Theory About Exoplanets

simulated view of an exoplanet in its environment, with its mother-star. picture site 'Amateur Astronomy' from Celestia

more than 3,700 exoplanets found up to day!

From the later observations made, there should me more planets in the Universe, than stars. Giant planet 51 Pegasi b was discovered in October 1995. It is about half the size of Jupiter and orbits its star in about four days as that groundbreaking discovery had been announced by the European team of Michel Mayor and Didier Queloz. But the news was met with some initial skepticism in the astronomical community. By a stroke of good luck, American astronomers Geoff Marcy and Paul Butler happened to have previously scheduled observation time on a 120-inch telescope at the Lick Observatory, atop California's Mount Hamilton. They spent four straight nights making their observations and confirmed a huge planet, at least half the size of Jupiter, was orbiting its host star more tightly than Mercury about the Sun, in just four days. The planet, called 51 Pegasi b, was to usher astronomy into a new view of our cosmic neighbourhood. The hunt for extrasolar planets -exoplanets, for short- had a poor track record, during fifty years, with false detections. The early 1990's had seen the actual detection of planets orbiting a pulsar but most scientists would reserve the first designation for a planet orbiting a normal star as a planet now known as Tadmor had been also detected in 1988, the discovery was withdrawn in 1992 but confirmed by 2002. Both the Mayor and Marcy teams were working with wobbles, or stars' radial velocity induced by the gravitational tugs of orbiting planets. The starlight wavelength was compressed, then stretched, as the star moved toward and away from the observer. First exoplanets had nothing in common further with known planets of our solar system. As soon as by 1996 Marcy had found 70 Virginis and 47 Ursae Majoris providing a bridge to our own solar system in terms of orbits and revolutions. The Marcy-Butler team then discovered at least 70 of the first 100 exoplanets found in the following years. A decade after, the hunt for exoplanets got into high gear with the launch of the NASA's Kepler Space Telescope in 2009, exploring a small patch of sky during four years and working with the planetary transit technique, looking for infinitesimally tiny dips in starlight that occur when a planet crosses the face of its star. The method only works for distant solar systems whose planets' orbits, from our perspective, are seen edge-on. European ESA COROT by 2006-2012 had already discovered numerous planets also using the transit method. Kepler was the brainchild of William Borucki of the NASA Ames Research Center in Moffet Field, California who had had, during the 1990's, his proposed designs rejected no less than four times. He finally won approva in 2001. No one then knew what Kepler might find, or even if it would find anything at all. The Kepler by 2014, found Kepler-186f, the first exoplanet discovered with a size similar to the one of the Earth and orbiting in the Goldilock zone of its star, a place favourable to the existence of liquid water. Kepler identified more than 4,600 candidate planets hundreds to thousands of light-years distant with 1,028 confirmed so far, of which Earth-sized planets that orbit within their star's so-called habitable zone, where liquid water can exist on a planet. The Hubble Space Telescope and the Spitzer Space Telescope are also used to characterize exoplanets atmospheres. Several other techniques of detecting exoplanets are now in used as by 2015 about 1,800 exoplanets have been confirmed found. Our Milky Way seems to be crowded with planets as next-gen telescopes using spectroscopy should reveal what data the light of exoplanets are conveying! Some of those atmospheric constituents might one day suggest the presence of life far beyond. The exosystem which looks the most to our own solar system is orbiting around the HD 10180 star, with 7 planets regularly spaced; that exosystem is found at 127 light-years away, in the constellation Hydrus, the Water Serpent as its parent star is similar too to our Sun. The most apt instrument today for the study of exoplanets in the HARPS spectograph which is installed unto the ESO 3.6-meter telescope, in La Silla, Chile. Exoplanets likely are to considered ubiquitous in the Universe, from our own Milky Way Galaxy to other galaxies or from earliest stars in the Universe to most recent, from youngest to oldest, and affecting any kind of them. Free-floating planets might be extremely common across our Milky Way Galaxy and even more numerous than common exoplanets! Our Galaxy, for example, could host billions of lone planets, mostly Neptune or Earth-sized as such light-weighed bodies are easier to eject from their parent-system. It might generally that the planetary architecture being redrawn in exosystems with the comet belts evolving, and planets gaining and losing their moons. Although nearly a thousand exoplanets have been detected, only a dozen exoplanets have been directly imaged up to now

A simulator view of a multiple planets exosystem with a planet seen transiting on the parent star. The researchers say that future Earths are more likely to appear inside giant galaxy clusters and also in dwarf galaxies, which have yet to use up all their gas for building stars and accompanying planetary systems. By contrast, our Milky Way galaxy has used up much more of the gas available for future star formation. picture site 'Amateur Astronomy'

