Astronomers utilizing the NASA/ESA Hubble Space Telescope have recently scrutinized the smallest known exoplanet where water vapor has been discerned in its atmosphere. With a diameter merely about twice that of Earth, GJ 9827d potentially exemplifies the existence of planets with water-rich atmospheres beyond our solar system.
Discovered by NASA’s Kepler Space Telescope in 2017, GJ 9827d completes its orbit around a red dwarf star every 6.2 days. Situated in the constellation Pisces, the star GJ 9827 resides approximately 97 light-years away from Earth.
According to team member Björn Benneke from the Université de Montréal, this observation marks a significant milestone in directly showcasing the existence of planets with water-rich atmospheres around distant stars. Understanding the prevalence and diversity of atmospheres on rocky planets is crucial, and this discovery offers a valuable glimpse into this aspect of exoplanetary science.
The findings, detailed in The Astrophysical Journal Letters, suggest the potential presence of a water-rich atmosphere on GJ 9827d. However, further investigation is warranted to ascertain whether Hubble’s spectroscopic analysis detected a minimal amount of water vapor within a hydrogen-rich atmosphere, or if the planet’s atmosphere predominantly consists of water remnants from a primordial hydrogen/helium atmosphere that dissipated under stellar radiation.
The lead author of the paper, Pierre-Alexis Roy from the Université de Montréal, highlighted the meticulous design of their observing program, which aimed not only to detect molecules within the exoplanet’s atmosphere but specifically targeted the search for water vapor. Roy emphasized the excitement surrounding either outcome: whether water vapor prevails or exists as a minor component within a predominantly hydrogen-dominated atmosphere.
Björn Benneke, adding to the discussion, underscored the significance of directly detecting the atmosphere of a planet as small as GJ 9827d. He noted the gradual transition in their observations as they delve into the realm of smaller planets, anticipating a shift away from hydrogen-dominated atmospheres toward compositions akin to Venus, dominated by carbon dioxide.
Given the planet’s scorching temperatures, equivalent to those of Venus at approximately 425°C, the prospect of a predominantly water vapor atmosphere suggests a steamy and inhospitable environment.
The current analysis leaves the team with two intriguing possibilities. GJ 9827d could retain a hydrogen-rich envelope interlaced with water, resembling a miniature version of Neptune. Alternatively, it might represent a warmer iteration of Jupiter’s moon Europa, potentially consisting of a significant water-to-rock ratio, with copious water vapor enveloping smaller rocky formations, as theorized by Benneke.
In one scenario, if the planet possesses a residual water-rich atmosphere, it likely originated farther from its host star, where temperatures were cold enough for water to exist in icy form. Over time, gravitational forces may have nudged the planet closer to its star, subjecting it to increased radiation. As a result, any hydrogen present would have been heated and potentially escaped, or could still be in the process of dissipating due to the planet’s relatively weak gravitational pull. Alternatively, there’s a possibility that the planet formed in close proximity to the hot star, with only traces of water in its atmosphere.
The observational program conducted by Hubble spanned 11 transits over three years, capturing instances where the planet passed in front of its host star. During these transits, starlight filtered through the planet’s atmosphere, revealing spectral signatures indicative of water molecules. If clouds exist on the planet, they likely hover at lower atmospheric levels, allowing Hubble to peer through and investigate water vapor concentrations above the cloud cover.
Hubble’s recent discovery sets the stage for more in-depth scrutiny of the planet. It emerges as a promising candidate for further investigation using the NASA/ESA/CSA James Webb Space Telescope, particularly for infrared spectroscopy aimed at uncovering additional atmospheric components beyond water vapor.
Resources
- ONLINE NEWS European Space Agency. (2024, January 25). Hubble finds water vapor in small exoplanet’s atmosphere. Phys.org. [Phys.org]
- JOURNAL Roy, P., Benneke, B., Piaulet, C., Gully-Santiago, M., Crossfield, I. J. M., Morley, C., Kreidberg, L., Evans, T. M., Brande, J., Delisle, S., Greene, T. P., Hardegree-Ullman, K. K., Barman, T., Christiansen, J. L., Dragomir, D., Fortney, J. J., Howard, A. W., Kosiarek, M. R., & Lothringer, J. D. (2023). Water Absorption in the Transmission Spectrum of the Water World Candidate GJ 9827 d. The Astrophysical Journal Letters, 954(2), L52. [The Astrophysical Journal Letters]
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APA 7: TWs Editor. (2024, January 26). Water Vapor is Detected on a Tiny Exoplanet by Hubble!. PerEXP Teamworks. [News Link]
