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    A Way Out of the "So What?" Paradox of Hunting Alien Earths

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    Michael Byrne

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    The James Webb Space Telescope

    Not sure I'd go as far as to say that we're "bombarded" with new Earth-like or habitable zone planet discoveries, but soon enough we'll reach a certain "so what?" plateau. You can get some idea of the planetary glut from the graphic below, courtesy of NASA's Kepler mission, one of two main projects in the extrasolar planet hunt. Note that all of those spheres look pretty blank.

    Based on the Kepler findings, which have been coming in a steady stream since 2010, we can now estimate that at least one in six stars has an Earth-size planet in its orbit, according to a presentation this month from Francois Fressin of the Harvard-Smithsonian Center for Astrophysics. Some of these are also lucky enough to orbit within a star's "habitable zone," the region most likely to have life-harboring capabilities. Which is just one of parameter of a great many that might suggest the possibility of alien life.

    Kepler has another three years of observation to go, and the project's most recent results only include findings up to 2011 (the mission churns out a whole lot of data). So, count on many, many more Earth-size and habitable zone planets to come. Meanwhile, the European Southern Observatory's HARPs project is also on the hunt, using a different technique, but also discovering Earth-size habitable zone at a good clip, including the much-vaunted Gliese 581 system. The difference between the projects is in how they detect planets: HARPs uses a technique looking for the "wobbles" of stars as a planet passes by, while Kepler looks as the dimming of stars as a planet passes in front (called a "transit").

    The catch is that these projects can't tell us a whole lot more about their Earth-size and habitable zone planet discoveries. Some other crucial stuff can be estimated, like density and temperature, but getting a read on a planet's atmosphere seems nigh impossible, even with foreseeable future instruments. A paper out earlier this month by Philip von Paris et al found that it's unlikely future missions will be able to get much of a read on atmospheric conditions around extrasolar planets beyond carbon dioxide detection. The spectral signature is just too weak.

    A new paper posted to the arVix preprint server this week is more hopeful, however. Using the forthcoming if beleagued James Webb Space Telescope, researchers should be able to get a slice of alien atmosphere at high enough resolution to tell if it contains oxygen, that perfect biomarker. This hope just depends on one crucial thing: that we'll find Earth-like planets around white dwarf stars.

    Currently, most planet hunting involves what're called main sequence stars, like our sun and most other stars we currently see in the universe. A main sequence star is simply one that's turning hydrogen into helium in its core through nuclear fusion; it's a star in the prime of its life. White dwarves, the hot remnants of stars like our sun after they've exhausted their fuel, are dimmer and smaller, about the size of an Earth-size planet itself. The upshot for habitable/inhabited planet hunting is that planets passing in front of white dwarves, instead of just causing a slight dimming, could block out nearly half of the star.

    Relative to bigger, brighter stars, this means having a larger band of atmosphere to perform spectral analysis on. In other words, a larger area of contrast compared to bigger stars where, to observers, the planet's atmosphere is obliterated by its too-bright background. What we're looking at is this tiny sliver of atmosphere being projected back at Earth as a planet passes between our telescope and its star. The planet itself blocks that starlight, but its atmosphere only changes it into different bandwidths that we can decompose here on Earth, in order to see what that atmosphere is made of (different materials have different bandwidth signatures). 

    According to the new paper, courtesy of authors Abraham Loeb and Dan Maoz of Harvard and Tel Aviv Universities, respectively, getting a good spectral analysis of that atmosphere would be "easily feasible." That's still limited to telling if the planet's atmosphere has oxygen and carbon dioxide, and not what its surface looks like or what it smells like, but it would still be a sign of life.  

    "Earth-mass planets in the habitable zones of WDs may offer the best prospects for detecting bio-signatures within the coming decade," the authors write. But the catch is that we haven't found any planets orbiting white dwarf stars, but there is evidence of them in the form of dust rings and debris disks around the stars, which are suggestive of planets. The paper suggests it's just a matter of taking a large sample of white dwarf stars, around 500, and spying on them with "small telescopes" for possible planetary transits. Once found, the JWST would presumably take over collecting data for a spectral analysis (for about six total hours of exposure time).

    The paper's in the preprint stages as of now, but with the grant backing of NASA and the National Science Foundation one might assume it isn't junk. We've focused so far on main sequence stars because that's where most of the planets should be, and planet searches are in the business of finding planets, less so hunting for relatively rare planets around tiny stars to be examined theoretically by future technology. But this does point at a future phase of the habitable planet hunt and, crucially, one that could take place in our lifetimes. Hope, in other words. Other worlds, too.

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