->100 Billion Planets in The Milky Way Galaxy!
A detailed statistical study by early 2012 of how much exoplanets our Milky Way Galaxy could hold is hinting to 100 billion planets, at a minimum of one planet for every star. About 1,500 planets should lie within just 50 light-years of Earth. A other result by that same study is that Earth-sized planets should be more numerous that Jupiter-like ones, with more than 10 billion terrestrial planets in the Galaxy. Recent finds by the Kepler mission are however showing how exosystems, albeit holding some planets of about the size of the Earth, are mostly packed inside the orbit of Venus in our solar system. Other results based on observations taken over six years since 1995 concludes that there are far more Earth-sized planets than bloated Jupiter-sized worlds and planets around stars are the rule rather than the exception. The average number of planets per star is greater than one. This means that there is likely, for example, to be a minimum of 1,500 planets within just 50 light-years of Earth. More than 10 billion terrestrial planets would exist across our Milky Way Galaxy. 20 percent of all stars, generally, host planets in their habitable zone

->The Kepler Mission Honing to a Real Earth-Like Planet
Kepler-22b, is the first confirmed habitable zone planet, from 54 candidates. As that exoplanet is about twice the size of the Earth and orbits a star similar to the Sun, that may be considered a milestone on the way to determine habitable planets more similar still to our Earth as also located in their star's habitable zone, which is the region of a star system at which liquid water could exist on a planet’s surface. The discovery is due to NASA's Kepler exoplanets-searching mission which tends to discover more Earth-sized planets than before. Planets one to four times the size of Earth may be abundant in the Milky Way Galaxy. The study leading to find Kepler-22b also proved that the habitable zone of a star might be pushed outwards for cause of the warming effect of planetary atmospheres, which would move the zone away from the star, out to longer orbital periods
Kepler, on a other hand, has discovered the first Earth-size planets orbiting a Sun-like star albeit too close to their star, or in a area under the orbit of our Mercury. Both planets reside in a five-planet system called Kepler-20, approximately 1,000 light-years away in constellation Lyra as that system is another example of how exo-systems may be different from each other, as the planets, that time, are organized in alternating size

->Multiple or Single-Planets Systems?
Our solar system, with multiple planets, likely is part of one third of all exo-solar systems in the Universe with the same characteristics. The tilt of planets orbit in such systems further, might well be a unique feature as mono-planet systems have a zero-tilt, which allows for a easier transit-discovery. That might due, in our solar system, to the influence of the giant gas planets, beginning with Jupiter, whose gravitational influence disturbs the orbits of inner planets. Insted of using the velocity method to pin down multiple systems, astronomers could use a transit timing variations, measuring how successive transits vary in time due to gravitational interactions between planets instead

->Orphan Planets by Billions?
By May 2011, a study has been announcing a new class of Jupiter-sized planets floating alone in the dark of space, away from the light of a star which likely originated like probably ejected from their developing, turbulent planetary systems, as some may have formed along with stars and orbiting planets from a same gas cloud and stars and planets formation process. Such so-called 'orphan planets' had been predicted by never observed as difficult to spot. The team which performed the observation estimates there are about twice as many of such planets as stars, and, generally, as common as planets orbiting a star! That would add up to four hundreds of billions of lone planets in our Milky Way galaxy alone. The survey was not sensitive to planets smaller than Jupiter and Saturn as theories also suggest that lower-mass planets, like Earth, should be ejected from their star more often still! Without a star to circle, these planets would move through the Galaxy as our Sun and other stars do, in stable orbits around the Galaxy's center. Such planets are detected throught the so-called microlensing technique, which allow to spot exoplanets through a increase of light produced by that planet which transits before a distant star

->Kepler Findings as Of February 2011!
Kepler, a NASA mission launched March 2009 and searching exoplanets, especially Earth-size and smaller planets in or near the habitable zone of their star, the zone where liquid water is allowed at the surface of those, has yielded the following results as of early 2011, based on observations conducted May 12-Sept 17, 2009 of more than 156,000 stars on the mission's field, covering 1/400 of the sky:
. a total of 1,235 planets have been found, of those 68 approximately Earth-sized, 288 super-Earth-sized, 662 Neptune-sized, 165 Jupiter-sized; and 19 are larger than Jupiter
. 170 of planets show evidence of multiple planetary candidates
. 53 of that total of 1,235 have been found in the habitable zone, some of which could have moons with liquid water. Among those 53, 5 are near Earth-sized as the remaining 49 range from super-Earth size -- which is up to twice the size of Earth -- to larger than Jupiter
Less than 1 percent of the stars observed are thus exosystem-candidates as Kepler also yielded the observation of planetary systems with two or more planets seen transiting their star. Kepler's ultra-precise camera measures tiny decreases in the stars' brightness that occur when a planet transits them. The size of the planet can be derived from these temporary dips. The distance of the planet from the star can be calculated by measuring the time between successive dips as the planet orbits the star. Small variations in the regularity of these dips can be used to determine the masses of planets and detect other non-transiting planets in the system. By observing several transits by each planet the time between successive transits could be analyzed and a detection of significant changes in the intervals from one planetary transit to the next, what is called transit timing variations can occur, hinting to the gravitational interaction between multiple bodies of a system. The Kepler mission, generally, have found that at least a third of the stars have planets and the number of planets in our Milky Way Galaxy must number in the billions. Kepler announced the discovery of the first unquestionably rocky planet outside the solar system, Kepler-10b, 1.4 times the size of Earth, in January 2011. Very crowded and compact planetary system – a star with multiple transiting planets- exist too

