Astro News Briefs: August 23–29

Astronomers Find Lightweight Exoplanet

August 26, 2004 | The European planet-hunting team led by Michel Mayor (Geneva Observatory, Switzerland) has announced a low-mass planet orbiting the 5th-magnitude southern star Mu Arae. The planet has a minimum mass of just 14 Earths, almost identical to the mass of Uranus. It races around the solar-type star every 9.5 days at an average distance of just 0.09 astronomical unit. Because we do not know the inclination of the planet's orbit, its most likely mass is actually about 17 or 18 Earths, similar to Neptune. This object is the lightest planet discovered outside the solar system other than the three terrestrial-mass bodies orbiting the pulsar B1257+12. Mu Arae also has a previously discovered, Jupiter-mass planet in a 650-day orbit, and there are strong indications of another planet even farther out.

Like the vast majority of the 140 or so known extrasolar planets, astronomers discovered this new object by tracking back-and-forth Doppler shifts in the star's spectrum induced by the planet's gravitational pull. The detection method does not reveal whether the planet is a large ball of rock and iron (a so-called "super-Earth"), a gas giant like Jupiter and Saturn, or a hybrid Uranus- or Neptune-like world with a massive rock/ice core and a moderate gaseous envelope. Mayor's group discovered the planet with its new super-high-resolution HARPS spectrograph on the 3.6-meter European Southern Observatory telescope at La Silla, Chile.

Space Rock Buzzes Earth

August 25, 2004 | Last March 31st, a small chunk of asteroid hurtled within 6,500 kilometers of Earth's surface, according to an orbital analysis announced this week. Now designated 2004 FU162, the object was spotted only hours before its close brush by the LINEAR survey telescope in New Mexico. Unfortunately, LINEAR recorded the speeding visitor on only four frames over a 44-minute period. By the time astronomers were alerted to its existence, the little asteroid had moved into the daylight sky. Despite these all-too-brief observations, dynamist Steven Chesley (Jet Propulsion Laboratory) managed to derive a reliable orbit — and the near miss — that was reported August 22nd by the Minor Planet Center in Cambridge, Massachusetts.

With an estimated diameter of less than 10 meters, 2004 FU162 is small enough that it would have exploded harmlessly had it collided with Earth's atmosphere. Alan Harris (Space Science Institute) notes that an asteroidal fragment of this size should pass just as close to Earth at least once per year. "This event is not particularly rare," comments impact specialist David Morrison (NASA-Ames Research Center), "except that LINEAR had the good fortune to notice it."

The closest known near-miss asteroid skimmed within 60 km of Earth on August 10, 1972, creating a dazzling daylight fireball seen along a 1,500-km-long trajectory over the Rocky Mountains. Its estimated diameter was also about 10 meters.

For details about the observations and orbit of 2004 FU162, see the Minor Planet Electronic Circular 2004-Q22.

Mars Odyssey Starts Extended Mission

August 25, 2004 | After 23 months of scrutinizing the red planet's surface and atmosphere from orbit, NASA's Mars Odyssey orbiter has officially completed its primary mission. The spacecraft was launched on April 7, 2001, and reached Mars 6½ months later on October 23rd. A full Martian year has passed since February 2002, when the orbiter began its scientific studies. In that time the spacecraft confirmed that Mars has vast reserves of water ice just below its surface, especially in and around the south pole, and its infrared spectrometer has compiled a global map of mineral abundances and surface textures. From this point forward Odyssey will carry out an extended scientific mission, as it continues to relay 85 percent of the images and other data transmitted by the twin rovers Spirit and Opportunity.

A complete overview of the 2001 Mars Odyssey mission is at

New Nebula's X-rays

August 25, 2004 | Backyard observer Jay McNeil's discovery of a nebula in January generated much excitement among amateur astronomers and considerable interest on the part of professionals. The previously invisible nebula suddenly lit up when the star embedded within it brightened, probably because of a sharp increase in the amount of matter falling onto it from its circumstellar disk. Just a few weeks after McNeil spotted the object in one of his CCD images, Joel H. Kastner (Rochester Institute of Technology) and his collaborators realized that, by chance, they had imaged the region with the Chandra X-ray Observatory in 2002, before the outburst began. They have observed it twice since then, recording a 50-fold increase in the object's X-ray luminosity that mirrors the brightening in optical and infrared wavelengths. X-ray emission is commonly seen in young stars, but there is an ongoing debate about what causes it. The fact that this X-ray outburst is occurring simultaneously with the optical and infrared eruption demonstrates that in at least some cases, the X-ray emission is due to matter accreting onto the star.

More information as well as images can be found at the Chandra Web site.

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