Are exoplanets real? (Intermediate)

How do we know we're really looking at exoplanets and are not getting fooled by other phenomenon?

For instance: for planets where a doppler shift has been detected and ascribed to a planet how do we know it isn't pulsations of the surface of the star?

This is a question many an astronomer had when the first planets were being announced in the 1990s (and one in 1989). Are these real? Exoplanets have a long history where scientists have claimed the discovery of a planet and been wrong.

You're also absolutely correct that pulsations of a star can occur regularly and mimic the effect or a planet's gravitational tug, however, there a many reasons to believe the claimed exoplanets out there. Here are just a few of them:

  • Some exoplanets that tug on stars to create the Doppler shift also cross in front of them -- "transiting planets" - both the Doppler shift of the star and the transit absorption are consistent with a Jupiter-size, Jupiter-mass object in orbit around there star. For pulsations to mimic this, the star would have to pulsate regularly with a red shift and blue shift in a sinusoidal pattern and then somehow develop a sunspot directly after blueshift that persists for exactly the same time it takes a planet to cross in front of the stellar disk.
  • High resolution imaging of planet host stars have revealed that they do not have nearby eclipsing binary stars which can also mimic the effect of a planet.
  • I think this is the most convincing reason. There are stars that host multiple planets and furthermore their planetary orbits have been mapped out in detail. Analysis of the orbits is consistent with gravitational interactions of the planets that pull on each other. In my mind it would be very hard to construct a scenario where stellar effects or orbiting binary stars would mimic two planets that gravitationally interact.

With nearly 2,000 discovered planets out there (this was revised in June 2015), there are possibilities for false positives and mistakes. However, I expect that almost all of the published discoveries such as in exoplanets.org are real.

 

This page was updated on June 27, 2015

About the Author

Everett Schlawin

Everett's research focus is extra-solar planets or exoplanets. These are the planets far, far beyond Neptune and Pluto, which orbit other star systems. He observers exoplanet atmospheres to learn about their composition. The colors of an absorbing exoplanet atmosphere tell astronomers what the atmosphere is made of, so he uses spectrographs to split up the colors of these star-planet systems and infer which gases make up its atmosphere. He also is building a new infrared spectrograph to go on the Blanco Telescope in Chile with the TripleSpec 4 team.

Everett's Website: http://astro.cornell.edu/~schlawin/

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