Kepler-452b: The Closest Exoplanet Match to Earth

July 23, 2015, was a red letter day for exoplanet astronomers -- it was the day that marked the announcement of the discovery of Kepler-452b, an alien world that possesses more Earth-like traits than any exoplanet discovered before it. Located around 1,400 light-years away, at first this extrasolar planet may not appear very "Earth-like." For starters, it is approximately 60 percent bigger than Earth and likely 5 times more massive. This means that the surface gravity on Kepler-452b would be twice that of Earth's.

But the world has a 385 day orbit around a very sun-like (G2-class) star, putting it right inside of the habitable zone for that star. Previous exoplanet discoveries have either been too big, too small, or have wide or extremely compact orbits; none have been approximately Earth-sized (and probably rocky) with an orbit within the habitable zone of a sun-like star. Shown here are six the small "habitable zone planets" discovered by NASA's Kepler Space Telescope. Each exoplanet orbits their star within the region that allows just the right amount of solar energy to allow water to exist in a liquid state on their surface (assuming they have water and if they even have a "surface"). As you can see, each is wildly different and none are approximately Earth-sized and orbit a sun-like star. Kepler-452b, however, ticks all the boxes, making it an intriguing exoplanet for rigorous study.

Kepler-452b has another fascinating characteristic: it's older than Earth. The star it orbits, Kepler-452, is approximately 1.5 billion years older than our sun, indicating that the star system is quite a bit older than our solar system. According to planetary formation theory, planets form soon after their stars are born. This means that Kepler-452b is an older world, likely existing inside its star's habitable zone for 6 billion years. If there are life's ingredients* on Kepler-452b's surface, including a bountiful supply of liquid water and a life-supporting atmosphere, life could thrive on this world. Of course, the question of life is pure speculation -- we don't know if there's any water or even an atmosphere -- but it is a tantalizing thought.

*Life "as we know it", in any case.

In this artist's impression, the size of Kepler-452's habitable zone is compared with that of our sun's, with the size of the planet's orbits overlaid. Also, the compact star system surrounding the M-dwarf Kepler-186 is included. As can be seen, the habitable zone surrounding Kepler-186 would fit well within the orbit of Mercury around the sun -- red dwarfs are smaller and cooler than G2-class stars, so their habitable zones will be a lot more compact.

To date, 1,030 exoplanets have been confirmed by the Kepler mission, but only a dozen less than twice the size of Earth that orbit within their star's habitable zones have been discovered. All of these approximately Earth-sized "habitable" worlds orbit 3 classes of star: G stars (like the sun), K stars and M stars (a.k.a. M-dwarfs). G-class stars are hotter and more massive than K-class stars and K-class stars are hotter and larger than M-dwarfs. As the star class becomes cooler, the habitable zone around each shrinks.

Currently, it is not known if Kepler-452b even has a rocky surface -- but planetary models suggest that it should have. If this is the case, the alien world would be a fascinating place, leading Kepler mission scientists to speculate that the exoplanet may have a thick atmosphere and, possibly, active volcanoes. But the big question is whether such a world could host life. Unfortunately we'll have to wait until the next generation of space telescopes to possibly observe Kepler-452b's hypothetical atmosphere. To find life, however, we may never know until we have the ability to send a probe there. Alas, at 1,400 light-years distant, we'll be waiting a long time for that to happen.