Arecibo Observatory took radar images of asteroid 1998 QE2 and its moon as the space rock sailed safely past earth. The sequence of images show a dark, cratered asteroid 3.0 kilometers across (1.9 miles) across with a companion moon about 750 meters (2500 feet) in size. Both the asteroid and its moon passed 6 million kilometers (3.8 million miles) from earth. This object has no chance of hitting the Earth, but comes close enough to study with a variety of telescopes, large and small.

Last week NASA’s Kepler mission added 1,924 new objects of interest to its list of 2,713 exoplanet candidates. The new data has not been completely analyzed yet and many of these objects might be attributed to non-planetary processes (false-positives). However, this new batch gives a general idea of what new types of planets could be announced in the future.

The Planetary Habitability Laboratory (PHL) did a basic analysis of this new Kepler data compared to the existing exoplanets data. The new data suggests the addition of up to 83 new potentially habitable exoplanet candidates, a big increase from the current 18 listed in the Habitable Exoplanets Catalog. Only four of these have been confirmed so far: Kepler-22b, Kepler-62e, Kepler-62f, and Kepler-61b.

Two types of potential exoplanets are particularly notable in this new Kepler data. There is a big increase of the number of warm and cold super-Earths. This is somewhat expected since the new data includes objects with longer periods. However, other larger objects did not increase as well but it might be too early to attribute this to the abundance of low mass planets.

The most interesting additions are the first potential Earth-like planets. All currently known potentially habitable exoplanets are quite larger than Earth and therefore not very Earth-like by definition. The new Kepler data suggests six new objects with the right size and distance from their star to be considered Earth-like worlds. Since Kepler is no longer operating, it will be very hard to confirm any of these objects with additional data in the near future.

The recent study by Everett et al. (2013) suggests that one fourth of the Kepler exoplanets candidates are 35% larger than expected, which could also affect the number of those considered potentially habitable. The new Kepler data is being used by the PHL and other groups to prioritize targets of interest for analyses and future observations. 

The PHL’s Habitable Exoplanets Catalog (HEC) uses a wide definition for potentially habitable exoplanets that includes planets with 0.1 to 10 Earth masses (or 0.4 to 2.6 Earth radii) orbiting the habitable zone of their parent star within the Venus-Mars empirical limits. Other research groups use narrower or wider definitions. A much wider definition that includes ‘dry planets’ and ‘hydrogen-rich planets’ will be implemented in the future.

Figure 1. Current potentially habitable exoplanets showing the new Kepler-62e and Kepler-62f. Kepler-62e is now the best candidate for an Earth-like planet based on measured parameters. However, the actual potential for life of any of these worlds depends on their atmospheric properties which are unknown at this time.

Figure 2. Current potentially habitable exoplanets showing the new additions, Kepler-62e and Kepler-62f.

Figure 3. Artistic representations of Kepler-62e and Kepler-62f compared with Earth. Both are shown with a similar terrestrial atmosphere thus making Kepler-62e slightly hotter than Earth and Kepler-62f a cold but still habitable world. Another possibility is that both have dense atmospheres. In that case the temperatures of Kepler-62f might be more suitable for life than on Kepler-62e.

The Planetary Habitability Laboratory (PHL) is now searching for Earth-like worlds. The PHL maintains the Habitable Exoplanets Catalog (HEC) in which exoplanets discoveries are classified and compared according to different habitability metrics. Previously the PHL has not been involved in making the initial exoplanet discoveries. Now, the PHL is using new algorithms based on pattern recognition to search for Earth-like worlds within the NASA Kepler Telescope data.

The idea of an Earth-like planet might suggest a world with oceans, breathable air, or even life. However, astronomers are far from getting this information because exoplanets are very far away for our current instruments. For astronomers the definition of Earth-like planets is limited by what we can measure now, their orbit and size. Any exoplanet around a Sun-like star with a similar orbit and size as Earth is considered an Earth-like world.

The new PHL’s project, Search for Potentially Habitable Exoworlds Resembling Earth (SPHERE), is exploring the NASA Kepler data for Earth-like exoplanets and exomoons. Since October 2012 the NASA Kepler data is publicly available for any research team to explore. Many other teams are using this data to discover exoplanets, including the NASA Kepler Team and scientific community projects such as the Hunt for Exomoons with Kepler and Planet Hunters.

Only a few of the nearly 900 confirmed exoplanets barely fit as candidates for potentially habitable worlds. All of these are superterran worlds (super-Earths) up to two times larger than Earth. Scientists are not confident of how habitable these larger worlds might be as compared to Earth. They are more interested in terran worlds (Earth-size) orbiting in the habitable zone of their star, which are more comparable to Earth.

