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A Way to Other Worlds |
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A Way to Other Worlds |
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Resp. Giuseppina Micela
INAF - Osservatorio Astronomico di Palermo
The study of atmospheres of exo-planets is today the frontier of
the planetary research. The goal is to understand the atmospheric
structure (composition and temperature), albedo, and biomarkers in
a variety of exo-planets.
The final objective is to identify planets with atmosphere composition
similar to that of the Earth atmosphere, i.e. rich in water
vapour, oxygen, ozone, and carbon dioxide. We are already able to
observe atmospheres of few hot Jupiters and Neptunians, and we
expect to observe in the next decade many atmospheres of small
planets as Super-Earths (1-10 Mearth) in temperate regions (with
temperature consistent with liquid water). Super-Earths are the
best candidates for hosting life and therefore to search for
bio-signatures.
Spectroscopic measurements of the atmospheres of transiting extrasolar
planets are a key tool towards understanding the planetary composition,
formation and evolution, that will eventually lead to identification
of chemical bio-signatures. The transit technique includes the
primary and secondary eclipse methods. With the primary transit
method, the thin atmospheric annulus surrounding the optically thick
disk of the planet can be observed, while the planet is transiting
in front of its parent star. The properties of the planet atmosphere
are obtained by studying the transmitted stellar spectrum.
In the
secondary transit method, we observe as first step the combined
spectrum of the star and the planet. Then, the stellar spectrum
alone, during the secondary eclipse, is measured. The difference
between the two measurements provides the planet's contribution.
Primary and secondary transit techniques have been tested in visible
and infrared ranges for few hot Jupiters and Neptunians from ground
and space (HST and Spitzer).
This WP will be focused on the observations and interpretation of
planetary atmospheres. The goal is the definition of an Italian
roadmap, coherent with the international context, aimed at
reconstructing the environmental conditions on exo-planets and
identifying those compatible with habitability and life.
In order to fulfil the goals discussed above we plan:
The study will be focused on spectrometric instrumentation able to
work in the range between the visible and the near-infrared (0.4-5micron).
We propose to develop prototypes of instrumental components that
involve the chain of the detection and the measure of the signals,
through the transformation of the acquired data to a scientific
product to be directly used by the scientists.
In order to obtain an high signal to noise ratio, crucial for the
detection of weak signals, the instrumentation should work at
temperature as low as 40-50 K. Moreover, the ability of working at
low temperatures would permit an easier integration (not as the
more commonly used and ''warms'' CCDs) with the other spectroscopic
modules working at longer wavelengths and therefore operating at
low temperature. Beside the definition and the characterization
of the detector performances at those challenging temperatures, a
chain of instrumental solutions for acquisition of the signal from
the detectors and its transfer to an on-board computer will be
studied. The success of the proposed activity will improve the
current European technology of detectors increasing their international
competitiveness. The responsible for this activity will be dr.
A. Adriani of INAF - IAPS. The temporal development on the project
will be on a time span of four years.
The primary scientific institutions involved in the technological
part of project will be INAF-Institute of Space Astrophysics and
Planetology from Rome and the INAF- Osservatorio Astronomico di
Arcetri in Florence with the contribution of the Department of
Physics and Astronomy of the University of Florence (prof. E. Pace).
The first year activity will be developed within the research
institutions with the support of scientific and industrial experts
and will consist in: a) the study and the definitions of the
requirements and specifications of the instrumentation; b) acquisition
of some key instrumentations and electronics components; c) definition
and construction of the laboratory set up for the tests of critical
elements; d) perform preliminary laboratory test on single hardware
elements; e) define the developing plan for the executive design
and construction of prototypal instrumentations and their
characterization.
The second year will see the direct involvement of an industrial
partner to develop the prototypal instrumentation and the development
of the different hardware and software subsystems with the support
of the scientific institutions.
The third and four years will see the test and characterization of
the instrumentation and the elaboration of the results of the study.
A feasibility study in conjunction with industry would assess the
scientific potential of such an approach, the optomechanical
complexity, and the cost of the technical implementation. The study
will also provide a roadmap for the possible implementation and the
evaluation of the impact that this would have to the Cherenkov
operations. The latter, although expected to be small as transit
and eclipses are of short duration and well known predicted in
advance, will be investigate analysing the real-life experience of
existing facility nearby astronomical telescopes (e.g.Magic close
to the Italian TNG telescope where the team members have considerable
experience in high SNR spectra ).
A preliminary analysis (based on real FORS-VLT observations) indicates
that with Magic we could reach a sensitivity of 2.4*10-4 on individual
transit, enabling us to detect at 3-sigma alkali elements in the
atmospheres of the majority of known transits. A detailed study of
the targets is needed, since the contamination from sky background
and nearby stars would be relevant. The resources needed for this
activity is one “assegno di ricerca”, and laboratory equipments and
materials for the feasibility analysis.
The activity will be conducted under the responsibility of dr.
Cecchi Pestellini .
