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Scientists from Puerto Rico get together to study habitable planets.
One of the main goals of exoplanet science is the detection and characterization of Earth-like planets. So far, there are about 30 exoplanets considered potentially habitable out of the nearly two thousands already confirmed. However, some or all of these planets might turn out not suitable for life depending on their bulk composition and atmospheric properties.
The Planetary Habitability Laboratory of the UPR Arecibo is organizing a workshop to work on present and future problems on the characterization for Earth-like worlds. The purpose of this workshop is to initiate new multidisciplinary research collaborations on the chemical and biological constraints for simple and complex life on Earth-like planets.
Scientists and graduate students from Puerto Rico interested in the search for life in the universe, especially those with a biology or chemistry background, are invited to this two-hour workshop. The workshop will be held on Thursday, April 30, 2015 from 10AM to 12 Noon at the ISMuL Conference Room of the University of Puerto Rico at Arecibo.
The workshop is sponsored by the Planetary Habitability Laboratory (PHL), the NASA Astrobiology Institute (NAI), the Integrated Science Multi-use Laboratory (ISMuL), and the Center for Research and Creation (CIC) at the University of Puerto Rico at Arecibo (UPR Arecibo).
More information is available on phl.upr.edu/press-release/ew2. Register online or send an email to firstname.lastname@example.org or call ISMuL at (787) 815-0000 x3680/3690 or (787) 817-4611. Spaces are limited.
Artistic representation of the potentially habitable Super-Earth Gliese 832 c against a stellar nebula background. Credit: PHL @ UPR Arecibo, NASA Hubble, Stellarium.
UPDATE: Check figure 5 for an alternative version.
Gliese 832 c is the nearest best habitable world candidate so far
An international team of astronomers, led by Robert A. Wittenmyer from UNSW Australia, report the discovery of a new potentially habitable Super-Earth around the nearby red-dwarf star Gliese 832, sixteen light years away. This star is already known to harbour a cold Jupiter-like planet, Gliese 832 b, discovered on 2009. The new planet, Gliese 832 c, was added to the Habitable Exoplanets Catalog along with a total of 23 objects of interest. The number of planets in the catalog has almost doubled this year alone.
Gliese 832 c has an orbital period of 36 days and a mass at least five times that of Earth's (≥ 5.4 Earth masses). It receives about the same average energy as Earth does from the Sun. The planet might have Earth-like temperatures, albeit with large seasonal shifts, given a similar terrestrial atmosphere. A denser atmosphere, something expected for Super-Earths, could easily make this planet too hot for life and a "Super-Venus" instead.
The Earth Similarity Index (ESI) of Gliese 832 c (ESI = 0.81) is comparable to Gliese 667C c (ESI = 0.84) and Kepler-62 e (ESI = 0.83). This makes Gliese 832 c one of the top three most Earth-like planets according to the ESI (i.e. with respect to Earth's stellar flux and mass) and the closest one to Earth of all three, a prime object for follow-up observations. However, other unknowns such as the bulk composition and atmosphere of the planet could make this world quite different to Earth and non-habitable.
So far, the two planets of Gliese 832 are a scaled-down version of our own Solar System, with an inner potentially Earth-like planet and an outer Jupiter-like giant planet. The giant planet may well have played a similar dynamical role in the Gliese 832 system to that played by Jupiter in our Solar System. It will be interesting to know if any additional objects in the Gliese 832 system (e.g. planets and dust) follow this familiar Solar System configuration, but this architecture remains rare among the known exoplanet systems.
Figure 1. Artistic representation of the potentially habitable exoplanet Gliese 832 c as compared with Earth. Gliese 832 c is represented here as a temperate world covered in clouds. The relative size of the planet in the figure assumes a rocky composition but could be larger for a ice/gas composition. Credit: PHL @ UPR Arecibo.
