Harvard Department of Astronomy <br>
Special Seminar<br>
<br>
Planet Formation in Astronomy, Experimental Physics, and Planetary Science: From Current Concepts to New Ideas and Further<br>
<br>
Dr. Gerhard Wurm<br>
U. Munster, Germany<br>
<br>
Thursday, March 23<br>
1:00 PM<br>
Phillips Auditorium<br>
60 Garden St.<br>
<br>
Abstract: Throughout the history of the Solar System, collisions have<br>
been an important, inevitable, vital and sometimes destructive part of<br>
it. In the standard model of planet formation, terrestrial planets and<br>
their precursor planetesimals are supposed to be built through<br>
sticking collisions. Easy enough, many assume that this sticking works<br>
whatsoever. Equally well justified, others exclaim it is impossible to<br>
do if particles collide at tens of m/s. So, who is right? Our recent<br>
impact experiments show that "high-speed" collisions between cm-size<br>
particles can lead to growth and - quite non-intuitively at first<br>
sight - might even promote growth better than slow collisions.<br>
However, this is only one part of the story and it has to be placed in<br>
the timeframe and location of protoplanetary disks. Recent<br>
observations especially by the Spitzer telescope give clear evidence<br>
for a very different evolution of gas and dust in protoplanetary<br>
disks. Large inner holes filled with gas but devoid of dust are<br>
observed at transitional stages. And it is only recently that we<br>
realized that this might be one of the most important phases in<br>
planetary formation.<br>
<br>
Long known, but completely forgotten in astrophysics, a force named<br>
photophoresis rules in these environments. In an elegant fashion<br>
already its simplest application provides means to explain the<br>
clearing of the inner disk from dust, to explain circumstellar dust<br>
rings, to explain the formation of comets and Kuiper belt objects, and<br>
to explain the formation of primitive asteroids and some of their<br>
descendent chondritic meteorites. New experiment on photophoresis<br>
result in still more interesting effects which, applied to<br>
protoplanetary disks, might prevent the loss of solid matter (m-size<br>
bodies) to the star. Terrestrial but also giant planet formation will<br>
not go unimpressed by this. And if this were not yet enough,<br>
photophoresis has other applications e.g. on the surface and in the<br>
atmosphere of the current Mars (and Earth) which are so obvious that<br>
it will easily fit within this talk without restraint.<br clear="all"><br>