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Exoplanets

Yale Astronomy

M-to-K

The goal of the M-to-K project is to discover the planetary companions to the lowest mass stars, and to understand the factors which determine how and why planets form around such stars.

M-to-K is a Doppler survey for exoplanets orbiting early M through late K type stars. These stars are smaller and lower in mass than our Sun. Low mass stars tend to be under-represented on current Doppler surveys because they are intrinsically faint and therefore large telescopes are required to pick up a strong enough signal. Yet these stars comprise the vast majority of stars in our Galaxy, far outnumbering G-type stars like the Sun.

We are especially interested in M and K stars because planets in the habitable zone should be more easily detected with our Doppler technique. Detectability is also enhanced because the astrophysical noise from M and K type stars is lower than that of G and F type stars. The most massive stars (A, B and O types) are not good candidates for Doppler planet searches because the high luminosity of the stars ionizes most atomic spectral lines and the stars rotate faster (broadening the few spectral lines); this reduces our ability to make precise measurements of spectral line shifts.

The habitable zone is a distance from the star where it is not too hot (so that liquid water evaporates away) and not too cold (so that liquid water is frozen). It is sometimes called the "Goldilocks zone" because the temperature is "just right" for retention of oceans of water.

The low intrinsic brightness of M and K type stars means that less energy is being emitted from the star. As a result, the "Goldilocks zone" where planets intercept enough energy for liquid water to exist is located closer to the host star. This proximity makes it easier for Doppler surveys to detect prospective planets for two reasons: (1) the orbital periods are shorter (shortens the length of time we need to look) and (2) the signal that we are trying to detect (the reflex velocity that a planet exerts on its star) is larger because the gravitational force of the planet on the star increases rapidly with decreasing distance. The Figure above depicts the relative distance between the host stars and planets for different star classifications.

We are taking advantage of the recent identification of more than 3,000 nearby K and M dwarfs from the SUPERBLINK survey of collaborator, Sebastien Lepine. The target stars have been pre-screened with low resolution spectroscopy to reduce the number of spectroscopic binaries and flare stars. The goal of this project is to screen up to 1,600 low mass stars for the presence of exoplanets.

Doppler measurements for the M-to-K program are carried out at Keck Observatory on Earth's largest telescope, located on Mauna Kea in Hawai'i. We gratefully acknowledge support for telescope time at Keck from Yale University and NASA.

Discoveries

  • SUPERBLINK has now discovered a few thousand M and K dwarfs which are closer than 30 parsecs (our stellar "back yard")
  • The M-to-K project has successfully discovered two planets around low mass stars, including a rare Jovian Mass companion to HIP 79432 (Reference)