Rocky habitable exoplanets

Searching for rocky planets in the habitable zone

Terrestrial planets in the habitable zone of their parent stars are one of the main scientific topics of the next decades in Astronomy, and one of the main science drivers for the new generation of extremely large telescopes. ESPRESSO, being capable of achieving a precision of 10 cm s-1 in radial velocity, will be able to register the signals of Earth-like and massive Earths in the habitable zones (i.e. in orbits where water is retained in the liquid form on the planet surface) around nearby solar-type stars and stars smaller than the Sun.

Since 1995, research teams using the radial velocity technique have discovered over 500 extrasolar planets, some of which with only a few times the mass of the Earth. Today, dozens of detected radial velocity extrasolar planets have masses estimated at below 10 Earth Masses, and most of them were identified using the HARPS spectrograph. The rate of these discoveries increases steadily. The HARPS high-precision radial-velocity program has shown that half of the solar like stars in the sky harbor Neptune-mass planets and super-Earths, a finding also supported by the recent discoveries of the Kepler satellite. These exciting discoveries were made possible thanks to the sub-m/s precision reached by HARPS. Given the faint magnitude of the target star and/or the tiny radial-velocity signal induced by the planet, most of the observed objects would have remained out of reach of existing facilities that were limited to 3 m/s. The most recent planet formation models support the current view that this emerging population is only the tip of the iceberg.

Figure 1. 10 years of Tau Ceti’s radial velocities as measured by HARPS. The overall dispersion in of 1 m/s. Time-binning of the data will reduce the dispersion as expected with the square root of the number of observations down to 20 cm/s. Most importantly, yet, is the absence of any long-term trend, proving thus the exquisite precision of HARPS.

Considering the observational bias towards large masses, on one hand, and the model predictions, on the other hand, one should expect a huge amount of still undiscovered low-mass planets, even in already observed stellar samples. ESPRESSO is designed to explore this new mass domain and charter unknown territory (see Figure 2). This goal can only be obtained by combining high efficiency with high instrumental precision. ESPRESSO will be optimized to obtain best radial velocities on quiet solar-type stars. A careful selection of these stars will allow focusing the observations on the best-suited candidates: non-active, non-rotating, quiet G to M dwarfs. The high efficiency of the instrument and observation scheme will permit the characterization of the demanding planetary systems and the stellar noise that frequently hampers planetary detections.

Figure 2. Detectability of planets orbiting a 0.2-Msol star (red solid line) and a 1-Msol star (green solid line) in the mass vs semi-major axis plane expected for ESPRESSO. The detectability curves have been calculated assuming a velocity amplitude of 10 cm/s (for the 1-Msol star) and 40 cm/s (for the 0.2-Msol star), null eccentricity, and sin i = 1. Known radial-velocity planets of solar-type stars are plotted as open circles, and the planets of the solar system (solid circles) are labeled. The “habitable zones” of 0.8-1.2 Msol and 0.2-0.3 Msol stars are indicated with blue and pink dotted lines, respectively. These are regions where rocky planets with a mass in the interval 0.1-10 Mearth can retain liquid water on their surface.

With a precision of 10 cm s-1 (about a factor of 10 better than HARPS), it will be possible to detect rocky planets of few Earth masses in the habitable zone of solar type stars – for comparison, the Earth imposes a velocity amplitude of 9 cm/s onto the Sun. By extending the sample towards the lighter M-stars, the task becomes even easier since the radial velocity signal increases with decreasing stellar mass. Given its efficiency, spectral resolution, and spectral domain, ESPRESSO will operate at the peak of its efficiency for stars up to M4 spectral type. The discovery and the characterization of this new population of very light planets will open the door to a better understanding of planet formation and deliver new candidates for follow-up studies by transit, astrometry, transit spectroscopy, Rossiter-McLaughlin effect, etc.

Figure 3 Phase-folded radial-velocity variation induced by the P = 58-days period around the K5 dwarf HD 85512.

Another important task for ESPRESSO will be the follow-up of transiting planets. It should be recalled that many KEPLER transit candidates are very faint and can be hardly confirmed by existing radial-velocity instrument. ESPRESSO will play a significant role with this respect. Most important, yet, is the fact that other satellites like GAIA, TESS and hopefully PLATO will provide us with many new transit candidates, possibly hosted by bright stars. ESPRESSO will be the ideal (and maybe unique) machine to make spectroscopic follow-up of Earth-sized planets discovered by the transit technique.

Besides being an exquisite radial-velocity machine, ESPRESSO will provide extra-ordinary and stable spectroscopic observations, opening new possibilities for transit spectroscopy and analysis of the light reflected by the exoplanet. Several groups are currently investigating to which extend this will be feasible in the visible and infrared spectral domain ESPRESSO should for sure be considered as an important intermediate step in view of high-precision spectrographs on ELT’s, as for instance HiReS@E-ELT.

Fast-cadence spectra of the most promising candidates will provide estimates of the maximum frequency of solar-like oscillations.  The resulting seismic constraints on the gravity of the host stars will allow us to improve the stellar parameters determination and, in turn, planet ones. In addition, ESPRESSO will be the ideal –and maybe unique– machine to make spectroscopic follow-up of Earth-sized planets discovered by the transit technique.

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