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Field Guide To New Planets - astronomers discover new planets
Discover, March, 2000 by Kathy A. Svitil
Understanding Doppler Plane hunters spot their prey by measuring tiny variations in light emitted by distant stars. As a planet orbits a star, its gravity tugs on the star, creating a slight wobble. When the star wobbles toward Earth, the light waves it sends our way are squeezed together like an accordion, causing a subtle shift toward shorter blue wavelengths. That's called a Doppler shift. When the star wobbles away, its light waves are stretched apart, shifting the spectrum toward red. The same effect makes a train's whistle rise in pitch as it approaches and then, as it hurries away, drop off to a low-pitched howl. With Doppler, astronomers can determine how long a planet takes to orbit its star, how far away it is, and what its minimum mass might be. They can also estimate temperature. The effects can't be measured unless a star is stable, limiting the number of candidates. Our sun's velocity is braked only 27 miles per hour by Jupiter's tugs. A planet the size of Jupiter will compress and expand the light from a star by about one part in 10 million, and plucking that signal out of the spectrum of a star that's trillions of miles away requires a precision of three parts in 100 million. Today's best instruments perform three times better, says astronomer Steven Vogt: "That's equivalent to detecting the change in the length of a two-inch ruler lying on a table vs. its length when standing on its end: It is shorter standing by 1/100,000,000 of its length, due to its own weight."
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Clear Skies These planets rotate from 7 million to about 80 million miles from their suns. They are too cool to have silicate clouds, but too warm for water clouds. Gas giants, they range in temperature from 900 [degrees] F down to a nearly tolerable 170 [degrees] F, estimates modeler Burrows. They may have clear or hazy skies of sulfides and chlorides, including table salt. If such a planet orbits a star like ours, its red wavelengths might be absorbed by the atmosphere, and blues would scatter.
Solar Revisionism
In the beginning our solar system was a gigantic whirling disk of gas and dust surrounding a primitive sun. Solid minerals condensed out of the gas and clumped together to form proto-planets. Small ones like Earth emerged close to the center; giant planets, big enough to grab gases in the disk, formed further out. The orbits in which they were born, some 4.6 billion years ago, have remained the same ever since.
Until recently that was the accepted scenario. But now the detection of extra-solar planets has forced astronomers to re-examine such notions, because they present us with a paradox. Many are so monstrous in size, and hug their stars so closely, that they could not have formed in their present positions. The searingly hot stars around which they circle would have melted their rocky cores before they got started. Instead, it's assumed that they coalesced some distance away, then barreled inward over millions of years. And if such chaos characterizes the birth of extra-solar planets, could not similar disorder have reigned closer to home?
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