CHANCES OF LIFE BEYOND THE SOLAR SYSTEM - PART 2
SEARCHING THE SKIES
So far, we have looked at how planets are formed, but how have we found them? I already said we've found them, so there must be a way. Actually, there are currently two ways of tracking down planets. The problem with doing this is that the planets are very small and only reflect light compared to stars, which can be even larger than our sun and emit light. It is extremely hard to find planets considering Pluto was not discovered until the 1930s. Also, the light from stars completely overwhelms the planets that orbit it, considering that our sun is one billion times brighter than the planets that orbit it. The two ways of tracking down planets consist of looking for tiny wobbles made by the stars and searching for a change in the Doppler effect in the light coming from a star. Both of these methods produce extremely small amounts.
To understand how the first method works, you must first understand how a star can actually wobble. We believe that planets rotate around the center of gravity of a star and that star stays stationary. This, in effect, is not entirely true. Since both the planet and the star produce a gravitational field, both the planet and the star tug on each other to produce this wobble. Both the planet and the star orbit around their center of gravity. To visualize this, imagine an adult and a child holding hands and spinning around in a circle. Their feet do not trace out the exact same pattern on the ground. Now, using this, image the adult at the sun and the child as Jupiter, and you have how the star wobbles. Since the stars are so massive compared to the planets, their movement is extremely small compared to that of the planet.
The movement of our sun is traced out mostly by Jupiter. Since Jupiter is about one-thousandth the mass of the sun, the sun's orbit is one-thousandth that of Jupiter's. The sun revolves around a center of gravity just beyond its surface.
Since the movements of stars are so small, they are extremely difficult to detect. Although this method is much simpler than just searching for planets since the stars radiate light, so its movement would be much easier to see than a planet's. These movements are so small that a star whose size was that of our sun with a planet the size of Jupiter 10 light years away would move about the width of a piece of hair from 2 miles away. Many times over, these wobbles that were detected actually turned out to be observational errors.
Searching for planets by using the Doppler shift in light coming from a star is much different. Scientists use this method to study the changes in light coming from a star to see if it is moving toward us and away from us in a cyclical manner. There are still searching for the wobble, but by using light waves. As the star moves about its circle, the frequency of the wavelength of the light coming from the star shifts. This is much more accurate than observing movements. Astronomers can actually measure the velocity of a star toward or away from us down to an accuracy of seven miles per hour. This level of precision allows us to find Jupiter-sized gas giants.
Both of these techniques are good for finding systems with large wobbles but not with small ones, which means we cannot find Earth-sized planets using these techniques since those shifts would be much smaller than a shift from a Jupiter-sized planet and star.