138 research outputs found
Habitable Zones of Post-Main Sequence Stars
Once a star leaves the main sequence and becomes a red giant, its Habitable
Zone (HZ) moves outward, promoting detectable habitable conditions at larger
orbital distances. We use a one-dimensional radiative-convective climate and
stellar evolutionary models to calculate post-MS HZ distances for a grid of
stars from 3,700K to 10,000K (~M1 to A5 stellar types) for different stellar
metallicities. The post-MS HZ limits are comparable to the distances of known
directly imaged planets. We model the stellar as well as planetary atmospheric
mass loss during the Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB)
phases for super-Moons to super-Earths. A planet can stay between 200 million
years up to 9 Gyr in the post-MS HZ for our hottest and coldest grid stars,
respectively, assuming solar metallicity. These numbers increase for increased
stellar metallicity. Total atmospheric erosion only occurs for planets in
close-in orbits. The post-MS HZ orbital distances are within detection
capabilities of direct imaging techniques.Comment: Published in The Astrophysical Journal (28 pages, 7 figures, 8
tables
Atmospheres and UV Environments of Earth-like Planets Throughout Post-Main Sequence Evolution
During the post-main sequence phase of stellar evolution the orbital distance
of the habitable zone, which allows for liquid surface water on terrestrial
planets, moves out past the system's original frost line, providing an
opportunity for outer planetary system surface habitability. We use a 1D
coupled climate/photochemistry code to study the impact of the stellar
environment on the planetary atmospheres of Earth-like planets/moons throughout
its time in the post-main sequence habitable zone. We also explore the ground
UV environments of such planets/moons and compare them to Earth's. We model the
evolution of star-planet systems with host stars ranging from 1.0 to 3.5
M throughout the post-main sequence, calculating stellar mass loss and
its effects on planetary orbital evolution and atmospheric erosion. The maximum
amount of time a rocky planet can spend continuously in the evolving post-MS
habitable zone ranges between 56 and 257 Myr for our grid stars. Thus, during
the post-MS evolution of their host star, subsurface life on cold planets and
moons could become remotely detectable once the initially frozen surface melts.
Frozen planets or moons, like Europa in our Solar System, experience a
relatively stable environment on the horizontal branch of their host stars'
evolution for millions of years.Comment: 23 pages, 7 figures, 10 table
Colors of extreme exo-Earth environments
The search for extrasolar planets has already detected rocky planets and
several planetary candidates with minimum masses that are consistent with rocky
planets in the habitable zone of their host stars. A low-resolution spectrum in
the form of a color-color diagram of an exoplanet is likely to be one of the
first post-detection quantities to be measured for the case of direct
detection. In this paper, we explore potentially detectable surface features on
rocky exoplanets and their connection to, and importance as, a habitat for
extremophiles, as known on Earth. Extremophiles provide us with the minimum
known envelope of environmental limits for life on our planet. The color of a
planet reveals information on its properties, especially for surface features
of rocky planets with clear atmospheres. We use filter photometry in the
visible waveband as a first step in the characterization of rocky exoplanets to
prioritize targets for follow-up spectroscopy. Many surface environments on
Earth have characteristic albedos and occupy a different color space in the
visible waveband (0.4-0.9 microns) that can be distinguished remotely. These
detectable surface features can be linked to the extreme niches that support
extremophiles on Earth and provide a link between geomicrobiology and
observational astronomy. This paper explores how filter photometry can serve as
a first step in characterizing Earth-like exoplanets for an aerobic as well as
an anaerobic atmosphere, thereby prioritizing targets to search for atmospheric
biosignatures.
