Astronomers Find Grains of Sand Around Distant Stars

In a find that sheds light on how Earth-like planets may form, astronomers this week report finding the first evidence of small, sandy particles orbiting a newborn solar system at about the same distance as the Earth orbits the sun. The report will be published online this week by the journal Nature.

"Precisely how and when planets form is an open question," says study co-author Christopher Johns-Krull, assistant professor of physics and astronomy at Rice University. "We believe the disk-shaped clouds of dust around newly formed stars condense, forming microscopic grains of sand that eventually go on to become pebbles, boulders and whole planets."

In previous studies, astronomers have used infrared heat signals to identify microscopic dust particles around distant stars, but the method isn't precise enough to tell astronomers just how big they become, and whether the particles orbit near the star, like the Earth does the sun, or much further away at a distance more akin to Jupiter or Saturn.

In the new study, Johns-Krull and co-authors in the United States, Germany and Uzbekistan used reflected light from the sand itself to confirm the Earth-like orbit of grainy particles around a pair of stars called KH-15D in the constellation Monoceros. The stars are about 2,400 light years from Earth in the Cone Nebula, and they are only about 3 million years old, compared to the sun's 4.5 billion years.

"We were attracted to this system because it appears bright and dim at different times, which is odd," Johns-Krull says.

The researchers found that the Earth has a nearly edge-on view of KH-15D. From this perspective, the disk blocks one of the stars from view, but its twin has an eccentric orbit that causes it to rise above the disk at regular intervals.

"These eclipses let us study the system with the star there and with the star effectively not there," Johns-Krull says. "It's a very fortuitous arrangement because when the star is there all the time, it's so bright that we can't see the sand."

The team conducted both photometric and spectrographic analyses of data collected during the past 12 years from a dozen observatories, including the McDonald Observatory in west Texas, the Keck Observatory in Hawaii and the VLT on Mount Paranal in Chile.

"Because of how the light is being reflected there are opportunities to make observations about the chemical composition of these sand-like particles," says co-author William Herbst, an astronomer at Wesleyan University in Middletown, Conn. "That's very exciting because it opens up so many doors for new type of research on this disk."

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Astronomers Monitor Asteroid to Pass Near Mars

Astronomers funded by NASA are monitoring the trajectory of an asteroid estimated to be 164-feet wide that is expected to cross Mars' orbital path early next year. Observations provided by the astronomers and analyzed by NASA's Near-Earth Object Office at the Jet Propulsion Laboratory in Pasadena, Calif., indicate the object may pass within 30,000 miles of Mars at about 6 a.m. EST on Jan. 30, 2008.

"Right now asteroid 2007 WD5 is about half-way between the Earth and Mars and closing the distance at a speed of about 27,900 miles per hour," says Don Yeomans, manager of the Near Earth Object Office at JPL. "Over the next five weeks, we hope to gather more information from observatories so we can further refine the asteroid's trajectory."

NASA detects and tracks asteroids and comets passing close to Earth. The Near Earth Object Observation Program, commonly called "Spaceguard," plots the orbits of these objects to determine if any could be potentially hazardous to our planet.

Asteroid 2007 WD5 was first discovered on Nov. 20, 2007, by the NASA-funded Catalina Sky Survey and put on a "watch list" because its orbit passes near the Earth. Further observations from both the NASA-funded Spacewatch at Kitt Peak, Ariz., and the Magdalena Ridge Observatory in New Mexico gave scientists enough data to determine that the asteroid was not a danger to Earth, but could potentially impact Mars. This makes it a member of an interesting class of small objects that are both Near Earth Objects and "Mars crossers."

Because of current uncertainties about the asteroid's exact orbit, there is a 1-in-75 chance of 2007 WD5 impacting Mars. If this unlikely event were to occur, it would be somewhere within a broad swath across the planet north of where the Opportunity rover is.

