The paper selected for discussion at the second meeting of the astronomy twitter journal club (8pm UK time, 7pm UT, Thursday 23rd June) is Planet Occurrence within 0.25 AU of Solar-Type Stars from Kepler (Howard et al. 2011). As the title suggests, it exploits the large amounts of data collected by the Kepler satellite to investigate statistically how likely it is for a (solar-type) star to have a planet orbiting close to it (within 0.25 AU). I was planning to write a summary of this paper, as I did last week, but while researching it this evening I found that the excellent astrobites blog had beaten me to it.
The main criticism of the first meeting was that the discussion needed to be more guided. With that in mind, here are what I think are the key points/questions that arise in this paper.
- Are we convinced by the treatment of the potential sources of bias in the planetary sample, as well as the justification for assuming that all possible planets were detected in the Kepler pipeline?
- Is it surprising that the ‘planet desert’ turns out to be full of planets i.e. was this a known limitation of the simulations?
- The period dependence of planet occurrence is in contradiction with the previous radial velocities surveys; does the explanation that this is a result of Kepler’s increased sensitivity to smaller planets make sense?
- Is the metallicity bias a reasonable explanation for the observation that cooler, smaller stars have a higher occurrence of close-in planets? Is the result itself convincing?
This list isn’t meant to be exhaustive, but it should hopefully help to focus the debate. If there’s something that I’ve missed please add it in the comments, and don’t forget to come to the discussion itself on Thursday!
Here’s some more discussion points from Meg Schwamb the proposer of this week’s paper:
1. What are the biases of only using SNR=10 selection for Kepler stars included in the sample ?
2. What are interpretations that the “occurrence of hot Jupiters in the Kepler field is only 40% that in the Solar neighborhood?” Kepler results show a pile-up of hot Jupiters with three day periods but not much more significant than for other orbital periods – for radial velocity surveys this pile-up is a factor of three above the background. Can COROT help answer that?
3. Observations and analysis appear to confirm the Howard et al (2010) radial velocity results that the abundance of short period planets (periods less than 50 days) increases with decreasing planet radius
4. The lack of close in Neptune-mass planets seen in the radial velocity measurements does not appear to be as visible in the Kepler results – especially Mpsini=100 earth radii – what does that mean?
5. There appears to be a increase in planet occurrence with smaller effective temperature (ie smaller mass stars)? What does that say about accretion models and conditions for terrestrial planet formation?
6. The “planet desert” – is this a limitation of accretion models or we beginning to detect other effects that shape the evolution of planetary systems and modify the orbital distribution of planets once formed (ie planet-planet scattering, planetesimal driven migration)