About Exo-planets

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

The search for exoplanets began in the 1990s when astronomers became able to look for them. They are mainly working about the wobbles planets yield about their parent star's motion and the redshift hence seen in star's light

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

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. 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 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. 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. Debris and particles of the disc collide, mix, and form planetesimals about 300 mi accross. Such planetesimals accrete in turn. These protoplanets sweep the disc, yielding a structure a bit similar to Saturn' rings, gathering or clearing debris. 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. 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

"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

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

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 exo-solar 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 exo-solar 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

How Much Exo-Systems 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

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))
. The nearest exoplanet found is at about 10 light-years from the Earth

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

Exoplanets Count As of May 2008: 291

any other planets in the process. Such a closeness might prevent such planets from having rings or moons. 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)

The nearest exoplanet found is lying at 10.5 light-years from us, around Epsilon Eridani, a Sun-like 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

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