You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
the kepler space telescope is now the most successful planet - finding mission to date .
most of its thousands of planets ( and planet candidates ) have been found around cool stars @xcite , while exoplanets orbiting a stars remain elusive .
after the announcement of an additional 1284 _ kepler _ exoplanets in 2016 may @xcite , still fewer than 20 a stars have confirmed planets .
radial velocity observations of cool sub - giants , most notably via the ` retired a stars ' project @xcite , suggest that planet occurrence rates reach a maximum for stars of @xmath3m@xmath4 @xcite .
this coincides remarkably with the masses of main - sequence a stars in the classical instability strip , where delta scuti ( @xmath5sct ) pulsators are common .
the apparent planet deficit around these main - sequence stars can be explained as an observational selection effect , caused by problems in the application of the most successful planet - hunting methods to these types of stars .
the transit method has difficulty because of the pulsational luminosity variations , which amount to several mmag , and because planets occupy wider orbits around a stars ( @xcite , cf .
@xcite ) , resulting in a lower transit probability . the radial velocity method , on the other hand
, is particularly hindered by the nature of a - type spectra .
a stars are typically fast rotators , with the mean of the equatorial rotational velocity distribution exceeding 100kms@xmath6 @xcite .
their high effective temperatures lead to fewer , shallower absorption lines , and these lines can be distorted by pulsation .
therefore the wavelengths of their spectral lines are not a precise standard of measure , and the state of the art is limited to 12kms@xmath6 precision @xcite .
fortunately , the same pulsations that limit rv and transit surveys of a stars can themselves be used as precise clocks for the detection of orbital motion @xcite .
the pulsations of @xmath5sct stars are particularly well suited to this task @xcite .
no planets have yet been discovered orbiting main - sequence a stars via the motion of their host stars , namely , via the rv , astrometric , or pulsation - timing methods .
discoveries have been made using two other methods : transits and direct imaging .
the former category includes _ kepler _ planet hosts ( kepler-13 , 462 , 516 , 959 , 1115 , 1171 and 1517 ; @xcite ) , plus wasp-33 @xcite , hat - p-57 @xcite and kelt-17 @xcite .
the orbital periods of these systems are all under 30d , except for kepler-462b whose period is 85d .
these planets are strongly irradiated and would have high surface temperatures .
the imaged planets in the nasa exoplanet archive have very wide orbits .
these include three objects ( hd100546b , hip78530b and @xmath7andb ) with masses above the canonical 13-m@xmath0 boundary between planets and low - mass brown dwarfs , which orbit b - type stars .
exoplanets orbiting a stars include fomalhautb @xcite , @xmath8picb @xcite , hd95086b @xcite , the multi - planet system hr8799b , c , d , e @xcite , and the planet in the triple system hd131399ab @xcite , all of which have orbits ranging from 9 to 177 au .    finally , there are some planets orbiting evolved hot stars , such as those discovered from eclipse timing variations of the white - dwarf binary nnser @xcite , the companion to the sdb star v0391peg from pulsation timing variations @xcite , and companions to other sdb stars kic10001893 @xcite and the now - doubtful case of kic5807616 ( @xcite , cf .
@xcite ) from orbital brightness modulations .    in this paper
we present the first planet discovered around a main - sequence a star by the motion of the host star , and the first to be in or near the habitable zone .
we describe the observations and our method in sect.[sec : obs ] .
the planet s habitability and the implications for planet occurrence posed by this discovery are discussed in sect.[sec : habitability ] .
the host star , kic7917485 , was observed for the full four years of the _ kepler _ mission in long - cadence mode ( 30-min sampling ) .
our analysis used the multi - scale map data reduction of this data set @xcite .
the time series spans 1461d with a duty cycle of 91percent .
this @xmath9mag star was observed spectroscopically with lamost @xcite .
the stellar atmospheric parameters were extracted by @xcite , showing kic7917485 to be a main - sequence a star , in agreement with the photometric characterization by @xcite . its position on a @xmath10@xmath11 diagram is shown in fig.[fig : tlogg ] , and atmospheric parameters are given in table[tab : atmos ] .
the star is located in the @xmath5sct instability strip and has a mass of approximately 1.63m@xmath4 .
@xmath11 diagram , according to lamost spectroscopy ( filled circle ) and revised kic photometry ( open square ) .
the solid and dashed blue lines delineate the @xmath5sct and @xmath12dor instability strips , respectively .
evolutionary tracks of solar metallicity and mixing length @xmath13 are also shown @xcite , with corresponding masses in m@xmath4 written where the tracks meet the zero - age main - sequence ( black line ) .
the peak of the planet occurrence rate distribution , at @xmath3m@xmath4 @xcite , crosses the center of the @xmath5sct instability strip.,scaledwidth=49.0% ]    .global atmospheric parameters from lamost spectroscopy @xcite and revised kic photometry @xcite .
we evaluated these quantities against evolutionary tracks to calculate a mass and luminosity , which are given below the mid - table break . [ cols="^,^,>,<,^",options="header " , ]     we detected the planet by phase modulation ( pm ) of the stellar pulsations @xcite .
a fourier transform of the light curve ( fig.[fig : ft]a ) is dominated by two oscillation modes at frequencies @xmath14 and @xmath15d@xmath6 , where the uncertainty on the final digit has been given in parentheses .
these two modes were used in the pm analysis .
other significant peaks have amplitudes that are at least an order of magnitude lower .
their signal - to - noise ratios are too low to add usefully to the pm analysis , but their presence adds unwanted variance to the data .
we therefore subtracted all peaks above 50@xmath16mag from the data , except for @xmath17 and @xmath18 , by fitting their frequencies to the light curve with a nonlinear least - squares algorithm .