->The Latest View About How an Exosystem Forms!
Planets are forming about a star when the latter is reaching to between 1 to 3 million years old. The materials present in the protoplanetary disc determine what kind of planets are forming. The gas giant planets are the first fo form, as they form wery quickly, in about 2 million years only! That swiftness is due to that they are only accreting layers of gas around a small rocky core. Gas giants further are impacting on how the terrestrial planets form, as such planets, in any case, are taking longer -about 100 million years- to form as they have to weld planetesimals together. Through what the shape of their orbit is, the gas giants are shepherding the material of the protoplanetary disk in such or such shape, determining where the terrestrial planets can form! The question of water on the terrestrial then: it's brought to the planets likely through asteroids (and not through comets like previously thought). At last, another factor about the exosystems are the 'hot Jupiters'. Hot Jupiters are gas giants forming far away in the system and then moving inwards. As the migration may be a risk for the terrestrial planets already in formation, it's too bringing renewed protoplanetary material from the disc closer to the star, triggering a second generation of terrestrial planets formation. Such second generation terrestrial planets are of the 'hot Super-Earth' type, with a size barely larger than our own Earth. The migration of the hot Jupiter, on the other hand, is rejecting material towards the outside of the disk too, bringing to that terrestrial planets are forming there too, with the pecularity of owning a lot of water! Such a renewed scenario of how the exo-solar systems are forming brings to that the possibility of terresterial planets outside there is increased! Hot Jupiters once migrated remain in fairly stable orbits which turned circularized -the reason for their stability- for billions of years, until the day comes when they may ultimately get eaten by their own star. A crystal-carbon exoplanet -thus made of pure diamond- has been discovered around a minute star

->More About a Young Stellar System!
A study, in 2009, is well showing the early life of a stellar system! As they studied the formation, in a protoplanetary disk of silicate crystals, from non-crystallized silicate particles, astronomers found that the young, nascent star, was experiencing outbursts every few years, from the material gathering up unto the star, from the surrounding disk, as major eruptions are occurring every 50 years or so. During such episodes, the star is brightening to a staggering 30 to 100 times brighter than usual! Such outbursts, thus, are interacting with the innermost parts of the disk, in a radius comparable to where the terrestrial planets formed in our own solar system. As silicate crystals, or forsterite, are found too in comets, the process could reach the faraway regions of the disk too, where the materials for comets are found, or such regions might have been closer to their star and then migrated outwards
Non-crystallized silicate is part of the mix of gas and dust constituting a protoplanetary disk. Being converted into crystals is always due to that the molecular bonds of the material break and then re-form, changing the material's physical properties, a mechanism called 'annealing'. Astronomers now know that three different processes may be involved in that transformation. Long exposure to heat. Shock waves induced by a large body within the disk and heating dust grains suddenly to the right temperature, after which the crystals would cool nearly as quickly. The outbursts of a nascent star, like previously described. About 1,000 Kelvin (1,340 degrees Fahrenheit, 727 Celsius) and now higher than 1,500 Kelvin (1,040 degrees Fahrenheit, 1,227 degrees Celsius) is the temperature needed. The dust grains would evaporate above. At 10 light-years away in the constellation Eridanus, the epsilon Eri exosystem is the closest to our own solar system. It is much younger however, as some 800 million years old only. Those views are showing that exo-solar system in details, as the solar system is given to scale the diagram. One planet each seems to be associated with two asteroid belts and sheperding those, some like Jupiter is for the asteroid belt of the solar system. A dense, outer comet belt is extant too as it will thin out with the system aging and turn into something like our Kuiper Belt! Its comets will collide with each other and break up, or get pushed out of the ring by the gravitational influences of planets

click to a detailed view of the exosystem of epsilon Eri. site 'Amateur Astronomy'

->The 'Super Earths', Beyond the Hot Jupiters!
With observations techniques advancing, scientists are beginning now to discover more and more exoplanets of a type called 'Super Earths'. A Super Earth is considered a planet that's up to bout 10 times the mass of the Earth. Exoplanets bigger than that tend to be gas giants, like Uranus or Neptune. Super Earths are small enough, thus, to have terrestrial surfaces or liquid oceans and thence could be able to support Earth-like life! The way astronomers may differentiate between a rocky-type planet and a gas giant is when the exoplanet is transiting in front of its star. With a gas giant, the light of it is dimming gradually as starlight passes through thicker and thicker layers of gas. And with a Super Earth, the dimming is occurring more quickly as such planets has just a thin atmosphere. Astronomers, from that, are now looking to discover a Super Earth with the right chemical composition and at the good range from its mother-planet to harbour life as the starlight passing through the Super Earth atmosphere is being able now to be analyzed in search of chemical clues for the existence of life
A lot of super-Earths might well be the remnant rocky core of some evaporated gas giant in the system, recent studies are showing when such planets are close to their mother star, tidally locked and bearing scorching temperatures of about 4,000° Fahrenheit. Such oddities might be named 'super-Ios' as they are enduring tidal-generated -from their sister planets- volcanism. Those planets would sport a vaporized rock atmosphere as they are gradually disappearing and that might be the fate of hot Jupiters along!
Super-Earths, according to some astronomers would be more hospitable to life, as they would have retained more energy and cooled slower. They would feature oceanic planets with varied atmospheres and abundant volcanic heath. One such planet already was found by late 2009

