HAT-P-32b

Lore & Background

The planet HAT-P-32b was first suggested as a candidate in 2004 by the HATNet Project, which uses six telescopes to search for transiting planets. However, a high level of jitter—random, shaky deviations in radial velocity measurements—made confirmation difficult. Bisector analysis, the common technique for such confirmation, could not yield certainty. Between August 2007 and December 2010, twenty-eight spectra were collected using the High Resolution Echelle Spectrometer at the W.M. Keck Observatory; twenty-five were used to deduce radial velocity. Stellar activity, not undiscovered planets, was identified as the cause of the jitter. Photometric observations with the KeplerCam CCD on the Fred Lawrence Whipple Observatory's 1.2m telescope produced a light curve showing a dimming at the expected transit time. The Blendanal program ruled out false positives such as a hierarchical triple star system or blended light from a binary background star, though the possibility of a dim binary companion could not be entirely excluded. The discovery was reported alongside that of HAT-P-33b in the Astrophysical Journal.

The host star HAT-P-32 is a double star: the primary is a G-type or F-type dwarf, and the secondary is an M-type dwarf. The primary is larger and more massive than the Sun, slightly hotter, younger, and metal-poor with an iron content 69% that of the Sun. A companion was found at 2.9 arcseconds using adaptive optics at the MMT Observatory. The planet's orbit is nearly polar relative to the star's rotation, with a misalignment of 85°, as determined by the Rossiter–McLaughlin effect in 2012.

HAT-P-32b is a hot Jupiter with 0.68 Jupiter masses and 1.776 Jupiter radii, completing an orbit every 2.15000820 days at a distance of 0.0340 AU. Its effective temperature is 1962 K. The planet's radius varies with wavelength, suggesting an atmosphere with a Rayleigh scattering haze above a thick cloud deck (clouds up to 0.4–33 kPa). Water was detected in the atmosphere in 2020. The planet shows evidence of Roche lobe overflow and rapid mass loss of 13 million tons per second.

Reader's Guide

HAT-P-32b is significant primarily for its extreme radius, which at discovery was among the largest known for any exoplanet. Its bloated nature, despite being less massive than Jupiter, exemplifies a class of inflated hot Jupiters whose expansion mechanism remains poorly understood. The planet's radius cannot be explained by temperature alone, as demonstrated by comparison with the hotter but smaller WASP-18b. This puzzle has driven studies of atmospheric structure and dynamics. The detection of water and a layered cloud deck in 2020, along with wavelength-dependent radius variations, provides constraints on atmospheric models. The planet's near-polar orbit (85° misalignment) adds to the diversity of orbital architectures. The high jitter in the host star's radial velocity, ultimately attributed to stellar activity, illustrates the challenges of confirming planets around active stars. The use of Blendanal to rule out false positives parallels techniques used for Kepler discoveries. The ongoing mass loss (13 million tons per second) and Roche lobe overflow make HAT-P-32b a valuable laboratory for studying planetary evaporation and atmospheric escape.

Did You Know?

• HAT-P-32b has one of the largest radii known among exoplanets, nearly twice Jupiter's size despite being less massive.
• The planet's discovery was delayed by three years due to high radial velocity jitter in its host star.
• HAT-P-32b orbits its star in a nearly polar orbit, misaligned by 85° relative to the star's rotation.
• The planet loses mass at a rate of 13 million tons per second due to Roche lobe overflow.

The Long Road to Confirmation

In 2004, the six-telescope HATNet Project, a dedicated search for transiting planets, first flagged a candidate world orbiting the star HAT-P-32. What followed was not a quick confirmation but a grueling multi-year struggle. The star's spectrum exhibited a high level of jitter—random, shaky deviations in radial velocity measurements—that defeated the standard bisector analysis technique astronomers rely on to verify planetary signals. To wrestle a reliable signal from the noise, researchers turned to the digital speedometer at Arizona's Fred Lawrence Whipple Observatory and later the High Resolution Echelle Spectrometer at the W.M. Keck Observatory in Hawaii. Between August 2007 and December 2010, twenty-eight spectra were gathered, twenty-five of which were used to extract radial velocity data. This was a far larger sample than typical for a planetary candidate, a deliberate strategy to average out the jitter. The team ultimately attributed the instability to stellar activity rather than to hidden planets. The discovery, paired with that of HAT-P-33b, was formally submitted on 6 June 2011 and published in the Astrophysical Journal.

A Young, Metal-Poor Star and Its Faint Companion

HAT-P-32, catalogued as GSC 3281–00800, sits roughly 950 light years from Earth and is in fact a binary system. The primary is a G-type or F-type dwarf that outweighs the Sun by about 26 percent and stretches to 1.23 solar radii. Its effective temperature of roughly 6,407 kelvin makes it a touch hotter than our own star, while its luminosity—about 2.27 times solar—reflects that extra energy output. The star is notably young at an estimated 2.7 billion years and metal-poor, carrying only 69 percent of the Sun's iron abundance. Its apparent magnitude of 11.197 renders it invisible without optical aid. Adaptive-optics imaging at the MMT Observatory revealed a faint M-type companion 2.9 arcseconds away, 3.4 magnitudes dimmer, with a temperature near 3,565 kelvin and a mass likely below half that of the Sun. That dimmer star may be responsible for the pronounced jitter seen in the primary's spectrum, a complication that complicated the planet-hunting effort.

A Bloated World in a Tilted Orbit

HAT-P-32b is a textbook hot Jupiter in terms of its orbital distance—just 0.0340 AU from its host star, roughly 3.4 percent of the Earth-Sun gap—yet it defies expectations in size. At 0.68 Jupiter masses it is actually lighter than our solar system's giant, but its radius swells to 1.776 times Jupiter's, making it one of the most inflated worlds known at the time of discovery. Its effective temperature sits at about 1,962 kelvin, sixteen times hotter than Jupiter's equilibrium temperature, a consequence of its 2.15-day orbit. The planet's orbital inclination of 89 degrees means it crosses directly in front of its star as seen from Earth, producing the transit signal that first revealed it. A 2012 study employing the Rossiter-McLaughlin effect found the orbit is nearly polar, tilted 85 degrees relative to the star's rotation axis. The extreme inflation, also seen in WASP-17b and HAT-P-33b, suggests that factors beyond simple stellar heating are driving these planets to such extraordinary sizes.

Eliminating the Impostors

Because radial velocity alone could not confirm HAT-P-32b, the team turned to photometry. Using the KeplerCam CCD on the 1.2-meter telescope at FLWO, they built a light curve for HAT-P-32 and identified the subtle dimming consistent with a transiting body. But a transit-like dip can also be produced by unrelated stellar configurations, so the astronomers ran the candidate through Blendanal, a software tool designed to strip away false-positive scenarios. The analysis excluded a hierarchical triple-star system and the possibility that the signal arose from blended light between a bright foreground star and a background binary. One scenario—HAT-P-32 being a binary with a dim secondary nearly indistinguishable from the primary—could not be fully excluded, yet the weight of evidence still favored a genuine planet. Given the persistent jitter, researchers suggested that a Spitzer Space Telescope occultation observation would be the most productive next step for gathering additional data on the planet.