Review: Planet Occurrence within 0.25 AU of Solar-Type Stars from Kepler

For the second Astronomy Twitter Journal Club meeting we discussed a paper from the Kepler team: Planet Occurrence within 0.25 AU of Solar-Type Stars from Kepler (Howard et al. 2011). Technical problems with twitter took up most of the first half hour of the meeting, so the discussion was a little shorter than it should have been, but it still raised some interesting points.

The paper uses the new data from the Kepler satellite to investigate

“…the distribution of planets as a function of planet radius, orbital period and stellar effective temperature for orbital periods less than 50 days around Solar-type stars”.

After carefully accounting for incompletenesses in their sample, the authors find a larger number of smaller planets with these close-in orbits. This result disagrees with simulations of planet formation which predict a ‘desert’ of planets of this size in this region. They also see fewer hot Jupiters in their sample than previous surveys which used the radial velocity technique. For a fuller summary of the paper, see this post over at the astrobites blog.

Hot Jupiter fraction mismatch

The first major point that came up for discussion was the discrepancy in the number of hot Jupiters between Kepler and the radial velocity (RV) surveys:

so I think 1 of the interesting things is that the hot jupiter fractions don’t match between the RV estimates and Kepler – @megschwamb

occurrence of hot Jupiters in the Kepler field is only 40% that in the Solar neighborhood (ie corrected RV values ) – @megschwamb

One solution put forward to explain this is that the two methods probe different regions of the Galaxy, containing different populations of stars. However, this solution is unsatisfactory:

no they can’t. That would imply large abundance variations over small scales in galactic terms – @Matt_Burleigh

As is often the case in astronomy, this is something that is likely to be answered with new instruments:

I’m interested to see what Corot sees – because they observe more fields at different pointings – @megschwamb

so if it’s a Kepler field effect we should see the Corot obs agree with the RV numbers – I think? – @megschwamb

The tip of the iceberg?

The discussion then moved onto how planets migrate, and what we think happened in our own Solar System (SS):

we’ve had planetesimal driven migration in the SS – (aka Nice model) giant planets were in a compact configuration and then Jupiter and Saturn cross a resonance pump up eccentricities and start scattering planetesimals driving all the giant planets to move – jupiter moves inward everybody moves outward Neptune moved as much as ~10 AU – @megschwamb

The Kepler observations haven’t been going on long enough to detect even closer in planets so it’s possible there are many more planets still to find:

I think that’s the question we all want to answer but it will take 3 years to really start to believe 1 AU planet transits – @megschwamb

The high precision of the data may even be good enough to detect something Earth sized at an Earth distance.

Issues with the sample selection

One of the potential problems with the results presented in the paper is that planets could have been missed from the sample, since the efficiency at which Kepler could detect this sort of planet comes from assuming a high signal to noise (SNR) threshold:

there are no efficiency estimates currently from injecting simulating transits into the kepler lightcurves and seeing what fraction are detected – this paper assumes a SNR 10 threshold and calculates the detection efficiency from that – @megschwamb

The problem with this assumption is that any variability in the stars being studied could affect how many planets could be detected around them. However the Planet Hunters project, where the public can search for planets using the Kepler lightcurves, could help with understanding any biases that might be present:

…@planethunters can give an independent estimate of the planet/radii frequency in the Kepler field – @megschwamb

Exactly — so if it’s something in the Kepler pipeline, @planethunters may find it? – @vrooje


The paper was well received overall, and I think we’re all looking forward to more exciting Kepler results in the near future:

I think this paper is an impressive demonstration of Kepler’s potential to really nail down the stats in this field – @astronomyjc

and we’ve just scratched the surface – this is the first 4 months of data – @megschwamb

I feel like I learned a lot; this was a good paper. Still have questions about metallicity/formation models. – @vrooje

I agree that this is an exciting starter. I’ll be good to see the diversity of papers that come from these data – @brucesibthorpe

I’ll end here with the final sentence from the paper:

“Clearly an extended mission of an additional ~3 yr is needed to bring planets of 1 Earth-radius to SNR > 7.”

Which, as @vrooje pointed out,

…echoes Debra Fischer at #aas218 : the point isn’t planets, but life.

Full transcript of meeting

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