The "Rare Earth hypothesis" is starting to look extremely unlikely, according to a new report.
How common are planets like Earth? This has been in debate for long amounts of time. Some believe in the Mediocrity principle; which states that planets like Earth are very common, as we are in an infinite universe, and others believe in the Rare Earth hypothesis, which states Earth-like planets, if there are any, are extremely rare. However, thanks to the Kepler spacecraft, the question can finally be put to rest. One in five Sun-like stars in our galaxy have Earth-sized planets which may possibly harbor extraterrestrial life, according to the recent study of Kepler data.
The closest star with an Earth-sized planet in its habitable zone may be as close as 12 light years away, and you may even be able to see this star from Earth, says UC Berkeley graduate studenet Erik Petigura, who led the analysis of the Kepler and Keck Observatory data.
The Kepler telescope's mission was to try and find small, rocky planets with the potential for hosting liquid water, and perhaps the ingredients needed for biology to take hold. For four years, this space telescope monitored the brightness of more than 150,000 stars, recording a measurement every 30 minutes.
For a recent focused study, scientists concentrated on 42,000 sun-like stars, (G and K type stars, respectively), looking for periodic dimmings that occur when a planet transits - or crosses in front of - its host star. A team of scientists from the Kepler mission and the Keck telescope in Hawaii have announced that, from the survey, they found approximately 603 planets, 10 of which are Earth sized and orbit in the stars habitable zone, where conditions permit liquid water on its surface.
Since there are approximately 200 billion stars in the Milky Way galaxy alone, with 40 billion of them similar to our Sun, this gives us approximately 8.8 billion Earth-sized planets in the habitable zone, in the Milky Way alone.
Although this does not necessarily mean these planets are hospitable. Even if they orbit in the habitable zone of a star where the temperature is not too hot and not too cold, the atmospheres of these planets may be thick, making it so hot on the surface that DNA-like molecules could not survive. Others however, may have rocky surfaces that could harbor liquid water suitable for living organisms.
All of the potentially habitable planets found in their survey are around K stars, which are cooler and slightly smaller than the sun.
If the stars in the Kepler field are representative of the stars in the solar neighborhood, then the nearest Earth-sized planet is expected to orbit around a star that is less than 12 light years away from Earth, and can be seen by the unaided eye.