Howe many planets around M dwarf stars? The study: “HADES RV Programme with HARPS-N at TNG. XV. Planetary occurrence rates around early-M dwarfs” of M. Pinamonti (INAF-OATo) recently appeared on A&A

M dwarf stars, with temperature ranging between 2400 and 3900 K and mass between 0.08 and 0.7 solar masses, are ideal targets for the search of exoplanets. This because the most important techniques to search for exoplanets are more effective when applied to stars of this spectral type than to more massive stars. For instance, the method of radial velocity consists in searching for small oscillations of the stars around their rest position during the orbit of their planets. These oscillations are due to the gravitational interaction between star and planet, and it is quite intuitive that the larger is the mass ration between planet and star, the wider are these oscillations.

 

For these reasons, a specific projects has been set up to search exoplanets around M dwarf stars, called HADES (Harps-n red Dwarf Exoplanet Survey). The objective of HADES is to find exoplanets in 106 stars with spectral type M0-M3, e.g. with mass ranging from 0.3 to 0.7 solar masses. The program is based on observations performed with the HARPS-N spectrograph, mounted on the Telescopio Nazionale Galileo. These observations allowed the astronomers the discovery of 11 planets to date. This is significantly larger than the expected number of exoplanets in the HADES sample, predicted to be 3-4. This suggests that something is lacking in our knowledge of the frequency of exoplanets around M dwarf stars.

 

The study led by the astronomer M. Pinamonti (INAF – Astrophysical Observatory of Turin) aimed at obtaining a reliable estimate of the probability for M0-M3 dwarf stars of hosting planets. To this aim, the team has analyzed in a homogeneous way, and adopting a complicated statistical analysis, the HARPS-N spectra of all the HADES stars (which on average were observed 77 times). The results suggest a quite large fraction of M0-M3 dwarf stars hosting low-mass planets (1-10 Earth masses) with orbital period between 10 and 100 days, equal to 85%. Lower probabilities were found for low-mass planets with shorter periods (1-10 days, about 10%) and for massive planets (100-1000 Earth masses, less than 2%).The astronomers also found clues suggesting the presence of 5 new planets, whose extistence will be the subject of future analysis. The study is described in “HADES RV Programme with HARPS-N at TNG. XV. Planetary occurrence rates around early-M dwarfs” , recently appeared on the journal Astronomy & Astrophysics. The team of authors counts 20 astronomers of Italian and Spanish institutions, among whom the astronomers J. Maldonado, L. Affer, G. Micela, and A. Maggio of INAF – Astronomical Observatory of Palermo.

 

The image (click here to visualize the entire figure) shows the 11 planets discovered in the framework of HADES to date. The planets are indicated with red circles whose dimensions depend on the mass of the planets. The green stripes indicate the size of the habitable zone around each star (the habitable zone is the region around a given star where rocky planets can host liquid water on their surface).

 

Mario Giuseppe Guarcello  ( follow mguarce) ( youtube)

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