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SPIRou : detecting earth-like planets in the habitable zone of low-mass dwarfs

Par Jean-Francois Donati - 12/09/2008


SPIRou : detecting earth-like planets in the habitable zone of low-mass dwarfs

90% of the ~300 known exoplanets were discovered with the radial velocity (RV) method, which for Sun-like stars is limited to planets with masses larger than 10 Earth masses. The reason is that the Earth-mass planets induce RV wobbles (a few cm/s around a Sun-like star) that are too small to be detectable with existing instruments. To investigate thoroughly the properties of Earth-like planets, and in particular those located in habitable zones (ie with surface temperatures allowing for the presence of liquid water), low-mass dwarfs are an extremely promising option. The RV wobble induced by an Earth-mass planet is an order of magnitude larger for an M dwarf than for a G star, not only because the planet/star mass ratio is larger but also because the habitable zone is closer to the star (to ensure a similar stellar flux at the planet surface).

Low-mass dwarfs vastly dominate the stellar population (8 of 9) in the solar neighborhood and are likely the hosts of most planets in our Galaxy. Systematic RV observations of nearby M dwarfs should thus provide a broader view on the diversity of planetary formation and yield a quantitative estimate of the fraction of habitable Earth-like planets about the Sun. However, M dwarfs will be difficult to observe with future space missions aimed at investigating habitability of extrasolar planets due to their intrinsic faintness. Ground pre-launch preparation is thus essential for selecting the best few M dwarfs on which Darwin could concentrate. While ground photometry can reveal a number of potential candidates through transits (eg the MEarth project, Irwin et al 2008, arXiv:0807.1316), spectroscopic observations are absolutely necessary both to obtain detections of non-transitiing systems (only 1% of habitable planets in M dwarfs transit their star) and confirm the planetary nature of photometric candidates showing transits. Similarly, follow-up spectroscopic observations will be needed for all Kepler transit detections.

Presently, dwarfs with masses lower than 0.25 Msun are mostly out of reach of current optical extra-solar planet RV surveys as a result of their small sizes and low surface temperatures (fluxes of late M dwarfs peak at ~1.5μm); exploring them therefore requires a high-resolution nIR echelle spectrograph providing simultaneously high RV accuracy (1m/s), high throughput (15%) and wide single-shot spectral coverage (0.98-2.4μm, ie the YJHK bands, to maximise the line content). Low mass stars apparently feature a rich nIR spectrum of atomic and molecular lines whose strengths increase at lower temperatures (eg McLean et al, 2007, ApJ 658, 1217), ensuring that potential planets generate detectable RV wobbles. The impact of telluric lines on the estimated RVs is minimised by observing from the dryest non-polar observatory on Earth (ie MaunaKea) and can be further reduced a posteriori (through subtraction techniques).

By monitoring simultaneously the magnetic field topology of the host star (using the polarimetric capabilities of SPIRou), we can filter out efficiently the activity noise from the RV signal. Preliminary studies indicate that the activity-induced RV jitter of M dwarfs is smaller in the nIR (<4m/s) and roughly constant with spectral type (eg Endl et al, 2006, ApJ 649, 436). With the proposed filtering scheme, RV accuracies of 1m/s should be attainable.

A nIR spectropolarimetric survey of 800 slowly rotating M dwarfs, observed with a peak spectral quality of S/N=250, can be obtained with about 150n/yr on a timescale of 7yr. This corresponds to average observing times of about 0.5hr per star (J=10) and per visit, with an average of 25 visits per star. Extrapolating from the (small) number of very-low-mass planets detected with HARPS/ESO indicates that at least 80 planetary systems hosting planets less massive than 20 Earth masses could be detected with SPIRou. Giant planets around early L brown dwarfs should also be detectable.

SPIRou will also be very useful for detecting planets around young low-mass stars, and young M dwarfs in particular, yielding improved constraints on timescales of planet formation. Previous attempts at detecting planets around young stars have failed or produced false planet claims (eg Setiawan et al 2008, Nature 451, 38) due to their high level of intrinsic activity (magnetic fields & cool spots). Going to the infrared (where the spot/photosphere contrast is much lower) will provide a drastic improvement by strongly reducting the activity jitter in the RV curve (by at least a factor of 5 between the V and H band, Huelamo et al 2008, arXiv:0808.2386). Several tens of K and M classical T Tauri stars featuring narrow spectral lines and mK<10 should be accessible with SPIRou for such investigations.



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