A Scale-Height Estimate of the Mean Molecular Weight of K2-18 b from JWST NIRISS SOSS and NIRSpec G395H Transmission Spectra

In the vast catalog of worlds orbiting other stars, a few stand out not just for their size or distance, but for the possibility that they might harbor conditions suitable for life. Among these is K2-18 b, a planet roughly twice the width of Earth, circling a dim red star within a region where liquid water could theoretically exist on a surface. Astronomers have long been fascinated by this world because it sits in a gray area between a rocky planet and a gas giant. It is large enough to hold a thick atmosphere, yet small enough that it might possess a solid surface beneath the clouds. The central mystery is the nature of that atmosphere. Is it a light, fluffy envelope of hydrogen and helium, similar to a miniature gas giant? Or is it a heavy, dense layer of gases like carbon dioxide and nitrogen, more akin to the thick air of Venus or the thin air of Mars? The answer determines whether the planet is a "Hycean" world with a global ocean beneath a hydrogen sky, or a different kind of exotic environment entirely. To solve this, scientists look at how starlight filters through the planet's atmosphere as it passes in front of its star, searching for the chemical fingerprints of the gases present and measuring how much the atmosphere swells the planet's apparent size.

A recent study by Muhammad Ibraheem takes a fresh look at data collected by the James Webb Space Telescope to address this specific question about the atmosphere's weight. The researcher did not attempt to build a complex computer model to simulate every detail of the planet's weather or chemistry. Instead, the work focuses on a simpler, more direct measurement: the height of the atmosphere's "skin." When a planet passes in front of its star, the atmosphere absorbs some of the star's light, making the planet look slightly larger at certain colors of light than at others. The difference in size between the deepest part of an absorption feature and the surrounding baseline tells scientists how far the atmosphere extends. This vertical distance is known as the scale height. By measuring how much the atmosphere swells at the wavelengths where methane and carbon dioxide absorb light, one can work backward to find the average weight of the molecules making up that air. Lighter molecules create a puffier, more extended atmosphere, while heavier molecules create a tighter, more compressed one.

Ibraheem analyzed the raw data from two instruments on the space telescope, which had previously been used to detect methane and carbon dioxide on K2-18 b. The study focused on six specific bands of light where these gases are known to absorb energy. By comparing the depth of the planet's shadow at these specific colors against the neighboring colors where the atmosphere is transparent, the researcher measured an excess depth of 56.1 parts per million. This tiny difference, equivalent to the thickness of a human hair seen from a mile away, represents the physical extent of the atmospheric features. The researcher then converted this measurement into a scale height, which describes the vertical thickness of the atmosphere, and used it to calculate the mean molecular weight. This calculation is not a single number but a range that depends heavily on the temperature of the atmosphere and an assumption about how many scale heights the observed features represent.

The results show that the atmosphere's average molecular weight falls somewhere between 2.0 and 5.3, depending on the assumptions made about the temperature and the structure of the atmosphere. If the atmosphere is relatively cool, between 120 and 160 Kelvin, the calculated weight is between 2.0 and 3.3. This range is significant because it overlaps with the weight of a hydrogen-rich atmosphere, which would be around 2.3, but it also reaches into the territory of heavier, enriched atmospheres. The data are consistent with both a light, hydrogen-dominated world and a heavier, more complex atmosphere. Crucially, the study finds that the result is not driven by measurement noise or a single controversial detection. Even if the signal for carbon dioxide is removed from the analysis, the conclusion remains the same. The uncertainty in the final answer comes not from the quality of the telescope data, but from the unknown temperature of the atmosphere and the difficulty of knowing exactly how many layers of atmosphere are being probed.

The paper explicitly avoids claiming that the planet is definitely a Hycean world or a mini-Neptune. While the results allow for a hydrogen-rich envelope, they do not rule out a heavier atmosphere, nor do they provide enough evidence to distinguish between a planet with a liquid ocean and one with a magma surface. The researcher notes that the current data cannot separate these possibilities because both scenarios can produce a similar atmospheric weight. Furthermore, the study highlights that previous debates about the presence of certain gases, such as dimethyl sulfide, are not resolved by this specific measurement. The work serves as a clear, straightforward estimate that confirms the atmosphere has features consistent with a light gas but leaves the door open for heavier compositions. It underscores that while the James Webb Space Telescope has provided incredibly precise measurements, the true nature of K2-18 b's atmosphere remains a question of temperature and structure that requires more observations and deeper modeling to answer definitively.