Can multicellular life be distinguished from single cellular life on an exoplanet? We hypothesize that abundant upright photosynthetic multicellular life (trees) will cast shadows at high sun angles that will distinguish them from single cellular life and test this using Earth as an exoplanet. We first test the concept using Unmanned Arial Vehicles (UAVs) at a replica moon landing site near Flagstaff, Arizona and show trees have both a distinctive reflectance signature (red edge) and geometric signature (shadows at high sun angles) that can distinguish them from replica moon craters. Next, we calculate reflectance signatures for Earth at several phase angles with POLDER (Polarization and Directionality of Earth's reflectance) satellite directional reflectance measurements and then reduce Earth to a single pixel. We compare Earth to other planetary bodies (Mars, the Moon, Venus, and Uranus) and hypothesize that Earths directional reflectance will be between strongly backscattering rocky bodies with no weathering (like Mars and the Moon) and cloudy bodies with more isotropic scattering (like Venus and Uranus). Our modelling results put Earth in line with strongly backscattering Mars, while our empirical results put Earth in line with more isotropic scattering Venus. We identify potential weaknesses in both the modeled and empirical results and suggest additional steps to determine whether this technique could distinguish upright multicellular life on exoplanets.
The nature of aerosols in hot exoplanet atmospheres is one of the primary vexing questions facing the exoplanet field. The complex chemistry, multiple formation pathways, and lack of easily identifiable spectral features associated with aerosols make it especially challenging to constrain their key properties. We propose a transmission spectroscopy technique to identify the primary aerosol formation mechanism for the most highly irradiated hot Jupiters (HIHJs). The technique is based on the expectation that the two key types of aerosols -- photochemically generated hazes and equilibrium conden…
Before about 500 million years ago, most probably our planet experienced temporary snowball conditions, with continental and sea ices covering a large fraction of its surface. This points to a potential bistability of Earth's climate, that can have at least two different (statistical) equilibrium states for the same external forcing (i.e., solar radiation). Here we explore the probability of finding bistable climates in earth-like exoplanets, and consider the properties of planetary climates obtained by varying the semi-major orbital axis (thus, received stellar radiation), eccentricity and ob…
Circumbinary Planets (CBPs) can be misaligned with their host binary stars. Orbital dynamics, simulations, and recent observations of proto-planetary disks all suggest that the planet can stably orbit in a plane perpendicular to that of an eccentric host binary star (i.e., a polar orbit). No solid claim of detection of such a configuration has been made; the nine systems detected by the transit technique are nearly coplanar, but their discovery is also biased towards that configuration. Here, we develop Eclipse Timing Variations (ETVs) as a method to detect misaligned CBPs. We find that since …
Upcoming biosignature searches focus on indirect indicators to infer the presence of life on other worlds. Aside from just signaling the presence of life, however, some biosignatures can contain information about the state that a planet's biosphere has achieved. This additional information can be used to measure what fractions of planets achieve certain key stages, corresponding to the advent of life, photosynthesis, multicellularity, and technological civilization. We forecast the uncertainties of each measurement for upcoming surveys, and outline the key factors that determine these uncertai…
Light curves show the flux variation from the target star and its orbiting planets as a function of time. In addition to the transit features created by the planets, the flux also includes the reflected light component of each planet, which depends on the planetary albedo. This signal is typically referred to as phase curve and could be easily identified if there were no additional noise. As well as instrumental noise, stellar activity, such as spots, can create a modulation in the data, which may be very difficult to distinguish from the planetary signal. We analyze the limitations imposed by…
Characterizing the oxidation state of rocky exoplanets with the Large Interferometer for Exoplanets (LIFE)
Kapteyn Astronomical Institute, University of Groningen（格罗宁根大学卡普坦天文研究所）；SRON Netherlands Institute for Space Research（荷兰空间研究研究所）；Freie Universität Berlin, Institute of Geological Sciences（柏林自由大学地质科学研究所）；NASA Goddard Space Flight Center（美国国家航空航天局戈达德太空飞行中心）；ETH Zurich, Institute for Particle Physics & Astrophysics（苏黎世联邦理工学院粒子物理与天体物理研究所）；ETH Zurich, Department of Earth and Planetary Sciences（苏黎世联邦理工学院地球与行星科学系）
The future biosphere on Earth (as with its past) will be made up predominantly of unicellular microorganisms. Unicellular life was probably present for at least 2.5 Gyr before multicellular life appeared and will likely be the only form of life capable of surviving on the planet in the far future, when the ageing Sun causes environmental conditions to become more hostile to more complex forms of life. Therefore, it is statistically more likely that habitable Earth-like exoplanets we discover will be at a stage in their habitable lifetime more conducive to supporting unicellular, rather than mu…
The next generation of ground and space-based telescopes will image habitable planets around nearby stars. A growing literature describes how to characterize such planets with spectroscopy, but less consideration has been given to the usefulness of planet colors. Here, we investigate whether potentially Earth-like exoplanets could be identified using UV-visible-to-NIR wavelength broadband photometry (350-1000 nm). Specifically, we calculate optimal photometric bins for identifying an exo-Earth and distinguishing it from uninhabitable planets including both Solar System objects and model exopla…
In our solar system massive outer planets dominate in terms of both mass and angular momentum, but few such planets are among the more than 4000 currently confirmed exoplanets, and none of these have accurately determined densities. Here I examine the detection rate of single-transit events anticipated from the LSST deep drilling fields for such planets with periods of 2 - 25 years. Using conservative detection criteria, I anticipate 3-30 such detections over the 10-year period of the project, though distinguishing detections from false positives may prove challenging. However, these few detec…
Recent and upcoming stabilized spectrographs are pushing the frontier for Doppler spectroscopy to detect and characterize low-mass planets. Specifications for these instruments are so impressive that intrinsic stellar variability is expected to limit their Doppler precision for most target stars (Fischer et al. 2016). To realize their full potential, astronomers must develop new strategies for distinguishing true Doppler shifts from intrinsic stellar variability. Stellar variability due to star spots, faculae and other rotationally-linked variability are particularly concerning, as the stellar…
[Abridged] To simulate the kinds of observations that will eventually be obtained for exoplanets, the Deep Impact spacecraft obtained light curves of Earth at seven wavebands spanning 300-1000 nm as part of the EPOXI mission of opportunity. In this paper we analyze disc-integrated light curves, treating Earth as if it were an exoplanet, to determine if we can detect the presence of oceans and continents. We present two observations each spanning one day, taken at gibbous phases. The rotation of the planet leads to diurnal albedo variations of 15-30%, with the largest relative changes occuring …
After Earth's origin, our host star, the Sun, was shining 20 to 25 percent less brightly than today. Without greenhouse-like conditions to warm the atmosphere, our early planet would have been an ice ball and life may never have evolved. But life did evolve, which indicates that greenhouse gases must have been present on early Earth to warm the planet. Evidence from the geologic record indicates an abundance of the greenhouse gas CO2. CH4 was probably present as well, and in this regard methanogenic bacteria, which belong to a diverse group of anaerobic procaryotes that ferment CO 2 plus H2 to…
Future space telescopes may be able to directly image $\sim$10 - 100 planets with sizes and orbits consistent with habitable surface conditions ("exo-Earth candidates" or EECs), but observers will face difficulty in distinguishing these from the potentially hundreds of non-habitable "false positives" which will also be detected. To maximize the efficiency of follow-up observations, a prioritization scheme must be developed to determine which planets are most likely to be EECs. In this paper, we present a Bayesian method for estimating the likelihood that any directly imaged extrasolar planet i…
