For millennia human kind has looked to the stars with wonder and awe. One prevailing question above all else is ingrained into the minds of those who dare to ask. Are we alone? Now more than any other time in history mankind is poised to answer that question. New technologies, amazing theories, and revolutionary visionaries in the field of Astronomy, Physics, Chemistry and Biology are leading the charge in the search for Extraterrestrial life in the Universe. But if there is to be life in the universe then they will definitely need a place to live. This is where the hunt for extra-solar planets come in.

The first planets (Greek for “wandering star”) were discovered in ancient times, presumably by the Greeks, who noticed their eccentric motion, and their occasional retrograde path across the ecliptic. Ever since then they have been studied in great detail, culminating with the first landings by rovers in the 1970s. We have a deep intimate knowledge of how, and why planets form in our solar system, and presumably every other possible system in the universe. But, until approximately seven years ago we had no confirmation of their existence around other stars. What seemed like absolute common sense, there being planets around other stars, was in fact, extremely difficult to prove. So, and this goes without saying, many a people were excited by the first discoveries. But, how were the discoveries made?

It all goes back to a relatively simple physical law, the conservation if momentum. Isaac Newton described momentum, in its simplest form, as the product of an objects mass and velocity, i.e. p=mv. It is also established than in a multiple body system the total momentum is always conserved., i.e. m1v1=m2v2. This is the same principle that governs a game of billiards, or the violence of a car crash. It is one of the fundamental properties of the universe, and it cannot be avoided. So, it would go to reason that this property is fundamental in every part of the universe, and would exist between a parent star and its planets. Again, since the gravitational force exists between the two bodies in question (i.e., a star and its planet(s)) then the total momentum of the system is conserved. Using this fact we can determine the motion of the star in question caused by their planets, if they have them. This is all well and good, but how do we determine if the star is moving or not since it is so far away? By analyzing the light coming from it.

There is a simple occurrence in everyday life that illustrates this. Listen to an ambulance as it approaches you. The sirens are heard at a much higher pitch, that is until the ambulance passes and the pitch drops dramatically. This is because sound is propagated as a wave, so when the source of the sounds moves with the wave it literally pushes them forward making them have a much higher pitch. But how does this apply to light? Well, light is propagated in waves just like sound. So, if the source of the light is moving with the waves, they are pushed forward in frequency and vice versa. This shift in frequency, called the Doppler Shift, is seen when the natural spectrum of a star is pushed towards the blue or red end of the color spectrum.

When the star is moving towards us it is blue shifted, and pulled to the red side when moving away. This is illustrated in the picture below. Furthermore, by calculating the velocity and the mass of the star in question we can make determinations about the planet we are trying to find.

The spectrum at the top is the normal of a star in question. Notice, in the bottom graph, the shift of all the absorption lines towards the 7000 Angstrom side of the graph.

So, now that you know how scientist find planets around other stars, how many have we actually found? According to Harvard’s Exosolar Planet catalog there have been 91 Planetary systems found, with a 105 total planets discovered as of March 26, 2003. This may not seem like much, but keep in mind that ten years ago, we didn’t know of any planets around other stars.

However, all the planets we have discovered have been extremely large in mass. Many are several times more massive than Jupiter. As shown in the graph below.

This fact relegates these planets to the realm of gas giants, like Sol System’s Jupiter and Saturn. They have no rocky cores to speak of and are totally inhospitable to life. So while these planets are nice to find, the real meat of the quest is the discovery of small terrestrial planets, those that are rocky like Earth and Mars.

As of today, there has been no such luck. At this present time we do not have the ability to detect low mass, terrestrial planets. So, what do scientist have planned to overcome this? Enter, the Space Interferometry Mission (SIM).

Scheduled for launch in the year 2009 the SIM will be the first optical telescope capable of detecting planets to within a few Earth masses around other stars. It will enter into a trailing orbit following the Earth around the sun. Each year the space craft will gain .1 AU distance from the Earth and reach its maximum communications range of 95 million miles from Earth in a bout 5.5 years. It is at this time that we will, at last, be able to answer the question about if there are Earthlike planets out there.

These few words that I have written here have barely scratched the surface on the search for exosolar planets. If you would like more information on the upcoming missions, and the current search for planets please use the links listed below:

Harvard Exosolar Planets Catalogue:
http://cfa-www.harvard.edu/planets/catalog.html

Extrasolar Visions:
http://www.extrasolar.net/mainframes.html

C&C Planet Search:
http://exoplanets.org/

Space Interferometry Mission Site:
http://planetquest.jpl.nasa.gov/SIM/sim_index.html