Figure 2. Orbital analysis of Gliese 832 c, a potentially habitable world around the nearby red-dwarf star Gliese 832. Gliese 832 c orbits near the inner edge of the conservative habitable zone. Its average equilibrium temperature (253 K) is similar to Earth (255 K) but with large shifts (up to 25K) due to its high eccentricity (assuming a similar 0.3 albedo). Credit: PHL @ UPR Arecibo.
Figure 3. The Habitable Exoplanets Catalog now has 23 objects of interest including Gliese 832 c, the closest to Earth of the top three most Earth-like worlds in the catalog. Credit: PHL @ UPR Arecibo.
Figure 4. Stellar map with the position of all the stars with potentially habitable exoplanets including now Gliese 832 (lower left). Credit: PHL @ UPR Arecibo, Jim Cornmell.
Figure 5. Artistic representation of the potentially habitable Super-Earth Gliese 832 c with an actual photo of its parent star (center) taken on June 25, 2014 from Aguadilla, Puerto Rico by Efrain Morales Rivera of the Astronomical Society of the Caribbean. Original annotated image available here. Credit: Efraín Morales Rivera, Astronomical Society of the Caribbean, PHL @ UPR Arecibo.
Artistic representation of the potentially habitable world Kapteyn b with the globular cluster Omega Centauri in the background. It is believed that the Omega Centauri is the remaining core of a dwarf galaxy that merged with our own galaxy billions of years ago bringing Kapteyn's star along. Credit: PHL @ UPR Arecibo, Aladin Sky Atlas.
The planets around the nearby red-dwarf Kapteyn's star are over twice as old as Earth
An international team of astronomers, led by Guillem Anglada-Escude from Queen Mary University, reports two new planets orbiting a very old and nearby star to the Sun named Kapteyn's star. One of the newly-discovered planets, Kapteyn b, is potentially habitable as it has the right-size and orbit to support liquid water on its surface. What makes this discovery highly interesting is the peculiar story and age of the star. Kapteyn b is likely over twice the age of Earth and the oldest known potentially habitable planet listed in the Habitable Exoplanets Catalog.
The Super-Earth Kapteyn b orbits the star every 48 days and has a mass at least five times that of Earth's. The second planet, Kapteyn c, is a more massive Super-Earth with an orbit of 121 days and too cold to support liquid water. At the moment, only a few properties of the planets are known: minimum masses, orbital periods, and distances to the star. By measuring their atmospheres with future instruments, scientists will try to find out whether some of these planets are truly habitable worlds.
Kapteyn b is probably colder than Earth given a similar atmosphere. However a denser atmosphere could easily provide for equal or even higher temperatures. Based on its stellar flux (45% that of Earth's) and mass (≥ 4.8 Earth masses) the Earth Similarity Index (ESI) of Kapteyn b is comparable to Kepler-62f and Kepler-186f. Given its old age (~11.5 billion years), Kapteyn b has had plenty of time to develop life, as we know it.
The astronomers used new data from HARPS spectrometer at the ESO's La Silla observatory in Chile to measure tiny periodic changes in the motion of the star. Using the Doppler Effect, which shifts the star’s light spectrum depending on its velocity, the scientists worked out some properties of these planets, such as their masses and orbital periods.The study also combined data from two more high-precision spectrometers to secure the detection: HIRES at Keck Observatory and PFS at Magellan/Las Campanas Observatory.
About Kapteyn's Star
Discovered at the end of the 19th century and named after the Dutch astronomer who found it (Jacobus Kapteyn), Kapteyn's is the second fastest moving star in the sky and belongs to the galactic halo, an extended cloud of stars orbiting our Galaxy in very elliptic orbits. With a third of the mass of the Sun, this red-dwarf can be seen in the southern constellation of Pictor with an amateur telescope.
Typical planetary systems detected by NASA's Kepler mission are hundreds of light-years away. In contrast, Kapteyn's star is the 25th nearest star to the Sun and it is only 13 light years away from Earth. It was born in a dwarf Galaxy absorbed and disrupted by the Early Milky Way. Such a galactic disruption event put the star in its fast halo orbit. The likely remnant core of the original dwarf galaxy is Omega Centauri, an enigmatic globular cluster 16,000 light years from Earth which contains hundreds of thousands of similarly old suns. This sets the most likely age of its planets at 11.5 billion years; which is 2.5 times older than Earth and 'only' 2 billion years younger than the Universe itself (~13.7 billion years).
