SETI Institute Principal Investigator
As astronomer Angela Cotera grapples with the second and third terms of the Drake equation, fp and ne, she seeks to understand what fraction of stars have planetary systems (fp), and how many planets in those systems are capable of supporting life (ne). The key, in her view, is to understand how dust grains in the interstellar medium are transformed into habitable, Earthlike planets. While much of the substance of the interstellar medium consists of gas, almost all of this gas goes into stars, with the remainder making up gas giant planets. Cotera’s research, however, focuses on the dust grains from the interstellar medium that provide the foundation for rocky planets like our own.
Stars like our Sun are formed in several stages. First, denser regions of the interstellar medium prompt a gravitational collapse that leads to protostellar objects. These protostars are enveloped by material that begins to heat and rotate. As one of these young stars evolves, much of the material in its circumstellar envelope falls into the star. The remainder of the material collapses into a disk that rotates around the star. Current theory holds that all planets are formed from such circumstellar disks.
Within the last ten years, high resolution images taken by the Hubble Space Telescope have clearly shown the existence of these disks around very young, low mass stars. In Cotera’s view, studying how these circumstellar disks evolve is the key to understanding how the diffuse interstellar medium is transformed into planetary systems. In particular, at what point do the small dust grains begin to disappear from the interstellar medium, and where do they go? “The grains must grow to larger and larger sizes before planets can form,” says Cotera, “but how soon in the star formation process does that occur?” As she attempts to determine whether grain growth occurs whenever there are circumstellar disks, and then establish a time scale for that growth, she will gain a better understanding of planet formation – and a more accurate estimate of two critical terms in the Drake equation.
- SETI Institute Explorer, Special Edition 2005