Key Words: Color-color, Habitability, Extrasolar terrestrial planet, Extreme
environments, Extremophiles, Reflectivity.Comment: Published in Astrobiology (see Journal reference); 25 pages, 5
figures, 1 table; Minor language updates from version 1 to match published
versio
Exploring atmospheres of hot mini-Neptunes and extrasolar giant planets orbiting different stars with application to HD 97658b, WASP-12b, CoRoT-2b, XO-1b and HD 189733b
We calculated an atmospheric grid for hot mini-Neptune and giant exoplanets,
that links astrophysical observable parameters- orbital distance and stellar
type- with the chemical atmospheric species expected. The grid can be applied
to current and future observations to characterize exoplanet atmospheres and
serves as a reference to interpret atmospheric retrieval analysis results. To
build the grid, we developed a 1D code for calculating the atmospheric thermal
structure and link it to a photochemical model that includes disequilibrium
chemistry (molecular diffusion, vertical mixing and photochemistry). We compare
thermal profiles and atmospheric composition of planets at different semimajor
axis (0.01a0.1AU) orbiting F, G, K and M stars. Temperature and UV
flux affect chemical species in the atmosphere. We explore which effects are
due to temperature and which due to stellar characteristics, showing the
species most affected in each case. CH and HO are the most sensitive to
UV flux, H displaces H as the most abundant gas in the upper atmosphere for
planets receiving a high UV flux. CH is more abundant for cooler planets.
We explore vertical mixing, to inform degeneracies on our models and in the
resulting spectral observables. For lower pressures observable species like
HO or CO can indicate the efficiency of vertical mixing, with larger
mixing ratios for a stronger mixing. By establishing the grid, testing the
sensitivity of the results and comparing our model to published results, our
paper provides a tool to estimate what observations could yield. We apply our
model to WASP-12b, CoRoT-2b, XO-1b, HD189733b and HD97658b.Comment: 14 pages, 9 figures, Accepted for publication in Ap
How surfaces shape the climate of habitable exoplanets
Large ground- and space-based telescopes will be able to observe Earth-like
planets in the near future. We explore how different planetary surfaces can
strongly influence the climate, atmospheric composition, and remotely
detectable spectra of terrestrial rocky exoplanets in the habitable zone
depending on the host star's incident irradiation spectrum for a range of
Sun-like host stars from F0V to K7V. We update a well-tested 1D
climate-photochemistry model to explore the changes of a planetary environment
for different surfaces for different host stars. Our results show that using a
wavelength-dependent surface albedo is critical for modeling potentially
habitable rocky exoplanets.Comment: Published in MNRAS 11 February 2020 - 12 pages, 10 figure
Atmospheric mass loss and evolution of short-period exoplanets: the examples of CoRoT-7b and Kepler-10b
Short-period exoplanets potentially lose envelope masses during their
evolution because of atmospheric escape caused by the intense XUV radiation
from their host stars. We develop a combined model of atmospheric mass loss
calculation and thermal evolution calculation of a planet to simulate its
evolution and explore the dependences on the formation history of the planet.
Thermal atmospheric escape as well as the Roche-lobe overflow contributes to
mass loss. The maximum initial planetary model mass depends primarily on the
assumed evolution model of the stellar XUV luminosity. We adapt the model to
CoRoT-7b and Kepler-10b to explore the evolution of both planets and the
maximum initial mass of these planets. We take the recent X-ray observation of
CoRoT-7 into account and exploring the effect of different XUV evolution models
on the planetary initial mass. Our calculations indicate that both hot super
Earths could be remnants of Jupiter mass gas planets.Comment: 7 pages, 7 figures, accepted for publication in MNRA
Refraction in planetary atmospheres: improved analytical expressions and comparison with a new ray-tracing algorithm
Atmospheric refraction affects to various degrees exoplanet transit, lunar
eclipse, as well as stellar occultation observations. Exoplanet retrieval
algorithms often use analytical expressions for the column abundance along a
ray traversing the atmosphere as well as for the deflection of that ray, which
are first order approximations valid for low densities in a spherically
symmetric homogeneous isothermal atmosphere. We derive new analytical formulae
for both of these quantities, which are valid for higher densities, and use
them to refine and validate a new ray tracing algorithm which can be used for
arbitrary atmospheric temperature-pressure profiles. We illustrate with simple
isothermal atmospheric profiles the consequences of our model for different
planets: temperate Earth-like and Jovian-like planets, as well as HD189733b,
and GJ1214b. We find that, for both hot exoplanets, our treatment of refraction
does not make much of a difference to pressures as high as 10 atmosphere, but
that it is important to consider the variation of gravity with altitude for
GJ1214b. However, we find that the temperate atmospheres have an apparent scale