Original Research: Guillem Anglada-Escudé (email@example.com)
Habitable Exoplanets Catalog: Abel Méndez (firstname.lastname@example.org)
Figure 1. Artistic representation of the potentially habitable exoplanet Kapteyn b as compared with Earth. Kapteyn b is represented here as an old and cold ocean planet with a network of channels of flowing water under a thin cloud cover. The relative size of the planet in the figure assumes a rocky composition but could be larger for a ice/gas composition.
Figure 2. Comparison of the relative size of the orbits and the planets of Kapteyn's Star and the inner planets of our Solar System. The two planets of the Kapteyn's star fit within the orbit of Mercury. Planets are magnify x100 and stars x10 with respect to the orbit scale for clarity. The size of the corresponding optimistic (light green) and conservative (dark green) habitable zones are shown.
Figure 3. Details of the orbits of the two planets around the Kapteyn's star. Only one planet is shown in each diagram using a different orbital scale for clarity. The eccentricity of the planets correspond to the upper 99% confidence level, but they are more likely close to circular orbits.
Many Worlds with Complex Life (Credit: PHL @ UPR Arecibo, NASA, Richard Wheeler @Zephyris)
One Percent of All Exoplanets May Be Suitable for Complex Organisms
The number of planets on which complex life could exist in the Milky Way may be as high as 100 million, according to a study published this week by two former University of Texas at El Paso (UTEP) professors and their colleagues in the on-line journal, Challenges.
“This constitutes the first quantitative estimate of the number of worlds in our galaxy that could harbor life above the microbial level, based on objective data,” according to the lead author of the peer-reviewed study, Dr. Louis Irwin, Professor Emeritus and former Chair of Biological Sciences at UTEP.
Irwin and his colleagues surveyed the growing list of more than a thousand known exoplanets (planets in other solar systems). Using a formula that considers planetary density, temperature, substrate (liquid, solid, or gas), chemistry, distance from its central star, and age, Irwin’s team computed a “biological complexity index (BCI)”, which rates planets on a scale of 0 to 1.0 according to the number and degree of characteristics assumed to be important for supporting multiple forms of multicellular life.
The BCI calculation revealed that 1 to 2 percent of exoplanets showed a BCI rating higher than Europa, a moon of Jupiter thought to have a subsurface global ocean which could harbor different forms of life. Based on a very conservative estimate of 10 billion stars in the Milky Way Galaxy, and assuming an average of one planet per star, this yields the figure of 100 million. It could be over 10 times higher if we consider a larger number of stars in our galaxy.
Irwin emphasized that the study does not indicate that complex life exists on that many planets – only that the planetary conditions that could support it do. He also noted that complex life doesn't mean intelligent life (though it doesn't rule it out), or even animal life, but simply that organisms larger and more complex than microbes could exist in a number of different forms, quite likely forming stable food webs like those found in ecosystems on Earth.
“Other scientists have tried to make educated guesses about the frequency of life on other worlds based on hypothetical assumptions, but this is the first study that relies on observable data from actual planetary bodies beyond our solar system,” Irwin said.
Despite the large absolute number of planets that could harbor complex life, the Milky Way is so vast that, statistically, planets with high BCI values are very far apart. One of the closest and most promising extrasolar systems, known as Gliese 581, has possibly two planets with the apparent capacity to host complex biospheres, yet the distance from the sun to Gliese 581 is about 20 light years. One light year is the distance that light travels in one year.
Most planets with a high BCI are much further away. If the 100 million planets that Irwin’s team says have the theoretical capacity for hosting complex life were randomly distributed across the galaxy, they would average about 24 light years apart.
“On the one hand,” according to Irwin, “it seems highly unlikely that we are alone. On the other hand, we are likely so far away from life at our level of complexity, that a meeting with such alien forms is extremely improbable for the foreseeable future.”