Figure 1: ESI for Solar System bodies with radius greater than 100
km (orange) and 258 known extrasolar planets (blue). Only some of
the most notable bodies are labeled. The ESI scale makes a distinction
between those rocky interior (light red area) and temperate surface
(light blue area) planets. Only planets within these two categories
can be considered Earth-like planets (light green area). The dotted
lines represent constant ESI values. If confirmed, presently only
Gliese 581 g is in the Earth-like category together with Earth (from
phl.upr.edu)
Step a) will be based on the results of detection techniques of
planets at the right distance from the host stars. Albedo analysis
from spectra in optical bands will be used to account for the
greenhouse effect. Note that even if giant gaseous planets cannot
host life, their satellites can be considered habitable - as for
Jupiter and Saturn in our Solar System giants. In figure 5 we report
the ESI values for a sample of known planets.
In Step b) we will verify if a habitable planet from step a) has
an adequate environment to sustain life. In particular, although
still not well understood, we know that stellar activity has
definitely an influence on the habitability of a planet. Thanks to
their small sizes M dwarfs are the best candidates to search for
habitable planets. However, the current efforts to find planets in
their habitable zone does not account for the fact that these stars
may provide an extreme case of habitability. M dwarfs tend to be
very active with large UV and X-ray fluxes, and frequent coronal
mass ejections. This will clearly influence both the evolution of
the planetary atmosphere, and any life forms that can develop on
the planet surface. Theoretical work investigating such influences
will be undertaken to assess whether bio-signatures of planets in
the habitable zone of M dwarfs can even exist. This work is relevant
not only for planets around M dwarfs. The early Sun was itself very
active at young age, and this enhanced activity may have had an
influence in the terrestrial life origin and evolution.
In Step c) we will evaluate the detectability with the future
instruments of the so-called biomarker molecules that are expected
to be present in an environment hosting life. As for the Earth
biomarkers include ozone (O3), molecular oxygen (O2), and nitrous
oxide (N2O). Figure below shows the spectra of Earth, Venus, and
Mars as seen from space. The difference is striking: the Earth
spectrum is much more complex than the spectra of the other two
planets, showing clear lines of biomarker molecules.Theoretical
studies have begun to explore the wide range of potential biomarker
spectral signatures, assuming a planet evolution similar to the
Earth but varying parameters such as planetary and atmospheric mass,
biosphere, star temperature and gravity. The results, so far obtained,
suggest that biomarkers responses strongly depend upon the physical
properties of the central star.
Figure 2: Infrared spectra of Venus, Earth, and Mars. All three
spectra show absorption lines due to carbon dioxide. However, only
Earth's spectrum shows additional lines due to water and ozone.
Step d) is the most interesting but also the most challenging. Life
perturbs disequilibria that arise due to kinetic barriers and can
impart unexpected structure to an abiotic system. As a consequence,
a bio-signature is an object, substance and/or pattern whose origin
specifically requires a biological agent. This is a broad definition,
that may be unfortunately misleading because our concepts of life
and bio-signatures are inextricably linked. For instance, certain
specific mechanisms of our biosphere, e.g., DNA and proteins, might
not necessarily be mimicked by other examples of life elsewhere in
the cosmos. On the other hand, basic principles of biological
evolution might indeed be universal. Thus, bio-signatures must
reflect fundamental and universal characteristics of life, and they
should not be restricted solely to those attributes that represent
local solutions to the challenges of survival. Terrestrial based
bio-signatures include cellular and extracellular morphologies,
biogenic minerals, chirality, biogenic stable isotope patterns in
minerals and organic compounds, atmospheric gases, and remotely
detectable features on planetary surfaces. On Earth, bio-signatures
also include those key minerals, atmospheric gases and crustal
reservoirs of carbon, sulfur and other elements that collectively
have recorded the enduring global impact of the utilization of free
energy. To be relevant to an astronomical search such features must
be sufficiently complex and/or abundant so that they retain a
diagnostic expression of some of life's universal attributes. Care
is needed to distinguish true biomarker signals from so-called
''false- positives'' i.e. cases where planetary atmospheres ''mimic''
life due to inorganic chemical processes producing biomarkers - for
example, strong CO2 photolysis eventually leading to molecular
oxygen production. Ozone has a strong infra-red absorption band
at 9.6 micron that, being highly saturated is not a useful indicator
of its abundance. Sources of nitrous oxide (N2O) into Earth's
atmosphere are almost exclusively associated with microbial activity.
It absorbs mostly in the troposphere with bands at 7.8 and 3.9 micron.
It is an excellent biomarker because, as far we know on the Earth,
its inorganic contribution is negligible, implying that false-positives
are unlikely. However, its absorption features are weak for today
Earth abundances and the measurements are extremely challenging.
Expected results at the end of the first year are:
Pragmatically, we can hope to constrain an operational definition
of biomarker (in other words a protocol) approaching the problem
in the laboratory, in the field on Earth, and in (solar) planetary
studies (both in remote and in situ). We must define life in
universal, measurable terms, remembering that bio-signatures are
present over various spatial and temporal scales. The key ''properties''
in such a research are structure, chemistry, replication, energy
budget, and environmental conditions. Driven by exo-chemistry
studies, laboratory experiments may be performed in other regions
of the parameter space, testing other ''extreme'' physical and chemical
conditions. Exoplanets studies offer a glimpse of worlds with a
fundamentally different chemistry from Earth. Such studies may open
new paths for the study of geochemistry and geophysical processes,
from which we can learn about e.g., planet's thermal evolution and
plate tectonics in conditions far from the evolution of our planets.
Since, we know that life is strongly affected by environmental
conditions, exoplanetary research enlarge the knowledge of possible
chemistries that may be open the way to the construction of systems
of ever increasing complexity.