HAT-P-32b

HAT-P-32b is a planet in the orbit of the G-type or F-type star HAT-P-32, which is approximately 860 light years^[1] away from Earth. HAT-P-32b was first recognized as a possible planet by the planet-searching HATNet Project in 2004, although difficulties in measuring its radial velocity prevented astronomers from verifying the planet until after three years of observation. The Blendanal program helped to rule out most of the alternatives that could explain what HAT-P-32b was, leading astronomers to determine that HAT-P-32b was most likely a planet. The discovery of HAT-P-32b and of HAT-P-33b was submitted to a journal on June 6, 2011.

The planet is considered a Hot Jupiter, and although it is slightly less massive than Jupiter, it is bloated to nearly twice Jupiter's size. At the time of its discovery, HAT-P-32b had one of the largest radii known amongst extrasolar planets. This phenomenon, which has also been observed in planets like WASP-17b and HAT-P-33b, has shown that something more than temperature is influencing why these planets become so large.^[6]

Discovery

It had been suggested that a planet was in the orbit of the star HAT-P-32 as early as 2004; these observations were collected by the six-telescope HATNet Project, an organization in search of transiting planets, or planets that cross in front of their host stars as seen from Earth. However, attempts to confirm the planetary candidate were extremely difficult because of a high level of jitter (a random, shaky deviation in the measurements of HAT-P-32's radial velocity) present in the star's observations. High-level jitter prevented the most common technique, that of bisector analysis, from revealing the star's radial velocity with enough certainty to confirm the planet's existence.^[6]

The spectrum of HAT-P-32 was collected using the digital speedometer on Arizona's Fred Lawrence Whipple Observatory (FLWO). Analysis of the data found that HAT-P-32 was a single, moderately rotating dwarf star. Some of its parameters were also derived, including its effective temperature and surface gravity.^[6]

Between August 2007 and December 2010, twenty-eight spectra were collected using the High Resolution Echelle Spectrometer (HIRES) at the W.M. Keck Observatory in Hawaii. Twenty-five of these spectra were used to deduce HAT-P-32's radial velocity. To compensate for jitter, a greater number of spectra than that the usual for planetary candidates was collected. From this, it was concluded that stellar activity (and not the presence of yet-undiscovered planets) was the cause of the jitter.^[6]

Because astronomers concluded that the use of radial velocity could not, alone, establish the existence of planet HAT-P-32b, the KeplerCam CCD instrument on FLWO's 1.2m telescope was used to take photometric observations of HAT-P-32. The data collected using KeplerCam CCD helped astronomers constructed HAT-P-32's light curve. The light curve displayed a slight dimming at a point where HAT-P-32b was believed to transit its star.^[6]

The astronomers utilized Blendanal, a program used to eliminate the possibilities of false positives. This process serves a similar purpose to the Blender technique, which was used to verify some planets discovered by the Kepler spacecraft. In doing so, HAT-P-32's planet-like signature was found to not be caused by either a hierarchical triple star system or by a mixture of light between a bright single star and that of a binary star in the background. Although the possibility that HAT-P-32 is actually a binary star with a dim secondary companion nearly indistinguishable from the primary companion could not be ruled out, HAT-P-32b was confirmed as a planet based on the Blendanal analysis.^[6]

HAT-P-32b had one of the highest radii known amongst planets at the time of its discovery. Like planets HAT-P-33b and WASP-17b, which are similarly inflated, the mechanism behind this is unknown; it is not solely related to temperature, which is known to have an effect. This is especially clear when compared to WASP-18b, a planet that is hotter than the aforementioned HAT and WASP planets, because despite its temperature its radius is far lower than that of its counterparts.^[6]

Because of the high jitter of the star, the best way to collect more data on HAT-P-32b would be to observe an occultation of HAT-P-32b behind its star using the Spitzer Space Telescope.^[6]

HAT-P-32b's discovery was reported with that of HAT-P-33b in the Astrophysical Journal.^[6]

Host star

HAT-P-32, or GSC 3281-00800, is a G-type or F-type dwarf star^[6] located some 260 parsecs (860 light years) away from Earth.^[1] With 1.176 solar masses and 1.387 solar radii, HAT-P-32 is both larger and more massive than the Sun. HAT-P-32's effective temperature is 6,001 K, making it slightly hotter than the Sun, although it is younger, at an estimated age of 3.8 billion years, not long after the Archean eon started 4.031 ± 0.003 billion years ago.^[3] HAT-P-32 is metal-poor; its measured metallicity is [Fe/H] = -0.16, which means that it has 69% the iron content of the Sun.^[3] The star's surface gravity is determined to be 4.22, while its luminosity suggests that it emits 2.43 times the amount of energy that the Sun emits.^[6] These parameters are adopted given the condition that the planet HAT-P-32b has an irregular (eccentric) orbit.^[6]

HAT-P-32 has an apparent magnitude of 11.197, which makes it invisible to the naked eye.^[2] A search for a binary companion star using adaptive optics at the MMT Observatory discovered a companion at a distance of 2.9 arcseconds that is 3.4 magnitudes dimmer than the primary star.^[7]

A very high level of jitter has been detected in the star's spectrum. Because this jitter disrupts the ability to determine radial velocity measurements with high-precision accuracy, there is a possibility that HAT-P-32 is actually a binary star system, where the brighter companion visually masks the presence of a secondary, dimmer companion. If this is the case, then HAT-P-32's dimmer constituent probably has a mass that is under half of the Sun's mass.^[6]

Other planets with orbital periods that are smaller than that of HAT-P-32b's orbit may be present in this system. However, when the discovery of the planet was published, not enough radial velocity measurements had been collected to determine if this was the case.^[6]

Characteristics

Size comparison

Jupiter	HAT-P-32b

HAT-P-32b is a Hot Jupiter that has 0.941 Jupiter masses and 2.037 Jupiter radii. In other words, HAT-P-32b is slightly less massive than Jupiter is, although it is nearly twice Jupiter's size.^[3] The planet's average distance from its host star is 0.0344 AU, or approximately 3% of the mean distance between the Earth and the Sun. It completes an orbit every 2.150009 days (51.6 hours).^[3] HAT-P-32b has an equilibrium temperature of 1888 K,^[6] which is fifteen times hotter than Jupiter's equilibrium temperature.^[8]

Many of the described characteristics are derived on the assumption that HAT-P-32b has an orbit that is elliptical (eccentric). The best fit for HAT-P-32b's orbital eccentricity is 0.163, denoting a slightly elliptical orbit, although the jitter effect observed in its host star has made the planet's eccentricity difficult to accurately find. The discoverers have also derived the planet's characteristics assuming that the planet has a circular orbit, although they have given preference to the elliptical model.^[6]

Because of HAT-P-32b's orbital inclination with respect to Earth is 88.7º, the planet is seen almost edge-on with respect to Earth.^[3] It has been found to transit its host star.^[6]

References