Top 4 exoplanets that have the best chance for life beyond our solar system.

Here's a list of 4 planets we know of that are most likely to host alien life, according to the University of Puerto Rico at Arecibo.

Kepler-186f

Kepler-186f (also known by its Kepler object of interest designation KOI-571.05) is an exoplanet orbiting the red dwarf Kepler-186, about 582 light-years (178.5 parsecs, or nearly 5.298×10¹⁵ km) from the Earth.

• The force of gravity on Kepler 186f could be very similar to Earth's.
• There are four other planets in the Kepler 186 system, all of which orbit extremely closely to their host star
• It is the first truly Earth-size exoplanet ever found in its host star's habitable zone. The alien world, which is just 10 percent bigger than our own planet and is almost certainly rocky.

Gliese 581g

This planet is a controversial find. It was discovered in 2010, but there has been difficulty in getting it confirmed. Still, the University of Puerto Rico at Arecibo calls Gliese 581 the top candidate for alien life. If confirmed, this rocky world is about 20 light-years away from Earth's sun, and is two to three times as massive as Earth. It orbits its parent star, Gliese 581, about every 30 days in the constellation Libra.

•Gliese 581g is one of the most Earth like planets detected in the habitable zone of its star.

Gliese 581g Statistics:-

Host Star: Gliese 581
Host Star Type: Red Dwarf - M Class
Host Star Mass: 31% of Sun
Constellation: Libra

Distance from Earth: 20.3 light years
Planets Detected in System: 4 confirmed, 2 unconfirmed

Discovery Date: September 2010
Detection Method: Radial Velocity

Exoplanet Type: Super Earth
Mass: Around 3.5 x Earth
Diameter: Around 130% to 200% greater than Earth depending on composition
Distance from host star: 13.6 million miles (22 million km)
Orbital Period: 37 days

Atmosphere: Unknown, models suggest an atmosphere composed of one fifth carbon dioxide would provide the most habitable environment.
Temperature: Unknown, potential temperatures vary dramatically depending on atmospheric conditions.
Gravity: Around 10% to 70% stronger than Earth's gravity.

Possibility of Life: Depending on atmospheric and surface conditions it is possible life could exist on Gliese 581g.

Gliese 667Cc

Another "super-Earth", Gliese 667Cc is also close by Earth: about 22 light-years away in the constellation Scorpius. 

Star: Gliese 667A
Star Type: Orange Dwarf - K Class
Host Star Mass: 73% of Sun

Star: Gliese 667B
Star Type: Orange Dwarf - K Class
Host Star Mass: 69% of Sun

Distance from Earth: 22 light years
Planets Detected in System: 6 confirmed, 1 unconfirmed

Discovery Date: November 21st 2011
Detection Method: Radial Velocity
Exoplanet Type: Super Earth

Mass: Around 4 to 5 x Earth
Diameter: Unknown, around 1.6 to 2.2 x Earth with similar composition

Distance from host star: 11.5 million miles (18.5 million km)
Orbital Period: 28 days

Atmosphere:Unknown, a similar atmospheric composition to Earth would produce temperatures slightly higher than on Earth, a thicker atmosphere may produce a runaway greenhouse effect.
Temperature:Unknown, possibly around around 30C (86F) with a similar atmospheric composition to Earth.
Gravity: Unknown, 1.6 to 2.2 x Earth's gravity with a similar composition to our own planet.

Possibility of Life: With a similar atmosphere and composition to Earth it is possible life could exist on GJ 667Cc.

Kepler-22b 

Kepler 22b is 620 light years from Earth .

The planet was discovered in 2009 and confirmed in December 2011.

 •It was the first confirmed discovery by the Kepler Mission of a planet known to orbit in the habitable zone. 

Kepler 22b Statistics:-

Host Star: Kepler 22
Host Star Type: Yellow Dwarf - G Type Main Sequence
Host Star Mass: 98% of Sun
Constellation: Cygnus

Distance from Earth: 620 light years
Planets Detected in System: 1

Discovery Date: 5th December 2011
Detection Method: Transit Method
Exoplanet Type: Super Earth

Mass: Around 6 to 14 x Earth depending on composition
Diameter: 2.4 x Earth

Distance from host star: 79 million miles (128 million km)
Orbital Period: 290 days

Atmosphere: Unknown.
Temperature: Unknown, With a similar atmospheric composition to Earth surface temperatures would be around 22C (72F).
Gravity: Unknown, an Earth like composition would result in gravity around 2.4 x Earth.

Possibility of Life: With an Earth like composition and atmosphere it is possible life could exist in its oceans.