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3-Jul-2014

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Planet's signals are tricks created by starry noise



The Sun, imaged through calcium K (blue) and hydrogen alpha (red) filters. Prominences are shown inverted for visibility. The calcium line is commonly used as a proxy for stellar activity. However, for red dwarf stars such as Gliese 581, redder lines such as hydrogen alpha may detect stellar signals not easily seen in calcium. In the case of Gliese 581, the hydrogen line reveals that the signals previously attributed to two planets in the Habitable Zone are actually caused by stellar activity.
[Credit: Alan Friedman]

Regions of strong activity coming from stars have made scientists think they are planets, a new study reports in the 4 July issue of the journal Science reports, when in reality, they are not.

In this study, researchers led by Pennsylvania State University's Paul Robertson studied Gliese 581, a cool red star located about 22 light years from Earth. Gliese 581, also known as GJ 581, is one of the most well-known stars in the history of exoplanets, or planets outside our Solar System.

Previous scientific studies have suggested that as many as six planets orbit around GJ 581, some of which do so at distances that leave them able to sustain liquid water (not too hot, not too cool). Thus, as the search to discover the first life-sustaining exoplanet continues, these planets have been widely discussed.

Already, the existence of three planets around GJ 581 -- e, b, and c -- is well-established. But, whether or not the other three planets -- d, f, and g -- exist, has been debated.

It may be that the measurements attributed to these planets are just due to dark spots on the star itself, some scientists say. That would mean they are not planets after all. Indeed that was shown to be true for planet f, which scientists no longer believe exists.

Here, to help resolve the debate over whether d and g are real, Paul Robertson and colleagues studied the activity of GJ 581. They looked carefully at lines radiating from the star, discovering that the measurements around GJ 581 thought to represent planets d and g are in fact due to regions of intense starry activity that had created false planet signals for d and g. The two are not real planets.

Meanwhile, with the same methods, the team was able to confirm that planets e, b and c are real.

The results help put a planetary debate to rest, provide a method with which to more reliably look for habitable exoplanets going forward, and reveal how scientists' knowledge of the physics of star formation continues to grow.

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