How to Tell the Difference Between an Extrasolar Planet and a Star Spot

-- | September 18, 2013

Question:  I am a retired PhD chemist..interested in amateur astronomy.  Recently a lot of exo planets have been discovered using a “transit” method of observing brightening and dimming of stars.  How do we know these are planet transits, and not the stellar equivalent of sun spots????  — T. Engle

Answer:  Let me first point you to a very nice description of the techniques used to detect extrasolar planets (called “exoplanets”) by my colleagues at the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado.  There is also a very nice description of the transit method for exoplanet detection from the Las Cumbres Observatory Global Telescope (LCOGT) network.  As you pointed out, one of the methods used to detect exoplanets uses measurements of the dimming of a star’s light as a planet transits across the face of the star.  Stars also have “spots”, much like those found on our Sun (“sunspots”), which are cooler dark spots that could conceivably be mistaken for an exoplanet.  Starspots rotate with the star and cause relatively slow changes in the brightness of the star, while a transiting exoplanet crosses the star in a much shorter time, often as short as a few hours, and cause quick dips in the brightness of the star.  A much more in-depth answer to this question can be found at the Planet Hunter’s Blog.

Jeff Mangum