Co-authors of the study include Dirk Schulze-Makuch, formerly an Associate Professor of Geological Sciences at UTEP, now at Washington State University, Alberto Fairén of Cornell University, and Abel Méndez, of the University of Puerto Rico at Arecibo.
Not surprisingly, higher BCI values tend to be correlated with higher ESI values, but there are some exceptions. “Planets with the highest BCI values tend to be larger, warmer, and older than Earth,” said Irwin, “so any search for complex or intelligent life that is restricted just to Earth-like planets, or to life as we know it on Earth, will probably be too restrictive.”Science Contacts
Louis Irwin, University of Texas at El Paso (email@example.com)
Figure 1. Biological complexity (BCIrel) relative to Earth similarity (ESI), as calculated in Schulze-Makuch et al. (2011), for Solar System planets (orange squares) and satellites (yellow squares), and for 365 exoplanets for which BCIrel > 0. The vast majority of exoplanets known to date are gas giants (green circles), but the ones with highest BCI values are likely rocky-water worlds (purple circles).
Kepler-186 is a stellar system of five planets with an Earth-size world in the habitable zone.
Simulated comparison of a sunset on Kepler-186f and Earth. On Kepler-186f the star looks dimmer but slightly larger.
All the known potentially habitable exoplanets so far are superterran worlds (aka super-Earths) somewhat larger than Earth. The potential of life of these worlds is difficult to relate to Earth since there are no planets in the Solar System of comparable size and we know very little of them. Now a team of scientists led by Elisa V. Quintana from the SETI Institute and NASA Ames report the discovery of Kepler-186f, the first terran world (Earth-size) in the habitable zone of a star.
Kepler-186f has a similar size to Earth and it is most likely a rocky world. It orbits the M-dwarf star Kepler-186 along with four other inner planets, which are as old as the Solar System (>4 Gyr), in the constellation Cygnus 500 light years away. Kepler-186f receives less stellar flux (~32%) than presently does Mars (~43%). It could have a temperate climate if it has an atmosphere much denser than Earth. Even Earth probably experienced at least one episode of global glaciation with just a slightly lower stellar flux than today, 650 million years ago. However, early Mars had running surface liquid water with a similar stellar flux as Kepler-186f.
Kepler-186f was added to the Habitable Exoplanets Catalog with a low Earth Similarity Index (ESI) of 0.64 due to its potential colder climate. Still, it could be a more Earth-like world if it is experiencing a much higher greenhouse effect than Earth. Nevertheless, Kepler-186f is also the best candidate now of a rocky world in the habitable zone compared to the other known potentially habitable worlds.
Figure 1. Artistic representation of Kepler-186f as a cold world with shallow oceans as compared to Earth. Other possible interpretations of Kepler-186f are as a snowball frozen world (Hoth-like) or a dry cold world (Mars-like).
Figure 2. Orbital distribution of planets in the stellar system Kepler-186 (top) compared to Kepler-62 (bottom). Both planets 'f' of Kepler-186 and Kepler-62 receive about the same stellar flux.
Figure 3. Analysis of the orbit of Kepler-186f. Its equilibrium temperature is around 192 K for a similar terrestrial albedo. For comparison, Mars has an equilibrium temperature of 210 K. Kepler-186f also orbits just in the outer stellar zone for tidally locked-planets but its rotational state is uncertain. The diagram does not show the orbits of the other inner planets of Kepler-186. More details about this figure are available in the Exoplanet Orbital Catalog.
Figure 4. The new lineup of up to 21 potentially habitable exoplanets according to the Habitable Exoplanets Catalog. Kepler-186f is the most Earth-size planet now but it also receives one third the energy from its star than Earth. This significantly lowers its observable similarities with Earth as compared with other planets in the catalog.
Four new nearby potentially habitable planet candidates, two of them in the same star system.
