About Extra Solar Planets

Are we alone in the universe? To begin to answer this question, we could first ask if Earth is unique in the universe. And to begin to answer that, we could look for solar systems around other stars. In the mid-1990s, astronomers found them.

Astronomers search for extrasolar planets with the Hubble Space Telescope by looking for planetary "shadows" passing in front of stars. No large, Jupiter-like planets were detected in this "swarm" of 35 000 stars. Image courtesy of Hubble Space Telescope (STScI and NASA)

It wasn’t easy. An extrasolar planet cannot be seen directly in a telescope, because the reflected light from the planet would be overwhelmed by the glare of the light produced by the star. Since such a planet could not be observed directly, astronomers sought a way to infer the planet’s existence through its gravitational pull on the star.

Imagine you are holding on to another person and you both are turning. If you both have the same mass, you both turn about a point halfway between the two of you. Now suppose you are turning and holding onto a small child. The child moves in a large circle and you move in a small one. You move in response to the force the child is exerting on you, the equal and opposite reaction force.

With gravity, it works in the same way. Imagine a double-star system, with both stars of equal mass. They orbit each other. The drawing shows two such stars in a circular orbit, each moving in a circle about the center. Now imagine a star orbited by a Jupiter-sized planet, with about 1/1000 the mass of the star. In this case, the star moves in a small orbit, whose center is well inside the star.

Suppose we look at a star that has a planet whose orbit is edge-on, as shown in the diagram. We cannot see the planet itself. What we can observe is the slight motion of the star as described in the paragraph above, which we see as a slight side-to-side wobble.

Two stars of equal mass in a circular orbit around each other

A planet’s orbit (straight line) around a star; the orbit is edge-on to our line-of-sight to the star.

The arrow shows the wavelength of the light.

Astronomers compare the shifting absorption lines from the star with fixed emission lines from a laboratory light source. The shift is extremely small, only about one part in 100 million. Analysis of these shifts yields much information, including the period of the motion, the masses of star and planet, and the shape of the orbit.

The Doppler effect: The black dot is the source of the waves, which is at rest in the drawing on the left and moving to the right in the drawing on the right. The circles are wavefronts. For example, each circle could represent a wave crest. If the source moves towards the observer, the wavefronts pile up, reducing the observed wavelength. If the source moves away from the observer, the wavefronts spread out, increasing the observed wavelength.