Since the first extrasolar planet was discovered around a main sequence (MS) solartype star, 51 Peg, in 1995 (Mayor & Queloz 1995), a total of ~ 200 new planets have been detected by the Doppler surveys using a dozen ground-based telescopes with various sizes, from 0.6 meters to 10 meters, and different Doppler instruments (e.g. Frink et al. 2002; Vogt et al. 2000; Butler et al. 2006; Mayor et al. 2003; Cochran et al. 2007; Sato et al. 2007). Except the planet HD 102195b, which was detected by the recently developed DFDI method (Ge et al. 2006), all of the RV planets were detected with single object high resolution echelle spectrographs. Due to limited telescope resources, the detection rate for RV surveys has reached a plateau of about 30 new planets per year in very recent years (Fig. 2.4). Here we summarize the main results from past RV planet surveys. Details for most of the conclusions can be found in recent review articles (Marcy et al. 2005; Udry, Fischer & Queloz, 2007).
2.3.1 Main Conclusions on Giant Planets
Despite the heterogeneity of the RV planet data sets obtained by many different groups, using different target samples, instruments, telescopes, and data reduction techniques, some basic conclusions based on the existing sample of giant planets can be drawn. For instance,
- The planet mass distribution for giant planets with (Msini > 0.2 Mj) has shown a clear power-law with dN/dM « Mfor the FGKM survey stars (Marcy et al. 2005).
- About 1% of FGK stars in both the Lick+Keck+AAT and CORALIE survey samples host hot Jupiters (orbital period, P<10 d) (Marcy et al. 2005; Udry, Fischer & Queloz, 2007). It appears that single giant planets have a 3 day period pileup, while giant planets in multiple planet systems do not have the 3 day pileup (Wright et al. 2007).
- There are a total of 179 planets detected so far within 200 pc with RV methods and a total of 212 planets detected using Doppler techniques to date.
- About 5-7% (or higher percentage) of FGK stars in both the Lick+Keck+AAT and CORALIE survey samples have giant planets within 5 AU.
- Extrasolar giant planets have much larger eccentricities than the planets in the Solar System. The eccentricities range from 0 to 0.93. The median of the eccentricities of the extrasolar giant planets with orbit radii > 0.1 AU is about 0.25 (Marcy et al. 2005). The eccentricities in multiple planet systems are not higher than that in single planet systems (Wright et al. 2007).
- There appears to be a lack of giant planets with masses larger than ~ 2M jup with intermediate and short periods (of < 100 d) around single stars (Udry, Fischer & Queloz, 2007). Most of the massive planets with short and intermediate periods are found in multiple star systems (Zucker & Mazeh 2002). It appears that the maximum planet mass increases with the distance from the host star (Udry, Fischer & Queloz, 2007).
- There is a strong correlation between the giant planet occurrence and the planet host star metallicity. (e.g. Gonzalez 1997; Santos et al. 2001; Fischer & Valenti 2005). In the Lick-Keck-AAT survey sample, the occurrence of planets is consistent with being proportional to the square of the metallicity of the host stars (Fischer & Valenti 2005). However, the recent detection of giant planets around metal poor stars (Cochran et al. 2007) suggests that the occurrence of planets may be constant at low metallicity (e.g. [Fe/H]^ —0.5; also see Santos et al. 2004).
- Among stars with planets, at least 14% of them have multiple planets. Including those with RV trends, the percentage may be as high as 34% in the Lick+Keck+AAT sample (Wright et al. 2007). However, this high percentage may be partially due to detection bias. A large fraction of multiple planets are in resonant orbits.
- Evolved A type stars (subgiants with 1.1-2.0 M©) have a higher giant planet percentage than solar-type stars (Johnson et al. 2007). G giants have more mas sive planets than solar-type stars (e.g. about 2% of G giants have planets with >5Mj versus about 1% for solar-type stars (Sato et al. 2007)). - Fewer than 1.3% of M dwarfs host giant planets (Endl et al. 2007). The occurrence of low mass planets (super-Earth masses) is much higher than giant planets around M dwarfs (Udry, Fischer & Queloz, 2007). Most of the planets around M dwarfs are in multiple planet systems (Mayor et al. 2007).
In the last 3 years, a new population of low mass planets in the 5-21 Earth mass range with a few day periods have been detected with high precision cross-dispersed echelle spectrographs (Santos et al 2004; McArthur et al. 2004; Butler et al. 2004; Rivera et al. 2005; Bonfils et al. 2005; Vogt et al. 2005; Udry et al. 2006; Lovis et al. 2006; Udry et al. 2007; Melo et al. 2007; Pepe et al. 2007). This becomes possible due to the improvement in Doppler sensitivity of planet survey instruments from 3-10 m/s before 2003 to 1-3 m/s post-2003. So far a total of 13 such planet systems orbiting nearby stars have been announced. Most of them are detected around M dwarfs, which may be due to observational biases.
Although the sample of super Earth mass and Neptune-mass planets is still relatively small, their properties show different trends than the known giant planets. Unlike the giant planets, whose frequency scales as the square of the host-star metallicity (Fischer & Valenti 2006), the Neptune-mass planet frequency seems to weakly depend on the host-star metallicity (see Udry, Fischer & Queloz 2007; Mayor et al. 2007). There also appears to be a lack of the 3-day orbital period pile up for these low-mass planets, unlike the giant planets (Marcy et al. 2005). In addition, the discovery of two low-mass planets with microlensing indicates that cool Neptune-mass planets may be common (Beaulieu et al. 2006; Gould et al. 2006).
All of these early results indicate that they may belong to a distinct planet population whose formation and evolution may be very different from that of the giant planets. For instance, they may be formed without accumulating a substantial amount of gaseous material, unlike the gas giant planets; i.e. they may be terrestrial rocky planets. Their lower mass may make them less capable of opening up a gap in the protoplanetary disk, so they may have undergone a very different migration history than the gas giants, and some may have been formed in situ.
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