Detecting Earthmass planets

The threshold of the lowest-mass planet detectable by the Doppler technique keeps decreasing. Today, with the currently achieved precision of about 1 ms-1, Neptune-mass planets can be discovered. Nobody has yet explored in detail the domain below the 1ms~1 level. The measurements on ^ Ara have demonstrated that it is possible to beat the stellar pulsation noise by investing sufficient observing time. However, one open issue remains unsolved: the behavior of the stars on longer timescales, where stellar jitter and

Figure 10. The mass-radius diagram from stars to planets. The low-mass end of the diagram can be investigated by the combination of precise transit search and radial-velocity measurements. With the combination of satellites like COROT and Kepler with a HARPS-like instrument, the Neptune- to Earth-mass domain will become accessible. The inserted diagram on the top left shows a simulation of the radial-velocity variation caused by two Earth-mass planets on a four-day orbit around a solar-type star. With the HARPS precision of about 60cms_1, only 50 measurements are needed to determine the planet mass with an accuracy within 10%.

Figure 10. The mass-radius diagram from stars to planets. The low-mass end of the diagram can be investigated by the combination of precise transit search and radial-velocity measurements. With the combination of satellites like COROT and Kepler with a HARPS-like instrument, the Neptune- to Earth-mass domain will become accessible. The inserted diagram on the top left shows a simulation of the radial-velocity variation caused by two Earth-mass planets on a four-day orbit around a solar-type star. With the HARPS precision of about 60cms_1, only 50 measurements are needed to determine the planet mass with an accuracy within 10%.

spots may impact the final achievable accuracy. In this case, an accurate pre-selection of the stars may help focus on 'good' candidates and optimize the observation time. In addition, bisector analysis and follow-up of activity indicators such as log RHK, as well as photometric measurements, would allow for the identification of potential error sources.

Nevertheless, to discover an extrasolar planet by means of the Doppler technique requires that the radial-velocity signal induced by the planet be significantly higher than the dispersion, or alternatively, requires that a large number of data points be recorded. This is particularly important to rule out artifacts, given the relatively high number of free parameters in the orbital solution, especially for multi-planet systems. A large number of measurements could overcome this problem, but would demand an enormous investment of observing time.

The many ongoing transit surveys may provide another interesting route to the characterization of very low-mass planets. If one considers a transit signal with a known orbital period, it is obvious that measuring its mass is less demanding, both on the number and the accuracy of the radial-velocity measurements. For example, a 2 M§ planet on a 4-day orbit would make a radial-velocity amplitude of about 80cms-1. Given the present precision of HARPS, which is estimated to about 60cms-1, it may be possible to detect and measure the amplitude of the radial-velocity wobble with only few radialvelocity measurements—provided that the period of the system is known in advance (see Figure10 and included plot).

The COROT (2007-) and Kepler (2009—) space telescopes will provide many Neptune-sized and even Earth-sized planet candidates with orbits similar to, or smaller than, that of j Arac. The radial-velocity follow-up of these candidates will deliver their precise mass and orbit. When this information is combined with the transit-observation parameters, one obtains the mass-radius relation of planets in the domain of very low masses. This combined approach is currently being successfully carried out for the Optical Gravitational Lensing Experiment (OGLE) planetary candidates of about the mass and the size of Jupiter (Figure 10; see e.g., Pont et al. 2005). The almost mandatory radial-velocity follow-up of the COROT and Kepler planetary candidates is clearly within reach of the capabilities of a HARPS-like instrument. In this context, the most exciting aspect is the opportunity to explore mass-to-radius relation down to the Earth-mass domain.

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