I realize that this discovery was made some time ago, however I stumbled upon it again as being the closest (22 light years) Earth-like exoplanet in its star's "habitable zone."
First things first, I want to be certain that when I refer to the "habitable zone" of a star, I am referring to only a mean average surface temperatures above 0°C and below 100°C. Where water can be in a liquid state, not gaseous (like on Venus), and not ice (like on Mars). That is
ALL it means. It does not mean the planet is habitable for humans.
Gliese 667 Cc is a classic example:
More talk about Gliese 667 Cc, the ‘Holy Grail’ of exoplanets | Space oddities
Beyond Earthly Skies: Potentially Habitable Planets around Gliese 667C
Gliese 667Cc: A new ‘Super-Earth’ basking in the light of three Suns | the II-I- blog
Even some very nice artwork.
File:Gliese 667 Cc sunset.jpg - Wikipedia, the free encyclopedia
Too bad they are all wrong.
Let's look at reality. First, the star Gliese 667 C itself. It is an M1.5V red dwarf, with a surface temperature of 3,700 ± 100° K, a mass of 0.31 M☉, a radius of 0.42 R☉, and a luminosity of 0.0137 L☉. The metallicity of the star is much less than Sol, –0.59 ± 0.10 dex.
Because Gliese 667 C has a spectral type "V" it is a variable star, like Sol (G2V), which means it is prone to solar flares, CMEs, and increasing or reducing its luminosity. Granted, this star is going to live virtually forever (trillions of years), but its light would be much more toward the infrared than Sol.
Now lets consider the planets. Two have been confirmed, and a third is suspected but unconfirmed.
The first confirmed planet is Gliese Cb. It has a mass of ≥5.68 ± 0.23 M⊕, and a semi-major axis of 0.049 AU. One AU equals 149,597,871 kilometers. Which means this planet is only 7,330,296 km (4,554,835 miles) from the surface of its star.
The second confirmed planet is Gliese Cc. It has a mass of ≥4.54 ± 0.38 M⊕, and a semi-major axis of 0.123 ± 0.02 AU. Which means this planet is only 18,400,538 km (11,433,564 miles) from the surface of its star.
The third unconfirmed planet is Gliese Cd. It has a mass of ≥5.65 ± 0.54 M⊕, and a semi-major axis of 0.235 AU. Which means this planet is 35,155,500 km (21,844,615 miles) from the surface of its star.
By comparison, Mercury is 57,909,100 km (35,983,047 miles) from the surface of Sol.
Granted, Gliese 667 C is only 0.31 the mass of Sol, but it would still mean that both the confirmed planets b and c would be tidally locked, with only one side facing their star at all times. If we assume planet c has the same Albedo (0.30) and Infrared Emissivity (0.77) as Earth, then at the distance it is from its star it would have a maximum mean surface temperature of 9.68°C and a minimum mean surface temperature of 8.98°C. Cooler than Earth, but still warm enough to support liquid water.
Even though the mass of planet c is 4.54 times the mass of Earth, because of the star's low metallicity it is reasonable to presume any of its planets will be proportionally less dense. However, size in this case does work toward the advantage of the planet considering its proximity to its parent star. A large rocky planet it is more likely to also have a large iron/nickel molten core capable of generating a magnetic field. Possibly strong enough to retain some of its atmosphere. If a planet loses most of its atmosphere (like on Mars) then water can no longer remain in a liquid state.
If there is liquid water on Gliese 667 Cc, then it will not be on the side of the planet perpetually facing its star. Nor will water be in a liquid state on the far side of the planet that is perpetually in darkness (only in the form of ice). Liquid water could only appear in a band of a few degrees stretching from pole to pole, where the star is perpetually caught between dusk or dawn.
With only 1.37% of the light from the star in the visible spectrum, it will be very dark and gloomy from a human's perspective. It would also have some very interesting weather, assuming it still has an atmosphere.
As far as the artist's rendering, both Gliese 667 A and B orbit each other 230 AU away from Gliese 667 C. That is almost five times the distance Pluto is from our sun. They would appear as pin-pricks, just another two distant stars, barely noticeable.
I understand the human need to sensationalize new discoveries, but it creates misconceptions and ultimately does science a disservice by making it less credible. It is great to be excited by such discoveries, I certain am, but lets keep it real.