HAT-P-33b

Lore & Background

HAT-P-33b was first suspected as a planetary candidate as early as 2004, based on observations by the HATnet collaboration. However, high levels of stellar jitter—random variations in the star's spectrum—made radial velocity measurements unreliable. Initial follow-ups were postponed for several years. Between September 2008 and December 2010, twenty-two spectra were collected using the HIRES instrument at the W. M. Keck Observatory, but even this larger-than-usual dataset could not confirm the planet via radial velocity alone. The planet was ultimately confirmed after photometric observations with the 1.2-meter telescope at the Fred Lawrence Whipple Observatory produced a light curve showing a transit-like dimming. The Blendanal program, similar to the Blender technique used for Kepler planets, ruled out false positive scenarios such as a hierarchical triple star or a background blend, though the possibility that HAT-P-33 is an unresolved binary star could not be entirely eliminated.

The planet has 0.764 Jupiter masses and 1.827 Jupiter radii, making it less massive but much larger than Jupiter. Its orbit is very close to its star, at an average distance of 0.0503 AU, completed every 3.474474 days. The best-fit orbital eccentricity is 0.148, but this value is difficult to constrain because of the star's jitter. The planet's equilibrium temperature is 1,838 K. Its host star, HAT-P-33, is an F-type dwarf with 1.403 solar masses, 1.777 solar radii, an effective temperature of 6,401 K, and a metallicity 12% higher than the Sun's. The star is younger than the Sun, at an estimated age of 2.4 billion years, and has an apparent magnitude of 11.89, too dim to be seen with the naked eye.

Reader's Guide

HAT-P-33b, along with HAT-P-32b and WASP-17b, raised a key question in exoplanet science: what factors besides temperature cause some hot Jupiters to become so inflated? The puzzle is highlighted by the contrast with WASP-18b, which is far hotter than these planets yet has a much smaller radius. The discovery paper, submitted in June 2011, suggested using the Spitzer Space Telescope to observe the planet's occultation behind its star to better characterize it. The planet's confirmation was unusually difficult because stellar jitter prevented the radial velocity method from providing a clear signal; instead, the Blendanal process was essential to rule out false positives. The discovery also left open the possibility of additional shorter-period planets in the system, though insufficient radial velocity data existed at the time to determine this. The case of HAT-P-33b illustrates how stellar activity can complicate exoplanet detection and how multiple observational techniques must sometimes be combined to confirm a candidate.

Did You Know?

• Its confirmation required the Blendanal program to rule out false positive scenarios such as a hierarchical triple star.
• The planet's host star, HAT-P-33, experiences high jitter that made radial velocity measurements unreliable.
• HAT-P-33b orbits its star every 3.474474 days at an average distance of 0.0503 AU.

The Long Road to Confirmation

HAT-P-33b's existence was first hinted at by the HATnet collaboration, a six-telescope survey program scanning the sky for planets crossing in front of their host stars. As early as 2004, astronomers noticed something unusual in the data around HAT-P-33, but the star's spectrum was plagued by what researchers call jitter—an erratic, noisy wobble that muddied every radial velocity measurement. This made the standard confirmation pathway nearly impossible. Over the following years, the team turned to multiple facilities: the 1.5-meter telescope at Fred Lawrence Whipple Observatory in Arizona, the SOPHIE spectrograph at France's Haute-Provence Observatory, and ultimately the HIRES instrument at Hawaii's Keck Observatory, where twenty-two spectra were gathered between September 2008 and December 2010. That far exceeds the number typically collected for a single candidate, a deliberate strategy to average out the jitter. When radial velocity alone proved insufficient, photometric observations with the KeplerCam instrument captured the faint dimming of a transit. The Blendanal algorithm then eliminated most false-positive scenarios, and the discovery was formally reported in the Astrophysical Journal in June 2011.

The Inflation Puzzle

One of HAT-P-33b's most striking features is the mismatch between its mass and its size. The planet weighs roughly three-fourths as much as Jupiter, yet its radius stretches nearly eighty percent beyond Jupiter's. That kind of extreme puffiness places it in a small and puzzling club alongside WASP-17b and HAT-P-32b, all of which are far larger than their masses would naively predict. The standard explanation for hot-Jupiter inflation is intense stellar irradiation—hotter planets should be more bloated. But the comparison with WASP-18b breaks that simple picture: WASP-18b is considerably hotter than both HAT-P-33b and WASP-17b, yet it has a much smaller radius. This contradiction has prompted researchers to ask what other physical mechanisms, beyond temperature alone, might be driving these planets to expand so dramatically. In their discovery paper, the authors also recommended follow-up observations with the Spitzer Space Telescope, specifically to watch HAT-P-33b disappear behind its star during an occultation event, a technique that could sharpen the planet's atmospheric and thermal profile and help resolve the inflation mystery.

The Host Star

HAT-P-33, also catalogued as GSC 2461-00988, is an F-type dwarf star situated roughly 401 parsecs—about 1,310 light-years—beyond our solar system. It is a noticeably more massive and larger star than our Sun, packing 1.403 solar masses and spanning 1.777 solar radii, which translates to roughly forty percent greater mass and seventy-seven percent greater size. Its effective temperature sits at 6,401 kelvin, making it warmer than the Sun, and its metallicity, measured at [Fe/H] = 0.05, indicates it carries about twelve percent more iron than our star. At an estimated age of 2.4 billion years, HAT-P-33 is considerably younger than the Sun. Its surface gravity is 4.09, and its apparent magnitude of 11.89 means it is far too faint for unaided human eyes. The star's spectrum, however, carries persistent jitter attributed to stellar activity rather than unseen companions, a complication that clouded every radial velocity measurement and kept the binary-star hypothesis alive for years. An adaptive-optics search at the MMT Observatory failed to detect any dim secondary companion.

Ruling Out the Imposters

Confirming HAT-P-33b required more than simply detecting a periodic dip in brightness. The SOPHIE spectrograph data at Haute-Provence raised a troubling alternative: the radial velocity anomalies might have been caused by background distortion from an unrelated stellar system rather than a genuine planet. To address this, the team employed Blendanal, a statistical tool modeled on the Blender technique used to validate Kepler discoveries. The algorithm systematically tested whether the observed signal could instead be explained by a hierarchical triple-star configuration or by the blended light of a foreground star superimposed on a background binary. Both of those scenarios were ruled out. One possibility, however, could not be fully excluded: that HAT-P-33 is itself a binary pair in which the secondary companion is so dim it is visually indistinguishable from the primary. If true, that hidden star would need a mass below 0.55 solar masses. The team also noted that the jitter in the data stemmed from stellar activity rather than additional planets, and they acknowledged that shorter-period companions might still lurk in the system, undetected because not enough radial velocity data had yet been accumulated to constrain them.