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

Facebook

Twitter

The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
	Founded in 1956, the NRAO provides state-of-the-art radio telescope facilities for use by the international scientific community. NRAO telescopes are open to all astronomers regardless of institutional or national affiliation. Observing time on NRAO telescopes is available on a competitive basis to qualified scientists after evaluation of research proposals on the basis of scientific merit, the capability of the instruments to do the work, and the availability of the telescope during the requested time. NRAO also provides both formal and informal programs in education and public outreach for teachers, students, the general public, and the media.
	The National Science Foundation (NSF) is an independent federal agency created by Congress in 1950 "to promote the progress of science; to advance the national health, prosperity, and welfare; to secure the national defense..."
	AUI collaborates with the scientific community and research sponsors to plan, build, and operate cutting-edge facilities. We cultivate excellence, deliver value, enhance education, and engage the public.