An international team of astronomers led by Mikko Tuomi from the University of Hertfordshire announced the discovery of four nearby potentially habitable super-Earth worlds among eight new planet candidates. They orbit the habitable zone of the nearby stars, Gliese 180, 422, and 682.
These new four planets increase the number of potentially habitable worlds of the Habitable Exoplanets Catalog to twenty, among fifteen stellar systems. Just last week four NASA Kepler planets were also added. The catalog went from twelve to twenty planets in less than a week. Twelve of these are in the Solar Neighborhood, within 50 light years from Earth.
Gliese 180 is 38 light years away and it is the fourth stellar system found with multiple potentially habitable planets, after Gliese 581, Gliese 667, and Kepler-62. The orbital proximity of its two planets is quite remarkable and very rare. The inner planet, Gliese 160 b, has a minimum mass of 8.30 ME and a radius of ~1.8 RE if rocky in composition, just like Earth. Gliese 160 c has a minimum mass of 6.40 ME and a radius of ~1.7 RE if rocky.
Gliese 442 is 41 light years away. Its only planet, Gliese 442 b, has a minimum mass of 9.9 ME and a radius of ~1.9 RE if rocky. This planet could easily be instead a mini-Neptune, rather than a rocky world, due to its large mass, but there is no way to tell at the moment.
Gliese 682 is 17 light years away and the second closest system found with potentially habitable planets, after Tau Ceti. Its habitable zone planet, Gliese 682 b, has a minimum mass of 4.4 ME and a radius of ~1.5 RE if rocky. There is also a second outer and larger super-Earth planet in the Gliese 682 system.
We only know the minimum masses of these new planet candidates but they could be much more massive and therefore non-habitable. The unique orbital architecture of the two particular planets of Gliese 180, if confirmed, better constrain their maximum mass thus improving their chances of being the right size for life. This also improves their chances of being transiting planets and prime targets for atmospheric characterization by future observatories such as the JWST.
Without any information about their size there is no way to tell if these planets are indeed rocky worlds or small gas planets. There is also no guarantee about the habitability of any potentially habitable world as we know very little of them, some even need to be confirmed as true planets. They are only objects of interest for additional observations.
Stellar systems with multiple habitable planets seem to be common. So far, four out of the fifteen (~27%) known stellar systems with potentially habitable planets have more than one. Astronomers have been struggling to determine how common are stellar systems with Earth-like planets in the universe. Today a new question is emerging about how common are stellar systems with multiple Earth-like planets.
Other members of the research team are Hugh R. A. Jones and John R. Barnes from the University of Hertfordshire, Guillem Anglada-Escude ́ from the University of London, and James S. Jenkins from the Universidad de Chile.
Figure 1. Artistic representation of the habitable zone super-Earth planets Gliese 180 b and c, and Gliese 422 b and Gliese 682 b. They are represented here as worlds with thick cloud covers that look pinkish due to the reddish light of the red dwarf stars they orbit. Only their minimum masses are known but they are shown with sizes corresponding to rocky worlds, just like Earth. They could be twice as big, almost like Neptune, if they are instead non-habitable gas worlds. Earth, Mars, Neptune, and Jupiter are shown for size comparison. Credit: PHL @ UPR Arecibo, NASA.
Figure 2. These images show the star fields around the new three stellar systems with four potentially habitable planet candidates discovered by astronomers led by Mikko Tuomi from the University of Hertfordshire. The field of view is about the size of the Full Moon. A small telescope is necessary to see these since they are dim red dwarf stars. Credit: PHL @ UPR Arecibo, CDS/Aladin.
Figure 3. The individual orbits of the four new potentially habitable planets discovered by astronomers led by Mikko Tuomi from the University of Hertfordshire. Two are around the star Gliese 180, and the other two around Gliese 422 and Gliese 682. Note that each frame shows only the orbit of one planet at a time for simplicity. Click the frames to enlarge. Credit: PHL @ UPR Arecibo.