A potentially habitable exoplanet is not necessarily the same as an Earth-like exoplanet. In astrobiology a habitable exoplanet refers to any world suitable for surface or subsurface life of any type, as we know it. This extends to a series of possibilities that are viable for the most environmentally stress tolerant life forms (i.e extremophilic life) but not necessarily by most complex life such as plants, animals, and humans. In theory, an Earth-like planet might be viable for most terrestrial surface life, but future ground and space observatories are needed to confirm the actual habitability of any of such bodies.

The main tool of the SPHERE project is the transit detection software package K-SPHERE. It consists of a series of IDL software tools designed to recognize the faint signals of potential habitable worlds out of the Kepler data using different pattern recognition algorithms. K-SPHERE has been successfully tested identifying previously known confirmed and candidate exoplanets. It is also identifying new signals that are currently under study.

Pattern recognition techniques are widely used in many scientific, engineering, and computer science applications to perform the ‘most likely’ matching of a pattern to a given input. Common day experiences like recognizing faces, understanding words, and reading characters are natural examples of this process. One of its most popular computer applications is in biometrics where a computer is able to distinguish the subtle differences between two similar faces.

Identifying an Earth-like world in the Kepler data is not much harder mathematically than identifying a face in a picture. Consumer cameras do that all the time even matching persons. The problem is that as humans and computers are sometimes confused with similar faces, even with inanimate objects (pareidolia), the signals of Earth-like worlds can be confused with other unrelated stellar, planetary, or instrumental phenomena. It is very challenging to discern false-positives from the Kepler data at these scales.

The SPHERE project is optimized to search for Earth-size exoplanets and exomoons within the habitable zone of stars. It takes advantage of similarity indices such as the Earth Similarity Index (ESI), a measure of Earth-likeness, to optimize the search for interesting targets within all the Kepler data. SPHERE expects to sort out any Earth-like world hopefully within this year, but it takes a lot of additional effort to validate any discovery.

Thanks to the NASA Kepler Telescope, astronomers are not only searching for another ‘Pale Blue Dot,’ as Carl Sagan used to refer to Earth, but more likely for another ‘Pale Blue Sphere,’ since Kepler reveals the size of these worlds. Earth-like worlds are not anymore a dot in space but featureless ‘spheres’. The surface color and features of these ‘spheres’ are a problem for future observatories.

SPHERE is a project of the PHL of the University of Puerto Rico at Arecibo (UPR Arecibo) with the international collaboration of scientists from other institutions. The NASA Kepler Telescope is a statistical mission that uses the transit method to detect exoplanets, especially those terrestrial ones in the habitable zone of their stars where liquid water and possibly life might exist.

Science Contact: Abel Méndez, PHL @ UPR Arecibo, abel.mendez@upr.edu

Caption: Simulated transit to scale of some of the planets of the Solar System as seem from far away. Earth with the Moon (center left of the Sun) and Saturn with Titan (center right from the Sun) are show transiting the Sun. Note that Earth and the moons are about the size of many of the sunspots thus making them harder to recognize from the Sun's background. Jupiter (left) and Neptune (right) are also shown to scale but not transiting. CREDIT: PHL @ UPR Arecibo, NASA, A. Fuji.

A team of astronomers from Penn State led by Ravi Kumar Kopparapu and Ramses Ramírez, also PHL collaborators, announced a redefinition of the classical stellar habitable zone. Their study has strong implications on how we search and study potentially habitable exoplanets.

The main conclusion of their study is that the habitable zones are actually farther away from the stars than previously thought. Interestingly, Earth appears to be now situated at the very inner edge of the habitable zone.

The new study builds on the previous work of James Kasting, Evan Pugh Professor of Geosciences at Penn State, also one of the co-authors. Currently, all definitions of the habitable zone were based on Dr. Kasting's pioneering work.

Estimates with the new habitable zone suggest that some exoplanets previously believed to be in the habitable zone may not be. However, the study has not as yet taken into account the complicated feedback of clouds, which also help to stabilize climates.

The PHL’s Habitable Exoplanet Catalog (HEC) will certainly be impacted by the new definition of the habitable zone. This will affect the classification of a number of potentially habitable exoplanets out of those nearly 900 confirmed and the over 2,700 NASA Kepler exoplanets candidates.

Recently, the Earth Similarity Index (ESI), a measure of Earth-likeness, and other habitability metrics were updated at the PHL. The ESI does not rely on the definition of the habitable zone but other considerations were used to improve it.

