I compare three methods for transmitting signals over interstellar distances: radio transmitters, lasers and artificial transits. The quantitative comparison is based on physical quantities depending on energy cost and transmitting time L, the last p
arameter in the Drake equation. With our assumptions, radio transmitters are the most energy-effective, while macro-engineered planetary-sized objects producing artificial transits seem effective on the long term to transmit an attention-getting signal for a time that might be much longer than the lifetime of the civilization that produced the artifact.
An important goal within the quest for detecting an Earth-like extrasolar planet, will be to identify atmospheric gaseous bio-signatures. Observations of the light transmitted through the Earths atmosphere, as for an extrasolar planet, will be the fi
rst step for future comparisons. We have completed observations of the Earth during a Lunar eclipse, a unique situation similar to that of a transiting planet. We aim at showing what species could be detected in its atmosphere at optical wavelengths, where a lot of photons are available in the masked stellar light. We present observations of the 2008 August 16 Moon eclipse performed with the SOPHIE spectrograph at the Observatoire de Haute-Provence. Locating the spectrograph fibers in the penumbra of the eclipse, the Moon irradiance is then a mix of direct, unabsorbed Sun light and solar light that has passed through the Earths limb. This mixture essentially reproduces what is recorded during the transit of an extrasolar planet. We report here the clear detection of several Earth atmospheric compounds in the transmission spectra, such as ozone, molecular oxygen, and neutral sodium as well as molecular nitrogen and oxygen through the Rayleigh signature. Moreover, we present a method that allows us to derive the thickness of the atmosphere versus the wavelength for penumbra eclipse observations. We quantitatively evaluate the altitude at which the atmosphere becomes transparent for important species like molecular oxygen and ozone, two species thought to be tightly linked to the presence of life. The molecular detections presented here are an encouraging first attempt, necessary to better prepare for the future of extremely-large telescopes and transiting Earth-like planets. Instruments like SOPHIE will be mandatory when characterizing the atmospheres of transiting Earth-like planets from the ground and searching for bio-marker signatures.
The so-called Vegetation Red-Edge (VRE), a sharp increase in the reflectance around $700 nm$, is a characteristic of vegetation spectra, and can therefore be used as a biomarker if it can be detected in an unresolved extrasolar Earth-like planet inte
grated reflectance spectrum. Here we investigate the potential for detection of vegetation spectra during the last Quaternary climatic extrema, the Last Glacial Maximum (LGM) and the Holocene optimum, for which past climatic simulations have been made. By testing the VRE detectability during these extrema when Earths climate and biomes maps were different from today, we are able to test the vegetation detectability on a terrestrial planet different from our modern Earth. Data from the Biome3.5 model have been associated to visible GOME spectra for each biome and cloud cover to derive Earths integrated spectra for given Earth phases and observer positions. The VRE is then measured. Results show that the vegetation remains detectable during the last climatic extrema. Compared to current Earth, the Holocene optimum with a greener Sahara slightly increases the mean VRE on one hand, while on the other hand, the large ice cap over the northern Hemisphere during the LGM decreases vegetation detectability. We finally discuss the detectability of the VRE in the context of recently proposed space missions.
The detection of exolife is one of the goals of very ambitious future space missions that aim to take direct images of Earth-like planets. While associations of simple molecules present in the planets atmosphere ($O_2$, $O_3$, $CO_2$ etc.) have been
identified as possible global biomarkers, we review here the detectability of a signature of life from the planets surface, i.e. the green vegetation. The vegetation reflectance has indeed a specific spectrum, with a sharp edge around 700 nm, known as the Vegetation Red Edge (VRE). Moreover vegetation covers a large surface of emerged lands, from tropical evergreen forest to shrub tundra. Thus considering it as a potential global biomarker is relevant. Earthshine allows to observe the Earth as a distant planet, i.e. without spatial resolution. Since 2001, Earthshine observations have been used by several authors to test and quantify the detectability of the VRE in the Earth spectrum. The egetation spectral signature is detected as a small positive shift of a few percents above the continuum, starting at 700 nm. This signature appears in most spectra, and its strength is correlated with the Earths phase (visible land versus visible ocean). The observations show that detecting the VRE on Earth requires a photometric relative accuracy of 1% or better. Detecting something equivalent on an Earth-like planet will therefore remain challenging, moreover considering the possibility of mineral artifacts and the question of red edge universality in the Universe.
