ASTR 3 Lecture Notes - Lecture 10: Gravitational Lens, Minimum Mass, White Dwarf
1995: astronomers discovered the first "failed star" or brown dwarf object - the missing link between
small stars and giant planets like Jupiter - using infrared
• Thousands of brown dwarfs are now known
In the same year, astronomers discovered the first planet orbiting another Sun-like star
• 51 Pegasi, in the constellation Pegasus
• The planet is called 51 Pegasis b
• Thousands of "exoplanets" are now known
• The term exoplanet means a planet that orbits another star; it is a contraction of the term,
extrasolar planets
Why are extrasolar planets hard to detect?
• Contrast: a sun-like star is about a billion times brighter than the light reflected from its planets
• Angular Separation: planets are very close to their stars, relative to the distance from us to the
star
• Putting concepts together…
o Trying to observe a planet around even the nearest star would be like standing in LA and
looking for a moth near a bright spotlight in DC
o Impossible!
How do we detect extrasolar planets?
1. Direct: pictures or spectra of the planets themselves (isolated from their "sun")
2. Indirect: precise measurement of a star's properties may indirectly reveal orbiting planets
We will review both approaches starting with the indirect methods, which have provided all of the initial
successes
Two indirect methods are…
1. Astrometric
2. Doppler
Both rely on observing motion of the star caused by gravitational tugs from orbiting planets
• Jupiter may seem to orbit the sun, but it actually orbits every 12 years around the center of mass
of the solar system
• The sun therefore orbits around the same center of mass with the same period as Jupiter
Our Solar System Center of mass
• Center of mass is just outside the surface of the Sun
• The sun's motion around the solar system's center of mass depends on gravitational tugs from all
the planets
o The sun's position wobbles on the sky because of our planetary system
Apparent motion on sky includes proper motion, parallax and motion about center of mass
The Doppler Technique
• An easier way to use the gravitational tug of a planet on the star it is orbiting is through the
doppler effect
• The star moves alternately slightly toward and away from us as part of its tiny orbital motion
• So the star's spectral lines will shift alternately to the blue and toward the red
o This is how 51 pegasis was discovered
