Union University
Union University Department of Physics

The Science Guys

Science Guys > December 2002

December 2002

How do astronomers detect planets outside our solar system?

Our solar system consists of a central star- our Sun- with a mass of 20 million, trillion, trillion kilograms. Moving in orbits around the Sun are nine planets as well as a myriad of asteroids and cometary material. The question asked for years has been, "Is our solar system unique, or are there other solar systems?"

Only since 1995 have we proven that our solar system is but one of many. At the time of this writing, over 90 planets in 80 plus different solar systems are recorded. A planet orbiting a star other than our Sun is referred to as an "extrasolar planet."

Extrasolar planets are found by several different techniques. We will focus on the most often used technique, Doppler spectroscopy. We have all heard a horn vary in pitch as a car or train passes us. As the horn approaches us, a relatively high pitch is heard and then, as it recedes the pitch drops. This change in pitch (frequency) was not due to the horn itself- it was due to the relative motion between the source and the observer. The key thing to realize is that the wavelength and frequency measured by the observer depends on the motion of the source relative to the observer. This effect is often termed the Doppler Effect.

Thanks to gravity, a planetary companion will tug on a star. This tug will actually cause the star to wobble slightly! In reality, planets do not orbit their sun in perfect circles; rather the planets and sun all orbit a common point termed the center of mass (CM). The CM for a solar system is typically inside the diameter of the star and off-center, making it seem that the planets orbit the sun. However, the star itself orbits or wobbles about this off-center point.

This wobble is very slight if the planetary companion’s mass is a lot less than the mass of the star itself - which is typically the case. Nevertheless, the wobble will introduce subtle changes in the light we receive from the star! Sometimes the wobble will be towards us, and sometimes away. Therefore, the wobble will create a Doppler shift in the frequency of the light we receive. Moreover, if the shift is due to a planetary companion the wobble will exhibit a distinct periodicity- that is, the star will wobble in an ever-repeating cycle. It is like a fire truck with the siren blowing moving around a circular track. When the truck approaches us in the grandstand, the frequency (pitch) appears higher than normal. Similarly, when the truck recedes from the grandstand, the frequency appears below normal.

Only in recent years have we possessed both the hardware and software necessary for the delicate analysis of starlight necessary to detect this periodic shift. Now that we have this ability, extrasolar planets are being discovered quite often.