For intelligent life to exist in the Universe, stable planets must exist.
Over the last 8 years, substantial progress has been made in detecting the existence of other solar systems beyond our own. As of 2003, there are approximately 100 known extrasolar planetary systems and more are being discovered each year.
We will go through and simulate that discovery process as an exercise later today. For now we start out with some basics.
As mentioned before, planetary formation will occur after there has been sufficient time for the Galaxy to be seeded with heavy elements created through the process of stellar evolution and supernova ejection.
Thus we expect to find planetary systems around stars that have been created within the last few billion years because in that case the protostellar cloud will have been enriched in heavy elements. This expectation is clearly borne out by the data:
The technique used to discover extrasolar planets is called the radial velocity variation technique or doppler wobble .
Consider the case of our Sun and Jupiter. Jupiter is sufficiently massive that it actually exerts a small but noticeable gravitational tug on the Sun.
To an external observer, the Sun will be observed to have a small change in its radial velocity. For the mass of Jupiter and the distance between Jupiter and the Sun, the amplitude of this change in velocity of +/- 12 meters per second.
This is shown below. At position 1, the observed velocity of the sun will be +12 meters per second while at position 2 it will be -12 meters per second. At positions 3 and 4 the observed velocity will be zero, as there is no radial component to the motion of the star.
Observations of this system will then yield the following kind of radial velocity curve :
The amplitude of this curve depends upon the mass and separation from the star of the perturbing planet. The period of the curve is the orbital period of the planet.
In this particular case, the orbital period is about 3 years and many orbital periods have been detected.
The shift in radial velocity is due to the well known doppler effect . This effect occurs for motion along the line of site (this is what we call radial motion). Motion along the line of site towards and observer causes the wavelength to be compressed and the frequency to shift to higher values, while motion away form the observer cause the wavelength to stretch and the frequency to lower.
Therefore, if we can monitor nearby stars for radial velocity variations at the level of a few meters per second, then we can potentially detect Jupiter mass planets in orbit about them.
Until recently, however, this was easier said than done.
To produce a detection, precision radial velocity observations are needed. Until about 1990, this was not possible with instrumentation. For instance, the typical precision was +/- 25 meters per second, and such an instrument clearly would not detect a system like Jupiter and our Sun.
In addition, one generally needs to observe a full orbital cycle to be sure that you have detected the presence of a perturbing planet.
So, for instance, even an external observer was monitoring the radial velocity variation of our Sun over a 6 month period, they would not be able to detect the presence of Jupiter, since its orbital period is 5 years.
The Press Release Race
Finding bonafide evidence of another planet in orbit about a nearby star would be a big deal. In fact, it wasn't until 1995 that this discovery was made. This exercise is designed to simulate the scientific process behind this initial discovery.
In the exercise linked below, you will go to the star catalog called "Solar Type Stars". These are 197 stars like the sun (e.g. 1 solar mass). Around each star is a planet that is causing a radial velocity variation.
However, not all planets are going to be detectable using the assigned observing/detector conditions. For each star you select, you are to try and find a solution of the system in terms of the mass of the orbiting planet and its separation from its host 1 solar mass star. For many systems a solution may not be possible, but you will have to determine that based on the quality of the data.
If you think you have a solution, then publish it to the global viewer and we will see if you are the winner for the particular detector plus observataional combination.
Detector plus Observing Conditions
Within the simulation, you can set this 3 items.
For each case, be the first on your block to publish a bonafide detection of an extra solar planet.
Observing/Dectector Cases
Case I:
Case II:
Case III: