Earth’s Cousin


Since its launch in 2009, NASA’s Kepler Space Telescope has been hunting planets that could be other Earths. Each discovery is a significant milestone in humanity’s efforts to find evidence of life elsewhere in our galaxy; astronomers are looking for a planet that mimics the Earth’s characteristics to be able to support life.

Some of the previously discovered planets were found in the habitable zone of its solar system, but are all at least 40% larger than Earth’s size. The habitable zone is the region around a star within which a planet can sustain liquid water on its surface given the right atmospheric conditions; being in this zone does not mean that the planet is habitable though.

Life on a planet requires the presence of liquid water; so a planet with the potential for life would not be too close to the star, which would be too hot that the water vaporizes, yet not too far than the star, where it would be too cold that the water freezes. Size is a critically important factor; scientists said that if, for example, a planet is about 50% wider than Earth, and it is packing a lot of mass, its gravity could attract a hydrogen-helium envelope, shrouding the surface in a gassy atmosphere that is too thick for Earth-like life.

Several months ago, NASA announced that its Kepler Telescope has uncovered a new solar system about 500 light years away from Earth and they gave it the name Kepler 186; this star is an M-dwarf, also called a red dwarf. M-dwarf stars are usually cooler and dimmer than sun-like stars; that is why their habitable zones are located much closer to the star. These stars also gravitationally interact with their planets, causing tides that heat the planet and often cause their rotations to be tidally locked, which means one side always faces the star and the other faces the old open space, much like our moon is tidally locked with Earth.

Researchers had previously thought that tidal locking would make a planet inhospitable to life; as such a world would have one scorching-hot hemisphere and another freezing cold hemisphere. More-recent modeling work suggests that air or water currents could transport heat around such planets, evening out their temperatures.

There are five planets orbiting the M-dwarf star Kepler 186; four of them are too close to the star, therefore they are too hot to have liquid water. The outermost planet, Kepler-186f, is the first planet with a size similar to Earth to be discovered in the habitable zone of another star. According to the astronomers’ observation, Kepler-186f takes only 130 days to orbit its star, which means it is on a tighter orbit than Earth is.

This planet receives one-third of the energy from its star as what the Earth receives from our Sun; it would still be warm enough to prevent liquid water from freezing, in case it has an atmosphere that provides a substantial greenhouse effect. At high noon, the planet’s surface is only about as bright as the Earth is at sunset.

The planet’s size influences the strength of its gravitational pull, and its ability to pull in abundant gases as hydrogen and helium. At this size, Kepler-186f has a small chance that it could have gathered up a thick hydrogen and helium envelope, so there is a good chance that it does have a rocky surface as Earth.

Actually, it is not easy to know if the planet is habitable; astronomers need to get a sense of the atmosphere and its greenhouse effect. Still, Kepler-186f, nicknamed “Earth’s cousin”, is considered a huge discovery that proves the existence of worlds that might be similar to our own, and will undoubtedly shape future investigations of exoplanets that could have terrestrial surface environments.


*Published in SCIplanet, Winter 2015 Issue "Bare Necessities".

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SCIplanet is a bilingual edutainment science magazine published by the Bibliotheca Alexandrina Planetarium Science Center and developed by the Cultural Outreach Publications Unit ...
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