It is now possible to detect planets orbiting stars other than the Sun. One surprise is a class of planets known as 'Hot Jupiters' which seem to be fairly common. These are planets with a mass similar to Jupiter yet orbit their parent star closer than Mercury does around our Sun.
Astronomers say there could be billions of Earths in our galaxy, the Milky Way. Their assessment comes after the discovery of the 100th extra solar planet. [BBC News]
Astronomers now believe it is more likely that the strange
object is a background star whose light has been dimmed and
reddened by interstellar dust, giving the illusion that it is in
the vicinity of the double star system in which it was initially
believed to have been a planet.
NASA's Hubble Space Telescope photographed the mysterious object, called TMR-1C, in 1997. The picture shows a bright dot at the end of a long streamer of reflective dust stretching 135 billion miles (225 billion kilometers) back to the binary star located 450 light-years away in the constellation Taurus the bull. A light year is about 6 trillion miles.
In 1998, astronomer Susan Terebey of the Extrasolar Research Corp., Pasadena, CA, reported her observation at a scientific meeting as a possible young and hot "protoplanet" several times the mass of Jupiter. Because of its potential importance and the compelling nature of the image, NASA also released the picture to the public with the caution that future observations would be critical in verifying whether or not this object actually is a planet.
Tereby initially proposed that the object had been ejected from a double star system via a "slingshot" effect (interaction with one of the stars or another giant planet). Since then she has conducted follow-up observations with the 10-meter Keck telescope in Mauna Kea, HI, to test her hypothesis.
Now, in results to be published in the May Astronomical Journal, Tereby reports, "The new data do not lend weight to the protoplanet interpretation and the results remain consistent with the explanation that TMR-1C may be a background star. Although the Hubble image is striking, there is the alternate possibility that TMR-1C is an unrelated background star, seen, by chance, projected close to the young star system. Finding a clearer answer is difficult for an object as faint as TMR-1C."
To better understand the nature of this faint object, Tereby used the Keck telescope to measure TMR-1C's temperature by dissecting its light through spectroscopy. Much like the way a prism disperses sunlight to make a colorful rainbow, a spectrum breaks apart the light from the observed object. The relative amounts of red and blue light help tell the object's temperature.
Tereby and colleagues then constructed models of dust- obscured objects to compare with the spectrum of TMR-1C and found a corresponding temperature of greater than 4400 degrees Fahrenheit (2700 degrees Kelvin) for TMR-1C. This is hotter than the predicted temperatures of young giant planets.
"However the models are not yet reliable at such young ages, so this test by itself is not conclusive," Tereby cautions. "The idea remains alive and well that there may be runaway planets and brown dwarfs (small stars that failed to sustain nuclear fusion) which formed via ejection from multiple star systems. Theoretical models by several groups support this idea, and new searches -- including ours -- are finding many new candidates in star-forming regions. However at this time there is no strong evidence that TMR-1C itself is a protoplanet."
The Hubble image of TMR-1C and the initial press release (from May 28, 1998) are available at: http://oposite.stsci.edu/pubinfo/pr/1998/19/
Planet-hunting astronomers have crossed an important
threshold in planet detection, with the discovery of two planets
that may be smaller in mass than Saturn.
Of the 30 extrasolar planets around Sun-like stars detected previously, all have been the size of Jupiter or larger. The existence of these Saturn-sized candidates suggests that many stars harbor smaller planets, in addition to the Jupiter-sized ones.
Finding Saturn-sized planets reinforces the theory that planets form by a snowball effect of growth from small ones to large, in a star-encircling dust disk. The 20-year-old theory predicts there should be more smaller planets than large planets, and this is a trend the researchers are beginning to see in their data.
"It's like looking at a beach from a distance," explained Geoff Marcy of the University of California at Berkeley. "Previously we only saw the large boulders, which were Jupiter- sized planets or larger. Now we are seeing the 'rocks,' Saturn- sized planets or smaller. We still don't have the capability of detecting Earth-like planets, which would be equivalent to seeing pebbles on the beach."
Jupiter alone is three times the mass of Saturn. This has left the nagging possibility open that some of the extrasolar planets might really be stillborn stars, called brown dwarfs, which would form like stars through the collapse of a gas cloud. But now researchers are better assured these "Jupiters" are only the tip of the iceberg, and there are many more planets to be found that are the mass of Saturn or smaller.
"Now we are confident we are seeing a distinctly different population of bodies that formed out of dust disks like the disks Hubble Space Telescope has imaged around stars," said Marcy.
