How many habitable planets does each Sun-like star have?
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The classification system of stars by color and magnitude is very useful. By surveying our local region of the Universe, we find that only 5% of stars are as massive than our Sun is. It is thousands of times as luminous as the dimmest red dwarf star, but the most massive O-stars are millions of times as luminous as our Sun. About 20% of the total population of stars out there fall into the F, G, or K classes.
And M-class stars emit copious amounts of X-rays, possibly enough to sterilize the surface of any terrestrial planet orbiting it.All inner planets in a red dwarf system will be tidally locked, with one side always facing the star and one always facing away, with a ring of Earth-like habitability between the night and day sides.
The first thing that should be emphasized, however, is that there's no disagreement over the Kepler data itself! When a planet is fortuitously aligned with its parent star and our line-of-sight, it will transit across the face of the star once per orbit, blocking a fraction of the star's light for a small amount of time. The more transit events we build up, the stronger the signal gets. Owing to Kepler's mission, we've discovered thousands of stars with exoplanets around them.
A second source of uncertainty arises from the big question of"where is the habitable zone?" We typically define this as the range of distances an Earth-sized planet with an Earth-like atmosphere could exist from its parent star and still have liquid water on its surface. The answer to this question is much more difficult to obtain.
However, the largest source of uncertainty might come from failing to adequately estimate which worlds are Earth-like based on their radius alone.
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