->An Improved Scenario for the Planets Formation
Latest accurate studies are leading to think that between 3 and 300 millions years could be the time frame for planet formation around Sun-like stars. A difference should be made between planets formation from massive, or less massive protoplanetery disks. The massive disks are swiftly producing their planets are their wimpiest counterparts take more time -about 10 to 100 times longer. Those considerations brings that astronomers now estimate that about 45 percent of Sun-like stars would harbour rocky, Earth-like planets

->Cluster Formation, Likely the Mainstream for Solar Systems Birth!
Astronomers think that the formation of stars from a same interstellar cloud, likely brings to that the forming stars are several, and grouped into a cluster. Over time, stars in these turbulent regions disperse and spread out. Meanwhile, however, this proximity has an impact about the formation of planets around the stars. The externalmost part of the protoplanetary disks are likely volatilized by the strong solar winds expelled by the neighbouring, forming stars. That means that more Earth-like exoplanets would exist as gas giants, forming at the borders of the disk being blown out!

->Globular, Non-Globular Clusters, Metallicity, Hot Jupiters
Hot Jupiters, with their orbit at least 3 to 4 times closer to their host stars than Mercury to the Sun might be quickly destroyed with the gravitational pull of the planet on the star creating a tide or bulge on the star. The bulge on the star points a little bit behind the planet and essentially pulls against the orbital motion. The planet thus orbits less swiftly and moves a little closer to the star, with the bulge becoming bigger, etc. with a cumulative effect. After billions of years the planet eventually crashes into the star or is torn apart by the star's gravity, beginning with its outer layers of atmosphere. When applied to a globular cluster environment, such a model leads to a destruction of most of the hot jupiters present there as such clusters, until now, had been believed to harbor numerous exoplanets. 96 percent of the planets would be gone when the cluster is coming to a age of 11 billion years and 33 percent by the age of a billion years. It might that less cramped clusters might provide for more lifetime to planets. In clusters, generally, only the smaller and more distant planets would survive. Gravitational interactions also include those by nearby stars which can kick out a planet from its parent star. Globular clusters at last have been found with a low metallicity (poor in metals, or elements heavier than hydrogen and helium), which are the raw materials for making planets

How do Solar Systems Form?

->Two Protoplanetary Disks About a Same Star!
Nasa's Hubble Space Telescope interestingly showed, in June 2006, that Beta Pictoris is actually featuring two protoplanetary disks. The newest found one has been found inclined by about 4 degrees to the previous one, and extending far away, possibly beyond 24 billion miles! This is an important discovery as, planets forming in both -or more disks- might explain the slight differences of their orbits' planes between the planets of our solar system. It might that two or more planes of disk be the norm in the formation of any solar system. An explanation for such various disks might be that one planet forms and then steals dust and matter from the primordial disk, forming a new one

