# HAT-P-67

HAT-P-67 is a binary star system, made up of a F-type subgiant and a red dwarf star, which is located about 1,200 light-years away in the constellation Hercules. There is a hot Saturn planet orbiting the primary star, which is named HAT-P-67b.

## Stellar system

The stellar system consists of the F class primary star with a red dwarf companion separated by 9 arc-seconds or about 3400 astronomical units. According to measurements taken by the Gaia spacecraft the two stars have nearly identical parallax and proper motions confirming that they are a binary system.
The primary star is a rapidly rotating subgiant star with a radius 2.65 times that of the sun and a mass 1.64 times that of the sun.
Little is known of the secondary star other than it is a M-dwarf.

## Planetary system

There is one known planet orbiting HAT-P-67A. HAT-P-67b is a gas giant planet transiting its parent star every 4.8 days, at an orbital distance of 0.065 astronomical units (9,700,000 km). It is one of the largest and lowest density planets known as of 2024.

### Discovery

Transits of HAT-P-67b were discovered by the Hungarian Automated Telescope Network (HATNet), using small, wide field telescopes, located at the Fred Lawrence Whipple Observatory in Arizona and at the Mauna Kea Observatory in Hawaii. Observations were made in 2005 and 2008, analysis of the obtained data revealed the periodic transits of HAT-P-67b. Follow-up photometry of the transits were obtained using the 1.2 m telescope at the Fred Lawrence Whipple Observatory. A full transit was observed on 2012 May 28, and five partial transits were observed in 2011, 2012 and 2013.
The high rotational velocity of the star made initial attempts to confirm the planet using radial velocity measurements difficult, with data from 2009 showing that the transiting object was less massive than a brown dwarf. Using measurements taken from 2009 to 2012 the Keck telescope was able to determine that the mass of the planet was less than 0.59 that of Jupiter. In 2016 Doppler tomography was used to confirm the planet.

### Characteristics

With a radius of over double that of Jupiter's HAT-P-67 b is one of the largest exoplanets known to date. It also one of the least dense at approximately 0.05 grams per cubic centimeter, a density lower than that of marshmallows.
An analysis of a radial velocity time series obtained at the Galileo National Telescope detected the Rossiter–McLaughlin effect and determined the projected spin-orbit angle to be 2.2 ± 0.4°. The calculated value suggests an aligned planetary orbit, indicating that the planet likely migrated to its present orbit through tidal interactions with a protoplanetary gas disk.

### Atmosphere

Gas giants with masses less than Jupiter's, and temperatures greater than 1800 K, like HAT-P-67 b, which has an equilibrium temperature of approximately 1900 K, are so inflated and puffed out that they are all on unstable evolutionary paths which eventually lead to roche lobe overflow and the evaporation and loss of the planet's atmosphere.
A team of astronomers led by Aaron Bello-Arufe used the CARMENES spectrograph at the Calar Alto Observatory to study the atmosphere of HAT-P-67b. Based on this data, the planet's atmosphere seems to be highly ionized and may be escaping at a rate of 10 million tons per second. The team detected sodium and ionized calcium in the atmosphere of HAT-P-67b. Ionized calcium is typically found in hotter planets; however, it was detected quite prominently in the spectrum of HAT-P-67b.
The data also revealed absorption in the hydrogen and helium lines, typically a sign that part of the atmosphere is escaping into space. In the case of HAT-P-67b, these signals were detected before and after the planet's transit, suggesting the possibility of a vast cloud of gas escaping far beyond the planet. A different team led by Michael Gully-Santiago performed a multiyear spectroscopic survey of HAT-P-67 b, using the Habitable Zone Planet Finder on the Hobby–Eberly Telescope. They observed a prominent leading tail and a significantly fainter trailing tail, which they interpreted as direct evidence of preferential mass loss on the dayside. A third team using an average of many spectra acquired after transits found a clear absorption signal. They estimated an effective planetary radius 6 times that of Jupiter, indicating that the planet's atmosphere is evaporating.
The HAT-P-67 planetary system :
| Companion (in order from star) | Mass | Semimajor axis (AU) | Orbital period (days) | Eccentricity | Inclination | Radius |
| b | 0.418±0.012 M$_J$ | 0.0615±0.0022 | 4.8101088(2) | 0 (assumed) | 85.01+0.35 −0.32° | 2.038+0.067 −0.068 R$_J$ |