Mini-Neptunes

Mini neptunes, the gaseous dwarf exoplanets

A mini-Neptune is an exoplanet, 2 to 10 Earth masses with a density less than 1. Mini-Neptunes are dwarf gas that have a liquid ocean surrounded by a thick atmosphere of hydrogen and helium and a small rocky core. Detection methods exoplanets, are becoming more sophisticated and more precise.
The radial velocity method or the oscillation method is an indirect method for finding exoplanets by observing the Doppler shifts in the spectrum of the star. By measuring these variations, we can calculate the movement described by the star and deduce the presence and characteristics of any planets that accompany it. In our solar system we observe a slight oscillation of the Sun on a 12-year cycle, which corresponds to the cycle of gravity of Jupiter.
The transit method allows telescopes to measurement of radiance to confirm the presence of planets around a star because to each transit of the planet in front of the star darkening takes place. Cyclical variations in brightness reveal the passage of a planet between the Earth and the star.
The astrometric method consists in measuring a star's absolute position in the sky and its movement. When the star describes a regular ellipse in the sky, is that it is influenced by one or more of its planets.
Direct detection of exoplanets is based on a high-resolution imaging and high contrast using adaptive optics.
Detection of gravitational microlensing effect occurs when the gravitational field of a star warps space-time, which deflects the light from a distant star behind. This effect is only visible if the two stars are aligned with respect to the Earth. If the star that acts as a lens has a planet, the field of the planet can have a small but detectable effect.
Since March 6, 2009, Kepler Space Telescope is specialized in the search for extrasolar planets or exoplanets and more specifically exoterres small size, 2 to 20 times the size of Earth. Kepler mission must determine whether there are habitable planets outside our solar system. Kepler will observes more than 100,000 stars in the Milky Way, rather in the regions of Cygnus and Lyra. Kepler observes continuously, two areas of the Milky Way, rich in stars and monitors tens of thousands of stars simultaneously.
Kepler discovered a wide variety of planets that scientists call Jupiter hot, super-Jupiter, planet of helium, super-earth exoterre, subterranean planet, dwarf planet gaseous, planet of transition, gaseous dwarf, planet ocean, planet of metal, iron planet, dwarf of gas, hot Neptune, Neptune cold, giant of ice, carbon planet, silicates planet, metallic planet or mini-Neptune.

A mini-Neptune is a gaseous planet or dwarf of transition. This type of planet is smaller than Uranus (14.5 Earth masses) and Neptune (17.1 Earth masses), about 2 to 10 Earth masses. Scientists believe that these mini-Neptunes have a thick atmosphere of hydrogen and helium, deep layers of ice and rock, liquid water oceans or ammonia or a mixture of both with a small core of matter volatile low density. Theoretical studies of these planets are usually based on the knowledge that one has planets Uranus and Neptune. Without a thick atmosphere, these planets would be kind of planet ocean. These mini-Neptunes do not turn on an orbit close to their stars, if not their thick atmospheres would be blown away by stellar winds.
Properties that differentiate the rocky planets to gaseous planets, are the diameter and the mass. As regards the diameter, the transition is made from two terrestrial diameters, and for the mass this can vary greatly depending on the composition of the planet, it is 2 to 20 Earth masses. Based on the above indicators, several intermediate planets, or mini-Neptunes, were discovered. Currently with the Kepler Space Telescope, 70% of exoplanets discovered by the transit method looks like mini-Neptunes whose size is comprised between that of our planet and that of Neptune. Neptune has a mass equivalent to 17.1 Earth masses and a density of 1638 kg/m3 is density (relative to water) of 1.638. Its atmosphere is composed of 80% of hydrogen, 19% helium, 1% methane.
Example of mini-Neptunes:
Kepler-11f has a mass of 2.3 Earth masses and a density of 0.69, the same as that of Saturn whose mass is 95 Earths. These properties class, this exoplanet in the category of mini-Neptunes or gaseous dwarf which have a liquid ocean surrounded by a thick atmosphere of hydrogen and helium and a small rocky core. 
Kepler-11c has a mass of 2.9 Earth masses and a density of 0.66. Its period of revolution around its star (Kepler-11) 191.231 days.
Kepler-11e has a mass of 8 Earth masses and a density of 0.58. Its period of revolution around its star (Kepler-11) 31.9996 days.
Kepler-16b has a mass of 8.45 Earth masses and a density of 0.964. Its period of revolution around its star (Kepler-16) 13.0241 days.
Kepler-87c has a mass of 6.4 Earth masses and a density of 0.15. Its period of revolution around its star (Kepler-87) in 191.231 days.
Kepler-109c has a mass of 2.22 Earth masses and a density of 0.65. Its period of revolution around its star (Kepler-109) 21.2227 days.

Image: Diagram showing the discovery of exoplanets per year until 2013. The colors indicate the discovery method, radial velocity (dark blue), transit (dark green), astrometry (dark yellow), direct imaging (dark red), microlens (dark orange).
The radial velocity method or the oscillation method is an indirect method for finding exoplanets by observing the Doppler shifts in the spectrum of the star.
The transit method allows telescopes to measurement of radiance to confirm the presence of planets around a star because to each transit of the planet in front of the star darkening occurs. A repeated cycle confirms an object passing in front of the star.
The astrometric method consists in measuring a star's absolute position in the sky and its movement. When the star describes a regular ellipse in the sky, this is certainly due to movement induced by an exoplanet.
Direct detection of exoplanets is based on a high-resolution imaging and high contrast using adaptive optics.
Detection of gravitational microlensing effect occurs when the gravitational field of a star warps space-time, which deflects the light from a distant star behind. This effect is only visible if the two stars are aligned with respect to the Earth. If the star that acts as a lens has a planet, the field of the planet can have a small but detectable effect.
Credit: Open exoplanet catalogue 10/01/2014.