Planet-building is messy and violent, astronomers say

<body leftmargin="0" bgcolor="#000060" text="#FFFFFF" link="#FFFF00" vlink="#FF9900"> <b> <p class="MsoNormal"><font face="Arial" size="4">Planet-building is messy and violent, astronomers say</font></p> </b> <p class="MsoNormal"><font face="Arial" size="1">Posted Oct. 14, 2004<br> Courtesy NASA<br> and World Science staff</font> </p> <b> <p class="MsoNormal"><font size="2" face="Arial">Astronomers traditionally believed planets are built in a gradual, calm process. In this picture, small dust particles and rocks gradually float together and stick together under the influence of gravity, piecing together a monolithic world bit by bit.</font></p> <p class="MsoNormal"><font size="2" face="Arial">But researchers are learning that the bits might be bigger than expected, and their rendezvous, more violent. A group of astronomers announced last week that they believe planets are built over a long period of massive collisions between rocky bodies as big as mountain ranges.</font></p> <p class="MsoNormal"><font size="2" face="Arial">New observations from NASA's Spitzer Space Telescope reveal surprisingly large dust clouds around several stars. These clouds most likely flared up when rocky, embryonic planets smashed together, the researchers said. The Earth's own moon may have formed from such a catastrophe.</font></p> <p class="MsoNormal"><font size="2" face="Arial">"It's a mess out there," said George Rieke of the University of Arizona, Tucson, first author of the findings and a Spitzer scientist. "We are seeing that planets have a long, rocky road to go down before they become full grown."</font></p> <p class="MsoNormal"><font size="2" face="Arial">Spitzer was able to see the dusty aftermaths of these collisions with its powerful infrared vision. When embryonic planets, the rocky cores of planets like Earth and Mars, crash together, they are believed to either merge into a bigger planet or splinter into pieces. The dust generated by these events is warmed by the host star and glows in the infrared, where Spitzer can see it.</font></p> <p class="MsoNormal"><font size="2" face="Arial">The findings will be published in an upcoming issue of the research journal Astrophysical Journal. They mirror what we know about the formation of our own planetary system. Recent observations from studies of our moon's impact craters also reveal a turbulent early solar system. "Our moon took a lot of violent hits when planets had already begun to take shape," Rieke said.</font></p> <p class="MsoNormal"><font size="2" face="Arial">According to the most popular theory, rocky planets form somewhat like snowmen. They start out around young stars as tiny balls in a disc-shaped field of thick dust. Then, through sticky interactions with other dust grains, they gradually accumulate more mass. Eventually, mountain-sized bodies take shape, which further collide to make planets.</font></p> <p class="MsoNormal"><font size="2" face="Arial">Previously, astronomers envisioned this process proceeding smoothly toward a mature planetary system over a few million to a few tens of millions of years. Dusty planet-forming discs, they predicted, should steadily fade away with age, with occasional flare-ups from collisions between leftover rocky bodies.</font></p> <p class="MsoNormal"><font size="2" face="Arial">Rieke and his colleagues have observed a more varied planet-forming environment. They used new Spitzer data, together with previous data from the European Space Agency's Infrared Astronomical Satellite and the joint NASA, United Kingdom and the Netherlands' Infrared Space Observatory. They looked for dusty discs around 266 nearby stars of similar size, about two to three times the mass of the sun, and various ages. Seventy-one of those stars were found to harbor discs, presumably containing planets at different stages of development. But, instead of seeing the discs disappear in older stars, the astronomers observed the opposite in some cases.</font></p> <p class="MsoNormal"><font size="2" face="Arial">"We thought young stars, about one million years old, would have larger, brighter discs, and older stars from 10 to 100 million years old would have fainter ones," Rieke said. "But we found some young stars missing discs and some old stars with massive discs."</font></p> <p class="MsoNormal"><font size="2" face="Arial">This variability implies planet-forming discs can become choked with dust throughout the discs' lifetime, up to hundreds of millions of years after the host star was formed. "The only way to produce as much dust as we are seeing in these older stars is through huge collisions," Rieke said.</font></p> <p class="MsoNormal"><font size="2" face="Arial">Before Spitzer, only a few dozen planet-forming discs had been observed around stars older than a few million years. Spitzer's uniquely sensitive infrared vision allows it to sense the dim heat from thousands of discs of various ages. “Spitzer has opened a new door to the study of discs and planetary evolution,” said Michael Werner, project scientist for Spitzer at NASA's Jet Propulsion Laboratory, Pasadena, Calif.</font></p> <p class="MsoNormal"><font size="2" face="Arial">The findings "give us new insights into the process of planetary formation, a process that led to the birth of planet Earth and to life," said Anne Kinney, director of the universe division in the Science Mission Directorate at NASA Headquarters, Washington.</font></p> </b> <p class="MsoNormal"> <b> <font size="2" face="Arial"><a href="http://groups.yahoo.com/group/world-science">http://groups.yahoo.com/group/world-science</a></font> </b> </p> </body>