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False Starts: Potentially Habitable Exoplanets

posted Oct 28, 2014, 10:57 PM by Abel Mendez   [ updated Oct 28, 2014, 10:59 PM ]

 Here is a list of the first claims of potentially habitable exoplanets that later were found to be incorrect. Now we know that Gliese 581 c is too hot, Gliese 581 d and g do not exist, and HD 85012 is too hot. Only Gliese 667C c survives today as the first confirmed potentially habitable exoplanet. Exoplanet detection and characterization is a very hard science. Original press release and paper links included.Gliese 581 c (too hot): Astronomers find first habitable Earth-like planet. (24-Apr-2007). PaperGliese 581 d (does not exist): Gliese 581: 1 planet might indeed be habitable (13-Dec-2007). PaperGliese 581 g (does not exist): Newly discovered planet may be first truly habitable exoplanet (29-Sep-2010). PaperHD 85512 b (too hot): 50 new exoplanets discovered by HARPS (12-Sep-2011). PaperGliese 667C c (so far ok): New super-Earth detected within the habitable zone of a nearby cool star (2-Feb-2012). PaperThis list was created as part of a presentation on the history of the search for habitable worlds.

The Map to the Stars

posted Oct 6, 2014, 3:00 PM by Abel Mendez   [ updated Oct 6, 2014, 3:23 PM ]

 "There will certainly be no lack of human pioneers when we have mastered the art of flight ...In the meantime, we shall prepare, for the brave sky-travellers, maps of the celestial bodies."This is a map of the over one thousand stellar systems with known exoplanets. The map helps to visualize the relative distance and location of exoplanets systems with respect to Earth using a flattened polar projection (i.e. zero declination) with a logarithmic distance scale. Those systems with potentially habitable exoplanets are highlighted with a red circle. You will need to enlarge to see details (probably something good for a Prezi presentation). The map can be printed 27" x 27" @ 300dpi. Check the original poster from 2011 for comparison.

We should prepare the map for the future space explorers

posted Oct 2, 2014, 2:27 AM by Abel Mendez   [ updated Oct 6, 2014, 3:21 PM ]

 Map of the constellations around Cygnus published on 1825, the first NASA Kepler Field of View. Credit: LoC.Here are two translations from a passage of an open letter from Kepler to Galileo published in the Conversation with the Star Messenger on April 19, 1610. We are still creating the maps of the universe for the future space explorers after four hundred years."There will certainly be no lack of human pioneers when we have mastered the art of flight. Who would have thought that navigation across the vast ocean is less dangerous and quieter than in the narrow, threatening gulfs of the Adriatic, or the Baltic, or the British straits? Let us create vessels and sails adjusted to the heavenly ether, and there will be plenty of people unafraid of the empty wastes. In the meantime, we shall prepare, for the brave sky-travellers, maps of the celestial bodies – I shall do it for the moon, you Galileo, for Jupiter." (1)"But as soon as somebody demonstrates the art of flying, settlers from our species of man will not be lacking. Who would once have thought that the crossing of the wide ocean was calmer and safer than of the narrow Adriatic Sea, Baltic Sea, or English Channel? Given ships or sails adapted to the breezes of heaven, there will be those who will not shrink from even that vast expanse. Therefore, for the sake of those who, as it were, will presently be on hand to attempt this voyage, let us establish the astronomy, Galileo, you of Jupiter, and me of the moon." (2) (1) Koestler, A., & Butterfield, H. (1959). The Sleepwalkers: A History of Man's Changing Vision of the Universe. Arthur Koestler. Introduction by Herbert Butterfield, Hutchinson.(2) Kepler, J., & Rosen, E. (1965). Kepler's Conversation with Galileo's Sidereal messenger. 1st complete translation, with an introd. and notes. New York, Johnson Reprint Corp., 1965., 1.