Figure 4. The new lineup of twenty potentially habitable exoplanets according to the Habitable Exoplanet Catalog including four new ones from NASA Kepler and the four new ones discovered by astronomers led by Mikko Tuomi from the University of Hertfordshire. Six out of these twenty planets are still unconfirmed.
Figure 5. Location in the night sky of the now known fifteen stellar systems with potentially habitable worlds. New ones are Gliese 180 in the constellation of Eridanus, Gliese 422 in Carina, and Gliese 682 in Scorpius. Click the image to enlarge. Credit: PHL @ UPR Arecibo, Jim Cornmell.
The milestone of 1,000 confirmed exoplanets was surpassed today after twenty-one years of discoveries. The long-established and well-known Extrasolar Planet Encyclopedia now lists 1,010 confirmed exoplanets. Not all current exoplanet catalogs list the same numbers as this depends on their particular criteria. For example, the more recent NASA Exoplanet Archive lists just 919. Nevertheless, over 3,500 exoplanet candidates are waiting for confirmation.
The first confirmed exoplanets were discovered by the Arecibo Observatory in 1992. Two small planets were found around the remnants of a supernova explosion known as a pulsar. They were the surviving cores of former planets or newly formed bodies from the ashes of a dead star. This was followed by the discovery of exoplanets around sun-like stars in 1995 and the beginning of a new era of exoplanet hunting.
Exoplanet discoveries have been full of surprises from the outset. Nobody expected exoplanets around the remnants of a dead star (i.e. PSR 1257+12), nor Jupiter-size orbiting close to their stars (i.e. 51 Pegasi). We also know today of stellar systems packed with exoplanets (i.e. Kepler-11), around binary stars (i.e. Kepler-16), and with many potentially habitable exoplanets (i.e. Gliese 667C).
The discovery of many worlds around others stars is a great achievement of science and technology. The work of scientists and engineers from many countries were necessary to achieve this difficult milestone. However, one thousand exoplanets in two decades is still a small fraction of those expected from the billions of stars in our galaxy. The next big goal is to better understand their properties, while detecting many new ones.
* planet candidate CREDIT: PHL @ UPR AreciboBy Dirk Schulze-Makuch
According to the Exoplanet Catalog maintained by the Planetary Habitability Laboratory (PHL) of the University of Puerto Rico at Arecibo, the number of confirmed planets outside our own solar system is approaching 1,000, while another 3,500 exoplanets—most of them detected by NASA’s Kepler mission—are yet to be confirmed. We’re not talking only about Jupiter- or Neptune-like gas giants, but also Super-Earths (terrestrial planets several times the mass of Earth) and Earth-size planets.
From this growing list, the PHL, directed by Abel Méndez, has identified the top 12 potentially habitable exoplanets based on an Earth Similarity Index (ESI). Their top choice is Kepler 62e, with an ESI value of 0.83 (an ESI of 1.0 would be a 100 % match with Earth in terms of astronomical parameters). Kepler 62e is a Super-Earth in the Constellation Lyra, with an estimated mass of 3.6 Earth masses and an estimated radius 1.6 times that of Earth. Its surface temperature is estimated at 31°C (88°F) and it is 7 billion years old, significantly older than Earth. It’s also very far away—1,200 light years, meaning that we won’t be visiting it any time soon.
How much like our own world is Kepler 62e, really? We should be careful to distinguish Earth similarity from planetary habitability. In many respects the Moon has very similar astronomical values to Earth, yet we know it’s a dead rock. On the other hand there’s Saturn’s moon Titan, a top candidate for finding primitive extraterrestrial life in our solar system. But Titan couldn’t be more different from Earth—an icy moon with liquid methane/ethane lakes on its surface, a nitrogen-methane atmosphere, and temperatures well below Earth’s arctic regions.
