Distant Nature: Astronomy Exercises
Objectives
This exercise will demonstrate the techniques and methods used to
identify planetary bodies orbiting around stars other than our Sun. These
planets are referred to as extra solar planets, or "exoplanets".
Equipment/Materials Needed
Stellarium, Calculator.
Subject Introduction
The first confirmed detection was in 1992 by Wolszczan and Frail, where
several planets were found orbiting a pulsar designated as PSR B1257+12. The
first confirmed detection of an exoplanet orbiting a main-sequence star was
made in 1995 by Mayor and Queloz. Named 51 Pegasi b, it was observed to be in a
four-day orbit around 51 Pegasi (G spectral type). These original discoveries
were made using ground based instruments. Today, however, there is now a
dedicated satellite, "Kepler", looking for exoplanets, with the
official count at over 3400 as of December 2016. Kepler has identified over 4600
more candidates, yet to be confirmed. For an updated list:
http://planetquest.jpl.nasa.gov/newworldsatlas/
It should also be noted that most known exoplanets orbit stars on the
main-sequence with spectral types of F, G, or K.
There are two basic methods used in detection that have accounted for most of
the detections:
Radial Velocity Method (Doppler)
As a planet orbits a star, the star also moves in its own small orbit around
the system's center of mass. Variations in the star's radial velocity, that is,
the speed with which it moves towards or away from Earth, can be detected from
displacements in the star's spectral lines due to the Doppler Effect. The
spectrum shifts to the red for increased speed away from the observer and blue
for decreased speed away from the observer. This has been by far the most
productive method of discovering/confirming exoplanets.
Transit Method
If a planet crosses (or transits) in front of its parent star's disk, then the observed brightness of the star drops by a small amount. The amount by which the star dims depends on both the size of the star and of the planet, as well as other factors. This has been the second most productive method of detection, though it suffers from a substantial rate of false positives and confirmation from another method is usually considered necessary.