Exoplanets

Are we alone ?

"Are we alone in the universe?".
This question running through the heads of our astronomers for a long time.
To answer this question we must know whether or not there other planets that could develop other forms of life from which the search for exoplanets.
Exoplanets are by definition outside of our solar system. Given the large number of existing galaxies and the large number of stars they contain, in search of these exoplanets or extrasolar planets, is endless.
Detect such planets is not easy.
They are very remote from us, do not produce light and the star around which they revolve, blinds us.
Even the Hubble Space Telescope can not see Pluto in the form of a small task.
How to find a planet outside solar, millions of times more distant than Pluto if we can not see it.
The Earth orbits the Sun but in reality, if a star is accompanied by one planet, one and one turn around the center of mass of the gravitational system they form. Given the much larger mass of the star, the center is much closer to the latter that of the planet.

Even if it is inside the star, it is not exactly in the center of it. The star therefore shows some variation caused by the presence of the planet.
This technique is to discern variations in the spectrum of light emitted by the star.
Doppler, it appears redder if it moves away from the observer, bluer if it comes close.

NB: The Hubble telescope is an optical instrument United States and Europe to observe the light in the visible, infrared and ultraviolet.
It is placed in orbit around the Earth at an altitude of 580 km thus avoiding disturbance to the Earth's atmosphere.
It includes a concave parabolic primary mirror that reflects the light incident on a convex hyperbolic secondary mirror.
These two mirrors are placed in a so-called Cassegrain configuration. The light collected by cameras or spectrographs is then returned to Earth for study.

Image: number of exoplanets discovered since 1989 (file dated June 10, 2009). Note: Exoterra objects are covered with a crust, with a radius between 2.5 and 20 times that of Earth and operating within the habitable zone of the parent star. In this region, water could persist on the surface of the planet which would make the planet potentially habitable.

Presence of planets

By measuring these variations, we can calculate the movement described by the star and to deduct the presence from it and the characteristics of the possible planets which accompany it.
In our solar system we observe a light oscillation of the Sun on a cycle of 12 years, what corresponds to the cycle of gravitation of Jupiter. Another technique consists in observing the cyclic variations of brightness of certain stars. The passage of a planet between the Earth and the star leads a modification in the light which reaches us from this last one. We can also base themselves on this fact in the search for extra solar planets. These two observations are going to be able to bring to light the presence of planets around a star, it does not more remain than to look its indications.

Image: Spectrography: Study of specters by means of a spectrograph, a device intended for the photographic recording of a brilliant specter. The spectrometry is the analysis of the specter of a radiation. The term radiation is to be taken in the broad sense, because he can involve a brilliant radiation, in a wider way an electromagnetic radiation, but also a mechanical wave as the sound or the seismic waves, or still particles.The movement of the dark lines in the specter of the star is the signature of the movement of the planet and the star around their centre of gravity. Credit: look-out observatory of Paris / ASM Emmanuel Pécontal

The hunting in exoplanets

The hunting in exoplanets thus consists in tracking down the oscillations of stars in the characteristics identical to the Sun. This hunting limits itself at the moment that to the gaseous giants witch turn around their star. More and more new stars are discovered with cyclic oscillations. Another characteristic of the presence of planets around a star is the variation of brightness of this star. When a planet walks past a star its brightness shows a small variation. This variation can reveal the size and the other characteristics of the planet.
Telescopes with measure of brightness, the precision of which allows to detect until 1 % of variation, confirm the presence of planets around a star because in every passage of the planet in front of the star a darkening with place.

A repeated cycle confirms a passage of object in front of the star.
More than 70 huge planets were so detected.

Image: Photo of Mercury passing in front of the Sun Among the bodies of the solar system large, only the Moon, Mercury and Venus can pass before the sun for an observer on earth. If, in the case of the phenomenon moon (solar eclipse) is common, it is not the same for Mercury and Venus with the phenomenon of passing before the sun is scarce. It is less than spectacular solar eclipse because the maximum apparent diameter of Mercury is about 1/200ème of the Sun and Venus is about 1/30th.

An unexpected discovery

Scientists are left in search of stars whose oscillation extends over a decade, as our Sun.
The star 51 Pegasi, which is 50 light years away, oscillates with a cycle of 4.2 days wonder.
So there is a huge planet orbiting with a cycle of 4.2 days. The discovery of this planet is a real thunderbolt for scientists around the world.
New stars are discovered one after the other and they like the giant planets, these are all the planets with short orbital periods.
On the star HD209458, located not far from Pegasus, a darkening occurs every 3.5 days, which confirms the presence of a gas giant.
It has a radius 1.6 times larger than Jupiter and a mass of 60% that of Jupiter.
It is located very close to its star.
This is a hot Jupiter.
Other planets in eccentric orbit have been discovered since, and their system is very different from ours. These observations show that massive planets can rotate around their stars with very short cycles, which calls into question our understanding of the formation of the solar system from a nebula of gas and matter.

The formation of planets at a distance so close to the star (≈ 0.05 AU), requires that the temperature of the circumstellar protoplanetary disk is sufficiently low to allow condensation of rocky material, and a surface density high enough.

Image: 51 Pegasi b, is a hot Jupiter. 51 Pegasi b, is much closer (0.05 AU) from its star as Jupiter and so its temperature is higher.
Its period of revolution around 51 Pegasi, is 4.23 days and the eccentricity of its orbit is equal to 0.
image: Digitized Sky Survey

Importance of the giants

The quantity of dust and gas determines the size and the quantity of planets which are going to constitute from this disk. With a more important quantity of dust and gas: another giant builds up himself, in time the gravitational strength of the third planet perturbs the others. With a less important quantity of dust and gas, there is no gaseous giant, only smaller planets with stable circular orbit. Simulations by computer, formation of a solar system show a surprising characteristic. If we introduce the third giant into the solar system, the system becomes unstable and eventually loses one of the huge planets.

Graver still it also perturbs the planets of ground type which eventually absorb by the central sun. It shows that the mass of gases and matter of the nebula has a vital importance for the suite of the evolution. If there is less dust and gas, certain researchers think that the life cannot abound there because it is Jupiter which protected our planet.
By being situated in the external part of our system, Jupiter decreased bombardment of the Earth by the numerous objects coming from the reservoir of comets.

The objects which change orbit and which goes to the center undergo the attraction of Jupiter. This attraction is so big as these objects are absorbed. Jupiter has guard's role for our ground world. These giants in borders of the systems thus behave as shields preventing the collisions with the planets of ground type. Instead of occurring all 60 000 000 of years, the collisions would occur every 60 000 years, only a microscopic life could then develop there.

Simulation of the third giant in a system

It follows itself a disturbance of orbits

The disturbance is more and more big

... until eject a giant

An incredible chance...
The simulation also shows that in one or two huge planets the system remains stable.
The planets of ground type can be only in a small percentage of stars.
The presence of Jupiter allowed the Earth to become a planet where abound thousands of alive sorts.
The importance of a huge planet is thus essential in a solar system, by looking one the other Jupiter we can find another earth.
The constitution of our solar system is the result of an incredible chance.