Unlocking the Secrets of Exoplanet GJ 9827 d: A Deep Dive with BBC Sky at Night Magazine

The GJ 9827 system, located roughly 100 light-years away in the Pisces constellation, encompasses three recognized super-Earth planets orbiting a nearby K dwarf star.

Among the trio, GJ 9827 d stands out as the most massive, nearly double that of Earth, and possesses the broadest orbit.

The two innermost companion planets are exceedingly dense, leading to the conclusion that they are largely rocky.

In contrast, GJ 9827 d exhibits a reduced density, suggesting that it likely harbors a considerable quantity of ‘volatiles’ such as water, and potentially a substantial atmosphere.

Insights into GJ 9827 d’s atmosphere

GJ 9827 d orbits in relative proximity to its sun, with a measured temperature around 350°C (662°F).

This indicates that its atmosphere – unlike most exoplanets of a similar size – cannot be primarily composed of hydrogen, as its elevated temperature would cause light gases to escape swiftly.

Earlier observations have only reinforced this expectation, as there was no evidence of hydrogen or helium escaping from the planet.

However, merely knowing the density of such a planet doesn’t enable astronomers to deduce its atmospheric conditions.

While much of it may have escaped, there could still exist a thin layer of hydrogen enveloping the planet.

Alternatively, it might possess a denser atmosphere comprising heavier gas molecules along with some hydrogen.

To gain a precise understanding of what is occurring, spectroscopic analysis of the atmosphere is essential to directly identify the gas molecules present.

Investigations

While clouds can significantly obstruct such observations of worlds like this, as they hinder the view of the lower atmosphere, data from the Hubble Space Telescope in 2023 suggested that molecular characteristics can be detected for this planet.

The superior capabilities of the James Webb Space Telescope (JWST) should further reveal the atmospheric composition of GJ 9827 d.

During her doctoral studies in physics at the University of Montreal, Caroline Piaulet-Ghorayeb endeavored to accomplish precisely that.

She employed the near-infrared spectrometers on JWST to observe GJ 9827 d during two transits across its star, allowing starlight to filter through its atmosphere.

Piaulet-Ghorayeb and her team identified the unmistakable spectral signature of water molecules within the planet’s atmosphere.

Indeed, Piaulet-Ghorayeb estimates that water vapor constitutes at least 30% of the atmosphere, signifying that GJ 9827 d is a volatile-rich ‘steam world’.

This planet’s temperature is so high that this water would not condense into liquid droplets in its atmosphere or on its surface.

Instead, under increased pressures, the water would exist in a supercritical state, where it blends effectively with hydrogen gas.

And remarkably, despite the planet’s elevated temperature, this thick atmosphere composed of heavier molecules would remain stable.

This aligns with the absence of detected escaping hydrogen or helium.

Piaulet-Ghorayeb and her associates project that additional anticipated JWST observations of the planet should be able to identify other elements of the GJ 9827 d atmosphere, such as carbon monoxide or carbon dioxide, and potentially sulfur dioxide.

Such findings would offer insights into how this planet developed such a volatile-rich atmosphere.

Lewis Dartnell was reading JWST/NIRISS unveils the Water-rich “Steam World” Atmosphere of GJ 9827 d by Caroline Piaulet-Ghorayeb et al. Read it online at: arxiv.org/abs/2410.03527.

This article was featured in the January 2025 edition of BBC Sky at Night Magazine.