Wednesday, June 24, 2015

High Precision Photometry of Transiting Exoplanets as I currently know it : Part II

Background &
 Non-Transit and Transiting Exoplanets  continued...

Exoplanets that do not transit their parent star are found using photometry and verified with spectroscopy.  The same applies for transiting exoplanets; however, with modern technology and current methods utilized in the new field of research, much more information can be extracted and extrapolated from the data of a transiting exoplanet.  For those that do not transit, astronomers can find, at most, the orbital parameters and lower limits on the planet's mass.  For transiting planets, we can find those parameters and even the physical properties of the individual planet.  These properties include radius, equilibrium temperature, mass, density, surface gravity, and more.

Radial Velocity Curves

A very common method of detecting hints of a revolving exoplanet is to observe if radial velocity measurements of a star insinuate that the star is wobbling.  Although the planet revolves around the star, the star also revolves around the barycenter, or center of mass, of the stellar-planet system.  However, because stars are typically immensely more massive than their revolving planets, the star makes very small movements around the barycenter due to the tug of the planets relatively weak gravity.  These movements can be so small that the star it is more accurately described to be a wobbling motion as opposed to a revolving motion.  This is also indicative of the fact that stars "revolve" around their barycenter that is, many times, within the radius of star itself.  The following "gif" illustrates this.  (However, there are many cases where the barycenter is outside of the stellar radius and the star visually does revolve around a common center of mass shared with the star's orbital companion, which may well be another star.)

In this motion-picture, the star and planet move around a common center of mass.  This is an apt illustration to compare to our Sun.  The barycenter for the Sun, its eight revolving planets, and all of the other orbiting material in the Solar System is just beneath the Sun's surface.  This illustration demonstrates stellar wobble due to a planet's (weak) gravitational pull.

Because the data astronomers analyze never look as clear and unambiguous as that gif, we use plots.  Oh! How vast in number the plots we Make!  Radial velocity plots, such as the one below, give evidence when stellar wobble due to a planet occurs.

Radial velocity curve for the star 51 Pegasi.  The points indicate the measurements obtain by a telescope and spectrograph.  The line indicates the fitting done on the data points that best represents the continuous trend that the star may be following.  As implied, the star's velocity toward the observer (Earthlings) can reach up to 50 meters per second.  When going away from the observer, the speed can be up to 70 m/s.

Doppler Shift

The following gif superbly illustrates how the velocity measurements in the above radial velocity curve were detected.

to be continued...

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