Astrobiology @ AGU Fall Meeting

posted Jul 30, 2014, 2:09 PM by Abel Mendez   [ updated Jul 30, 2014, 2:10 PM ]

What is habitability and how is it measured?

posted Jul 8, 2014, 10:45 AM by Abel Mendez   [ updated Jul 8, 2014, 5:24 PM ]

 Three very distinct habitable environments. Puerto Rico's El Yunque Rain Forest and Guánica Dry Forest, and Chile's Atacama Desert. Credit: PHL @ UPR AreciboNot everywhere on Earth is equally habitable. From deserts to rain forest there is an obvious habitability gradient, from worst to best for life. We are using the presence of life as a ‘proxy for habitability’ to recognize similar pattern, assuming that a place with more life is more habitable than the other, an assumption not always correct. This type of patterns helps to correlate what environmental factors control conditions which support the presence of more life. Scientists use this type of information to create models to predict from the environment how much life they can potentially support.A habitable environment is just an environment that might support some form of life, not necessarily one with life. Earth today is not that good for life if we consider its extensive areas of dry and cold deserts compared to rain forests. Mars is a certainly a desert planet but Earth today is more like a dry forest planet, on average. Imagine a rain forest planet, where most of its land areas support abundance life. That will be more habitable than Earth, again using the abundance for life as a proxy for habitability.How exactly do we measure or quantify habitability? Habitability metrics is an emerging field within astrobiology, or more correctly a re-emerging field since the basis for it were established more than three decades ago. One of the most frequent questions in the astrobiology field is how to measure habitability. Some people even take the concept as difficult to define as life. The true is that biologists already tackled this problem successfully during the ’70 and ’80 but is still seldom known by the astrobiology community. There are various reasons for this.First, habitability metrics originated within the field of ecology and population dynamics to understand the distribution of wild animals and plants. This seems to have no relation to astrobiology since it focuses more on microbial life. Second, this is a very specialized field within theoretical ecology and even not taught and used by all ecologists. Third, biologist calls it differently. We use the generic word ‘habitability’ but it is formally called ‘habitat suitability’ by biologists. So if an astrobiologist tries to look for scientific references on how to measure habitability he/she would probably miss the ‘habitat suitability’ concept or seem as irrelevant since it focus now on animal and plant life.The definition and core mathematical framework of ‘habitat suitability models’ is something that can be extended to all forms of life, including microbial life, and to the astrobiology field. That is precisely one of the reason we established the Planetary Habitability Laboratory on 2010, to adapt and apply this framework to the astrobiology field, as we call it ‘habitability metrics for astrobiology.’ Our first application was the Earth Similarity Index (ESI), a measure of Earth-likeness for planets based on a given set of planetary parameters. This index was inspired by the diversity and similarity indices used in ecology to compare populations. Similarity indices are also used in many other applications such as pattern recognition (e.g. face recognition). Still, this approach is an indirect measure of habitability and we want more direct measures.Habitability or ‘habitat suitability’ is defined as the suitability of an environment for life. This definition has three components, an environment, a life, and a suitability (see figure). All three need to be defined for a proper assessment of habitability. The ‘environment component’ is a description of the physical, chemical, or even biological location of life under consideration, the habitat. It is constrained by some space and time limits (e.g. surface of Earth today). This is the astronomy, planetary science, or geology part of the metric. The other two components contain the biology. The ‘life components’ requires the selection and knowledge of an individual species or community (i.e. aggregate of two or more species) as the test subject for the habitat. Therefore, given some habitat any habitability measure is always relative to the species or community under consideration. Finally, the ‘suitability component’ is the tricky part because it defines the connection between the environment and life. This is the ‘proxy for habitability’.The suitability for life, or ‘proxy for habitability’, could be direct or indirect. An indirect