The discovery was made by planet-sleuths Marcy, Paul Butler of the Carnegie Institution of Washington, and Steve Vogt of the University of California, Santa Cruz, using the mighty Keck telescope in Mauna Kea, Hawaii. They discovered a planet at least 80 percent the mass of Saturn orbiting 3.8 million miles from the star HD46375, 109 light-years away in the constellation Monoceros, and a planet 70 percent the mass of Saturn orbiting 32.5 million miles around the star 79 Ceti (also known as HD16141), located 117 light-years away in the constellation Cetus.
These planets are very close to their stars and so have short orbits. They whirl around their parent stars with periods of 3.02 days and 75 days respectively. This allowed for their relatively rapid discovery.
The astronomers detected the small wobble of a star caused by the gravitational tug of the unseen planets. For the past five years Marcy and Butler have used this technique successfully to catalog 21 extrasolar planets. Boosted by the light-gathering power of Keck, they have steadily increased the precision of their measurements so they can look for the gravitational effects of ever-smaller bodies. In this latest detection, the change in a star's velocity -- rhythmically moving toward and then away from Earth -- is only 36 feet per second, a little faster than a human sprints.
The Saturn-mass planets are presumably gas giants, made mostly of primordial hydrogen and helium, rather than the rocky material Earth is made of. They are so close to their parent stars they are extremely hot, and are not abodes for life as we know it. The planet orbiting 79 Ceti has an average temperature of 1530 degrees Fahrenheit (830 degrees Celsius). The planet orbiting HD46375 has an average temperature of 2070 degrees Fahrenheit (1130 degrees Celsius).
They probably formed at a farther distance from the star, where they could accumulate cool gas, and then migrated into their present orbits. Along the way they would have disrupted the orbits of any smaller terrestrial planets like Earth. These "marauding" gas giants seem more the rule than the exception among the planets surveyed so far, because Marcy and Butler's detection technique favors finding massive planets in short-period orbits. This seems to be the case for approximately six percent of the stars surveyed so far.
Their research is part of a multi-year project to look for wobbles among 1,100 stars within 300 light-years of Earth. The project is supported by NASA and the National Science Foundation.
The artist's impression above assumes that the newly discovered planet has rings. However, just because it is a similar size to Saturn does not infer a ring system. - Astronews.
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National Science Foundation News August 15, 2001
A team of astronomers has found a Jupiter-sized planet orbiting a faint nearby star similar to our Sun, raising intriguing prospects of finding a solar system like our own.
The planet is the second found orbiting the star 47 Ursae Majoris in the Big Dipper, also known as Ursa Major or the Big Bear. The new planet is at least three-fourths the mass of Jupiter and orbits the star at a distance that, in our Solar System, would place it beyond Mars but within the orbit of Jupiter.
"Astronomers have detected evidence of more than 70 extrasolar planets," said Morris Aizenman, a senior science advisor at the National Science Foundation (NSF). "Each discovery brings us closer to determining whether other planetary systems have features like those of our own."
"For the first time we have detected two planets in nearly circular orbits around the same star," said team member Debra Fischer of the University of California at Berkeley. "Most of the 70 planets people we have found to date are in bizarre solar systems, with short periods and eccentric orbits close to the star. As our sensitivity improves we are finally seeing planets with longer orbital periods, planetary systems that look more like our Solar System."
The planet-search team, which is supported by NASA and the National Science Foundation, Arlington, VA, has been instrumental in finding a majority of the planets outside our Solar System (also called extrasolar planets). Besides Fischer, the team includes Geoffrey Marcy, also of Berkeley, Paul Butler of the Carnegie Institution of Washington, Steve Vogt of the University of California at Santa Cruz and Gregory Laughlin of NASA's Ames Research Center, Mountain View, CA. Their report on the new planet has been submitted to the Astrophysical Journal.
A few years ago, Marcy and Butler discovered a planet more than twice the mass of Jupiter in a circular orbit around the same star. The star is one of 100 that the scientists have targeted since 1987 in their search for evidence of extrasolar planets. Using telescopes at the University of California's Lick Observatory, they measure changes in the characteristics of light emitted by the stars. Those changes, they believe, signal the presence of a planet periodically pulling the star toward or away from Earth.
Fischer was able to see the periodic wobble from the second planet, smaller and farther from the star than the first, because of improved instrumentation.
The star is a yellow star similar to the sun, probably about seven billion years old and located about 51 light-years from Earth. A light-year, the distance light travels in one year, is approximately 6 trillion miles.
"Every new planetary system reveals some new quirk that we didn't expect. We've found planets in small orbits and wacky eccentric orbits," said Marcy. "With 47 Ursae Majoris, it's heartwarming to find a planetary system that finally reminds us of our solar system."
Or at http://exoplanets.org/ follow the link to Planet Almanac.
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