HIP 65426 b / Najsakopajk

HIP 65426 b is on the lower left of the image, with the circle representing what Neptune's orbit would look like around the star HIP 65426, represented by a small cross.
Discovery[1]
Discovered by	SPHERE consortium[2]
Discovery date	6 July 2017
Detection method	Direct imaging
Designations
Alternative names	Najsakopajk[3]
Orbital characteristics
Semi-major axis	87+108 −31 au[4]
Inclination	100+15 −6 deg[4]
Physical characteristics
Mean radius	Atmospheric model: 0.92±0.04 RJ[4] Evolutionary model: 1.45±0.03 RJ[4]
Mass	7.1±1.1 MJ[4]
Surface gravity	4.07±0.19 dex
Temperature	Atmospheric model: 1667+25 −24 K[4] 1560±100 K[5] Evolutionary model: 1282+26 −31 K[4]

HIP 65426 b, formally named Najsakopajk,^[3] is a super-Jupiter exoplanet orbiting the star HIP 65426. It was discovered on 6 July 2017 by the SPHERE consortium using the Spectro-Polarimetric High-Contrast Exoplanet Research (SPHERE) instrument belonging to the European Southern Observatory (ESO).^[6][7] It is 385 light-years from Earth.^[8] It is the first planet discovered by ESO's SPHERE instrument.^[9]

Nomenclature

In August 2022, this planet and its host star were included among 20 systems to be named by the third NameExoWorlds project.^[10] The approved names, proposed by a team from Mexico, were announced in June 2023. HIP 65426 b is named Najsakopajk and its host star is named Matza, after Zoque words for "Mother Earth" and "star".^[3]

Overview

The exoplanet HIP 65426 b orbits its host star HIP 65426, an A2V star with apparent magnitude 7.01 and a mass of 1.96±0.04 M_☉.^[11] This planetary system is located in the constellation Centaurus. The planet is around 14 million years old. However, it is not associated with a debris disk, despite its young age,^[7][8] causing it to not fit current models for planetary formation.^[12] It is around 92 AU from its parent star, with a possible dusty atmosphere.^[13] It was discovered as part of the SHINE program, which aimed to find planetary systems around 600 new stars.^[1]

In September 2022, HIP 65426 b became the first exoplanet directly observed by the James Webb Space Telescope.^[14]

Planetary atmosphere

The spectrum taken in 2020 has indicated that HIP 65426 b is carbon-poor and oxygen-rich compared to Solar System gas giants.^[5]

Spectral analysis of data from the James Webb Space Telescope revealed strong evidence of silicate clouds containing enstatite with no evidence of a dusty atmosphere.^[15]

James Webb Space Telescope observation

In August 2022, a pre-print of the James Webb Space Telescope (JWST) observations was published. The JWST direct imaging observations between 2-16 μm of HIP 65426 b tightly constrained its bolometric luminosity to ${\displaystyle \log(L_{bol}/L_{sun})=-4.23\pm 0.02}$, which provides a robust mass constraint of 7.1±1.1 M_J. The atmospheric fitting of both temperature and radius are in disagreement with evolutionary models. The team also constrained the semi-major axis and the inclination of the planet, but the new JWST astrometry of the planet did not significantly improve the orbit of the planet, especially the eccentricity remains unconstrained.^[4]

HIP 65426 b is the first exoplanet to be imaged by JWST and the first exoplanet to be detected beyond 5 μm. The observations^[16] demonstrate that the James Webb Space Telescope will exceed its nominal predicted performance by a factor of 10 and that it will be able to image 0.3 M_J planets at 100 au for main-sequence stars, Neptune and Uranus-mass objects at 100-200 au for M-dwarfs and Saturn-mass objects at 10 au for M-dwarfs. For α Cen A JWST might be able to push the limit to a 5 R_🜨 planet at 0.5 to 2.5 au.^[4]

HIP 65426 star map

HIP 65426 b viewed by the James Webb Space Telescope (August 2022)