suitability does not necessarily specify how exactly the environment component affects life. For example, we know that the environment requires liquid water but we don’t care about the specific differences on the quantity or quality of this water for life (e.g. salinity, temperature). This is the case of current efforts searching for habitable environments in planetary environments such as exoplanets. The occurrence of Earth-size planets in the habitable zone of stars (Eta-Earth) is in fact an indirect measure of stellar habitability, the suitability of stars for planets with life. The ESI is also an indirect measure, but of planetary habitability, the suitability of a planet for Earth-like life. Thus, indirect measures of habitability rely on occurrences (aka presence/absent in biology), a similarity, or probability of some necessary conditions for life. It is recommended that these values be expressed with a common scale as a fraction for consistency, where zero denotes a non-habitable environment and one denotes a highly habitable environment. Negative values could be used to rate the magnitude of the damaging effect of a non-habitable environment (e.g. both the surface of Mars and Venus are non-habitable, but Venus is worst). Values over one could represent super-habitable conditions.The hardest part is to define direct measures of habitability, which are more biologically meaningful. These require much more knowledge of the interaction of life and the environment. There are some specific universal biological quantities that can be used as the ‘proxy for habitability’ such as growth rate, carrying capacity, metabolic rate, or productivity. Therefore, to construct a direct measure of habitability requires knowing how the environment affects one of these biological quantities for some species or community. We don't need to specifically estimate these quantities but only how the environment proportionally affects them. For example, we know how temperature affects the productivity of primary producers such as plants and phytoplankton. Most require temperatures between 0° to 50° C, but they do better (i.e. highest productivity) near 25°C. Their ‘thermal habitability function’ looks like a bell-shaped curve centered at their optimum productivity temperature. Direct measures of habitability are also better represented as a fraction from zero to one.Another problem is how to combine the effect of many environmental variables into a single direct or indirect habitability index. These are called aggregation methods in theoretical ecology. There are many ways to do this. Probabilities are simple to combine since they are multiply to each other. Similarity indices are easier to construct and combine too. The use of any direct methods already defines how the environmental variables are combined since they are based on biophysical principles. In practice, we recommend biological productivity as the best ‘habitability proxy’ since we know how to calculate it for microbial to complex life, it is relatively easy to measure or estimate, and there are even ways to measure it via remote sensors. The NASA’s Terrestrial Ecology Program uses the TERRA and AQUA satellites to monitor the global land and ocean primary productivity of Earth. This is a measure of ‘global health’ or ‘terrestrial habitability’ since primary producers are the base of the food chain.Unfortunately, most applications of habitability metrics in astrobiology are limited to indirect measures of habitability. This is especially true for exoplanets since we don't have enough information about them to appropriately weight how terrestrial life, life as we know it, could be affected by their planetary environment. There is no single quantitative measure of habitability but a collection of metrics for different types of environments and life. Nevertheless, habitability metrics are easy to compare and combine since they use the same scale and meaning (e.g. a value between zero and one). It doesn't matter the application, everybody would understand the meaning of an environment with habitability close to one. The next logical question after this statement is what are the limits of this value, in other words, what is the environment, reference life, and selected suitability under consideration.Habitability metrics provide an excellent way to understand and compare habitable environments, and prioritize targets for exploration within Earth, the Solar System, and beyond. Biologists have been using them, as 'habitat suitability models' for more than three decades to understand the distribution of terrestrial complex life from local to global environments. It is only a matter of adapting this mathematical framework to the needs of the astrobiology science.— Abel Méndez (First Draft, July 8, 2014)

