Padding New-Planet Portfolios
(From the July/August 2000 issue of StarDate magazine)
Like Wall Street's dot-com darlings, the numbers on the most recent planet discoveries keep going up: 21 new extrasolar planets were added just this spring.
Of these 21 planets, 3 appear to be less massive than Saturn. The first two, discovered by Geoff Marcy, Paul Butler, and Steve Vogt, are 70 percent and 80 percent the mass of Saturn and are very close to their parent stars. This close distance shortens the time it takes for them to orbit. The scientists used the planets' brief orbits to note how the gravity of the unseen, orbiting planet tugged on its parent star, causing it to wobble. The wobble allowed them to determine the planet's mass and distance from its parent star.
The third Saturn-mass planet is one of eight low-mass companions -- six planets and two brown dwarfs -- discovered by astronomers at the European Southern Observatory. The planet, which is about 80 percent as massive as Saturn, orbits a star 140 light-years away in the constellation Scutum, the shield.
Two of the team's six other planets are slightly more massive than Saturn and orbit stars in the constellations Vela and Crux. Of all the new planets discovered, Vela's planet has both the shortest orbital period -- under three days -- and the smallest distance to its star, only 3.5 million miles (5.7 million km). The team also may have detected a second low-mass companion to this same star.
A third team of astronomers says it has discovered 13 "free-floating" objects that might be planets 1,600 light-years away in the Trapezium cluster in the Orion Nebula. The mass of the smallest of these objects may be as low as about eight Jupiters. They have been dubbed "free floating" because they do not orbit any star and drift by themselves, aglow with the residual heat of their birth. Early analysis of some of these planets suggests the presence of water vapor.
The astronomers also discovered more than 100 very young brown dwarfs -- objects that might have become stars had they accumulated more material. Consequently, they did not heat up enough inside to trigger the nuclear reactions that keep stars shining for long periods of time. Because brown dwarfs and free-floating planets cool quickly, they are easiest to find while they are young and retain some heat from the formation process. The objects in the Trapezium cluster are mostly one million years old -- infants compared with our 4.5-billion-year-old Sun.
Although astronomers added almost two dozen extrasolar planets to the roster, they were forced to drop one. They believe the object, TMR-1C, is a background star whose light has been dimmed and reddened by interstellar dust. This gave the illusion that the object is near a double star system and was ejected via a "slingshot" effect as it interacted with one of the stars. Researchers later found that TMR-1C is probably too hot to be a planet. -- Gary Harrison
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