Revisiting the Long-period Transiting Planets from Kepler

Abstract

Currently, we have only limited means to probe the presence of planets at large orbital separations. Foreman-Mackey et al. searched for long-period transiting planets in the Kepler light curves using an automated pipeline. Here, we apply their pipeline, with minor modifications, to a larger sample and use updated stellar parameters from Gaia DR2. The latter boosts the stellar radii for most of the planet candidates found by FM16, invalidating a number of them as false positives. We identify 15 candidates, including two new ones. All have sizes from 0.3 to 1 R J , and all but two have periods from 2 to 10 yr. We report two main findings based on this sample. First, the planet occurrence rate for the above size and period ranges is 0.70 +0.40 −0.20 planets per Sun-like star, with the frequency of cold Jupiters agreeing with that from radial-velocity surveys. Planet occurrence rises with decreasing planet size, roughly describable as dN/d log R ∝ R α with α = −1.6 +1.0 −0.9 , i.e., Neptune-sized planets are some four times more common than Jupiter-sized ones. Second, five out of our 15 candidates orbit stars with known transiting planets at shorter periods, including one with five inner planets. We interpret this high incidence rate to mean: (1) almost all our candidates should be genuine; (2) across a large orbital range (from ∼ 0.05 to a few astronomical units), mutual inclinations in these systems are at most a few degrees; and (3) large outer planets exist almost exclusively in systems with small inner planets.

Kojima-1Lb Is a Mildly Cold Neptune around the Brightest Microlensing Host Star

“…Therefore, although it is too early to argue statistically, the discovery of this low detection efficiency planet may imply that Neptunes are common rather than rare in this orbital region. This result is consistent with the recent findings with transit and radial velocity techniques that Neptunes are at least as common as (Kawahara & Masuda 2019) or more common than (Herman et al 2019;Tuomi et al 2019) Jupiters at large orbits comparable to the snow line. Kennedy & Kenyon (2008).…”

Mutual Orbital Inclinations between Cold Jupiters and Inner Super-Earths

“…Their sample includes 13 stars with long-period giant planets, of which five also have inner transiting systems consisting of SEs. Herman et al (2019) pointed out that this relatively high occurrence of transiting SEs around stars with transiting CJs is consistent with a picture in which almost all long-period giant planets are associated with inner compact systems of smaller planets, with a typical mutual inclination of 4 • between the inner and outer systems. Our analysis led to the inference of a somewhat higher mutual inclination of 11.8 +12.7 −5.5 degrees.…”

“…These two effects cancel out as long as the transit depth and stellar flux are measured in the same bandpass (which is true of Kepler and Gaia), and the measurement of the effective temperature is not significantly biased by the presence of the unresolved companion (which is true when the effective temperature is based on spectroscopy, or when the stars are nearly twins). Herman et al (2019) recently presented the results of an independent effort to determine mutual inclinations between the inner and outer parts of planetary systems. While our approach is based on counting transiting outer planets around stars with transiting inner planets, they took the opposite approach: counting the cases of transiting inner planets around stars with transiting outer planets.…”

“…Bearing all these difficulties in mind, we nevertheless tried applying our methodology to the sample of stars with transiting cold giants, adopting the assumption in Herman et al (2019) that all the "cold giants" have inner SEs. We assigned one inner SE to each star with a period drawn randomly from the broken powerlaw distribution inferred for the entire Kepler SEs (Petigura et al 2018).…”

On the Patterns Observed in Kepler Multi-planet Systems

“…The nearly flat period ratio distribution, arisen from either in situ formation (Petrovich et al 2013;Wu et al 2019) or breaking the chain of resonances after migration (Izidoro et al 2017), also points to a stage of dynamical instability. Finally, Kepler planets are shown to be strongly correlated with outer giant planets Bryan et al 2019;Herman et al 2019). The planet-planet scatterings that are respon- Distance to commensurability (×10 2 ) 1 R 2 R 4 R 8 R Figure 10.…”