Beer with BMSIS: Habitability Metrics for Astrobiology

posted Jun 30, 2014, 6:25 AM by Abel Mendez   [ updated Jul 3, 2014, 7:28 AM ]

4D-Exoplanets: A summary of known exoplanets in four dimensions.

posted Jun 27, 2014, 7:49 AM by Abel Mendez   [ updated Jun 29, 2014, 11:28 PM ]

 This plot summarizes all confirmed exoplanets known today (~1,800) as a function of distance from the Sun (light years), their stellar flux (i.e. the energy that they receive from the star where one equals the energy Earth receives from the Sun), their approximate size (see legend top left), and those in the habitable zone (green). Hot planets are red and cold planets are blue. Note that planets in the HZ are close to the 'Solar Flux' line. Click the image for larger version (warning, large graphic).Interactive web version of the plot above. Those exoplanets in the warm zone (i.e. habitable zone) are shown in green with those potentially habitable (i.e. < 2.5 Earth radii) in a darker shade of green (see legend top right).

Exoplanets: From Exhilarating to Exasperating

posted May 30, 2014, 3:29 AM by Abel Mendez   [ updated Jun 2, 2014, 9:06 AM ]

 Here are the abstracts of the exoplanets press-conference of the AAS 224 on Monday, 2 June, 2:15 pm EDT. All findings are embargoed until the time of presentation at the meeting.Rates of Large Flares in Old Solar-like Stars in Kepler ClustersOfer Cohen (Harvard-Smithsonian Center for Astrophysics)We hope to better estimate the rate of very strong (Carrington event-type) flares in the Sun by studying flares of stars in several open clusters with well determined ages using Kepler data. Here we derive white light flare distributions for a sample of near-solar-mass (G0-G5) dwarfs in NGC 6811 (age ~ 1 Gyr) and NGC 6819 (~ 2.5 Gyr). We compare these with solar white light flare rates and, by estimating X-ray emission from the same flares using a solar-based relationship, we compare the Kepler results to other solar and stellar X-ray flare data. We explore implications of our results for the rates of large solar flares. This research was supported by Kepler grant NNX13AC29G.Kepler 56: Present & Future Configuration & ObliquityGongjie Li (Harvard-Smithsonian Center for Astrophysics)Kepler-56 is an interesting multi-planet system with two coplanar inner planets that are highly misaligned with their parent star, and accompanied by an outer 3.3 Jupiter mass planet with an unknown inclination. Determining the true spin orbit angle and the inclination of the outer companion is valuable to study the physical processes that can produce the misalignment. Here, using the observed line of sight measurements, we can constrain the true spin orbit angle and the inclination of the outer companion. These depend on the initial configuration of the system. We consider two inherent scenarios: the first scenario assumes the stellar spin axis to be initially aligned with the inner two planets' angular momentum, and it favors a large range of mutual inclination (~20 -160 degree) between the inner planets and the outer companion. Tighter constraints can be achieved if the true spin orbit angle can be measured (from both asteroseismology and Rossiter-McLaughlin measurements). The second scenario assumes the stellar spin axis to be aligned with the total angular momentum of the system, and it favors the mutual inclination to be around ~40 or ~130 degree. Future observation of the mutual inclination may distinguish the two scenarios and uncover the formation processes of this system. In addition, by modeling the stellar evolution and its tidal effects on the system, we predict that the innermost planet will be engulfed within ~130 Myr.Three Distinct Exoplanet Regimes Inferred from Host Star MetallicitiesLars A. Buchhave (Harvard-Smithsonian Center for Astrophysics)The occurrence rate of exoplanets smaller than 4 Earth radii (RE) in short orbits is ~50%. Despite their sheer abundance, the compositions of planets populating this regime are largely unknown. The available evidence suggests the existence of a compositional range, from small high-density rocky planets to low-density planets consisting of rocky cores surrounded by thick H/He gas envelopes. Understanding the transition from the gaseous planets to Earth-like rocky worlds is important to estimate the number of potentially habitable planets in our Galaxy and provide constraints on planet formation theories. Here, we report the abundances of heavy elements (metallicities) of over 400 stars hosting 600 exoplanet candidates discovered by the Kepler Mission and find that the exoplanets can be categorized into three populations defined by statistically distinct (~ 4.5σ) metallicity regions. We interpret these regions as reflecting the formation regimes of terrestrial-like planets (RP < 1.7 RE), gas-dwarf planets with rocky cores and H/He envelopes (1.7 < RP < 3.9 RE) and ice/gas-giant planets (RP > 3.9 RE). These transitions resonate well with those inferred from dynamical mass estimates, implying that host-star metallicity – a proxy for the initial solid inventory of the protoplanetary disk – is a key ingredient regulating the structure of planetary systems.Press Release: 'Neapolitan' exoplanets come in three flavorsHARPS-N Contributions to the Mass-Radius Diagram for Rocky PlanetsDimitar Sasselov (Harvard-Smithsonian Center for Astrophysics)Science operations began with HARPS-N on the TNG in August 2012. About half of the 80 nights per year allocated to the HARPS-N Collaboration have been dedicated to follow-up observations of bright Kepler Objects of Interest that showed promise of being rocky. In this presentation we show how mass determinations from HARPS-N are improving our understanding of the mass-radius diagram for rocky exoplanets, including recent results for Kepler 78 and Kepler 10.Press Release: Astronomers find a new type of planet: The 'mega-Earth'

