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. In The Stars: Odd Stars, Odder Planets

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by Phil Berardelli
UPI Science & Technology Editor
Washington (UPI) Feb 10, 2005
Sir Arthur Eddington, the late English astronomer, once commented that "not only is the universe stranger than we imagine, it is stranger than we can imagine."

It might be interesting to learn how Eddington, who died in 1944, would regard some of the latest discoveries about this ordinary galaxy in which we live, because they definitely are pushing the limits of strangeness.

Take SDSS J090745.0+24507, a star so unremarkable that astronomers had not even given it a formal name. Yet, of all the stars populating the Milky Way's spiral arms and orbiting around its center - by latest estimates more than 400 billion - SDSS J090745.0+24507 is doing something unique: it is leaving.

It seems SDSS J090745.0+24507 is moving along at a tremendous clip - about 1.5 million miles per hour or 1,000 times faster, more or less, than a rifle bullet. It also is moving three times faster than the sun and solar system as they revolve around the galactic center and twice the speed necessary to escape the Milky Way's collective gravity.

Stars, even small or undistinguished ones, are massive bodies. They cannot be pushed around very easily. So what could have provided the incredible force necessary to take hold of SDSS J090745.0+24507 and fling it on a path that eventually will take it out of the Milky Way and into dark and forbidding intergalactic space?

Astronomers at the Harvard-Smithsonian Center for Astrophysics, who discovered the star's unusual trajectory, think they have the answer: SDSS J090745.0+24507 once was part of a binary star system that suffered a close encounter with the massive black hole at the Milky Way's center.

They reason that at some point in the distant past, the pair ventured too close to the monster, called Sagittarius A*, which promptly trapped its companion - either forcing it into a close orbit or even pulling it within its event horizon - but propelled SDSS J090745.0+24507 away at great speed using its tremendous gravitational energy, a cosmic batter hitting one literally out of the park.

The evidence is persuasive. First, SDSS J090745.0+24507 is traveling along a path almost directly away from the location of Sagittarius A*. Second, based on spectrographic analysis of the star's light, its composition is rich in metals, which means it is relatively young - maybe less than 100 million years old, so it probably originated in a metal-rich star-forming region near the galactic center.

"We have never before seen a star moving fast enough to completely escape the confines of our galaxy," said Warren Brown of the Harvard-Smithsonian center. "We're tempted to call it the outcast star because it was forcefully tossed from its home."

Then there is the case of another number in the astronomical catalogue: PSR B1257+12, a pulsar, the tiny but extremely dense remnant of a much, much bigger star that went supernova in the vicinity of the constellation Virgo about 1,500 light-years away.

Except this object has attracted attention before. In 1992, astronomer Alex Wolszczan of Penn State University discovered a planet orbiting PSR B1257+12, making it the first planetary body found outside the solar system.

Since then, more than 100 exoplanets, as they are called, have been discovered - including several more orbiting PSR B1257+12.

Now Wolszczan and colleagues not only have found another - albeit very small - member of the distant system, but they also have determined that the system's structure is remarkably similar to our own.

The PSR B1257+12 system includes three small planets, in orbits almost exactly proportional to Mercury, Venus and Earth. A fourth and more massive planet has an orbit about six times farther away than the third planet, which co-discoverer Maciej Konacki, of Caltech, said is orbiting at the same distance, proportionally, as the asteroid belt, which falls between Mars and Jupiter.

Even the newly discovered smallest planet matches the Earth-system model to some extent. Only about one-fifth the size of Pluto, it trolls the outer limits of the PSR B1257+12 environs.

"Surprisingly, the planetary system around this pulsar resembles our own solar system more than any extrasolar planetary system discovered around a Sun-like star," Konacki said.

The system's similarity, while beguiling, is not its novelty - it is that there are planets at all.

When the star that became PSR B1257+12 exploded, it quickly expunged any planets that had the misfortune to form and orbit the red giant. So the assumption was the resulting pulsar would become a loner, left behind by the massive cloud of debris ejected in the supernova.

Not so. Somehow, in the event's aftermath, a new planetary disk was formed, and from it condensed the system astronomers are detecting today, using a sophisticated method that deduces the existence of planets from tiny fluctuations in the pulsar's extremely rapid and otherwise precise rotation rate. They can use the technique to detect objects down to the size of asteroids.

"Despite the extreme conditions that must have existed at the time these planets were forming, nature has managed to create a planetary system that looks like a scaled-down copy of our own inner solar system," Wolszczan said.

Speeding stars and pulsar planets aside, the strangest celestial discovery of late may be the possibility that some planets are actually made largely of diamonds.

Based on the composition of some meteorites and spectrographic analyses of certain stars, Marc Kuchner, an astronomer at Princeton University, and colleagues think an unknown number of planets might contain so much carbon that part of their interiors have turned into diamond layers hundreds of miles thick.

Such planets would have a composition based on carbon instead of silicon, which is the stuff Earth and the other rocky planets of this solar system are made of, mostly.

The diamond planets would be created by explosions of carbon-rich stars that also produced low levels of oxygen, Kuchner told a group of astronomers and reporters gathered recently in Aspen, Colo. - or listening in via telephone. In these cases, he said, the resulting planet-forming disks would contain large quantities of compounds such as graphite, which if condensed and heated sufficiently would form diamond structures of planet-sized dimensions.

He said the best place to look for diamond planets might be near the center of the Milky Way, where many carbon-rich stars are located, not to be confused with the metal-rich stars such as SDSS J090745.0+24507, which at the moment is leaving its former neighborhood rapidly behind.

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Carbon World's Could Sparkle Like Diamonds
Moffett Field CA (SPX) Feb 09, 2005
Some extrasolar planets may be made substantially from carbon compounds, including diamond, according to a report presented this week at the conference on extrasolar planets in Aspen, Colorado.
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