Exoplanet News

Using data from the Kepler satellite, scientists have found the first truly Earth-sized planet within the habitable zone of its star.  The planet, dubbed Kepler 186f, orbits at about the same distance as Mercury does around our sun, but because Kepler 186 is a red dwarf, it emits much less light and heat as the Sun does.  Because of this, Kepler 186f is actually on the outer half of the habitable zone, and receives less light than Earth does.  Despite this, scientists believe liquid water would probably exist on the surface, since unlike Mars, Kepler 186f is slightly bigger than Earth, and has more gravity to sustain a thick atmosphere.

Drawing by NASA Ames/SETI Institute/JPL-CalTech

This is a significant discovery in many ways.  Kepler 186f is the first definitively rocky planet found within a star's habitable zone.  We've discovered many other planets in the habitable zone before, but they have all either been gas giants or super-Earths.  We don't understand super-Earths so well, but scientists believe that any planet with more than 1.5 times Earth radius becomes massive enough to hold a thick hydrogen atmosphere.  While these planets probably do have liquid water somewhere, they are also completely different to Earth, and not hospitable to life as we know it.  This planet is different.  With a radius of only 1.1 times that of Earth, Kepler 186f is too small to hold on to significant amounts of hydrogen.  This makes much more similar to the terrestrial planets of our own solar system than to the super-Earths previously discovered.  While scientists point out that because Kepler 186f orbits a red dwarf instead of a sun-like star and is more of an Earth cousin rather than an Earth twin, it is still by far, the most promising planet we've discovered in that regard.

Speaking of red dwarfs, up to this point scientists have not been entirely sure about their suitability for Earth-like planets.   The biggest problem is tidal locking.  The planets we've found within the habitable zone of red dwarfs before this have all been tidally locked to their star, which means they always present the same side to the star, like the Moon does with Earth.  These planets may be suitable for life, or they may not be, we just don't know.  Kepler 186f is far enough away from its star to maintain a rotation for billions of years, which makes it even more suitable for life as we know it.  This also means that red dwarfs have become legitimate places to look for Earth-like planets, which greatly increases the potential number, seeing as red dwarfs make up the vast majority of stars in the universe. 

We don't know yet if Kepler 186f is actually like Earth, we don't know what it looks like, we don't know what its atmosphere is made of, we don't know much about it except where it orbits and how big it is, and it may be a long time before we get any of those answers.  But now we've found one planet that may really be a lot like Earth, and this is only the beginning.  It's only been 20 years since the search for exoplanets really began, and look where we've come.  Just imagine how far we'll get in the next 20 years.

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