"We estimate such impacts occur on Mars every thousand years or so," says Steve Chesley, a scientist at JPL. "If 2007 WD5 were to thump Mars on Jan. 30, we calculate it would hit at about 30,000 miles per hour and might create a crater more than half-a-mile wide." The Mars Rover Opportunity is currently exploring a crater approximately this size.

Such a collision could release about three megatons of energy. Scientists believe an event of comparable magnitude occurred here on Earth in 1908 in Tunguska, Siberia, but no crater was created. The object was disintegrated by Earth's thicker atmosphere before it hit the ground, although the air blast devastated a large area of unpopulated forest.

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Light Gives Asteroids Spin

Astronomers have observed an asteroid change the rate at which it spins for the first time, and shown that this is due to a theoretical effect predicted but never before seen. The international team of scientists from Europe and the United States used a range of telescopes to find that the asteroid is rotating faster by 1 millisecond every year.

The acceleration in the rate of rotation is due to heating of the asteroid’s surface by the Sun. Eventually it may spin faster than any known asteroid in the solar system. The Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect is believed to alter the way small bodies in the solar system rotate. YORP is a torque due to sunlight shining on the surfaces of asteroids and meteoroids and warming their surfaces, leading to a gentle recoil effect as the heat is emitted. By analogy, if one were to shine light on a propeller over a long enough period, it would start spinning.

Although this is an almost immeasurably weak force, astronomers believe it may be responsible for spinning some asteroids up so fast that they break apart, perhaps leading to the formation of binary asteroids. Others may be slowed down so that they take many days to rotate once. The YORP effect also plays an important role in changing the orbits of asteroids between Mars and Jupiter, including their delivery to planet-crossing orbits.

Despite its importance, the effect has never been seen acting on a solar system body, until now. Using extensive optical and radar imaging from powerful Earth-based observatories, astronomers have directly observed the YORP effect in action on a small near-Earth asteroid, known as (54509) 2000 PH5. This work is reported in two companion papers, in the March edition of Science Express, by Stephen Lowry et al. (Queens University Belfast, UK) and Patrick Taylor et al. (Cornell University, Ithaca, NY, USA).

Shortly after its discovery in 2000, it was realized that this asteroid would be the ideal candidate for such a YORP detection. At just 114m in diameter, it is relatively small and so more susceptible to the effect. Also, it rotates very fast, with one day on the asteroid lasting just over 12 Earth minutes, implying that the YORP effect may have been acting on it for some time. With this in mind, the team of radar and optical astronomers undertook a long term monitoring campaign of the asteroid with the aim of detecting any tiny changes in the spin-rate.

Over a 4 year time span, Stephen Lowry, Alan Fitzsimmons and colleagues took images of the asteroid at a range of telescope sites including the 8.2m Very Large Telescope array and the 3.5m New Technology Telescope of the European Southern Observatory in Chile, the 3.5m telescope at Calar Alto, Spain, along with a suite of other telescopes from the Czech Republic, the Canary Islands, Hawaii, Spain and Chile. With these facilities the astronomers measured the slight brightness variations as the asteroid rotated.

Over the same time period, the radar team led by Patrick Taylor and Jean-Luc Margot of Cornell University employed the unique capabilities of the Arecibo Observatory in Puerto Rico and the Goldstone radar facility in California to observe the asteroid by ‘bouncing’ a radar pulse off the asteroid and analyzing its echo. With this technique astronomers can reconstruct a 3-D model of the asteroid’s shape, with the necessary detail to allow a theoretical YORP value to be derived and compared with the actual observed spin-rate change seen at optical wavelengths. With careful analysis of the optical data, the asteroid’s spin rate was seen to steadily increase with time, at a rate that can be explained by YORP theory. Most significantly, the effect was observed year after year.

Furthermore, this number was elegantly supported via analysis of the combined radar and optical data, as it was required that the asteroid increase its spin-rate at exactly this rate in order for a satisfactory 3-D shape model to be determined.

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