height significantly smaller than their actual density scale height at
densities larger than 1 amagat, thus increasing the difficulty of detecting
spectral features originating in these regions. These denser atmospheric
regions form a refractive boundary layer where column abundances and ray
deflection increases dramatically with decreasing impact parameter. This
refractive boundary layer mimics a surface, and none of the techniques
mentioned above can probe atmospheric regions denser than about 4 amagat on
these temperate planets.Comment: 16 pages, 15 figures, 4 tables, Accepted for publication in MNRA
The Vegetation Red Edge Biosignature Through Time on Earth and Exoplanets
The high reflection of land vegetation in the near-infrared, the vegetation
red edge (VRE), is often cited as a spectral biosignature for surface
vegetation on exoplanets. The VRE is only a few percent change in reflectivity
for a disk-integrated observation of present-day Earth. Here we show that the
strength of Earth's VRE has increased over the past ~500 million years of land
plant evolution and may continue to increase as solar luminosity increases and
the planet warms, until either vegetation coverage is reduced, or the planet's
atmosphere becomes opaque to light reflected off the surface. Early plants like
mosses and liverworts, which dominated on land 500-400 million years ago,
produce a weaker VRE, approximately half as strong as that of modern
vegetation. We explore how the changes in land plants, as well as geological
changes like ice coverage during ice-ages and interglacial periods, influence
the detectability of the VRE through Earth's geological past. Our results show
that the VRE has varied through the evolutionary history of land plants on
Earth, and could continue to change into the future if hotter climate
conditions became dominant, encouraging the spread of vegetation. Our findings
suggest that older and hotter Earth-like planets are good targets for the
search for a VRE signature. In addition, hot exoplanets and dry exoplanets with
some water could be the best targets for a successful vegetation biosignature
detection. As well as a strong red edge, lower cloud-fractions and low levels
of atmospheric water vapor on such planets could make it easier to detect
surface features in general.Comment: Published in Astrobiology (Free open access until October 2 2018:
https://www.liebertpub.com/doi/10.1089/ast.2017.1798
Search for Extra-Terrestrial planets: The DARWIN mission - Target Stars and Array Architectures
The DARWIN mission is an Infrared free flying interferometer mission based on
the new technique of nulling interferometry. Its main objective is to detect
and characterize other Earth-like planets, analyze the composition of their
atmospheres and their capability to sustain life, as we know it. DARWIN is
currently in definition phase. This PhD work that has been undertaken within
the DARWIN team at the European Space Agency (ESA) addresses two crucial
aspects of the mission. Firstly, a DARWIN target star list has been established
that includes characteristics of the target star sample that will be critical
for final mission design, such as, luminosity, distance, spectral
classification, stellar variability, multiplicity, location and radius of the
star. Constrains were applied as set by planet evolution theory and mission
architecture. Secondly, a number of alternative mission architectures have been
evaluated on the basis of interferometer response as a function of wavelength,
achievable modulation efficiency, number of telescopes and starlight rejection
capabilities. The study has shown that the core mission goals should be
achievable with a lower level of complexity as compared to the current baseline
configuration.Comment: PhD thesis 2004, Karl Franzens Univ. Graz, 177 pages, download at:
http://cfa-www.harvard.edu/~lkaltenegger
Lessons from early Earth: UV surface radiation should not limit the habitability of active M star systems
The closest potentially habitable worlds outside our Solar system orbit a
different kind of star than our Sun: smaller red dwarf stars. Such stars can
flare frequently, bombarding their planets with biologically damaging
high-energy UV radiation, placing planetary atmospheres at risk of erosion and
bringing the habitability of these worlds into question. However, the surface
UV flux on these worlds is unknown. Here we show the first models of the
surface UV environments of the four closest potentially habitable exoplanets:
Proxima-b, TRAPPIST-1e, Ross-128b, and LHS-1140b assuming different atmospheric
compositions, spanning Earth-analogue to eroded and anoxic atmospheres and
compare them to levels for Earth throughout its geological evolution. Even for
planet models with eroded and anoxic atmospheres, surface UV radiation remains
below early Earth levels, even during flares. Given that the early Earth was
inhabited, we show that UV radiation should not be a limiting factor for the
habitability of planets orbiting M stars. Our closest neighbouring worlds
remain intriguing targets for the search for life beyond our Solar system.Comment: This article has been accepted for publication in MNRAS, published by
Oxford University Press on behalf of the Royal Astronomical Societ
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