it has exhibited characteristics that make it similar to our own earth). This is because this exoplanet is found just at the edge of the Circumstellar Habitable Zone. This means that it is found in a region around its star where, with sufficient atmospheric pressure, this extrasolar planet can be able to maintain water in its liquid state just like earth can. This exoplanet can therefore be potentially favorable to support Earth-based life. It has an earth-like analog and it is one of the few that have been discovered over the years.
It orbits a K-type red dwarf star called Gliese 370 and it is found at a distance of 36 light years away from earth. It is situated within the Vela Constellation. The exoplanet’s discoverers deemed it to be a super earth exoplanet as it is around 3.6 earth masses (it is a super earth when it has a mass that surpasses that of earth but still has a smaller mass than the solar system’s gas planets that are known as Uranus and Neptune). It is one of the smallest exoplanets to be discovered at the very edge of the Circumstellar Habitable Zone (CHZ). It has a surface gravity of 1.4 g or more and an average temperature of 289 Kelvin; which is a very good range similar to Earth's. It is considered to have the same temperature as the Rhone-Alpes in Southern France, thus showing how similar it is to our own home planet.
It was considered to be the very best candidate for life habitability along with tGliese 581 d on 30th August of 2011, but in recent years it has been surpassed by Gliese 667 c, which is a mesoplanet (meaning that it has a surface temperature between zero degrees and fifty degrees). Gliese 667 c having this characteristic easily trumped HD 58812 b which is a thermo planet (exoplanets that have a surface temperature that is between 50 degrees and 100 degrees). With its given temperature HD 58812 d is able to stay hot enough to sustain water, and if it has a similar atmospheric composition to our own planet it will be a good alternative to earth.
Its parent red dwarf star has a solar mass of 0.69, an average temperature of 4715 Kelvin (give or take 102 K) and has an age of around 5 gigayears (this is a unit of time that refers to 1 billion earth years). HD 58812 d orbits its star at a distance of around 0.26 AU and it has an orbital period of fifty four days. It does this with a speed of around 94.913 kilometers per second.
HD 58812 d also went through a phenomenon during its formation that is known as tidal lock. This occurs when the gravitational gradient makes one side of a particular body such as a planet always face another. The phenomenona occurring there are myriad, and are yet to be fully discovered.
Kepler 62 E
This is another one of the many examples of extrasolar planets that have been discovered in the world today. What makes this exoplanet more unique than other extrasolar planets is that it is earth-like in so many aspects. This means that it shares or resembles the characteristics that Earth as a planet exhibits. These similar characteristics may be in relation to two aspects; either the extrasolar planets share a near similar topography to Earth or the same composition, which makes the exoplanet terrestrial, or the extrasolar planet is found within the Circumstellar Habitable Zone, which means it can support life. The Kepler 62 e falls into both categories of the characteristic spectrum making it a very strong candidate indeed for an Earth-like planet.
Kepler 62 e is a proven terrestrial extrasolar planet which means that physically it resembles Earth to the degree that it is nearly identical to Earth. This is evident in Kepler’s findings that this exoplanet ranks on the Earth similarity index at 0.83 out of 1. Kepler 62 e also has a rocky topography which is primarily composed of silicate and iron.
Kepler 62 e is also a super earth sized extrasolar planet which means that it has a larger mass than earth but it still has a significantly lower mass than the solar system’s gas giants Uranus and Neptune. It orbits around the Kepler 62 star and this whole system is found in the Lyra constellation. Kepler 62 e as well as its parent star and its sister planets are about 1200 light years away from earth. It was discovered on the 18th of April, 2013, along with its parent star and its other four sister planets. The discoverer, obviously owing to its name, was NASA’s Kepler mission spacecraft. Its other most notable and well known sister exoplanet is Kepler 62 f. The Kepler 62 e is located within the habitable zone of its planetary system. This simply means that it is an extrasolar planet that is found in a region around its parent star where a planet with sufficient atmospheric pressure develops the ability to maintain water in a liquid state on its own surface, and thus possesses a similar atmospheric composition to our planet Earth. Kepler 62 e is one of the few proven and confirmed extrasolar planets that exist within the habitable zone.
Kepler used a method known as the transit method to discover Kepler 62 and its planet: whereby the dimming effect an exoplanet causes as it crosses in front of its parent star is measured. Coupled with the transit method, they also made use of the transit timing variation. Kepler 62 e is a large exoplanet, averaging nearly 60% larger than earth’s size. It is able to orbit around its star in 122 days and has an age of around 7 gigayears. It has an age of around 7 gigayears with a stellar flux of around 1.2 the times of earth and its radius is around 1.61 solar mass. Kepler 62 e is a magnificent discovery and more of it is still being explored.
