Man's Next Migration

Finding Habitable Exoplanets: Habcat to Biosignatures

Some of us are very much familiar with the basic requirement for an exoplanet to be counted as habitable: it must reside in the habitable zone of its star. The star should favorably be in the same class as our sun in the main sequence although studies show that red dwarfs could host them as well. As such, much focus is given to searching and filing a catalogue for these types of stars. Enter the HabCat, or the Catalog of Nearby Habitable Systems, a list of about 17,129 stars made by astronomers Margaret Turnbull and Jill Tarter in 2002.

This list came from a much longer list of stars generated by the Hipparcos mission from 1989 to 1993 which took astronometric observations from 118,000 stars with the results published in 1997. Turnbill and Tarter studied this long list of stars to come up with the HabCat, a list of stars that are believed to possess characteristics suitable for life to inhabit the “goldilocks zone” of its planetary systems. It was further narrowed down to the nearest 5,000 stars closest to us in about a 100 light year distance. Turnbull moved on further to identify ten stars in 2006 that have a better likelihood of hosting habitable planets.

The first five stars are to be the focus of the Allen Telescope Array being constructed in San Francisco, California involving a whole hectare of land dedicated for 300 or more radio telescope dishes. The last five stars are the proposed targets for the cancelled Terrestial Planet Finder, a NASA mission which could have used advanced infrared detection techniques to ultimately search for terrestial exoplanets. The last five on the list includes Epsilon Eridani and Alpha Centauri which we have covered in recent articles.

To date, most astronomers have high hopes that the Kepler Mission would be able to fulfill its objective to find earth-like planets. In contrast with the HabCat's focus on habitable planets, Kepler has 13.2 million targets available in its field of view and a lot of work has to be done down to narrow down this list. Kepler uses the Transit Detection Algorithm developed by PlanetQuest originally for ground-based photometric telescopes. Unlike regular astrometry, radial velocity method or infrared sensing (as the ones being used by WISE, Herschel and the Spitzer space telescope), the TDA uses long-term photometry observations to determine the presence of exoplanets as they pass in front of the star. Thus, with Kepler's FOV, it will be able to photograph glimpses of planetary transits as they occur.

Because of the relatively vast amounts of data that will be generated by Kepler and other ground-based telecopes, work is underway to develop a system of classfying stars and exoplanets through the use of grid computing. It will utilize a network or grid of linked computers to provide the necessary computing power to process vast quantities of data. This project is currently being led by PlanetQuest and will use the BOINC platform (Berkeley Open Infrastructure for Network Computing) to tap into computers all over the world from millions of volunteer users and participants.

Most key factors in determining the habitability of discovered exoplanets are immediately identified while analysing and listing planetary characteristics such as size and temperature. The exoplanet must not be too hot or too cold. It must be in the size range close to earth's or it could be a super-earth, as long as it remains relatively dense. Very low densities could indicated a large gaseous exoplanet that may not be friendly for life at all. Usually the composition of stars could already dicate if the surrounding exoplanets could harbor life in which case high metallic content is desirable.

More elusive characteristics involve planetary composition and astrobiology. Last year, scientists concluded that habitable exoplanets is common throughout the galaxy after analysing the composition of asteroids orbiting other stars. Light from the star passing through asteroid debris were analysed through a spectograph aboard the Spitzer Space Telescope. The basic principle takes into consideration that various materials disperse light in different wavelengths depending on their composition. By taking a look at these wavelengths, scientists are able to determine the chemical composition of the asteroids. Their belief that habitable exoplanets are just as common as other exoplanets may hinge on the fact that asteroids are either the remnants of a long gone planet or just about to become the primary ingredients of planet formation. It's almost as if they're saying that the analysis of the composition of asteroids is an indirect analysis of the exoplanets themseleves residing in the same system.

The detection of water, a critical astrobiological ingredient for life, is also an important aspect of determining habitability though it proves difficult as of the moment. Recently though, an easy method albeit loaded with inaccuracy is the hunt for sun glints which if spotted on exoplanets, could be hinting at the presence of water. The glints occur when sunlight reflects of a shiny surface, in this case may either be water or ice. Periodic observation of these glints in exoplanets could provide further clues of its habitability. The “sunglint” became popular due to last month's release of the EPOCh (Extrasolar Planet Observations and Characterization) video capture of earth when NASA's Deep Impact spacecraft looked back to view the our planet as an exoplanet and determine what an exoplanet would look life from afar. It is part of an extended mission codenamed EPOXI using Deep Impact which originally launched an impactor that crashed into the Tempel I comet in 2005.

Another interesting characteristic scientist may look for in exoplanets would be plate tectonics. A study conducted in Harvard University last year reveals that plate tectonics resulting to geological acitivity distributes different elements and compounds throughout a planet in time. The process allows carbon dioxide, our own planet's natural thermostat, to be recycled keeping temperatures pleasant for biological creatures. It also enables complex chemistry to take place and the the resulting diversity is essential for life to proliferate across the face of the earth. The study's calculations conclude that terrestial super-earths naturally involve greater tectonic plate activity, all the more increasing the likelihood of them being habitable. The scientists insist that “bigger is better”, claiming that rocky super-earths are more habitable than any type of terrestial exoplanet and in fact would look very much familiar to earth's physical environment.

Perhaps the most important characteristic in determining if an exoplanet is habitable or not could be the detection of biosignatures. Biosignatures, signs of life, in other planets are mainly derived by studying and obseving our own earth, the only known habitable planet to date despite 400 or more exoplanets already discovered. Unique physical and complex chemical processes that occur natively on earth could not be found elsewhere in the universe and hence are considered to be biosignatures. As an example, the biochemical process that produce proteins or the replication of DNA are just some of the things that can only be found on earth. If any of these biosignatures could be detected by future technologies or deduced from the gigantic mountains of data taken by ground and space observatories, radio telescopes and other equipment; then we can perhaps truly say that we have found one. Habitable exoplanets could just be around the corner, waiting for us to finally discover them.

References

About Astrobiology. NASA: Astrobiology, Life in the Universe. Accessed 06-February-2010
BOINC. University of California. Accessed 06-February-2010
Earth: A Borderline Planet for Life?. Press Release: Harvard-Smithsonian Center for Astrophysics. Accessed 06-February-2010
Extrasolar Planet Detection. PlanetQuest Bibliography. Accessed 06-February-2010
HabCat. Wikipedia.org.  Accessed 06-February-2010
Habitable Exoplanets Could Be Common in Our Galaxy. Clara Moskowitz. Wired Science. Wired.com. Accessed 06-February-2010
Hipparcos Catalogue. Wikipedia.org. Accessed 06-February-2010
Sun Glints Seen from Space Signal Oceans and Lakes. Elizabeth Zubritsky. NASA Goddard Space Flight Center. Accessed 06-February-2010