As far as solar systems formation is concerned, it is agreed today that they are due to star formation. Stars are appearing among dense clouds of gas and dust. Density irregularities, or supernovae shockwaves, are bringing these clouds to collapse. The conditions favourable to the formation of a star occurs among dense clumps of matter which are appearing in interstellar gas clouds, like those seen in the famed Hubble picture of the Pilars of Creation. Such clumps likely result from the stirring endured by a interstellar cloud inside a galaxy due to varied forces, like radiations, gravity or magnetic fields as, when such a clump is shrinking, it then becomes denser and thus more able to shield its interior from the light and other radiations. In turn, those conditions make that the clump cools more easily and is collapsing faster upon itself! Collapse yields heat and spin yields a disk which forms in the equatorial plane of the star. The first hundred thousand years of a star are tremendous as it swallows a part of the disk, with huge magnetic arches bringing quantities of material from the disk unto the star's surface, creating hot spots as, before that, the stress yielded by the material collapsing is evacuated through polar jets. A state of equilibrium is eventually reached in about one million years with a definitively formed star and a disk of debris around it. The study of 'cores', those cold, dark clouds where stars are forming are releasing 'corshines', which are starlight from nearby stars bouncing off the cores, reveals information about their age and consistency. As one thought that the grains of dust making up the cloud were too small to deflect the starlight, the dust grains were found bigger -- about 1 micron instead of 0.1 micron -- and able to that. The more developed star-forming cores will have larger dust grains, so, using this tool, astronomers can better map their ages across our Milky Way galaxy as the deflected starlight is scattered in a way that is dependent on the cloud structures. That discovery of larger dust grains inside interstellar clouds can significantly change astronomers' models of star and planet formation on a other hand, as, for one thing, the larger grain size means that planets might take shape more quickly, with the tiny seeds for planet formation may be forming very early on, when a star is still in its pre-embryonic phase. The young star, on the other hand, is keeping having strong flares. The jets emanating from young stars, on the other hand, surely impact the protoplanetary disk with X-rays, having an influence on the formation of planets and maybe producing complex molecules in the disk. The material of the disk, thus, which comes to contribute to the increase in mass of the nascent star, is triggering there outbursts, which interact on the disk and transform amorphous particles into silicate, fosterite crystals through mechanisms of melting/cooling. It has been demonstrated that the interaction between the nascent star's magnetic field and the surrounding protoplanetary disk has as a result to slow the star's rotation. A slower rotation might be involved with planets' formation. Sun, nonetheless, was, as a young star, rotating 10 times faster than its current 28 days revolution. It looks like protoplanets may form in some areas of the protoplanetary disk with a higher density. Debris and particles of the disc collide, mix, and form planetesimals about 300 mi accross. Such planetesimals accrete in turn. These protoplanets are then sweeping the disc, yielding a structure a bit similar to Saturn' rings, gathering or clearing debris, sweeping all that along. When the formation process is over the environment is left with we a star, planets, and leftover rings. The asteroid belt, the Kuiper Belt, and the Oort Cloud at our Sun are such leftovers. Vast comet belts seem common among exo-solar systems with low mass planets as they might harbor 10 more comets than in our solar system's Kuiper Belt. That hints to that low mass planets likely preserved Kuiper Belts, and that comets and asteroids might also have been -or be- water purveyors to exoplanets at a continuous rate during longer that the Heavy Bombardment Period, in the order of over billions of years! There is a correlation between the presence of massive debris discs and planetary systems with no Jupiter-class planets. Giant planets likely yield relatively sparse Kuiper Belts, as systems with only low-mass planets often have much denser Kuiper belts. Planets formation generally takes about 100 to 300 million years to occur. Terrestrial type planets takes 3-10 (or 10-50) million years to form, as gas giants planets form in 10-20 million years. The core of the gas giants may form as quickly as 1,000 years or even less. On the other hand it's the gas giants which form first as the rocky planets form later. The protoplanetary disk is passing from an original state where most of it is made of light elements, like those found in comets, to the one when it can form planetoids. Then, it's mostly rocks and iron. Then it's just keeping the leftovers of the formation of a solar system. Much gas, further is in the disk at the moment when, during the formation phase, gas giants may form, as the gas, at about 10 million years of age of the star, dissipate and let the place to the formation of the rocky planets. In terms of planets size, the 'Neptunian Desert' is a region close to stars where Neptune-sized planets can't be found as they are blasted with radiation from stars, and can't maintain their gaseous atmosphere. The formation of small-sized planets, in the order of that of the Earth looked like they needed above all stars rich in heavy elements like iron, silicium for exemple. New observations however by 2012 have shown that such planet could form around stars with a varied content in terms of those elements, which increases the possibilities that exoplants exist in a very important number. As far as giant planets are concerned, with a small orbital period, they are neatly more associated to heavy elements-featuring stars. The laws of physics thus are yielding opportunistic, and prolific results taking paths one sometime may consider difficult

->"Core Accretion" Vs. "Gravitational Instability"
A 2005 study of an exoplanet transiting its mother star is showing that the "core accretion" theory about how the planets are forming is the most likely, compared to the "gravitational instability" model. In the "core accretion" model, it's thought that a planet forms by accretion around a small, primordial rock-icy core. In the second model, planets are forming more quickly as they are thought to form due to the rapid collapse of a dense cloud

As the protostar is blowing up the protoplanetary disc in 6-10 million years, some large collision events are occurring which replenish the disc. It's an event of this kind which formed the Moon from the Earth. Some discs may last 1 million years only as some may last more than a hundred millions years due to the collisions and replenishment process. On the other hand, young stars may be seen without a disc, meaning that stars may form without any disc forming. At last exosolar systems may be much wider (10 to 100 times) than our own planetary system. On another hand, a star formation process is clearly producing two important effects. First, the strong solar wind emanating from the central protostar is clearing off most light elements like hydrogen and helium. Such gas elements are found only farther from the star. Second, the nearer the star the more the water mix with solid particles; the farther, the most is remains under the form of vapor. This explains that any exosolar system will have inner, "terrestrial", rocky, planets. And outer, gas giants ones. This also is explaining the icy nature of the gas giants' moons, like the Jupiter's or Saturn's moons in our own solar system. 'Circumbinary' planets, planets orbiting about a binary star system, definitely were evidenced in 2011 proving the diversity of planets in the Universe. The formation of small worlds like Earth previously was thought to occur mostly around stars rich in heavy elements such as iron and silicon. However, new observations in 2012 have shown small planets form around stars with a wide range of heavy element content and thus increasing the possibility that exoplanets be widespread. Giant planets as far as they are concerned, with short orbital periods tend to be associated with metal-rich stars. Nature may be considered opportunistic and prolific, finding pathways we might otherwise have thought difficult