My own favorite candidate for a habitable exoplanet is Gliese 581d. A mere 20 light years from us, it’s #12 on the PHL list. The planet has an estimated mass of about seven Earth masses, with a radius about double Earth’s. Gliese 581d orbits a red dwarf star with an orbital period of 67 days, which is important. Why? A red dwarf has less energy output than our (yellow dwarf) sun, and an orbital period of 67 days would put the planet in a Mars-like orbit in terms of temperature. In our own solar system, Mars is cold and dry today because it was too small to retain a thick atmosphere, internal heating, and a magnetic field. However, if Mars had been a Super-Earth like Gliese 581d, it would surely still be heated from inside, have kept its magnetic field and thick atmosphere, and would likely still have liquid oceans (and perhaps even life!) on its surface.
Gliese 581d’s estimated surface temperature of -37°C (-35°F) should not concern us too much. If aliens were to observe Earth from afar, based just on the amount of incoming solar radiation they would estimate our surface temperature as -18°C (0°F). It’s only due to the greenhouse effect that Earth’s average temperature is actually a benign +15°C (59°F; yes, a limited greenhouse effect can be a good thing!). On a Super-Earth planet such as Gliese 581d, I would expect the effect to be quite a bit stronger than on Earth. And one last thing in this planet’s favor: Gliese 581d is nearly twice as old as Earth, which could have given evolution plenty of time to develop advanced, perhaps even technologically advanced, life forms.
This article was originally published in the Daily Planet Blog of the Air & Space Smithsonian. The author, Dirk Schulze-Makuch, is a professor of astrobiology at Washington State University, and has authored or co-authored seven books related to the possibility of life in the solar system and beyond.
The 'Bright Blue Marble' and the 'Pale Blue Dot' Together
Here are actual satellite images of Earth near the moment and the angle the pictures from Cassini and Messenger were taken from Saturn and Mercury on the Day the Earth Smiled, respectively. High resolution black and white images from the GOES East and Meteosat meteorological satellites were combined with color information from NASA Visible Earth to generate true-color images. Check here for additional details.
Figure 1. Earth taken from orbit and from Saturn on the Day the Earth Smiled (July 19, 2013). Earth from orbit is shown with a little more illuminated area at the moment of the Cassini picture from Saturn. The Moon is also visible to the right of Earth in the image from Saturn. Earth was visible to the naked-eye (+1.9 magnitude) as a dim star at the moment the image was taken from Saturn. This figure makes references to the iconic 'Blue Marble' and 'Pale Blue Dot' images of Earth from space. Credit: PHL @ UPR Arecibo, NASA/JPL-Caltech/Space Science Institute, NERC Satellite Station, Dundee University, Scotland.
Figure 2. Earth taken from orbit and from Mercury on the Day the Earth Smiled (July 19, 2013). Earth from orbit is shown with almost exactly the area and illumination at the moment of the Messenger picture from Mercury. The Moon is also visible to the right of Earth in the image from Mercury. Earth was as bright (-4.8 magnitude) as the maximum brightness of Venus at the moment the image was taken from Mercury. This figure makes references to the iconic 'Blue Marble' and 'Pale Blue Dot' images of Earth from space. Credit: PHL @ UPR Arecibo, NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington, NERC Satellite Station, Dundee University, Scotland.
Figure 3. On July 19, 2013 the NASA Cassini and Messenger spacecrafts took pictures of Earth from Saturn and Mercury, respectively. These photos provide some context as to the approximate appearance of Earth during these pictures as seen from geostationary weather satellites. Click image for larger version. Credit: PHL @ UPR Arecibo, NASA, NERC Satellite Station, Dundee University, Scotland.
Figure 4. Earth from the geostationary weather satellite GOES East on July 19, 2013 at 5 PM EST - 2 PM EDT (21 UTC). Click for high resolution version. Credit: PHL @ UPR Arecibo, NASA, NERC Satellite Station, Dundee University, Scotland.
Figure 5. Earth from the geostationary weather satellite GOES East on July 19, 2013 at 5 PM EST - 2 PM EDT (21 UTC). Click for high resolution version. Credit: PHL @ UPR Arecibo, NASA, NERC Satellite Station, Dundee University, Scotland.
The star Gliese 667C is now the best candidate for harboring habitable worlds.
