NASA Kepler mission spots tiniest exoplanet yet

A comparison of the planets orbiting Kepler-37 and planets from our own solar system. (Image: NASA/JPL)
Astronomers with NASA's Kepler mission announced today that they have discovered a new planetary system that is not only the one of the smallest they've found so far, but it contains the smallest exoplanet found orbiting a Sun-like star.

The three planets in the system — Kepler-37b, Kepler-37c and Kepler-37d — orbit Kepler-37 (if you didn't catch the pattern), a star 215 light years away that is in the same spectral class as our Sun, but slightly smaller and slightly cooler.

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Kepler-37b is the the smallest of the three, with a diameter of just 3865 kms (that's just slightly larger than the Moon!), and closest, with an orbital distance of 15 million kilometers (one quarter the distance Mercury orbits the Sun!). Surface temperature on the tiny planet is likely around 400°C and it orbits Kepler-37 in just 13 days!

Next out is Kepler-37c, with an orbital distance of 20 million kms (about one-third the distance that Mercury orbits) and it's about three-quarters the size of Earth.

The outermost planet in the system is Kepler-37d, which is about twice the size of Earth and orbits at roughly half the distance from Mercury to the Sun.

With new exoplanets being discovered all the time (this makes 105 for the Kepler mission), the excitement at these discoveries might be wearing off. However, what makes this particular discovery so cool is the fact that we can find planets so small when it wasn't too long ago that we were only able to see massive planets that were the size of Jupiter or larger.

The Kepler mission finds planets by looking for 'transits' — a dip in the intensity of the light detected from a star — due to a planet orbiting it blocking some of the light as it passes across our view of the star. Sometimes this is a daunting task, especially for small planets, because the activity of the star (starspots, stellar flares, etc) can make the brightness vary so much that it 'drowns-out' any transits that might be there. When a transit is spotted, the planet is confirmed using Earth-based telescopes to examine the star in the infrared spectrum. This can pick up the heat emitted from a planet, verifying that it is actually there and it isn't a 'false positive'.

Once a planet is confirmed, astronomers can tell how big it is by comparing the size of the star to how much light the planet blocked during its transit. However, in this case, the size of Kepler-37 wasn't known very well, so astronomers used a method known as asteroseismology — the study of how oscillations of the star's surface affect its brightness — to get better a better estimate how big it is. Since Kepler-37 is a fairly quiet star, with only small oscillations, they were able to get very accurate measurements of its size, which allowed them to get very accurate measurements of the sizes of the planets orbiting it.

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Another cool part of this discovery is that the asteroseismology study was crowd-funded. The White Dwarf Research Corporation did the work, using money from private sources through their Pale Blue Dot Project. People who sign on to the project can 'adopt' a star for $10, and all the money goes towards funding science. They pooled thousands of dollars from the project to do the asteroseismology work on Kepler-37 to figure out the exact sizes of these planets.

The adopted parent of Kepler-37, SERGE, must be pretty proud.

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