Most planets in the exosystems -like those in the solar system- have a direct motion on their orbit. They are orbiting counterclockwise around their star, as seen from the North pole of it. The direction of rotation of the planets is the same than the one of their parent-star, generally, because the protoplanetary disk is rotating in the direction to which the rotation of the nascent star on itself is determined. That motion, on the other hand, is just an amplification of the motion the original gas cloud is acquiring when it collapses to begin to form a star. A exoplanet was bizarely found with a retrograde -clockwise- orbit in 2009. That is likely due to a gravitational encounter with a larger planet in the system, with changed the direction of the orbit. The planet, further, a feeble density only (half a Jovian mass for two Jovian diameters) and the orbit is extremely elliptic which might yield, through gravitational tides, the planet to heat and to swell. Most diminutive moons in our solar system have retrograde orbits as they are often captured objects around a planet. Most recent observations by the Hubble Space Telescope are showing that a number of nearby stars are left, like our solar system, with a Kuiper Belt-like disk of icy debris once their planets formed. Such disks, over billions of years, has the debris colliding and grinding the KBO-type objects down to ever smaller pieces, in the order of 3,000 ft across, which means that such objects are comet-sized bodies. A well-aligned configuration of planets orbital plane in exosystems is not guaranteed as dynamical interactions may tilt planetary orbits, or stars may be misaligned with the protoplanetary disk through chaotic accretion, magnetic interactions or torques from neighbouring stars. The most recent observation, in 2012 of a orderly alignment in the Kepler-30 system suggests that high obliquities are confined to systems that experienced disruptive dynamical interactions, which is to be confirmed by further studies

->What for The Exosystems As far As Heavy Bombardment Periods are Concerned?
20 percent of nearby Sun-type stars are possessing Kuiper Belts, posing the question whether potential planetary exosystems there endure, or not, a 'Heavy Bombardment Period' at a moment in their life. It might that the gas giant planets in such systems play the same role they play in our own solar system, to contain gravitationally, that is, the dangerous bodies. Astronomers now think that a few percentage only of the exosystems are at risk of devastating impactors. They two think however that, should life-hospitable planets endure impacts every 20 million years due to 6 to 60-miles (10-100-km) wide impactors, life processes would not correctly evolve either because impactors would purely and simply forbid any beginning of the process leading to life, or because the intervals of time between the impacts would be too short to allow living organisms to recover, or, at last, that biodiversity would be decreased, reducing thus the possibilities of the evolution of life. Bombardment of comets and asteroids generally however, akin to the Late Heavy Bombardment at Earth also are occurring in exosystems

How Much Exosystems May be Found in the Universe?

Three star generations are needed before any potential that Sun-like stars harbouring planets: a first generation of super-massive and short-lived stars producing first heavy elements ending supernovae. A second generation of stars allowed to be Sun-like, due to that some of these heavy elements are cooling the star formation process. At last a third generation meeting the requirements of these cooling heavy elements as a surplus of them is providing the exoplanets material. These conditions were met between 500 million and 2 billion years after the Big Bang. First population of stars -mostly hydrogen and helium- is called Population II as following -heavy elements- is named Population I. Stars 3 times richer in heavy elements than the Sun have 20 percent chance to have planets as stars like it 5 to 10 percent chance only. Due to stars getting always richer in heavy elements along the generations, this might lead, still now, to a "planet boom". Even binaries may get solar systems, provided the both star system is close or distant apart enough
An amazing discovery in July 2003 has been made that a gas planet is orbiting a pulsar, in a globular cluster, and is dated 13 billion years old. This means that in such an environment, gas is the only material available for planets formation as heavy elements have not still been created, thus bringing to the idea that this is a new class of planets, related to the earliest generation of stars and that they might be numerous. This means too that planets may appear in densely populated environments like globular clusters or star clusters

->Some More Facts
. The number of exoplanets found the central bulge of the Galaxy by a study of NASA's Hubble Telescope has been found consistent with exoplanet detections made in our local solar neighborhood
. Planets have a tendency to more naturally form around stars abundant in elements heavier than hydrogen and helium, such as carbon which provide for the formation of the planets
. In a dense environment of stars' formation, some super hot, type O stars may strip -through their winds and radiation- the planetary prone environment around others, dissipating any protoplanetary disk in about one million years. The 'security zone' around a 0-star is 10 trillion miles. As a reference, the nearest star to our Sun, Alpha Centauri is located thrice that distance away
. The question of the hot Jupiters is resolved in that such planets close to their star either form around 'cold' stars like red dwarfs or they do migrate inside. In the first case, the proto-planetary disk may edge down the star enough, as, in the second, the migration inwards of the planet halts where the disk ends. When the planets are found really close to their star, they are called 'Ultra-Short-Period Planets' (USPPs) as their orbital revolution is under one day, with a distance of about 740,000 miles (1,190,000 km). Such planets orbiting so close to their star usually have a masse of about 1.6 Jupiters. Otherwise, the gravitational pull of their star might tear them apart. The temperature at such planets is high (about 3,000°. F -1,600° C)
. Protoplanetary disks, hence exoplanets may be found too in complex star systems (a double star system orbiting around another one, for example)...
. Water found in the atmosphere of hot Jupiters by a study during the summer of 2007, is leading to think that water likely is present should rocky planets exist too there. The question of the exoplanets' atmosphere brings too to that planets with an atmosphere might well have too extreme-high (about 2,000 miles -3,200 km) altitude haze enveloping the planet (the far reaches limits of the atmosphere on the Earth are just at about 800 miles (1,280 km)). Hot Jupiters, in any case, seem well to often posess water in their atmosphere
. Planets in especially tight double-star systems of the RS Canum Venaticorum (RS CVn) class, would have collisions frequent and be doomed as their parent stars are separated with 2 million miles (3.2 million kilometers) only. The stellar pairs orbit around each other every few days, with one face on each star perpetually locked and pointed toward the other, with a size similar to the Sun and 1 to few billion years old. Powerful magnetic fields, and giant, dark spots as strong stellar winds are slowing the stars down pulling those closer over time. Such a gravitational perturbance causes disturbances to planetary bodies orbiting around both stars, bringing to collisions of planets, comets and asteroids. Everything is ending into a dusty disk surrounding the couple
. The nearest exoplanet found is at about 10 light-years from the Earth
. Young planets, of the gas giant type, may feature a ring akin to the one which surrounded Jupiter before that coalesced to yield the four Galilean satellites
. Models of planetary atmospheres indicate that any world with the common mix of hydrogen, carbon and oxygen, and a temperature up to 1,000 Kelvin (1,340 degrees Fahrenheit) should have a large amount of methane and a small amount of carbon monoxide. Some observations however are showing that it is not necessarily the case, hinting to the variety of exoplanets

