Lesson Objectives:- Extrasolar planets
- Detecting planets by their gravitational tugs
- Transit method
The establishment of the nebular theory decades ago made it all but certain that distant stars must have their own planets. That is because the process that accompanies the birth of a star also naturally leads to the formation of other bodies around it.
It was not until recently, however, that the existence of extrasolar planets, or planets around other stars, could be proven.
Detecting extrasolar planets is a huge challenge because planets are tiny relative to the distances between stars and because stars are usually a billion times brighter than the light reflected by their orbiting planets.
Technology still does not allow for much in the form of direct detection; telescopes are not advanced enough yet to provide high-resolution images or spectra. The best they can do is provide a rough infrared image such as the example above. Planets emit their own infrared light and stars are usually dimmer in the infrared.
Most of our information about extrasolar planets, then, comes from indirect study.
The first indirect method for detecting planets is by observing the motion of a star to look for the subtle gravitational tugs of orbiting planets.
We usually think of a star as spinning in place while the planets orbit around it, but the reality is, a star has its own orbit. All of the objects in a star system, including the star itself, orbit around the center of mass of that system. Since the star is so massive compared to all of the other objects in its system, it does barely move, but it is not staying EXACTLY in the center.
What does this mean in practical terms? The Sun and every other star with orbiting planets must show some slight orbital motion based on the masses and distances of its orbiting planets.
The astrometric method looks for very small shifts in a star's position. If a star "wobbles" gradually around its average position, then there must be unseen planets around it.
This method is of limited use since the changes in position are so subtle, and current technology does not allow such precise measurements for more than the nearest stars. Also, the largest planets have long orbital periods, so it can take decades for a noticeable change in a star's position.
The other method for detecting gravitational tugs is the Doppler method. We previously learned about Doppler shifts -- the Doppler effect causes a blueshift when a star is moving toward us and a redshift when it is moving away. This means alternating blueshifts and redshifts for a star's spectrum indicate orbital motion around a center of mass.
The Doppler method has already been used to detect hundreds of planets, but it works best for finding massive planets that have small orbits since the greater the mass of the planet, and the closer it is to its star, the greater its gravitational tug.
Detecting gravitational tugs is the first indirect method for detecting planets. The other method is the transit method, which works when a planet's orbit happens to be aligned in such a way that it passes between us and its star once each orbit.
This only occurs for a small fraction of the star systems that have planets, but when it does, the result is a transit, where the planet moving in front of the star causes a small, temporary dip in the star's brightness. Then, when the planet moves behind the star, there is an eclipse, where the infrared brightness of the system dips as the planet's infrared light is blocked.
The transit method finds these transits and eclipses by monitoring the brightness of a star system over time. If it exhibits a pattern of transits and eclipses, then we can be confident that the same planet is passing in front of the star at the same time in each orbit.