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Kepler-38: the Galaxy's Count of Binary Stars with Planets is Increasing
Artist's depiction of Kepler-38.
An artist's depiction of Kepler-38b in transit across the primary star. The artist is Naomi Orosz.

When it comes to planets orbiting binary stars (where two stars are orbiting each other), there are "S-type" planets and "P-type planets." An S-type planet orbits one of the stars only, whereas a P-type planet orbits both stars (also called circumbinary).

With the acceptance of the paper The Neptune-Sized Circumbinary Planet Kepler-38b, by Jerome A. Orosz, William F. Welsh, et al, for publication in the Astrophysical Journal (, the count of confirmed Kepler binary stars with planets circling both stars (circumbinary planets) has increased to 4.

Photo of Jerry Orosz
Jerome A. Orosz

The latest discovery, Kepler-38b, is roughly Neptune-size, with a radius 4.35 times Earth radius. It is in a 15-week orbit (106 days, just shy of 4 months), around a binary star consisting of a larger star 1.76 times the radius of the Sun, but only 0.95 times the mass of the Sun, paired with a low-mass star only 0.27 times the radius of the Sun and ¼ the mass of the Sun.

All of the first four Kepler circumbinary planets, Kepler-16b, Kepler-34b, and Kepler-35b, and Kepler-38b, have radii smaller than Jupiter’s and orbits that are only modestly larger than what would be required for a stable orbit.

Since about ⅓ of all stars are either binary or multiple star systems, finding planets in binary star systems has very important implications not only for estimating the total numbers of planets that exist, but for how star-planet systems form as well. Binary stars are also highly valued by astronomers because measuring their orbital properties allows us to directly compute the masses of the component stars (using classical physics—the Universal Law of Gravitation and Newton's Laws of Motion). If the stars also eclipse each other, we can further determine other stellar properties such as the radius and density.

photo of William Wlesh
William F. Welsh

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