A major release of HEC, reflecting all these results, will be presented via a public lecture at the Arecibo Observatory on Saturday, February 16, 2013 @ 6 PM AST (5 PM EST) and available at the PHL’s site on Monday, February 18, 2013 @ 9 AM AST (8 AM EST).

Ravi Kumar Kopparapu, Penn State, ruk15@psu.edu

Ramses Ramírez, Penn State, rmr5265@psu.edu

Abel Méndez, PHL @ UPR Arecibo, abel.mendez@upr.edu

Figure 1: Extension of the new habitable zone as a function of distance for cool to hot stars. Credit: Chester Harman, Penn State

NASA Kepler released last month 18,406 planet-like detection events from its last three year mission to search for exoplanets (Kepler Q1-Q12 TCE). Further analysis is required by the NASA Kepler Team and the scientific community to extract and identify true planets, including those potentially habitable. The Planetary Habitability Laboratory @ UPR Arecibo (PHL) performed a preliminary analysis and identified 262 candidates for potentially habitable worlds in this dataset. These candidates become top priority for further analysis, additional observations, and confirmation.

The Kepler Threshold Crossing Event (TCE) dataset consists of a list of stars with 18,406 transit-like features that resemble the signatures of transiting planets to a sufficient degree that they are passed on for further analysis. Many of these objects are false positives caused by stellar transits or other physical and instrumental conditions not related to planets. Those that pass additional tests are added to the Kepler Objects of Interest (KOI) list, currently at 2,320 candidates, for further validation. Finally, those verified by more astronomical observations supplement the 132 Kepler confirmed planets so far.

Only the best TCE objects, those with more than three transit events, were selected for the analysis in accordance with the PHL’s Habitable Exoplanet Catalog (HEC) criteria. This reduced the sample to 15,847 objects eliminating a known instrumental bias for one-year period planets. Unfortunately, this also eliminated many interesting objects but more analysis will be required to sort out longer period planets. HEC identified and sorted with the Earth Similarity Index (ESI), a measure of Earth-likeness, 262 potentially habitable planet candidates. These include four subterrans (Mars-size), 23 terrans (Earth-size), and 235 superterrans (super Earth-size).

The preliminary analysis performed by the PHL helps to sort out and rank the best candidates for further exploration in NASA Kepler’s TCE. Twenty-four of these have an ESI over 0.90 and therefore are quite Earth-like according to what is measurable. For example, the best candidate is an Earth-size planet in a 231 days orbit around the star KIC-6210395, which receives about 70% of the light that Earth receives from the Sun. More are expected with a similar period to Earth but they will be added later to HEC after further analysis. It will still be remarkable if only 50% of these turn out to be real planets.

It is estimated that there are millions of Earth-like planets in our Galaxy. However, most of these are out of our observational abilities for the coming decades, and probably many centuries. Only a small fraction of these planets, the ones that transit their star, are good enough for better characterization and to confirm their potential for life. This result suggests that there are over 8,500 transiting very Earth-like planets within reach of NASA Kepler-like missions, assuming the Kepler field is representative of all the sky. This sample is enough to occupy astronomers for many years.

• HEC List of Potentially Habitable NASA Kepler TCE Objects.
• PHL's Exoplanet Catalog, now including the TCE sample.
• Habitable Exoplanets Catalog.
• The title of this article is a clear reference to the movies 2010 and Contact. Thanks to Tony B. ‏Tubsturtle for suggesting the Contact quote.
• You can follow @PlanetaryHabLab for updates

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Tau Ceti e and f have minimum masses of 4.3 and 6.6 Earth masses, respectively. They are both superterrans (aka super-Earths) orbiting within the habitable zone of their parent star. There is no information about their size so it is not possible to tell if they are rocky, water, or even gas worlds. However, if we assume that they are composed of a water-rock mix, their radius might be closer to 1.8 and 2.3 Earth radii, respectively.

The planet Tau Ceti e orbits close to the inner edge of the habitable zone. It receives about 60% more light than Earth from the Sun making it a hot planet probably only habitable to simple thermophilic (heat-loving) life. Its mean global surface temperature should be near 70°C assuming a similar terrestrial atmosphere. However, it is likely that superterran planets have much denser and heat trapping atmospheres and Tau Ceti e might be instead dominated by a strong greenhouse effect making it more likely a super-Venus than a super-Earth. Without any knowledge of its atmosphere we are not able to tell if it is a mildly hot planet tolerable for simple life forms or a very hot non habitable Venus-like world. Tau Ceti e has an Earth Similarity Index of 0.77 assuming a more terrestrial-like atmosphere.

