Visit Your Local PBS Station PBS Home PBS Home Programs A-Z TV Schedules Watch Video Donate Shop PBS Search PBS

Extrasolar Planets by Andrew Fraknoi

"A few planetary systems happen to be oriented so that the planets [can cross the face of] their home star as seen from Earth. When this occurs, the dimming of the star's light can be detected using an ordinary digital camera on a backyard telescope." — Timothy Ferris, Seeing in the Dark

Eight planets and a swarm of smaller objects orbit our star, the Sun. We can see these planets—some of them (Mercury, Venus, Mars, Jupiter, and Saturn) with our eyes, others (Uranus and Neptune) only through telescopes—because they reflect the light of the Sun. Astronomers long wondered whether the Sun's system (known as the solar system) was unique, or whether other stars also had planets circling them.

This was one of the most challenging questions in astronomy. The stars are so far away that the faint reflected light of their planets is too dim to see, even with the largest telescopes on Earth, and is lost in the glare of the much brighter star. Twenty years ago, the prospects for finding such distant planets were, literally, dim.

But astronomers have recently managed to find evidence of more than 250 planets orbiting other stars. How did they do it?

One approach was to examine the gravitational interaction between stars and their planets. Stars are much more massive than planets, but a star's planets tug on it gravitationally. (Think of a fat man walking a gaggle of small, struggling dogs: He continues on his walk, but the dogs pull him this way and that. In this illustration, gravity is the leash on each dog.) The effect is very subtle, because the star is big and the planet is much, much smaller, but by using high-resolution spectroscopes, astronomers can detect such a small changes via the so-called Doppler shift: When a star is tugged toward us, the lines in its spectrum are shifted slightly toward the blue; when the star is tugged away, the shift is toward the red. Our own Sun moves back and forth because of the pull of Jupiter (its largest planet) by only about 30 miles per hour, roughly the speed with which most of us drive around town, but existing spectroscopes can detect that small a variation on stars hundreds of light years away.

Because this technique works best for planets that are massive and close to their planets, since they tug most strongly on their stars, it preferentially identifies big planets in small orbits. Astronomers call such planets "hot Jupiters." But while hot Jupiters are the first planets typically discovered, closer examination of the spectra of nearby stars has shown that a number of them have 2 to 4 other, less massive, planets as well. And, as time passes, since the first hot Jupiter was discovered in 1995, planets much farther from their stars are also being found.

Planet transiting star
Planet transiting star resulting in a fractional reduction of star's brightness.

Other extrasolar planets have been detected by professional and amateur astronomers using something called the transit method. When a planet crosses the face of its star as seen from Earth, the star's light output drops by a tiny fraction. Such a mini-eclipse may reduce the star's apparent brightness by only a fraction of one percent, but that's enough to not only tell us that the planet exists but to estimate the planet's diameter and the length of its year, which in turn reveals the radius of its orbit. (In the vacuum of space, all objects fall at the same velocity in a given gravitational field—as Galileo first realized—and an orbit is just an endless fall. So if you know the mass of a star and the rate at which a given planet is "falling" in orbit around it, you can calculate the diameter of the planet's orbit.)

Only a small fraction of planets happen to be orbiting in the correct orientation so that we can see them transit their star, but if we look at a large number of stars we increase our chances of catching a transit. Amateur astronomers with CCD cameras on their telescopes can measure the light of stars very accurately, and so can participate in the search for transiting extrasolar planets. As described in Seeing in the Dark, this is just what Ron Bissinger did in 2005. The planet he observed is a hot Jupiter that transits its star at intervals of just under 3 days. This meant that Ron was able to observe not just one transit but three during his initial observations, allowing more accurate measurements of the planet's characteristics.

Evidence gathered by professional astronomers using the spectroscopic method, and by both amateur and professional astronomers studying transits, now indicates that planets are quite common among the stars. The stage is set for backyard observers to discover new worlds.

To keep track of discoveries about extrasolar planets, we recommend three web sites:

Planet Quest from the Jet Propulsion Lab:
http://planetquest.jpl.nasa.gov/index.cfm

Jean Schneiders's Extrasolar Planet Encyclopedia:
http://exoplanet.eu/

The California and Carnegie Planet Search:
http://exoplanets.org/

Back to top