StarChild Question of the Month for June 1999

Question:

Can you tell me about the other solar systems being discovered?

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Answer:

During the past few years, researchers have detected over a dozen planets orbiting sunlike stars. The first was reported in October 1995 by Michel Mayor and Didier Queloz of the Geneva Observatory in Switzerland. While observing the star 51 Pegasi, they noticed a change in the light from the star - its light repeatedly shifted back and forth between the blue and red ends of the electromagnetic spectrum. The timing of this Doppler shift implied that the star was "wobbling" a little because of a closely orbiting planet. In fact, the planet appeared to be revolving around the star every 4.2 days. Shortly thereafter, a survey of over a hundred other sunlike stars performed by the team of Geoff Marcy and Paul Butler at San Francisco State University and the University of California at Berkeley, turned up six more such planets. Of those, one planet circling the star 16 Cygni B was independently discovered by astronomers William D. Cochran and Artie P. Hatzes of the University of Texas McDonald Observatory. Since 1996, the announcement of the detection of new planets has become fairly routine....but always exciting!

Twin images of protoplanetary  disk. On the left side is a image taken of the Orion Nebula from above.  On the right side is an image of the Orion Nebula taken edge-on.

This image of sections of the Orion Nebula shows stellar formation under way. Located in the Milky Way about1,500 light-years from Earth, the nebula formed from collapsing gas clouds, yielding many new stars. Among the new stars at least 153 show "protoplanetary disks" which are believed to be embryonic solar systems. Shown are views of two of the disks: (left) one is seen from above and (right) one is viewed edge-on. Both images reveal gas and dust that should eventually form planets, circling million-year-old (new!) stars.

Image Credit: Mark McCaughrean Max Planck Institute For Astronomy, c. Robert O'Dell Rice University and NASA

Want to know how astronomers detected the extrasolar planets?

It is not easy to detect another planet so far away from Earth. Unlike stars which are fueled by nuclear reactions, planets only reflect the optical light of their stellar companion. In our solar system, for example, the Sun outshines its planets about one billion times in visible light. Because of the distant planets' faintness near the brightness of the nearby star, astronomers have had to devise clever methods to detect them. Currently, the most successful approach is based on the fact that a nearby planet will cause the star to wobble back and forth just a bit as the planet revolves around it. Astronomers can detect this tiny wobble and then calculate the orbit and mass of the object which is causing it. Even using this technique, however, it is still not easy to detect planets around other stars. Consider this: someone looking at our Sun from 30 light-years away would see it wobbling in a circle whose size would be about as big as a quarter viewed from 10,000 kilometers away!

Yet this tiny wobble of the star can be detected by the Doppler shift of the starlight. As the star sways to and fro relative to Earth, its light waves become cyclically stretched, then compressed--shifting alternately toward the red and blue ends of the spectrum. From that cyclical Doppler shifting, astronomers can retrace the path of the star's wobble and, from Newton's law of motion, compute their masses, orbits and distances from their host stars.

What have astronomers been able to detect so far?

Of 16 such planets found around solar-like stars, their masses range from about a half to five times that of Jupiter, their orbital periods span 3.3 days to about 15 years, and their distances from their host stars extend from less than one twentieth of Earth's distance to the Sun to about 2.5 times that distance.

In March 1999, Marcy and Butler announced the detection of the first true other "solar system" in which they found evidence of 3 planets orbiting a single star. The star is called Upsilon Andromedae. A schematic diagram of the orbits for the 3 planets detected around Upsilon Andromedae is shown below.

Graph of the orbits of 3 planets orbiting a single star, Upsilon Andromedae.

The red dots mark the orbits of planets b, c, and d. The dashed circles show the orbits of Mercury, Venus, Earth and Mars (inside to outside, respectively) overplotted to give the scale of the orbits.

Image Credit: G. Marcy, D. Fischer, R. Noyes, S. Korzennik, P. Nisenson, A. Contos, and T. Brown

Drawing from the data on planets found so far, scientists believe they will continue to discover other planets orbiting sun-like stars; many of these planets will be the size of Jupiter, some will be the size of Earth. It may be that as many as 10 percent of all stars in our Galaxy host planetary companions. Based on this estimate, 10 billion planets would exist in our Milky Way Galaxy alone!

For additional information on planets around other stars, see:

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