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What is the habitable zone? Fast facts What is the habitable zone?
When searching for possibly habitable exoplanets, it helps to start with worlds similar to our own. But most of these Earth-sized worlds have been detected orbiting red-dwarf stars; Earth-sized planets in wide orbits around Sun-like stars are much harder to detect.
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This infographic compares the characteristics of three classes of stars in our galaxy: Sunlike stars are classified as G stars; stars less massive and cooler than our Sun are K dwarfs; and even fainter and cooler stars are the reddish M dwarfs. Habitable zones potentially capable of hosting life-bearing planets are wider for hotter stars. Planets in a red dwarf's comparatively narrow habitable zone, which is very close to the star, are exposed to extreme levels of X-ray and ultraviolet UV radiation, which can be lookin to hundreds of thousands of times more intense than what Earth receives from the Sun.
The artist's conception shows a hypothetical planet with two moons orbiting in the habitable zone of a red dwarf star.
Where are we looking for life, and why? An old joke offers an answer: Asked why, on a dark night, he was looking for his missing car keys beneath a street lamp, the man answered, "because the light's better. But it makes sense, at least at first, to search for something more familiar.
Who we are looking for
Life as we know it should be easier to find. And "the light's better" in the habitable zone, or the area around a star where planetary surface temperatures could allow the pooling of water.
Other similarities to Earth come into sharper focus in the search for life. Yet these red-dwarfs have a potentially deadly habit, especially in their younger years: Powerful flares tend to erupt with some frequency from their surfaces.
These could sterilize closely orbiting planets where life had only begun to get a toehold. Because our Sun has nurtured life on Earth for nearly 4 billion years, conventional wisdom would suggest that stars like it would be prime candidates in the search for other potentially habitable worlds.
G-type yellow stars like our Sun, however, are shorter-lived and less common in our galaxy. Stars slightly cooler and less luminous than llooking Sun — called orange dwarfs — are considered by some scientists as potentially better for advanced life.
They can burn steadily for tens of billions of years. This opens up a vast timescape for biological evolution to pursue an infinity of experiments for yielding robust life forms.
And, for every star like our Sun there are three times as many orange dwarfs in the Milky Way. The K stars, especially the warmer ones, have the best of fot worlds. If you are looking for planets with habitability, the abundance of K stars pump up your chances of finding life. Image credit: NASA.