The success story of the Doppler technique

The advances made in the Doppler technique led to the discovery of brown dwarf companions of solar-like stars in the late '80s (Latham et al. 1989), reached (in 1995) into the hot-Jupiter domain with the discovery of the first planet orbiting a solar-type star (i.e., 51 Peg; Mayor & Queloz 1995), allowed the exploration of the full Saturn-to-Jupiter mass range up to several AU, and finally ended in 2004 with Neptune-mass candidates (Santos et al. 2004; McArthur et al. 2004; Butler et al. 2004b). As illustrated in Figure 1, the minimum masses of thus-discovered planetary companions decreased by almost three orders of magnitude in only 15 years. These huge advances go hand in hand with the continuous and remarkable increase of Doppler precision obtained on solar-like stars. This is best illustrated by the residuals of the fitted orbits to the data, as shown for some of the discovery milestones in Figure 2.

The impact of this amazing development has become evident during the past year: Table 1 shows planetary parameters of the seven Neptune-mass planets discovered in less than one year. Note the extremely low m2 sin i of 5.9 (Gl876d) and m2 sin i/m\ of 4.2 x 10~5 (p Arac) obtained on these objects. These discoveries not only confirm the existence of very low-mass planets around other solar-like stars, but also indicate that these objects must be very common. Therefore, new high-precision observations will help considerably to reduce the observational bias towards low masses and will deliver much data for the understanding of planet formation and evolution. Finally, we should remark that these recent results have made the astronomical community realize that Earth-like planets on short orbits are now within reach of the Doppler technique.

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Figure 1. Evolution of the minimum mass of discovered exoplanets as a function of time.

Figure 2. Evolution of the measurement residuals on orbits of discovered extrasolar planets as a function of time.

Figure 2. Evolution of the measurement residuals on orbits of discovered extrasolar planets as a function of time.

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