Water Vapor Detected in a Tiny Exoplanet’s Atmosphere

Image from NASA

New Exoplanet Atmosphere Revealed

In January of 2024 scientists from the Hubble Space Telescope announced they had discovered water vapor in the atmosphere of a small exoplanet, GJ 9827 d, which is approximately twice the size of Earth.

This is an important benchmark because most of the earlier atmospheric findings were on larger, gas planets. Water vapor on a smaller planet is bringing us one step closer to understanding about planets that might be like Earth.

The find was made by looking at starlight filtered through the planet's atmosphere for evidence of absorption features from water. The mass and composition of the planet place it mid-way between rocky Earths and gas mini-Neptune in terms of size and composition, and as such is a key test case for atmospheric evolution. For exoplanetary science, the result doubles the number of worlds with atmospheres and at least potential for habitability.

Though water vapor does not suggest the planet is earth-like, it suggests small planets can retain atmospheres and volatile elements. That has implications for the understanding of how atmospheric loss, stellar radiation, and planet size influence conditions. The January finding is thus one step toward the detection of truly Earth-sized worlds with atmospheres.

Habitability and the Future of Exoplanet Research

One of the main consequences is that atmospheric science is decreasing in scale—from large planets to smaller ones approaching Earth's size. That brings into play the possibility of studying in detail actually Earth-similar worlds for evidence of water, oxygen, or other biosignatures within their atmospheres. Finding water vapor in GJ 9827 d suggests retention of volatiles is possible under some circumstances even on smaller worlds.

This also guides our instrument and mission design activities in an even sharper way: to target yet smaller planets with yet thinner atmospheres, we need more sensitive telescopes, higher spectral resolution, and longer observation times. Upcoming missions (e.g., the James Webb Space Telescope and next-generation observatories) will benefit from this work.

On the bigger picture, every time we find a planet with an atmosphere and water vapor, we are edging closer to being able to quantify how common Earth-like conditions might be within the galaxy. While GJ 9827 d is not Earth, its atmosphere tells us something about the diversity of planets and what conditions can make a planet habitable.