Jimmy Westlake: Planets of other suns
March 23, 2010
While the debate rages on about what is and what isn't a planet in our solar system, astronomers continue to discover planets orbiting other stars. As of March 20, the official count of extrasolar planets stood at 443. When you stop to consider that only 15 years ago we knew of no planets other than the familiar nine (now eight) that orbit our sun, this is quite remarkable. Recent searches are confirming what astronomers have thought all along — planets are common in the universe. Just how common remains to be seen.
The most recent definition of a planet has three requirements: 1) the object must orbit its star directly; 2) the object must have enough mass for its self gravity to pull it into a spherical shape; and 3) the object must have cleared its orbit of debris and must be the dominant body in its region of space. By that definition, there are eight planets in our solar system.
There are two different methods astronomers use to search for extrasolar planets. The first involves looking for stars that show a slight periodic wobble because of the gravitational effects of an unseen planet. This has proven to be the most successful method, revealing 352 planetary systems to date. The second method involves watching for the telltale dimming of starlight as a planet periodically transits in front of its star. Seventy-one planetary systems have been found in this way.
The first extrasolar planet orbiting a normal star was discovered in 1995 around the star 51 Pegasi, a faint star near the limit of naked-eye visibility in the constellation of Pegasus. This planet has about half the mass of our planet Jupiter but orbits its star in only four days. Many more of these "hot Jupiters" have been discovered and challenge our understanding of how gas giant planets can form so close to their parent stars.
The nearest known extrasolar planet orbits the star Epsilon Eridani, about 10 light years from Earth. It has an estimated mass of 1.5 Jupiters and orbits its star in about seven years. You can see the star Epsilon Eridani with your unaided eye, not far from the bright star Rigel, which marks the foot of Orion.
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In an effort to gauge just how common (or rare) planets are around other stars, NASA has launched an orbiting telescope named Kepler whose mission, according to the Kepler Mission home page, is to "survey our region of the Milky Way galaxy to discover hundreds of Earth-size and smaller planets in or near the habitable zone and determine how many of the billions of stars in our galaxy have such planets." Kepler's telescope is staring at 100,000 stars in a patch of sky in the constellation of Cygnus and watching for the periodic dimming caused by planets transiting in front of their stars. It could take years to confirm suspected planets, but Kepler could find the holy grail of extrasolar planet hunters — another Earth.
At 7:30 p.m. March 31, the Colorado Mountain College SKY Club will host a special free public night astronomy program on the CMC Alpine Campus featuring a presentation by yours truly entitled "Planets of Other Suns." Afterward, telescopes will be available for public viewing, weather permitting. There will be a drawing for door prizes and a raffle for a beautiful framed photograph of the Milky Way. I hope to see you there.
Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. He is an avid astronomer whose photographs and articles have been published all around the world. Check out Jimmy's Web site at http://www.jwestlake.com.