Title: Planets and Stellar Activity: Hide and Seek in the CoRoT-7 System.
Authors: R. D. Haywood, A. Collier Cameron, D. Queloz, S. C. C. Barros, M. Deleuil, R. Fares, M. Gillon, A. F. Lanza, C. Lovis, C. Moutou, F. Pepe, D. Pollacco, A. Santerne, D. Segransan, and Y. C. Unruh
First author’s institution: School of Physics and Astronomy, St. Andrews, United Kingtom
Status: Accepted to MNRAS.
Separating Planets from NoiseThe CoRoT-7 system was uncovered in 2009 when the innermost planet, CoRoT-7b was found via the transit method. Follow-up radial velocity observations detected two additional planets. However, the host star is known to be very active, so Haywood et al. decided to analyze the system closely to ensure that all the planets are real, and not artifacts induced by starspots. As we said in the previous section, starspots will affect both the radial velocity data and transit photometry, making both data sets correlated: each data point depends not only on what the planets are doing, but time-sensitive properties of the noise (the starspots).To alleviate this problem, the authors obtained radial velocity data from HARPS at the same time they collected transit photometry from CoRoT. They then modeled the effects of both the planets and the noise using a Gaussian process, assuming the noise followed a simple functional form. They assumed the RV signal was composed of three components. One of these components is stellar variability, with a period similar to the rotation period. A second is changes in the RV signal due to changes in the convective properties of the star, which depends on the size of the spots. The third is a long-period effect caused by slow changes in the activity of the star. This noise term is used to account for additional astrophysical effects in the stellar atmosphere that may appear in the radial velocity data but not the photometry. If this noise is real its affects must change on the same timescale as the stellar rotation, although it can have a different phase; the authors tie the periodicity of this noise to their fit to the rotation period.By combining these three effects, the authors were able to develop a complete noise model for the star. They then simultaneously fit the RV and photometric observations to their model, which included two planets and the aforementioned noise parameters. By letting the parameters (mass, orbital phase, eccentricity, amplitude of each noise mode, stellar rotation period, even number of planets!) vary, the authors were able to find which model creates the most appropriate fit and how well we can measure each of these parameters.The authors try to fit several models to the data. They fit a model which includes just stellar activity, one with 1 planet, one with 2 planets, and one with 3 planets. They find the 0 and 1 planet models can not explain the data well, and that the 2 planet model is favored by a factor of 10 over the 3 planet model. Thus, with 90% confidence the authors conclude the 2 planet model (shown below) is the most appropriate model, suggesting planet d may not exist.
Since they have very similar effects on our observations, starspots can easily masquerade as small planets. To differentiate between the two, our best hope is to model the noise in our observations simultaneously with the planet parameters. This hasn’t always been done, but the recent controversies over a few supposed planets may encourage astronomers to carefully model their noise before announcing a new planet. Now that Haywood et al. are on the case, we might hear about more planets in the future that, like Alderaan and Krypton, are relegated to ex-planet status.