Not all habitable planets are equally ‘habitable’

posted Mar 12, 2014, 7:30 AM by Abel Mendez   [ updated May 8, 2014, 7:41 PM ]

 So far we know of up to 20 potentially habitable planets around other stars out of nearly 2,000 planets that have been detected and confirmed. We expect to find many more in the following decades. Unfortunately, we are very far from really understanding the potential for life of these planets since we know very little of them. We have a rough idea or their size and temperature and that tells us how likely are they to support liquid water, a necessity for life as we know it. However, we also need to understand their atmosphere, water content, climate, and many other factors that are still out of reach of our observational technology, these planets are very far away. A habitable planet does not mean that it is actually inhabited, only that it might support life, at least microbial life. It might be hard to imagine a planet able to support life yet not inhabited, but that is a possibility. Some might be better than others since their habitability depends on complex physical and chemical interactions. If some already have life, well, that is another complication.Almost Home is a 3D computer-animated family film by DreamWorks Animation scheduled for release on November 26, 2014.Spanish Version: Casi en Casa es una película familiar animada por computadora en 3D y creada por Animación DreamWorks para ser estrenada el 26 de noviembre de 2014.

Surface Temperature of Planets

posted Jan 4, 2014, 9:49 AM by Abel Mendez   [ updated Jan 7, 2014, 6:22 AM ]

The mean global surface temperature of a planet in a circular orbit is given by (adapted from Qiu et al., 2003):

 $\large T_{s} = T_o\left (\frac{fL(1-A)}{a^2(1-g)}\right )^ \frac{1}{4}$ (1)

where Ts = mean global surface temperature (K), L* = star luminosity (solar units), a = semi-major axis, f = atmosphere redistribution factor (e.g. = 1 for fast rotators and f = 2 for tidally locked planets without atmospheres)A = bond albedo, and g = normalized greenhouse effect), and To = 278.5 K. The normalized greenhouse effect is defined as (Raval & Ramanathan, 1989):

 $\large g = \frac{G}{\sigma T{_{s}}^{4}}=1-\left (\frac{T_{eq}}{T_s} \right )^4$ (2)

where G = greenhouse effect (W/m2) or greenhouse forcing, and Teq = equilibrium temperature (K). Both A and g are numbers between 0 and 1 that are necessary to understand the temperature of planets. They do not only depend on the surface and atmospheric properties of the planet but also on the surface temperature. For example, Raval & Ramanathan (1989) determined the terrestrial g for clear-skies globally, but for a particular month (April 1985), as:

 $\large g = -0.658 + 3.42\times 10^{-3}T_s$ (3)

where Ts = sea surface temperature (SST), and only valid for temperatures between 275 K to 300 K. Eq. 1 can be easily extended to elliptical orbits assuming that both A and g are nearly constants as function of eccentricity (i.e. constant with orbital changes of Ts). Table 1 show some approximate values of A and g for Venus, Earth, and Mars.

Table 1. Necessary data to calculate the surface temperature of Venus, Earth, and Mars from Eq. 1. Solar luminosity L = 1.0 and f = 1. This solution can be extended to exoplanets given appropriate estimates of A and g.

 Planet a, Semi-Major Axis (AU) A, bond albedo g, greenhouse Venus 0.723 0.750 0.990 Earth 1.000 0.300 0.397 Mars 1.524 0.250 0.086

References

• Qiu, J., Goode, P. R., Pallé, E., Yurchyshyn, V., Hickey, J., Rodriguez, P. M., ... & Koonin, S. E. (2003). Earthshine and the Earth's albedo: 1. Earthshine observations and measurements of the lunar phase function for accurate measurements of the Earth's Bond albedo. Journal of Geophysical Research,108 (D22), 4709.
• Raval, A., & Ramanathan, V. (1989). Observational determination of the greenhouse effect. Nature342 (6251), 758-761.

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