Astronomers have created a way to compare and rank exoplanets to help prioritize which of the thousands discovered warrant close inspection in the search for life beyond Earth. The Kepler Space Telescope has enabled astronomers to detect thousands of exoplanets, those beyond our solar system. The James Webb Space Telescope, set for launch in 2018, will be the first able to actually measure the atmospheric composition of a rocky, possibly Earthlike planet far off in space, and so vastly enhance the search for life.
Astronomers detect some planets when the worlds “transit” or pass in front of their host star, thus blocking some of the light. The Transiting Exoplanet Survey Satellite, or TESS, is scheduled to launch in 2017 and will find many more worlds in this way. But it’s the Webb telescope and its “transit transmission spectroscopy” that will really be able to study planets closely to hunt for life.
But access to such telescopes is expensive and the work is methodical and time-consuming. The Virtual Planetary Laboratory’s index is a tool to help fellow astronomers decide which worlds might have the better chance of hosting life, and so are worthy of focusing limited resources on.
Traditionally, astronomers have focused the search by looking for planets in their star’s “habitable / Goldilock’s zone. But so far that has been just a sort of binary designation, indicating only whether a planet is, or is not, within that area considered right for life.The new index is more nuanced, producing a continuum of values that astronomers can punch into a Virtual Planetary Laboratory Web form to arrive at the single-number habitability index, representing the probability that a planet can maintain liquid water at its surface.
In creating the index, the researchers factored in estimates of a planet’s rockiness, rocky planets being the more Earth-like + “eccentricity-albedo degeneracy,” which comments on a sort of balancing act between the a planet’s albedo – the energy reflected back to space from its surface – and the circularity of its orbit, which affects how much energy it receives from its host star.
The higher a planet’s albedo, the more light and energy are reflected off to space, leaving less at the surface to warm the world and aid possible life. But the more noncircular or eccentric a planet’s orbit, the more intense is the energy it gets when passing close to its star in its elliptic journey.
A life-friendly energy equilibrium for a planet near the inner edge of the habitable zone – in danger of being too hot for life – Barnes said, would be a higher albedo, to cool the world by reflecting some of that heat into space. Conversely, a planet near the cool outer edge of the habitable zone would perhaps need a higher level of orbital eccentricity to provide the energy needed for life.
Barnes, Meadows and Evans found the best candidates for habitability and life are those planets that get about 60-90% of the solar radiation the Earth receives from the sun, which is in keeping with current thinking about a star’s habitable zone.
http://www.washington.edu/news/2015/10/05/where-to-look-for-life-uw-astronomers-devise-habitability-index-to-guide-future-search/
The James Webb Space Telescope, a large infrared telescope with a 6.5-meter primary mirror, is scheduled to be launched on an Ariane 5 rocket from French Guiana in October of 2018 and will be the premier NASA observatory of the next decade, serving thousands of astronomers around the world. UW astronomers have created a “habitability index for transiting planets” to help guide the ongoing search for life beyond Earth. Credit: NASA
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