HAT-P-12b

Lore & Background

HAT-P-12b was discovered by the HATNet Project on April 29, 2009, orbiting the 13th magnitude K-type star HAT-P-12. It is a hydrogen/helium-dominated gas giant with a core mass of approximately 11.3 Earth masses, making it the least massive planet of its kind known at the time, surpassing Saturn in that regard. The planet is moderately irradiated by its low-metallicity host star.

In 2020, transmission spectra revealed that the atmosphere of HAT-P-12b is cloudy, with haze present above the cloud tops, and water was detected. However, the prevalence of clouds and hazes in its atmosphere was disputed in 2021, leaving the atmospheric composition under debate.

In August 2022, the planet and its host star were selected for naming by the third NameExoWorlds project. The approved names, proposed by a team from Hungary, were announced in June 2023: HAT-P-12b became Puli, and its star became Komondor, after the Hungarian dog breeds.

Reader's Guide

HAT-P-12b holds significance as a benchmark for understanding the transition between gas giants with substantial cores and those dominated by hydrogen and helium. At the time of its discovery, it was the least massive H/He-dominated gas giant known, providing a key data point for models of planetary formation and interior structure. Its core mass of about 11.3 Earth masses and total metal fraction of roughly 15% suggest a composition intermediate between Saturn and more massive hot Jupiters. The detection of water and clouds in its atmosphere, though later disputed, highlights the challenges in characterizing exoplanet atmospheres and the role of hazes in obscuring spectral features. The planet’s inclusion in the NameExoWorlds project and its naming after the Hungarian Puli dog breed reflect public engagement in exoplanet science. As a moderately irradiated planet orbiting a low-metallicity star, HAT-P-12b continues to be a target for atmospheric studies, particularly regarding the prevalence of clouds and hazes in such worlds.

Did You Know?

• HAT-P-12b is formally named Puli, after the Hungarian dog breed.
• Its host star is named Komondor, also after a Hungarian dog breed.
• Water was detected in its atmosphere in 2020, but the presence of clouds and hazes was disputed in 2021.

Discovery and Physical Profile

WASP-12b entered the catalog of known worlds in April 2008, when the SuperWASP wide-field transit survey identified it as a hot Jupiter circling its host star. Its orbital period is barely more than a single Earth day, a stark contrast to the roughly 365-day cycle our own planet completes around the Sun. The planet orbits at roughly 3.5 million kilometres (about 0.023 AU), which works out to roughly one forty-third of the Earth-Sun separation, and its orbital eccentricity matches that of Jupiter. Despite possessing 40 percent more mass than Jupiter, the planet's atmosphere has swelled to nearly three times Jupiter's radius, giving it one of the lowest bulk densities measured among exoplanets. Researchers attribute this extreme inflation to the relentless flood of stellar energy bathing its upper layers. The combination of a bloated envelope and a comparatively compact core makes WASP-12b a striking example of how close-in giant planets can be sculpted into sizes and densities that have no analogue in our own solar system.

Atmospheric Chemistry and the Carbon Question

The chemical makeup of WASP-12b's atmosphere has generated both excitement and confusion among astronomers. In December 2013, a team using the Hubble Space Telescope reported detecting water vapour in the planet's upper atmosphere. Yet just seven months later, in July 2014, NASA announced that WASP-12b, along with HD 189733b and HD 209458b, possessed a remarkably dry atmosphere, a finding that complicated the earlier water detection. A separate 2010 study, published in Nature, revealed that the planet's carbon-to-oxygen ratio sits near 1, roughly double the solar value of 0.54, marking it as a carbon-rich gas giant. The excess carbon resides in the atmosphere as carbon monoxide and methane. One of the study's authors noted that a world with more carbon than oxygen could, in principle, host solid bodies made of pure carbon such as diamond or graphite. Media outlets latched onto this, dubbing WASP-12b a "diamond planet," though the carbon is atmospheric rather than solid. The finding opened a window onto a class of carbon-dominated worlds that theory had long predicted but observation had never confirmed.

Tidal Distortion, Extreme Heat, and a Vanishing Atmosphere

Because WASP-12b is tidally locked to its star, one hemisphere is locked in perpetual daylight while the other endures endless night, much like the Moon's fixed face toward Earth. This asymmetry drives a massive circulation of heat from the scorching dayside to the cooler nightside, generating powerful atmospheric winds. Researchers Taylor Bell and Nicolas Cowan noted that hydrogen atoms on the irradiated face tend to ionise, then recombine into neutral atoms as they drift to the cooler hemisphere, amplifying the heat transport. The gravitational pull of the star stretches the planet into a prolate spheroid and strips its outer atmosphere at a rate of roughly six billion tons every second. Combined with tidal heating, the planet's surface temperature exceeds 2,500 kelvin (about 4,000 °F). In September 2017, Hubble-based measurements showed the world reflects only about six percent of incident starlight, earning it the descriptors "black as asphalt" and "pitch black," even though its extreme heat causes it to radiate a faint reddish glow.

Orbital Decay and the Countdown to Destruction

WASP-12b is on a slow but irreversible path toward destruction. A 2012 analysis using the Rossiter-McLaughlin effect revealed that its orbit is tilted by roughly 59 degrees relative to the star's equatorial plane, a significant misalignment. More critically, timing measurements of successive transits show the orbital period shrinking. A 2019 study measured a decrease of about 29 milliseconds per year since the 2008 discovery; a 2020 update refined this to 32.53 milliseconds per year, implying a remaining lifetime of roughly 2.9 million years. By 2022 the decay rate had been narrowed to 29.81 milliseconds per year, corresponding to an estimated 3.16 million years before the planet spirals fully into its star. This orbital shrinkage, driven by tidal interactions, proceeds far more rapidly than the decay seen in WASP-19b. On 20 May 2010, Hubble's Cosmic Origins Spectrograph captured what appeared to be the star actively engulfing the planet, the first unambiguous observation of a star consuming a planet. These observations confirmed theoretical predictions published by Shu-lin Li of Peking University in Nature in early 2009, and earlier estimates placed the planet's remaining lifespan between three and ten million years.