Kepler-1648 / KIC 7917485 - An msini=12 MJ companion near the habitable zone of an A-type star

A planet in an 840-d orbit around a Kepler main-sequence A star found from phase modulation of its pulsations
http://arxiv.org/abs/1608.02945

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We have detected a 12 MJup planet orbiting in or near the habitable zone of a main-sequence A star via the pulsational phase shifts induced by orbital motion. The planet has an orbital period of 840±20 d and an eccentricity of 0.15. All known planets orbiting main-sequence A stars have been found via the transit method or by direct imaging. The absence of astrometric or radial-velocity detections of planets around these hosts makes ours the first discovery using the orbital motion. It is also the first A star known to host a planet within 1σ of the habitable zone. We find evidence for planets in a large fraction of the parameter space where we are able to detect them. This supports the idea that A stars harbor high-mass planets in wide orbits.

There has been talk of using frequency modulation in Delta Scuti variables as a planet detection method for some time, but even with Kepler the required level of precision is only barely reached. This is a fairly important first.

Some further results are also indicated:

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KIC 7917485b is the least massive companion that we have found in Kepler data with the PM method (Murphy et al. 2016, in preparation).  We have also found two other stars with time delay variations consistent with planetary companions having periods longer than the 4-yr data set (KIC 9700322, KIC 8453431), but the finite duration of Kepler time-series [sic] does not allow the orbits to be fully parametrized.  These detections allow us to comment on the planet occurrence around A stars.

The detectability of low-mass companions is very sensitive to the noise in the Fourier transform of the time delays, which is determined by the pulsation properties (Murphy et al. 2016). We quantified this noise level for 2040 pulsating single A stars and found that only five of them had lower noise levels than KIC 7917485. Against the same sample, KIC 9700322 and KIC 8453431 ranked as the 9th lowest and 2nd lowest, respectively. In other words, we have been able to detect a planetary-mass companion in one of the nine stars with the lowest noise levels, and two others show variations that are consistent with planetary-mass companions.

With further information, it will be possible to properly compare the planet distributions of A-dwarfs and their evolved counterparts for the first time. We shall see if the apparent planet population observed around giant stars is mirrored by higher-mass dwarfs.

The phase modulation method is most comparable to astrometry, as it deals with displacement rather than velocity (though it is considered in time rather than space). Thus there is reasonable potential for combined detections by phase modulation and astrometry, particularly between Kepler and Gaia. This is also indirect support for the Gaia detection of planets around early-type stars.

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The phase modulation method is most comparable to astrometry, as it deals with displacement rather than velocity

Are you sure? Are we not measuring light travel delay? Displacement is just the integral of velocity. You still have the same sin i degeneracy because the phase modulation's O-C depends only on the distance range of the star. Without astrometry, you don't know how much of the true displacement you're seeing. Maybe I'm misunderstanding this?

The observable in phase modulation is the change in phase (in the general case, timing) of a periodic oscillation mode. The amplitude of the phase shift is equal to the semi-major axis of the primary, in light-units. Thus the physical change being observed is displacement. It is certainly true that displacement is the integral of velocity, but the basic observable here is displacement.

The sin i degeneracy is present because the observational axis is one-dimensional, which is also true for radial velocities. More usual astrometric observations are two-dimensional, which is why positional astrometry allows for the solution of i. I said that this method is most comparable to astrometry on the argument that the defining characteristic of astrometry is that it is measured in displacement, while I take it that your argument is that the characteristic of astrometry is that allows for the solution of i.

(It is worth noting that it is possible to freely convert between phase modulation amplitude and radial velocity amplitude as they both suffer from the sin i degeneracy in the same way, but this cannot be used to solve i)

The system has been assigned the designation Kepler-1648.