My first AAS is at a close. I’ve had a great time hearing about new areas of research, seeing old friends and meeting new people. On Tuesday, I attended still more talks on exoplanets, some of which I will discuss below.
Wednesday I spent standing by my poster, which is on the gravitational lensing work I did at UC Santa Barbara. It was really fun to talk about the work that I’ve done: I got to tell people about what I spent my time doing, the parts of my work I found especially interesting and all the exciting prospects for future research. I had entered my poster in the Chambliss student poster contest for no particular reason, but am very grateful I did because it gave me the opportunity to talk to several people in depth about my work. In particular, one of the two people who came to judge my poster was quite interested in the ways in which I tested my code. I’m fond of several plots I made to illustrate the tests I performed and I find some of the results very cute, but it’s not something most people are interested and in our paper we relegate it to the Appendix. But in this judge, I found someone who was pleased I had put in the effort to test my code and quite interested in hearing about it (or at least pretended to be!).
Although the Kepler Mission will probably be most remembered for its transiting planets, it is also a wonderful instrument for studying stellar properties. Two talks at this session discussed not the planets, but the stars.
Lucianne Walkowicz (UC Berkeley) focused on this aspect of Kepler in her talk on modeling star spots. Properties of star spots, such as the geometry and coverage, are dependent to the interior properties of the star and can be related back to stellar magnetic fields and dynamo theory. Their simple models of star spots will allow them to look at trends in the large sample of Kepler stars. Using just non-evolving analytic spots and differential rotation, the authors are able to obtain reasonable fits to their data. Their next step is to add spot evolution. One amusing fact: their code is called Cheetah, because it’s fast and has spots.
Another quite interesting project by Sumin Tang (Harvard/ CfA) looks at the long term variability of stars in the Kepler field of view. They are able to reach 100 year timescales by looking back at plates (which were used before CCDs and computers were developed, see the Harvard College Observatory) taken over the last century by Harvard astronomers. There’s one break in the data, a ten year gap around the 1960s: the director at that time decided not to take more. The authors have been working on digitizing the plates, which is quite a difficult process since each one must be calibrated independently and the calibration varies across the plate. One star they have looked at is Kepler 10, the star recently found to host a rocky planet. Over the last 100 years, they find that it has had constant brightness, at least within the error bars.
Habitability of a Terrestrial Planet
My favorite talk of this session was from Colin Goldblatt (U. Washington on behalf of the Virtual Planetary Lab), who discussed the habitability of Gliese 581g. There is some controversy over whether or not this planet really exists (there are multiple ways to interpret the data), but for now let’s assume that it does. The authors wonder whether Gliese 581g would actually be habitable, even though it’s located in what is referred to as the habitable zone (HZ). The HZ is usually defined to be the orbital radii within which the planet would have an equilibrium temperature allowing liquid water. In the Solar System, the Earth and Venus are located in the HZ. Of course, Venus isn’t habitable to life as we know it due to the large amounts of CO2 in its atmosphere so it is reasonable to ask whether Gliese 581g would be.
For this work, the authors define the HZ by the flux received and the amount of greenhouse gas in the atmosphere. The simplest model for the planetary temperature has many parameters: the flux at the top of the atmosphere, the amount of greenhouse gas, the albedo of the surface and clouds and the optical depth of the surface and clouds. Since only the flux is known for Gliese 581g, one really can’t say anything about the habitability of this planet. In order to gain the additional necessary information, one would likely need a spectrum of the planet. Because Gliese 581g is not a transiting planet (it was detected in a radial velocity survey), this is likely impossible. For fun, the authors assume Earth values; in this case the planet’s surface temperature is 258K–quite cold. To keep the planet from freezing over, a stronger greenhouse effect is needed. The authors calculate that 3 bars of CO2 are necessary (recall the surface pressure of Earth’s atmosphere is about 1 bar).