The Hunt for Terrestrial Planets

Radial velocity surveys have announced 5 MEarth planets in the solar neighbourhood. However, the radial velocity detection method is extremely mass dependent as its signal is proportional to the ratio of the planet and stellar mass. In order to detect terrestrial planets around stars via the Doppler technique, it is necessary to increase the sensitivity of the detection method and/or decrease the mass of the target primary stars. Both strategies are being pursued. Existing surveys teams expand their allocation of time and construct new dedicated or larger telescopes (e.g. HARPS-N, Automated Planet Finder). Alternatively, a number of groups are con-

Mean habitable zone distance (AU) 0.03 0.05 0.10 0.20 0.50 1.00 2.00

Mean habitable zone distance (AU) 0.03 0.05 0.10 0.20 0.50 1.00 2.00

. 10 MEx 6Me \

S/N break-even point between optical and NIR surveys is early- to mid-M SpT

7 N.

\ NIR RV

OPTICAL RV

: 3ME\

1Me """

MeaNntrinsc RV jitter ~ 4 m/s -

Less intrinsic RV"

itter in NIR?

M9V M6V

AsteroseigmologynoiseX M3V M1V G2V

Msta/M

Msta/M

Fig. 6.13. The plot (from John Rayner) indicates how the power of radial velocities maybe further extended by carrying out surveys in the infrared and around lower mass stars. Such surveys have the potential to detect close-orbiting planets down to terrestrial masses.

structing promising high precision infrared spectrographs (e.g., Fig. 6.13, Precision Radial Velocity Spectometer, www.roe.ac.uk/ukatc/prvs)

The impetus of discovery and characterisation means that exoplanet discovery should continue to increase as objects are found from a wide range of techniques. The power of characterisation using several techniques has already been proven for GJ436 and HD209458. A much deeper understanding of exoplanets and our Solar System should become apparent once such data exists for a large sample. In the near term the continuing powerful combination of radial velocity together with transit photometry and timing is very pomising. Astrometric (e.g., CTIO) and interfero-metric (e.g. Magellan Ridge) imaging measurements should also provide important constraints. Notwithstanding an increasing rate of discovery and characterisation of an even broader spectrum of exoplanets, the next few years should continue to bring dramatic improvements in the realism of exoplanet formation and evolution simulations.

The revolution in exoplanet science is just beginning. At the moment we are close to being sensitive to Earth-mass exoplanets. While we need a better idea of planets in general terms as a function of host star properties, planet mass, composition (gas, ice, rock, metal) and orbit parameters and the intercorrelation of these parameters the recent launch of COROT and impending launch of Kepler means that progress should continue to accelerate. It will be fascinating to see the importance of environment on exoplanets, not only on the major planets that we detect over the next few years but also the minor constituents such as comets and asteroids which are already been constrained with improved transit timings. Substantial research into chemical differentiation will be necessary and allow the serious investigation into the extent to which terrestrial-like planets have non-equilibrium atmospheres.

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