Gliese 876 D
This is a well known Earth-like extrasolar planet. It was discovered on 13th of June, 2005, by Eugenio Rivera and a team that he assembled at the California and Carnegie Planet Search. He asserted that Gliese 876 d was an earth-like exoplanet in many ways. The extrasolar planet clearly lied within the Circumstellar Habitable Zone. It was found in a region around its star whereby, with adequate atmospheric pressure, this extrasolar planet can maintain water in a liquid state just like on Earth. In this view this exoplanet can therefore be potentially favorable to support life due to water being essential for carbon based life as we know it. Gliese 876 D is therefore said to be an Earth-like analog. Furthermore, the exoplanet is also described to be a terrestrial one. This is due to it being a rocky planet just like Earth, suggesting that it has a similar topography and composition to earth’s surface.
Gliese 876 d is the third planet that is orbiting its star, the red dwarf star, Gliese 876, in its planetary system. It is still considered to be the exoplanet with the lowest known mass in the habitable zone, along with the extrasolar planet PSR B1257+12 which orbits a pulsar. Gliese 876 d and its planetary system are 15 light years away from earth in the Aquarius constellation. If you know anything about Western Astrology then you will know that Aquarius is the eleventh sign of the zodiac which means water carrier.
It was discovered by analyzing changes in its parent star’s velocity (Gliese876), as a result of Gliese 876 d’s gravity pull. However this technique is only able to measure the lower mass limit of the extrasolar planet. This was done by observation of the Doppler shift in Gliese 876’s spectral lines. This extrasolar planet orbits at a distance of 0.021 AU from its star with an orbital period of around two days. It has a mass of around 6.83 earth masses which definitively makes it a super earth sized extrasolar planet. This means that it has a mass that surpasses that of earth, but has a smaller mass than the solar system’s giant gas planets that are known as Uranus and Neptune. Gliese 876 d has an estimated temperature of 430 to 650 Kelvin. The temperature as well as the composition of the extrasolar planet and its radius is not specified due to the mode in which the planet was discovered. Since Eugenio and his team used the effects Gliese 876 d’s gravity pull had on its star, Gliese 876, clear calculations of the above units were hard if not impossible to do.
The discovery team also believed that due to its assumed mode of formation, this extrasolar planet would have a pressurized ocean of water and an atmosphere which contained water vapor and free oxygen, which would come from the breakdown of ultraviolet radiation. That is Gliese 876 d for you - a prime Earth-like planet canditate.
Gliese 581 C
This is an example of another earth-like exoplanet which has been recently discovered. This simply means that it has near similar condition to our own planet, Earth. It has been to be proven to be a terrestrial extrasolar planet (being rocky and having topography similar to earth) and is thus believed to have a similar composition to the surface of the Earth. It is also within the Circumstellar Habitable Zone (CHZ) of the star that it orbits, which is Gliese 581. This means that it has the ability to maintain water in a liquid state on its surface, given that it has enough atmospheric pressure to support liquid H20. Unlike its sister planet Gliese 581 d, Gliese 581 c has an atmosphere that may be able to support extramophile forms of life from earth (extramophiles can survive in very extreme conditions that are unfavorable to most animals). It is thus among the dozen or so exoplanets that have been confirmed from the 54 candidates by the various institutions such as CoRoT in France and NASA’s Kepler mission project. In this aspect it joins its well known sister planet Gliese 581 d.
Its discoverers were a team from the Geneva Observatory which was headed by Stephane Udry from Switzerland. This took place on the 27th of April 2007. They actually discovered it on the 4th of April that year but released their findings only on 27th. It is the smallest known extrasolar planet that is known to orbit around a main sequence star, Gliese 581, making it the 89th closest planetary system to the sun known to date. Using the HARPS (High Accuracy Radial Planet Search) instrument which is in the European Southern Observatory, Stephane and her crew took the 3.6 meter telescope to La Silla, Chile, to discover this extrasolar planet using radial velocity. This radial velocity method involves measuring the minute periodic movements around Gliese 581 which acts as a common center. They used what is called a Keplerian solution to measure Gliese 581 c’s mass and they determined it to be approximately 5.6 earth masses making it a super earth (it is a super earth when it has a mass that surpasses that of earth, but still has a smaller mass than the solar system’s gas planets Uranus and Neptune).
Gliese 581 c has an orbital radius of about 0.072993 AU which is about 11 million kilometers, about 7 percent that of earth’s. This exoplanet also has an orbital period of about 13 days. Its system has two working models of either four planets, or of six planets. With the four planets model Gliese 581 c would be the second planet and in the six planet model it would be the third planet. According to Udry if this exoplanet is terrestrial it will have a large iron core and a radius approximately 50% larger than earth’s.
The Gliese exoplanets are a fascinating bunch with a lot of information and discoveries still to offer in the future.
