Extra-Solar Planet Detection
Introduction
What we are searching is evidence for the existence of planets around stars other than our own. We already know that our own star, the sun, has nine planets orbiting it. An unanswered question which is often debated by many people is, "are we unique in the Universe?".
One first possible step which we can make towards answering this question is to determine whether or not there are any other planets out there. We are NOT searching for extra-terrestial intelligence. (For information on the search for life in the Universe, please see the SETI Home Page.) We are merely trying to determine if there are any other star systems out there such as our own. This kind of information may help us to better understand the formation of planetary systems, and possibly aid in the understanding of stellar evolutionary processes. It is possible that the formation of planetary systems is really not very well understood. After all, until recently we had only one example!
Problems with Looking for Planets
The problem we encounter when trying to look for planets, is that the planets themselves are not bright objects. They don't emit their own light, like the sun and other stars do. The only way you can even see a planet is by reflected light from the star it is orbiting.
The other problem is that planets are so small and are so close to a comparitively bright star, that we just can't see them by looking through conventional telescopes. To understand how difficult it is to see a small planet orbiting closely around a bright star, think of this: imagine placing a small coin just a few inches away from the leadlight of a car. Now, do this at night and stand a long distance away from the car and try to read the date on the coin. You won't even be able to see the coin!
Solution - Methods for Planet Detection
Since we can't actually see a planet orbiting a distant star, the way we can look for planets is by their influence on the star they orbit. This influence is due to the force of gravity. Even though a planet may be small in comparison to its star, it will nonetheless exert a gravitational force, gently "tugging" at its star. As the planet(s) orbit the star, they will tug at it from different sides. The net effect then, if we carefully watch this star for many years, is that we will see this star "wobble" back and forth!
There are two basic methods of detecting a planet's gravitational influence on its star:
Astrometric Detection
Radial Velocity Detection
The first method - Astrometry - was hinted at in the previous paragraph. We observe a star and measure its position relative to distant background stars (hence the word "metric" in the name). As an orbiting body tugs at its companion star, we observe a change in position of the star. Measurements of a periodic change in position, back and forth, can indicate that something is possibly orbiting the star. Careful analysis of these measurements can tell us about the orbiting object.
The second method - Radial Velocity - is a completely different kind of measurement, but relies upon the same principle. That is, gravity. As an orbiting body tugs on its companion star, the light from the star will experience a Doppler shift. If the planet pulls the star slightly away from us on Earth, the starlight will be shifted towards longer wavelengths and appear more red; pulled slightly towards Earth, the starlight is shifted towards shorter wavelengths to appear more blue. To accurately measure this Doppler shift, we chose a known spectral line and observe its shift from red to blue and back.
Results - Planets Found!
As of 1997 May 01, nine (9) objects of planetary mass have been found orbiting other stars. For a detailed and updated listing, please see the Extrasolar Planets Catalog - A listing which also includes brown-dwarfs and candidates to be confirmed.
Ongoing Research - NEW Instrumentation
The search for extrasolar planets continues, as new and better techniques are being developed in order to be able to detect smaller planets and hence planetary systems more like our own solar system.
The next phase in extra-solar planet detection will be to attempt direct detection. Plans are underway for the development of new instruments which will allow us to see directly signals from terristial planets. This will require space-bourne interferometers.
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Last modified 1997 June 10