K2-33b

Lore & Background

K2-33b was discovered in 2016 by two independent research groups, one from the California Institute of Technology and the other from the University of Texas at Austin, using data from the Kepler space telescope's Second Light mission. The planet was detected via the transit method, measuring the dimming of its host star every 5.424 days. Its host star, K2-33, is a low-mass M-type star with a surface temperature of 3540 K and a radius 1.05 times that of the Sun, but only 15% of the Sun's luminosity. The star is too dim to be seen with the naked eye, with an apparent magnitude of 14.3.

The planet's mass is not well constrained; while the upper limit is 3.6 Earth masses, its true mass may be much lower, possibly making it better described as a sub-Neptune. Its atmosphere, studied by 2022, appears hazy with no noticeable molecular bands in transmission spectrum, likely containing abundant carbon monoxide and dense tholin haze. A 2022 study found evidence that K2-33b may be surrounded by a dusty ring system, which could be confirmed by future observations with the James Webb Space Telescope.

Reader's Guide

K2-33b is significant primarily for its remarkably young age of 9.3 million years, making it one of the youngest transiting exoplanets known until the discovery of IRAS 04125+2902 b in 2024. Its youth helps address a fundamental question in exoplanet science: how close-in planets form. Since its discovery, several theories of planetary migration that require long timescales can be ruled out for this system. The most plausible formation scenario is that K2-33b formed farther from its star and then migrated inward through the protoplanetary disk, though in-place formation remains a possibility. Trevor David of Caltech noted that the planet demonstrates that close-in planets can exist at a very early stage. Understanding such young exoplanets may eventually help scientists understand planetary formation processes and provide clues about the origin of the Solar System and other planetary systems. The planet receives about 125 times as much sunlight as Earth does from the Sun, and its equilibrium temperature is 850 K.

Did You Know?

• K2-33b is about 9.3 million years old, making it among the youngest known exoplanets.
• The planet may have a dusty ring system, as suggested by a 2022 study.
• Its host star, K2-33, has only 15% of the Sun's luminosity and is too dim to be seen with the naked eye.
• K2-33b orbits its star every 5.424 days at a distance of 0.049 AU.