Exploring the universe

Earth-like planets around G-type stars

Kepler telescope was launched by NASA on 7 mars 2009 to monitoring a large number of stars in the constellations Cygnus, Lyra, and Draco. The telescope was placed in orbit around the sun. The main goal was to look for planets that were similar to Earth. The ideal Earth 2.0 will have the same size as Earth and orbiting a Sun similar as our and is in the so-called Goldilocks zone of its star, those planets could harbor life if the planet has the correct atmospheric conditions and liquid water. The thought was also by monitoring approximately 100 000 stars we will be able to get statistics on how many Earth-similar planets that orbit sun-like stars it is in our galaxy.

Kepler-22 is a star in the northern constellation of Cygnus 600 light years away. It is a sun-like G-type main-sequence star a so-called yellow dwarf. Kepler 22 is just is slightly smaller and cooler than our Sun. The first transit of a planet orbiting Kepler 22 was observed already on the third day after Kepler's service begun on 12 mars 2009. The third transit was observed on 15 December 2010. These measurements were confirmed by observations made by the Spitzer telescope and from Earth-based telescopes. On 5 December 2011, the announcement about the first potentially habitable exoplanet has been discovered was released.   

Kepler 22 b is more than twice the size of Earth it has a radius of 2.35 times Earth's radius and 20 times more massive. As gravity is proportional to the mass of the planet divided with radius squared.

$$ g\propto \frac{1}{r^{2}}\Rightarrow \frac{20.36}{2.35^{2}}  =3.68  $$

It will have about 3.6 times the gravity than Earth. Kepler 22 b has a similar orbit as Earth a year on Kepler 22 b, is 290 days long. Kepler 22 b is a so-called Super Earth. It could have oceans but it could have an environment that is closer to Neptune.  


Image credit: NASA/Ames/JPL-Caltech

The first-ever exoplanet found around a G2V star as our sun and at the same distance from its star as Earth is Kepler-452b. The planet is often quoted Earth 2.0 or Earth’s cousin. It is 1.63 times larger in radius than Earth and is 4.7 times more massive than Earth. The planet is a Super-earth. There are also indications that the planet has water on the surface, which increase the possibility of life. A year on Kepler-452b is 380 days long just 15 days longer than Earth. The planet is about 1,400 light-years away from the Solar System. The star 1.4 billion years older than the Sun so the aliens there should have a much longer time than us to evolve their civilization. As the planet has twice the gravity than Earth the aliens would be twice as strong as we are.

$$ g\propto \frac{1}{r^{2}}\Rightarrow \frac{4.7}{1.63^{2}}  =1.77$$

The planet was discovered on 23 July in 2015 by the Kepler telescope.

The closest star that has exoplanets and that is similar to our own Sun (G-type star) is called Tau Ceti it is only just 12 light years away. Tau Ceti mass is 78 percent of our Sun. Tau Ceti has 7 confirmed exoplanets. Tau Ceti e is on the inner border to the habitable zone where it might be too hot and Tau Ceti f is on the outer border to the habitable zone where it might be too cold, but it is still a very interesting object with high Earth similarity index.

The farthest potentially habitable exoplanet discovered is Kepler-1638 b at approximately 2,900 light-years from Earth and it orbits a G-type star. With similar size and age as our Sun. Kepler-1638 b has a mass 8 times earth and its radius is 1.87 times Earth radius and 2.26 times the gravity

$$ \frac{8}{1.87^{2}}  =2.28$$

it was discovered in 2016. Two similar planets were also discovered the same year Kepler-1606 b and Kepler-1090 b

These are all the potentially habitable planets that been discovered around G-type stars so far and Kepler-452 b is the most Earth-like planet discovered yet. Planets that have the same size as Earth has been discovered around red dwarfs stars, but those stars could have dangerous for life radiation and their planets have tidally locked orbits. The new space telescope TESS will now continue the search for Earth 2.0 now when the Kepler telescope has retired.


Kepler-1638 b Kepler-1606 b Kepler-1090 b Kepler-452 b tau Cet e tau Cet f Kepler 22 b