The planet Tau Ceti f orbits close to the outer edge of the habitable zone. It only receives about 27% the light of Earth from the Sun making it a cold planet probably only habitable to simple psychrophilic (cold-loving) life. Its mean global surface temperature should be near -40°C assuming a similar terrestrial atmosphere. However, it is likely that as Tau Ceti e, it is also dominated by a strong greenhouse effect making it even acceptable for complex life, which requires temperatures from 0°C to 50°C. Without any knowledge of its atmosphere we are not able to tell if it is a frozen Mars-like planet tolerable for simple life forms or even an Earth-like world. Tau Ceti e has an Earth Similarity Index of 0.71 assuming a more terrestrial-like atmosphere.

Both Tau Ceti e and f are very interesting planet candidates for astrobiology that need to be confirmed by more observations. Neither yet fit the Earth-like distinction based on the little information we have of them. It is hard to tell which is better for life at this point without more information about their bulk composition and atmospheric structure. There is one thing we can be certain of now. Tau Ceti is now the closest and most similar stellar system to our Solar System with potential habitable exoplanet candidates, displacing the notorious Gliese 581 stellar system.

The first confirmed extrasolar planets, or exoplanets, were discovered from the Arecibo Observatory twenty years ago, in 1992. These were extreme planets around PSR-B1257+12, a collapsed form of star known as a Pulsar. In 1995 the planet 51 Pegasi b was discovered around a more typical Sun-like star. Today scientists know of nearly 900 confirmed exoplanets with over 2,500 still awaiting confirmation. Only a few of those might be good candidates for life, as we know it.

Michio Kaku said in his 2006 article Star Makers that “Scientists may eventually compile an encyclopedia identifying the precise coordinates of perhaps hundreds of Earth-like planets.” Five years later, on December 2011, it all started to happen with the creation of the Habitable Exoplanets Catalog (HEC). Although there are many databases dedicated to extrasolar planets, HEC is the only one focused on Earth-like worlds.

HEC is as a project of the Planetary Habitability Laboratory (PHL) of the University of Puerto Rico at Arecibo. It was conceived out of the need to measure the suitability for life of these emerging worlds and as a way to organize them for the general public. Every time an Earth-size planet is described as a habitable world is confusing. There are nearly 80 confirmed exoplanets, and 250 Kepler candidates, with a similar size to Earth but only a few of those have the right distance from their star to support liquid water on the surface.

Several methods are used by HEC to classify and rank the habitability of exoplanets. One of the most popular ones is the Earth Similarity Index (ESI), a measure of Earth-likeness in a scale from zero to one, one being identical to Earth. The exoplanet classifications use the word terran instead of ‘Earth-size’ to avoid a strict comparison with Earth. Many of these worlds might have some physical similarities with Earth but might not necessary be Earth-like.

Seven potentially habitable exoplanets are now listed by HEC, including the disputed Gliese 581g, plus some 27 more from NASA Kepler candidates waiting for confirmation. These are objects of interest for further astronomical observations but there is no guarantee on the habitability or even the existence of many of these planets. It will take many new observations, and more ground and space observatories to verify and characterize these objects.

HEC was launched, December 5th, 2011, the same day that the NASA Kepler team announced the discovery of Kepler 22b during the First Kepler Science Conference in NASA Ames, California. It started with just two planets, Gliese 581d and HD 85512b. Later, Kepler 22b, Gliese 667Cc, Gliese 581g, Gliese 163c, and HD 40307g were added, in chronological order. All these exoplanets are superterrans, aka Super-Earths, and quite larger than Earth but still considered potentially habitable. Scientists have not yet found a true Earth analog.

HEC is now a very complex computer program, written in IDL, that incorporates data from many exoplanet databases, such as the Extrasolar Planet Encyclopedia and NASA Exoplanet Archive, plus many updates from other research groups. It receives most of the data of new relevant discoveries directly from the research teams, which is analyzed and added to the catalog at the moment of their public announcement.

Potentially habitable exoplanets are hard to detect, from two to seven in a year was a big and unexpected change. What will happen in 2013? HEC will be expanded with new models and analysis in the next year. This will impact somehow the objects listed in the catalog. Expect new visualizations and new habitability assessments. However, the biggest impact will come from new discoveries. A true Earth analog or a potentially habitable exomoon would be big discoveries. Certainly, this was the right time to start mapping the habitable universe around us.

Caption: Computer representations of Earth and Mars, and all the current potential habitable exoplanets: Gliese 581d, HD 85512b, Kepler 22b, Gliese 667Cc, Gliese 581g, Gliese 163c, and HD 40307g. Credit: PHL @ UPR Arecibo, HPCf @ UPR, and ESO/S, Mash Mix: SPACE.com. Music: Mark Peterson, Loch Ness Productions.