Searching for a Pale Blue Sphere
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.