Alpha Centauri
This is the nearest exoplanet that is known to humanity, and the Alpha Centauris system is conveniently also the best candidate for a twin Earth yet to be discovered. This extrasolar planet orbits the K-type main sequence star Alpha Centauri B and it and its planetary system are just 4.3 light years away from Earth; which is incredibly close by galactic standards. Its discovery was announced on the 16th October 2012 by a team that is made up of researchers from the Geneva Observatory, and the University of Porto, Portugal. This team was led by chief researcher and graduate student Xavier Dumusque. He classified this extrasolar planet as being Earth-like in that it was terrestrial (this means that it had a rocky topography, indicating that it was very similar to our own planet Earth). This was indicated by the mass of this extrasolar planet. It was however not within the habitable zone. The habitable zones are regions around a particular star where a planet with a sufficient amount of atmospheric pressure is able to maintain liquid water on its surface and can thus be a potentially life supporting planet. This is also known as the CHZ (Circumstellar Habitable Zone). It is too close to its star with an orbital radius of only 0.04 AU making it rather hot.
Alpha Centauri Bb has an orbital period of about 3.2357 days owing to its small orbital radius. Since its orbital radius is small, the temperatures at the surface of this exoplanet reach about 1200 degrees Celsius which is above the melting point of most of the silicate magma. This gives Alpha Centauri Bb a special name: the Lava World due to it molten state. It has a mass of about 1.13 earth masses which categorizes it as a super earth sized extrasolar planet. A super earth sized extrasolar planet is one having a higher mass than earth but a significantly lower mass than that of the solar system’s smaller gas giants Neptune and Uranus. It has stellar flux of 312 F.
Alpha Centauri Bb was discovered together with its sister exoplanet Alpha Centauri A. Its discovery was achieved through the radial velocity method. Using the HARPS (High Accuracy Radial Planet Search) instrument which is in the European Southern Observatory, Xavier and his crew took the 3.6 meter telescope to La Silla, Chile, to discover this extrasolar planet using radial velocity. This radial velocity method involves measuring the minute periodic movements around the Alpha Centauri star which acts as a common center.
The planets parent star has a mass of 0.934 solar mass, a radius 0.863 solar radius and a temperature of around 5124 Kelvin. It and its planetary system have an age of about 4.5 to 7 gigayears. (Gigayears refer to a billion earth years). This exoplanet is a remarkable one in that it has revolutionized our understanding of exoplanets, since it is so close to us.
The Future of Exoplanet Discovery
The past, present and future of extrasolar planets is very diverse and many factors have affected the development of this amazing field. This is due to the fact that there have been countless and rapid advancements recently made in the field of Extrasolar Planet discovery. As of now there are 1048 exoplanets that have been discovered in 794 planetary systems all over the Milky Way galaxy, and this was as of the 25th of November, 2013.
At first there were only a few extrasolar planets being discovered annually. This is due to the rapidly accelerating level of technology, equipment and detection techniques.
From the first detection of the first exoplanet in 1988 by the Canadian astronomer Bruce Campbell, Stephenson Young and G.A.H. Walker from the University of Victoria, to the most recent discoveries such as Kepler 62 e, Kepler 62 f and Kepler 69 c, the rate of discoveries has radically increased.
For example, the detected exoplanets that are orbiting Gamma Cephei and discovered by Bruce Campbell were discovered through radial velocity, which was very popular at the time but also primitive. However, more recent extrasolar planet discoveries have utilized high quality state-of-the-art technology such as that in use for NASA's Kepler mission. This machinery is able to search through the galaxy faster and at a better accuracy so as to get maximum results in a very short period of time. The average current detection rate is now 50 new extrasolar planets being discovered every year.
On the other hand, Earth like extrasolar planets are a very different story all together. These exoplanets are much more difficult to detect and classify. They all fall into the category of extrasolar planets first, and that is why there is a high rate of detection when it comes to simply exoplanets; but determining whether the extrasolar planet is viable Earth-like canditate becomes difficult due to the advanced methods required.
All the extrasolar planets that have also been categorized as Earth-like, have been classified during the past three years.
There are now over 54 Earth-like extrasolar planet candidates, but over these 3 years only about a dozen have been proven to actually qualify as an Earth like exoplanet. These include Gliese 876 d, Gliese 581 d, Gliese 581 c, HD 85512 b and Alpha Centauri Bb. Erik Petigura, a grad student of Berkeley used data from the Kepler mission project to decipher just how many possible earth like exoplanets there are out there, using advanced software.
This puts the average discovery of these earth-like planets at around 5 earth-like exoplanets being discovered every year, which is significantly lower than that of just exoplanets. However, this is 5 more per year than 3 years ago!
As all of our discoveries of Earth-like exoplanets have occurred in the last 3 years, it is possible to speculate that we are only at the very beginning of our journey of discovering extrasolar worlds similar to our own. As the discovery rate increases, our fascination with a galaxy that may be brimming with life may call to us and lead us to embark on an extrasolar mission sooner than we may think.