An artist’s conception of an M dwarf with a habitable planet (and moons!). Credit: David A. Aguilar (CfA)

The Frequency of (Habitable?) Planets Around M dwarfs

Title: The Occurrence Rate of Small Planets Around Small Stars
Authors: Courtney Dressing and David Charbonneau
Authors’ institution: Harvard-Smithsonian Center for Astrophysics

The Kepler Space telescope has spurred a truly incredible variety of research (and astrobites!) in both planetary science and stellar astrophysics. However, its real reason for existence is as a statistical mission. Kepler is designed to figure out the frequency of Earth-like planets in habitable Earth-like orbits around Sun-like stars: the so-called “eta-Earth.”

As it turns out, this is tricky business. In Kepler’s original 3-year mission, a true Earth analog around a Sun-like star would only have orbited 3 times. Complicating matters, Kepler’s precise photometry has shown us that other stars with the same mass as the Sun tend to be slightly more variable, on average, making it even harder to find planets. So we haven’t figured out eta-Earth yet.

The Advantage of M dwarfs

However, the census of planets for smaller stars—M dwarfs—is now basically complete. This is because, when finding transiting planets orbiting M dwarfs, we have several factors on our side. First, M dwarfs are small. This means that an Earth-sized planet crossing in front makes a deeper transit around an M dwarf than around a G dwarf like the Sun. Second, M dwarfs are much fainter (0.015 to 7.2% of the Sun’s luminosity) which means that the habitable-zone orbits closer to the star. A smaller orbit means that the planet crosses in front of the star much more frequently. Since Kepler finds small planets by phase-folding data (essentially adding the signal from all the observed transits together), observing ~5 times as many transits for each planet means that we’ve actually found most of the Earth-sized “habitable zone” planets transiting M dwarfs in the Kepler sample already.

In this paper, Courtney Dressing (an astrobites alum!) and Dave Charbonneau use this M dwarf advantage to determine the occurrence rate of small planets around M dwarfs.

Read more: The Frequency of (Habitable?) Planets Around M dwarfs | astrobites.

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