A Table of Planet Size Around Sun-Like Stars. picture site 'Amateur Astronomy' from a picture NASA

From the 150 exoplanets found so far, most are large-sized, Jupiter-like planets, close to their stars. Such large "hot Jupiters" are thought to have formed much further and have spiraled inwards, sweeping any other planets in the process. Such a closeness might prevent such planets from having rings or moons. Despite the harsh conditions surrounding their existence, hot Jupiter mostly last about a trillion years before evaporating. Hot Jupiters' atmosphere have been found with water, methane, carbon dioxide and carbon monoxide as hot Jupiters are affected with a 'hot spot', lying directly under the Sun-facing side with observations showing however that the hot spots may be shifted slightly away from this point likely pushed by fierce winds. Offset have been found larger putting that theory to risk and hinting to possible supersonic winds triggering shock waves that heat material up, and star-planet magnetic interactions like other explanations. Planets which are found further from their Sun have highly eccentric orbits certainly due to the fact of the formation and interaction of several planets around the star. Up to now, nothing alike our own solar system was found but it's certain that, as the search will broaden, it will eventually lead to such findings. It's likely that what will be found are solar systems looking like our, with one or two "hot Jupiters" inside the orbit of Mercury, and some faraway bodies with highly eccentric orbits. An Earth-like planet might be found between 2015-20 and 2040 according to whether it's close to us or further. It would take a multi-generational journey of 150 years to get there. Figures vary from 25 per cent of the Sun-like stars in the Milky Way having exoplanets to 100 of all the Milky Way Galaxy stars! Further 10 percent of stars in the "galactic habitable zone" would harbour life. The "galactic habitable zone" is mostly made of the Milky Way's spiral arms. The limit between planets and brown dwarfs -that is low-mass stars- is at 13 times the mass of Jupiter (brown dwarfs are considered such until 75 times Jupiter, beyond they are usual stars). Recentest studies by 2010 show that nearly one in four stars similar to the Sun may host planets as small as Earth and orbiting in the 'hot zone,' close to their star. This is against some theories which predict a planet desert there and the presence of planets only due to the migration of gas giants inwards. In our own Milky Way Galaxy 46 billion Earth-size planets are lying in that zone, not counting Earth-size planets that orbit farther away, in the habitable zone

->A Estimated 50 Billion Planets in our Milky Way Galaxy!
Science team of the NASA's Kepler mission, a mission searching exoplanets, especially Earth-size and smaller planets in or near the habitable zone of their star, have released their first estimate of how many planets might lurk in our Milky Way Galaxy by February 2011. And the numbers are of importance! At least 50 billion planets likely are present around stars in the Galaxy! With at least 500 million of those planets are in the so-called 'Goldilocks zone', that area with mild conditions prone to life from their star! Such numbers are extrapolated from the early results of the Kepler telescope in the first year of searching. 1 of 2 stars has a planet, as 1 of 200 stars has planets in the habitable zone. Such numbers further should be a minimum because stars in the Galaxy may be accompanied with more than one planet. The Kepler team stresses that if Kepler was 1,000 light years from our Sun, it would have noticed a Venus' transit but not necessarily the Earth, at a mere 1 to 8 chance to spot our planet. Those numbers at last are just available for our own Milky Way Galaxy, as the Universe should hold about 100 billion galaxies in total, leading to 5,000 billion of billions stars holding a planet, with 50 billion of billions planets featuring conditions favourable to life, in the Universe. The famed Drake equation which ponders the diverse factors for advanced civilizations in our Galaxy brings to a estimation of 1 million of them, which is the estimation made through the equation by astronomer Carl Sagan as science fiction writer Isaac Asimov calculated 670,000 and Drake himself a more conservative 10,000

