Abstract
The ∼1000 nearest ultra-cool stars (spectral type M6 and latter) represent a unique opportunity for the search for life outside solar system. Due to their small luminosity, their habitable zone is 30-100 times closer than for the Sun, the corresponding orbital periods ranging from one to a few days. Thanks to this proximity, the transits of a habitable planet are much more probable and frequent than for an Earth- Sun analog, while their tiny size (∼1 Jupiter radius) leads to transits deep enough for a ground-based detection, even for sub-Earth size planets. Furthermore, a habitable planet transiting one of these nearby ultra-cool star would be amenable for a thorough atmospheric characterization, including the detection of possible biosignatures, notably with the near-to-come JWST. Motivated by these reasons, we have set up the concept of a ground-based survey optimized for detecting planets of Earth-size and below transiting the nearest Southern ultra-cool stars. To assess thoroughly the actual potential of this future survey, we are currently conducting a prototype mini-survey using the TRAPPIST robotic 60cm telescope located at La Silla ESO Observatory (Chile). We summarize here the preliminary results of this mini-survey that fully validate our concept.
Highlights
Confined for centuries to the rank of pure speculation, the existence of life outside our solar system is at the edge of gaining its status of testable scientific hypothesis
A habitable planet transiting one of these nearby ultra-cool star would be amenable for a thorough atmospheric characterization, including the detection of possible biosignatures, notably with the near-to-come JWST
Even if only 10% of them has a habitable planet, there should be at least 2 to 3 of these planets waiting to be caught in transit and studied by JWST and other future facilities
Summary
Confined for centuries to the rank of pure speculation, the existence of life outside our solar system is at the edge of gaining its status of testable scientific hypothesis. In the very-near-IR, the water molecule and OH radical contributes a number of absorption bands (as well as significant emission for OH = airglow) which bring unavoidable important levels of red noise in photometric time-series For these reasons, a thorough assessment study based on a prototype survey was mandatory to validate our concept. TRAPPIST is equipped with a near-IR optimized CCD camera offering excellent quantum efficiencies from 300 to > 900nm, making it a good prototype for the envisioned survey This mini-survey is called UCDTS (Ultra-Cool Dwarfs Transit Survey), and its concept is to monitor in a wide near-IR filter each ultra-cool star during at least 50 hours spread over several nights, with the goals to assess the typical photometric precisions that can be reached for these stars on nightly timescales, the resulting detection thresholds for terrestrial planets, and to identify in the mean time the astrophysical and atmospheric limitations of the concept.
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