we also high - pass - filtered the light curve to remove any remaining instrumental signal and low - frequency oscillations , preserving all content at frequencies above 5d@xmath6 .
the fourier transform after the additional processing is shown in fig.[fig : ft]c .    mag level .
the horizontal axis is the same in each panel.,title="fig:",scaledwidth=49.0% ] mag level .
the horizontal axis is the same in each panel.,title="fig:",scaledwidth=49.0% ] mag level .
the horizontal axis is the same in each panel.,title="fig:",scaledwidth=49.0% ]    we divided the light curve into 10-d segments to look for shifts in the phases of @xmath17 and @xmath18 .
these phases were converted into delays in the light arrival time ( ` time delays ' ) following the method of @xcite .
time delays of @xmath17 and @xmath18 show identical periodic variation ( fig.[fig : tds ] ) , which we attribute to a sub - stellar companion .
values for the orbital parameters were initially obtained using formulae from @xcite , and then refined with an mcmc algorithm .
the mcmc analysis used a metropolis 
hastings algorithm @xcite with symmetric proposal distributions based on gaussian - distributed random numbers .
trial runs were made to determine appropriate step sizes in each of the five orbital parameters fitted : the orbital period , @xmath19 , projected light travel time across the orbit , @xmath20 , eccentricity , @xmath21 , the phase of periastron passage calculated relative to the first time delay observation , @xmath22 , and the angle of the ascending node , @xmath23 .
, rather than @xmath24 , because of the common use of @xmath24 to represent angular oscillation frequencies in asteroseismology .
] we fitted a linear term to the time delays as an additional parameter to correct for slight inaccuracies in the oscillation frequencies .
proposed steps in all six parameters were made simultaneously .
a total of 200000 steps were made , with a final acceptance rate of 0.20 .
for further details on the methodology , including its verification by radial velocities , see @xcite .
( red squares ) and @xmath18 ( blue circles ) .
error bars are the formal least - squares uncertainties .
weighted averages of the two measurements in each 10-d segment are indicated as black diamonds.,scaledwidth=49.0% ]    table  [ tab : orbit ] gives the best - fitting orbital parameters , which are compared with the observations in fig.[fig : tdmcmc ] .
c c c & units & + @xmath19 & d & @xmath25 + @xmath21 & & @xmath26 + @xmath22 & @xmath2701@xmath28 $ ] & @xmath29 + @xmath30 & m@xmath4 & @xmath31 +   + @xmath32 & m@xmath4 & @xmath33 + @xmath20 & s & @xmath34 + @xmath23 & rad & @xmath35 + @xmath36 & ms@xmath6 & @xmath37 +   + @xmath38 & m@xmath4 & @xmath39 + & m@xmath0 & @xmath40 + @xmath41 & s & @xmath42 + @xmath43 & au & @xmath44 + @xmath23 & rad & @xmath45 +    ) .,scaledwidth=49.0% ]
the minimum mass of the sub - stellar companion to kic7917485 ( 11.8m@xmath0 ) is close to the planet  brown - dwarf mass boundary .
although such a body would not be expected to have a solid surface , it could presumably host smaller exomoons .
it is therefore interesting to determine whether the planet lies within the habitable zone .
the habitable zone is broader for a stars than for cooler stars because the inner edge extends to higher incident surface fluxes for hotter stars @xcite .
their calculations indicate that , at the temperature of kic7917485 , the inner edge lies between @xmath461.2 and 1.4 times the surface flux at earth , depending on the mass of the potentially habitable body .
the mean projected separation between the components , @xmath47 , is 2.05au . at this separation ,
the luminous a star irradiates the companion to a surface flux ratio , @xmath48 , of 2.36 times the flux at earth . statistical correction for random inclination gives @xmath49 as the most probable value .
the luminosity of the a star is not well constrained , and is a strong function of main - sequence age . at the @xmath50 lower luminosity limit ( see table[tab : atmos ] ) , those values of @xmath48 reduce to 1.72 and 1.27 , respectively .
thus the position of kic7917485b is consistent with the habitable zone at the @xmath50 level .
the companion would have been closer to the center of the habitable zone earlier in the star s lifetime .
the luminosity will be refined substantially when the _ gaia _ mission provides a distance measurement , and the question of habitability can be reassessed .
kic7917485b is the least massive companion that we have found in _
data with the pm method ( 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 ( kic9700322 , kic8453431 ) , but the finite duration of _ kepler _ time - series 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 @xcite .
we quantified this noise level for 2040 pulsating single a stars and found that only five of them had lower noise levels than kic7917485 . against the same sample , kic9700322 and kic8453431 ranked as the 9@xmath51 lowest and 2@xmath52 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 .
this fact is in strong support of existing observations that intermediate - mass stars ( ` retired a stars ' ) tend to host high - mass planets in wide orbits @xcite .
we have found a planetary companion of near the inner edge of the habitable zone of kic7917485 .
this is the first planet orbiting a main - sequence a star to be discovered via the motion of its host star .
other planets orbiting a stars have been discovered via the transit method and have short periods , or been discovered by direct imaging and have very long periods .
our finding is particularly significant because no other method is presently capable of detecting non - transiting planets around these stars with periods of a few years , i.e. , near their habitable zones .
kic7917485 has particularly low noise levels in its light arrival time delays .
we analyzed other stars with similarly low noise and for two of them we also found evidence for planetary - mass companions with periods similar to the 4-yr time span of _ kepler _ data .
this fact strongly supports the idea that intermediate - mass stars tend to host high - mass planets in wide orbits .
we have detected a 12m@xmath0 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 @xmath1d 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 1@xmath2 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 .