Ways astronomers use to find exoplanets are of varied types. They can look for the light of faraway stars dim slightly as their planets pass in front of them (which is available for stars located at 5 AU from their star only). For an Earth-size planet transiting a Sun-like star, the change in brightness is only 84 parts per million or less than 1/100th of one percent. By measuring the depth of the dip in brightness and knowing the size of the star, scientists can determine the size or radius of the planet as the orbital period of the planet can be determined by measuring the elapsed time between transits. Once the orbital period known further, Kepler's Third Law of Planetary Motion is applied to determine the average distance of the planet from its star. Or they can use coronagraphy, which uses a mask to block the optical light of a star, making its surrounding planets more easily visible. They can also observe the gravity-induced "wobbling" of their host stars affecting the star's proper motion as studied via the Doppler shift (also available for the planets found at 5 UA from their star). Direct imaging too is a option albeit less efficient. Another new technique came to add, by 2009: astrometry, which consists into to precisely measure the wobbles of the proper motion of a star, visually, against the background of stars. That technique further, allows to detect exoplanets around M-dwarf type stars as the exosystems there harbour more 'cold Jupiters' hence likely are more similar to our own solar system, with terrestrial planets too. At last, the most promising technique -especially as far as exo-Earths or the search for life components on the exoworlds- is concerned is the technique called a 'nulling interferometer,' which consist into combining starlight captured in the infrared by four different telescopes, arranging the light waves from the star in such a way that they cancel each other out, and eventually turning the star light off. Most recent advances in the field makes that the light from the parent star appear 100 million times fainter. As far as the detection of multi-planets exosystems is concerned, they are made through a spectrometer which allows precise measurements of a star's radial velocity or motion along the line of sight from Earth. The gravitational tug of an orbiting planet causes periodic changes in it as multiple planets induce complex wobbles, and astronomers use sophisticated analyses over a long period of time to determine such planets' orbits and masses. About the question of how to sort objects which are planets and those which are not, the mass is the main criterium, with bodies above 13 Jupiter masses considered stars -brown dwarfs- and bodies below being planets. Exoplanets researchers might well now consider that any object seen associated to any dust disk around a star, being a planet even when -like the case for those finds- the mass of the planet may not be sufficiently inferred from its light only. The study of the emission of a exoplanet in the infrared also allows to better determine the characteristics of the exoplanets which do not transit in front of the disk of their star

Conclusion

Next-generation search tools to appear between 2005 and 2010 (like the Terrestrial Planet Finder, the Space Interferometry Mission, the Kepler mission, or the European COROT) will concentrate upon finding Earth-sized exoplanets. Some of the new exoplanets might well be water-worlds due to the possible inward migration of further ice-planets which would melt during the phenomenon. The infrared NASA's Spitzer Space Telescope is already providing a better insight about protoplanetary discs. On the other hand, the technique of gravitational microlensing is helping too. Microlensing is a technique related to galaxies gravitational lensing. In this case, a foreground star is bending the light of a background one. Should a planet orbit the lensing star, it either increases or decreases the brightening. An advantage of the microlensing technique is that it allows the finding of Eart-sized planets

->Can Astronomers Spot Terrestrial-Like Plant Life on Exoplanets?
Photosynthesizing plants, on Earth, is reflecting away the infrared wavelengths, presumably to avoid overheating as they are absorbing the visible part of the light spectrum. This is yielding, for a spectrograph -a tool analyzing the spectrum of the Earth- a notch at about 700 nanometers, right at the boundary between visible light and the infrared. That feature is called the 'vegetation red edge' (VRE). Absorption lines of the oxygen or the methane may be too hints that some biology is at work on a planet. Scientists further have found that, as far at the Earth is concerned, even episodes of an ice, or a warm age, would not erase the typical signature of the VRE! The VRE was smaller by 4 percent, or larger by 6 percent only respectively. A much larger ice age, like polar caps, however, descending down to China on Earth, for example, would shut the signal off. For being spotable, the signal would need a 6-meter telescope and 2 to 4 weeks of exposure time. Those characteristics would be in the range of the Terrestrial Planet Finder, an exoplanet searching mission currently in development

->Spitzer Helps to a Better Knowledge of Hot Jupiters As astronomers thought that the exoplanets of the hot Jupiter type had atmospheres with a lot of water, the Spitzer Space Telescope, as it managed to study such atmospheres from afar, showed that they have none or most likely hidden, with some having tiny sand grains in the atmosphere, forming high, dusty clouds at the top of the planet's atmosphere. Hubble already had seen that the hot Jupiters' atmospheres contain elements like sodium, oxygen, carbon and hydrogen

->The Hyper-Telescopes Able to Detail a Terrestrial Exoplanet Surface by 2040 Hyper-telescopes, which are a set of numerous optical systems disseminated on a large space area and reassembling a picture unto a receptor satellite, at the ressemblance of a interferometer network, should be able, by 2040 to image down to the surface details of terrestrial exoplanets, allowing for a better search of life and extra-terrestrial intelligence. Such networks would need some 77 square miles (200 km²) at the least as the larger the area, the smaller the details seen, down to the hints of some intelligent civilizations with a set of telescopes spread unto a distance like between the Earth and Moon