Our Solar System has only one habitable planet, or maybe two if you count Mars’ past when liquid water was running on its surface. More than one potentially habitable planet per star has been a very rare event in the known stars with planets. The well-known star Gliese 581 might have two and just recently the Kepler Space Telescope discovered two in the star Kepler-62. Now a team of scientists, led by Guillem Anglada of the University of Göttingen, reports the discovery of a star with three potentially habitable worlds.
Gliese 667C is part of a nearby triple system of stars 22 light years away. The team discovered that Gliese 667C has six planets, or even more, with three of them in the habitable zone and not much more massive than Earth. Gliese 667C is now the most interesting object for studying stellar systems with the potential for life.
About one hundred exoplanets, of the 898 detected so far, orbit their star at the right distance to sustain liquid water or the so-called ‘habitable zone’. However, most are big Jupiter-like worlds that are unable anyway to have a surface with liquid water. Only a few probably have the right size to be rocky worlds just like Earth.
Gliese 667C is the fourth known planetary system with at least six confirmed planets (Kepler-11, HD 10180, and HD 40307 also have six planets). Gliese 667C f and e will be added to the Habitable Exoplanets Catalog, now having a dozen objects of interest. Gliese 667C c was already added last year.
Figure 1. Artistic representations of the three potentially habitable planets around the star Gliese 667C as compared with Earth. The planets are shown assuming a rocky composition with surfaces mostly covered by water clouds. Credit: PHL @ UPR Arecibo.
Figure 2. The Habitable Exoplanets Catalog now list a dozen object of interest as potentially habitable worlds with the addition of two new planets, Gliese 667C e and f (Gliese 667C c was known since early 2012). Credit: PHL @ UPR Arecibo.
Figure 3. Simulated view of the star Gliese 667C from its three orbiting potentially habitable worlds. The Earth image is an unretouched photo of a beach sunset. The size of the star, and the brightness and color of the sky was carefully adjusted in subsequent frames to approximate the same view from each planet. The original photo is from a beach in Puerto Rico. Credit: PHL @ UPR Arecibo.
Figure 4. Gliese 667 is a nearby system of three stars that is easy to locate in the Scorpius constellation. The main two central stars Gliese 667AB are barely visible to the naked eye but easy to spot by binoculars or a small telescope. They are so close together that they appear as a single star. Gliese 667C is far enough from the central stars to be seen as separate star. However, it requires a larger telescope to be seem. Credit: PHL @ UPR Arecibo.
Figure 5. Orbits and approximate relative size of the planets around Gliese 667C (orbits and planets are not to scale with each other). All three potentially habitable planets (c, f, and e) orbit within the 'conservative habitable zone.' Credit: PHL @ UPR Arecibo.
Figure 6. This image shows the relative location of each potentially habitable planet of Gliese 667C with respect to the habitable zone. The location of Earth and Mars are shown for comparison. Note that Earth is not at the center of the habitable zone. Any planet close to the inner edge risk being a super-Venus while a frozen world closer to the outer edge. The star is not to scale with the planets.
Table 1: Planetary properties of the six planets around the star Gliese 667C plus one still unconfirmed (Gliese 667C h). Three of these (e, f, and e) are potentially habitable (highlighted in red). This table includes some estimated values that are not part of the original observations (see table notes below). Table 2 has similar values for the Solar System planets.
Planet Type - according to the PHL's Classification of Exoplanets.
Expected Mass - estimate assuming a probable inclination of 60°.
Expected Radius - estimate from an empirical mass-radius relationship.
Equilibrium Temperature - estimate assuming a bond albedo of 0.3.
Expected Surface Temperature - estimate assuming a scaled Earth-like atmosphere.
Habitable Zone Distance (HZD) - a measure of the orbital location of the planet with respect to the HZ. Planets with HZD values between -1 and +1. are within the HZ.
Earth Similarity Index (ESI) - using the new revised ESI which only considers observed parameters, stellar flux, and radius or mass.
Table 2: Planetary properties of the planets of the Solar System for comparison purposes. Description similar to Table 1.
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