K2-28b

Lore & Background

K2-28b was first noticed as a candidate extrasolar planet by Vanderburg et al. in 2016 during a search of 59,174 stars from the Kepler space telescope's first year of K2 observations, which yielded 234 planetary candidates. Shortly thereafter, the K2-ESPRINT Project confirmed that the candidate was a super-Earth sized planet in a close orbit around a red dwarf star.

The planet is a sub-Neptune sized body orbiting its star in only 2.26 days. Despite this short orbital period, its equilibrium temperature is a relatively low 500 Kelvin due to the low luminosity of the parent star. Because of the very small size of the parent star, K2-28b is a particularly favorable target for transmission spectroscopy by the James Webb Space Telescope, which should be able to determine if the atmosphere is cloudy or clear by observing roughly 5 transits.

Among a group of small and cool planets orbiting relatively bright M-dwarfs, its predicted secondary eclipse depth of 230 parts-per-million is second only to Gliese 1214 b. The star K2-28 has a companion star 5.2 arcseconds to the northeast, but that star has a different proper motion and is therefore physically unrelated, probably a background star.

Reader's Guide

K2-28b holds significance as a prime target for atmospheric characterization with the James Webb Space Telescope. Its small, dim parent star enables transmission spectroscopy to be performed after observing roughly five transits, allowing astronomers to determine whether the planet's atmosphere is cloudy or clear. This makes K2-28b a key object for understanding the atmospheric properties of sub-Neptune worlds. The planet's predicted secondary eclipse depth of 230 parts-per-million places it second only to Gliese 1214 b among a group of small, cool planets orbiting relatively bright M-dwarfs, highlighting its observational accessibility. The discovery process, beginning with its identification as a candidate among 234 planetary candidates from 59,174 stars in the first year of K2 observations and later confirmed by the K2-ESPRINT Project, underscores the systematic search methods used to find such planets. The unrelated background star 5.2 arcseconds away does not affect the planetary system. K2-28b's legacy lies in its potential to advance knowledge of exoplanet atmospheres in a regime where few such studies are possible.

Did You Know?

• K2-28b orbits its star every 2.26 days.
• Its equilibrium temperature is 500 Kelvin despite the short orbit.
• The predicted secondary eclipse depth is 230 parts-per-million.
• The star K2-28 has an unrelated background star 5.2 arcseconds to the northeast.

Place in the Exoplanet Atmosphere Research Landscape

The study of extraterrestrial atmospheres stands as one of the most active frontiers in modern astronomy, serving a dual purpose: advancing our understanding of other worlds while simultaneously illuminating the processes that shape Earth's own atmosphere. K2-28b exists within this broader research ecosystem, where the mere evidence that extrasolar planets can possess atmospheres has opened an entirely new chapter in planetary science. Unlike the well-characterized atmospheres of Solar System bodies—from the giant planets to smaller worlds like Mars, Venus, and even moons such as Titan and Triton—extrasolar atmospheres remain largely uncharted territory. The existence of an atmosphere on a distant world like K2-28b places it among the growing catalog of exoplanets where atmospheric signatures have been detected, contributing to a field that draws on both astronomical observation and the comparative science of atmospheric physics.

The CATS Initiative and the Search for Atmospheric Signatures

In September 2022, a new collaborative group called Categorizing Atmospheric Technosignatures, or CATS, was formed to systematically compile and organize the results of exoplanet atmosphere studies. The group's mission centers on cataloging findings related to biosignatures, technosignatures, and related atmospheric indicators across the exoplanet population. For a world like K2-28b, whose atmospheric properties are being examined within this expanding body of research, the CATS initiative represents an important organizational framework. It ensures that atmospheric data from diverse exoplanets is gathered in a structured, comparable format, enabling researchers to identify patterns, anomalies, and potential signatures of biological or technological activity. This effort builds on the foundational principle that comparing atmospheres across different worlds broadens our basic understanding of atmospheric processes, from greenhouse dynamics to chemical cycling, and positions K2-28b's atmospheric data as a contributing data point in a larger, increasingly systematic scientific enterprise.

Solar System Benchmarks for Atmospheric Comparison

Understanding the atmosphere of any extrasolar planet requires a framework of comparison, and the Solar System provides an extraordinary range of atmospheric reference points. From Mercury's thin, constantly refreshed exosphere of helium, sodium, potassium, and oxygen—gases derived from solar wind, radioactive decay, and meteor impacts—to Venus's crushing carbon dioxide envelope where surface pressure reaches 92 times Earth's and temperatures soar to approximately 470 degrees Celsius, the variety is staggering. Mars offers a contrasting case: a thin CO2-dominated atmosphere with surface pressures of only 0.6 to 0.9 kPa, subject to strong thermal tides and seasonal temperature swings from roughly minus 140 degrees to 20 degrees Celsius. The giant planets, meanwhile, present atmospheres of hydrogen and helium that blend seamlessly into liquid interiors with no clear boundary. K2-28b's atmospheric characteristics, when examined, are inevitably interpreted against this spectrum of known possibilities, from the nearly atmosphere-less to the deeply layered.

Atmospheric Processes Under Investigation

The comparative study of atmospheres—whether on Solar System bodies or extrasolar worlds—serves as a natural laboratory for understanding fundamental atmospheric processes. Key phenomena under investigation include the greenhouse effect, aerosol and cloud physics, and the broader domains of atmospheric chemistry and dynamics. On Venus, for instance, greenhouse gases warm the lower atmosphere while simultaneously cooling the upper layers, producing a compact thermosphere and eliminating, by some definitions, a distinct stratosphere. On Mars, the thin atmosphere results in low thermal inertia, driving strong thermal tides that can shift total atmospheric pressure by up to ten percent. Jupiter's cloud layer, roughly 50 kilometers deep, contains frozen ammonia crystals and trace compounds including methane, water vapor, ammonia, and silicon-based molecules. These processes, observed across the Solar System, form the theoretical backbone against which the atmospheric behavior of extrasolar planets like K2-28b is interpreted, revealing universal principles that transcend individual planetary contexts.