On December 20, 2011, NASA’s Kepler spacecraft Astronomers have discovered two Earth-size-planets revolving around a distant sun-like-star. These two planets have been named Kepler-20e and Kepler-20f. This is the first time that a planet with mass nearly equal to mass of Earth has been discovered. These are the first and the smallest exoplanets which have been found revolving around a sun-like-star. When these two planets passed in front of their star, some small dimming of light occurred, which finally led to their discovery.

Star: Kepler-20

About 950 light-years away in the constellation Lyra, there is a star and its planet-system similar to our solar system. This star has been given name Kepler-20. Its radius is 94% of the Sun’s radius and mass is 91% of the Sun’s mass. Its photosphere-temperature is 5,466 K (slightly cooler than the Sun). It is G-Type star similar to our Sun. Precisely speaking its stellar class is G8.

Its metallicity (presence of elements other than H or He) is roughly same as that of the Sun.

Kepler-20 has five planets. these planets are Kepler-20b, Kepler-20c, Kepler-20d, Kepler-20e and Kepler-20f.

These planets have been named after the German astronomer Johannes Kepler (December 27, 1571 – November 15, 1630). He is best known for his three laws of planetary motion.

All the five planets are very close to their parent-star and hence are extremely hot. Orbit radii of all these planets are less than that of Mercury around the Sun. Their time periods for one revolution around their parent-star are :

  • Kepler-20b – 3.7 Days
  • Kepler-20c – 10.9 Days
  • Kepler-20d – 77.6 Days
  • Kepler-20e -6.1 Days
  • Kepler-20f – 19.6 Days

If we arrange the five planets in ascending order of their distances from the parent-star, the order will be:


Earth-Sized Planets: Kepler-20e and Kepler-20f

Before you make any speculation, we should let you know that these two planets are too far from habitable zone (possibility of liquid water essential for life).

Comparing Kepler-20e and Kepler-20f with Venus and Earth

Comparing Kepler-20e and Kepler-20f with Venus and Earth

Both the planets are rocky and extremely hot. The surface temperatures of Kepler-20e and Kepler-20f are 760 °C (1,400 °F) and 427 °C (801 °F) respectively.

Radius of Kepler-20e (slightly smaller than Venus) is 0.87 times the Earth’s radius. Radius of Kepler-20f (slightly larger than Earth) is 1.03 times the Earth’s radius.

Star: Kepler-22

Kepler-22 is a distant G-type star in constellation Cygnus. It is roughly 600 light years away from Earth. Radius and surface temperature of Kepler-22 is slightly less than that of the Sun. Its metallicity (presence of elements other than H or He) is less than that of the Sun.

Habitable Zone Planet: Kepler-22b

On December 5, 2011, NASA’s Kepler spacecraft scientists declared the discovery of the planet Kepler-22b which revolve around the habitable zone of the star Kepler-22.

Artist's Conception of Kepler-22b

Artist's Conception of Kepler-22b

Its radius is 2.4 times Earth’s radius and mass is roughly 10 times Earth’s mass. Its time period for one revolution around its parent-star is 290 days. Its surface temperature is estimated to be 22 °C (72 °F) which is extremely favorable for liquid water (which is essential for life).

This is the first planet discovered ever which is within habitable zone of a star.

Opinion of Experts

Dr. Francois Fressin is one of the astronomers from the Harvard-Smithsonian Centre for Astrophysics in Cambridge, US. He is very excited with the discovery of Kepler-22b, Kepler-20e and Kepler-20f. His conclusions are published in the journal Nature of this month. He said:

The primary goal of the Kepler mission is to find Earth-sized planets in the habitable zone. This discovery demonstrates for the first time that Earth-size planets exist around other stars, and that we are able to detect them.

It is the first time humanity has been able to discover an object similar to the Earth around a star, so maybe we will be able to find others. This could be an important milestone. I think 10 years or maybe even 100 years from now people will look back and ask when was the first Earth-sized planet found. It is very exciting.

You could fit 13 Earths inside Kepler-22b. The most likely thing is that it’s simply a mini-Neptune, not suitable for life. Just because a planet lies within the habitable zone that doesn’t mean it is habitable.

An astronomer at the Harvard-Smithsonian Center for Astrophysics, David Charbonneau said:

In the solar system, rocky worlds and gas giants don’t mingle. But in the Kepler 20 system they apparently do.