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Starspots Mimic the Effect of Orbiting Exoplanet

Starspots big enough to change a star’s brightness cause astronomers to rethink the exoplanet claim.

  • June 25, 2009

Here’s an interesting confounding factor for those involved in the search for exoplanets.

In 2004, astronomers found a hot, Jupiter-size planet orbiting a star called TrES-1 in the constellation of Lyra some 500 light years from Earth. They found this planet using a technique to measure tiny changes in the brightness of the star, caused by a planet passing in front of it.

Other groups verified the discovery but reported additional changes in brightness that seemed to indicate the presence of another body. It wasn’t long before researchers suggested that another planet was causing the changes, but the evidence was hard to pin down. Whatever this companion was, it seemed to orbit at a vast distance, equivalent to our Kuiper belt objects. How could that be?

Today, the mystery is solved thanks to some neat work by Jason Dittmann and pals at the University of Arizona. They say that the change in brightness is not caused by a planet at all, but by one or more giant starspots that pepper the face of this star.

The evidence comes from similar brightness anomalies on consecutive transitions. This implies that the second planet must have an orbital distance of 745 AU and a period of 21,000 years. In that case, it would have to be huge to cause a measurable change in brightness during a transit, which cannot be possible. The only other option is a giant starspot, or several of them, causing a measurable change in brightness.

So the mystery is solved: it looks as if TrES-1 is suffering from a severe case of spots and is significantly spottier than the sun–something that other planet hunters would be wise to bear in mind.

Ref: arxiv.org/abs/0906.4320: Tentative Detection of a Starspot during Consecutive Transits of an Extrasolar Planet from the Ground

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