CAUP Researchers: Nuno C. Santos, Sérgio A. G. Sousa
The HARPS search for southern extra-solar planets
We present the discovery of three new giant planets around three metal-deficient stars: HD5388 b (1.96 MJup), HD181720 b (0.37 MJup), and HD190984 b (3.1 MJup). All the planets have moderately eccentric orbits (ranging from 0.26 to 0.57) and long orbital periods (from 777 to 4885 days). Two of the stars (HD181720 and HD190984) were part of a program searching for giant planets around a sample of ∼100 moderately metal-poor stars, while HD5388 was part of the volume-limited sample of the HARPS GTO program. Our discoveries suggest that giant planets in long period orbits are not uncommon around moderately metal-poor stars.
Soon after the discovery of the first extra-solar planets, astronomers found evidence for a positive correlation between the metallicity of the host stars (reflecting the amount of solid material in the proto-planetary disk) and the presence of a giant planet. This observational result suggests that these planets are formed more easily around metal-rich stars, a result that is consistent with the core accretion model for planet formation.
To better constrain the frequency of planets around metal-poor stars, a radial velocity survey for planets orbiting a sample of 104 metal-poor dwarfs started in 2003 making use of the HARPS spectrograph (in the context of the HARPS GTO program). Preliminary results of this survey indicate that at least three of these stars host a planet (including the one previously reported in ). Further results for this survey, which revealed a total of 3 giant planets and another promising candidate, were published in early 2011, in two separate papers,  and . In addition to this sample, in this highlighted paper, we also present the results for a star from a large volume-limited survey for giant planets, part of another HARPS GTO program.
The radial-velocity time series of each of the stars (example shown in Figure 1), obtained between September 2003 and September 2009, was studied to search for any signal present in the data that could indicate the presence of orbiting planets. This study detected the presence of three giant planets orbiting the stars HD 5388, HD 181720, and HD 190984 (Table 1).
These results indicate that the frequency of planets around stars with about 1/4 the metal content of the Sun should be at least as high as ∼3% and further analysis of the data allowed us to take this estimate to ∼11% . This suggests that overall, giant planetary systems around solar type stars may be more frequent that previously thought.
Interestingly, no short period giant planets were found orbiting stars of the sample. All giants have long period orbits. Whether this trend reflects a true physical correlation or is simply due to the small number statistics is still to be confirmed, but if real it may provide new constraints for the models of planet formation and evolution.
This project was led by the CAUP member Nuno C. Santos and the spectroscopic analysis of the stars in this survey was performed by Sérgio G. Sousa, also a CAUP member; both of them took part in the observation of the target stars.
 N. C. Santos, M. Mayor, F. Bouchy, F. Pepe, D. Queloz, S. Udry, The HARPS search for southern extra-solar planets. XII. A giant planet orbiting the metal-poor star HD 171028, Astronomy and Astrophysics, Volume 474, Issue 2, pp. 647-651 (2007)
 N. C. Santos, M. Mayor, X. Bonfils, X. Dumusque, F. Bouchy, P. Figueira, C. Lovis, C. Melo, F. Pepe, D. Queloz, D. Ségransan, S. G. Sousa, S. Udry, The HARPS search for southern extrasolar planets XXV. Results from the metal-poor sample, Astronomy and Astrophysics (accepted)
 S. G. Sousa, N. C. Santos, G. Israelian, C. Lovis, M. Mayor, P. B. Silva and S. Udry, Spectroscopic characterization of a sample of metal-poor solar-type stars from the HARPS planet search program, Astronomy and Astrophysics (accepted)