Astronomers have come up with a novel way to look for circumstantial evidence of rocky planets around sun-like stars. The "stellar fingerprinting" technique points to the nearest star to our sun, Alpha Centauri A, and one of the top contenders for possessing an Earth clone.
What has become apparent over the past few years is that stars with planets can be very slightly anemic –containing less iron — compared to stars where planets are not detected.
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The conventional wisdom is that a small fraction of these elements are locked up in rocky planets and asteroids that condensed out of a disk of dust and ice that encircled the newborn star. The elements never manage to fall into their star to chemically enrich it. Even a planet as massive as just a few Earths will leave a telltale deficiency in the star's chemistry.
With extraordinary precision, Ivan Ramirez of the University of Texas at Austin studied the makeup of 11 target stars. The exact mix of elements the stars yield could betray the presence of terrestrial and gas giant planets, and tell of the episodes in the star's birth they formed.
Ramirez's spectroscopic survey found that 15 percent of solar type stars have circumstantial evidence for terrestrial planets according to his chemical recipe based on our sun's spectral fingerprint. This is a bit lower than estimates by detection from NASA's prolific Kepler space observatory.
He used the binary stars 16 Cygni as a good test case. The one companion star where a planet was found has a slight deficiency of heavier elements as compared to its apparently planet-less twin star.
By far the most exciting stellar candidate is merely 4.3 light-years away, Alpha Centauri A, the largest star in the triple star system. Compared to 85 percent of solar twins studied in Ramirez's survey, it has the closest temperature and iron abundance as our sun.
Ramirez says that this strongly argues for the presence of one or more terrestrial planets circling the star. Searches are underway, but no evidence for planets have been teased out of the data yet.
Regardless, given Kepler's trawl of rocky planets it would be much more surprising not to find any rocky worlds orbiting Alpha Centauri A. Its closest companion star, Alpha Centauri B is far enough away to allow for stable planet orbits around either star.
Finding such a planet would send a seismic wave through the astronomical community. There would be a strong incentive to build ever-larger space telescopes to sample the planet's atmosphere, and in the much farther future, look for oceans and continents.
Because Alpha Centauri is as old as the sun, it would be natural to assume that a planet with atmospheric biotracers such as ozone, oxygen, methane and carbon dioxide would have time enough to evolve multicelled forms of life. If we found out that didn't happen on an Earth clone, it would strongly suggest that complex life is much more rare in the universe than some astrobiologists imagine.
This couldn't be answered without visiting such a world. And that could be accomplished within a reasonable amount of time if a probe could be built that could be accelerated to 10 percent the speed of light. Alpha Centauri A could be reached in a little over 40 years.
There is little doubt that microbes exist on other planets in our solar system. But being able to peruse a world to see Darwinian evolution played out over billions of years would be a scientific watershed of immeasurable impact.
The tremendous curiosity about such a place would only be offset by the cost of getting there, which would easily run many trillions of dollars. It could be the equvalent of the Apollo moon project for the next century — for any nation and or group of nations ambtious and rich enough to undertake such deep space exploration.
Image credits: NASA, NOAA