Luan Ghezzi
Research
Since the discovery of the first extrasolar planet around a solar-type star in 1995, thousands of other exoplanets were detected, and thousands of additional candidates await further confirmation. Despite these impressive numbers, we were still not able to build a model that fully explains the formation, evolution and architectures of all the discovered systems so far.
One key ingredient to develop such a model involves a little bit of "planetary genetics". When a child is conceived, genetics is able to tell us the probabilities that the individual will have certain features (for example, the color of the eyes) based on the parents’ characteristics. It seems so far that the same holds for exoplanets and their parent stars.
The evolution and fate of stars is determined, on a first approximation, by their masses and chemical compositions. Therefore it is reasonable to wonder if and how these two stellar properties could also influence planetary formation or affect the characteristics of planets. It is exactly here that lies my main research focus.
Observing at Gemini South. December 2006.
In my research, I use of spectra collected at some of the world’s most advanced telescopes, such as the 10-m Keck, 9.2-m HET, 8-m Gemini-South (figure above), and 2.2-m MPG/ESO (silver dome in the header figure). These high-quality observational data allow my collaborators and I to perform detailed determinations of the physical properties and chemical compositions of FGK stars with and without planets. Careful statistical analyzes of the resulting parameters provide us some insights into the possible connections between stars and planets. Some of them are highlighted below. For my complete research, please check the publications page.
Is There a Planet-Mass Correlation?
The figure from Ghezzi & Johnson (2015) shows the comparison between model-independent masses (collected from the literature for binary systems or isolated asteroseismic targets) and the values determined using grids of evolutionary tracks for a sample of 59 benchmark evolved stars. The overall good agreement confirms that model-dependent masses are not significantly affected by systematic errors that would end up overestimating them. Stellar evolution models are thus capable of providing accurate masses for isolated stars that already evolved past the main sequence. This result provides support for the studies that found evidence that the probability of a star hosting giant planets increases with stellar mass. More details can also be found here.
Can Refractory Elements Reveal the Formation of Terrestrial Planets?
The figure from Schuler et al. (2015) shows the abundances of refractory elements as a function of condensation temperatures for 5 main-sequence stars with small planets discovered by the Kepler space mission. The chemical signature that could be caused by the formation of terrestrial planets (i.e., depletion of refractory elements) was not observed in any of the cases although calculations based on models have shown that the analysis was precise enough to detect it. This is one of the first few direct tests of this controversial hypothesis.
Does Lithium Indicate the Presence of Giant Planets?
The figure from Ghezzi et al. (2010b) is one of the comparisons indicating no differences between stars with and without planets (red and blue symbols, respectively; open circles are precise measurements and filled inverse triangles are upper limits; the black star represents the Sun) when objects with very similar properties are compared. This result does not support the idea that the presence of giant planets could be connected to an enhanced Li depletion on the host stars.
Can Larger Stellar Masses Compensate Lower Metallicites?
The figures from Ghezzi et al. (2010c) show that the metallicity distributions of dwarfs (solid black line on both panels) and subgiants (red dotted line on the lower panel) are similar. This is evidence that pollution of the stellar atmospheres did not occur because the accreted material would be diluted when the stars evolve off the main-sequence onto the subgiant branch. Giants stars (red dotted line on the upper panel), on the other hand, are more metal-poor than both other samples, which suggests that their larger masses could compensate the lower metallicities and provide the necessary amount of metals to form planets.
Does Planetary Mass Depend on Stellar Metallicity?
The figure from Ghezzi et al. (2010a) reveals a visible displacement between the metallicity distributions of stars with Jupiter-like (black solid line) and Neptune-like or smaller (red dashed line) planets. This is observed using only our targets (upper panel) or increasing our relatively small sample with literature data (lower panel). Note also that we did not include any M dwarfs in our analysis. This result suggests a possible link between stellar metallicity and the mass of the most massive planet in the system, since stars with Neptune-like planets or smaller are more metal-poor, on average, than those that host Jupiter-like planets.