Planet Formation Colloquium: Questions and Possibilities

As the first of three colloquiums in the Physics department this semester, “How Quickly Do Planets Form?” a lecture by Professor Eric Jensen, addressed some of the ways in which physicists are attempting to answer this question. Incorporated in the presentation, Jensen was able to discuss some of the research that he and Swarthmore student collaborators have done. The first portion of JensenÕs lecture focused on theories regarding how planets formed, the second on how the age of stars may be measured.

There are currently two models for determining how planets form. One model is the gravitational instability model. In this model, planets are formed when a circumstellar disk breaks apart. A planet such as Jupiter would only need a few hundred years to form in this model. The more dominantly accepted idea, however, is the core accretion model, which theorizes that planets form when tiny particles in space collide randomly into each other until they eventually form a planet. Under a model such as this, Jupiter would require roughly eight million years to form.

Scientists believe that roughly eighty percent of young stars have potentially planet forming disks, but by three million years of age, these disks would appear to be gone, contradicting the core accretion model that would require eight million years for a planet the size of Jupiter. However, scientists also acknowledge that current techniques do not ensure that all planetary materials are detected, particularly not gaseous ones which will make up the majority of a planet’s mass.

To better understand this, another question must be addressed: how old are the stars being studied (the only star that we can date at the moment is the sun, at about 4.5 billion years of age). Currently, the most commonly used method of determining a star’s age is the HR diagram in which age is calculated based on luminosity and temperature. However there is no certainty that this model is accurate nor is it always possible to obtain the distance to a star, needed to calculate luminosity.

Jensen’s own research has focused on developing a method of dating stars based on lithium depletion, as it would be a distance-independent indicator of age. Over time, increasing temperatures on a star’s surface should cause lithium to decay. However, Jensen’s efforts thus far to date stars based on lithium decay have encountered problems as binary systems (formed at the same time) do not seem to have the same age when using the lithium model. Ultimately, Jensen concluded that as “current models are not self consistent” the search for a strong indicator of star age with which to study planet formation goes on.

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