In a significant advancement, astronomers have pinpointed the tiniest exoplanet where the unmistakable presence of water vapor has been observed in its atmosphere. With a diameter roughly twice that of Earth, the planet GJ 9827d could serve as an illustration of potential planets with atmospheres abundant in water elsewhere in our galaxy.
Astronomers have previously detected water vapor on more giant planets, but smaller ones are challenging because they’re tricky to observe. To see a gaseous world like this, telescopes must observe it. At the same time, it passes in front of its star. As it does, specific colors of the starlight are dimmed or blocked by the water vapor, making it easier to discern the underlying atmospheric composition.
Using the Hubble Space Telescope, which has been operating for almost two decades, and the Spitzer telescope, which provides infrared images of distant objects, the researchers observed a planet named HAT-P-11b. The planet resembles a hot Venus with more than 1,000 degrees Fahrenheit temperatures. It also has clear skies, meaning astronomers can observe the underlying atmospheric composition.
The team used a technique known as transit spectroscopy, which involves measuring the change in brightness of a star as a planet passes it. The resulting spectrum is then searched for the chemical signature of water molecules. The team found 11 transits of the star with GJ 9827d and were able to detect the spectral signal that is characteristic of water vapor in its atmosphere.
“Water is one of the most important molecules in the Universe,” said lead author Pierre-Alexis Roy, a scientist with the Trottier Institute for Research on Exoplanets at the University of Montreal. It acts as a superior solvent that allows for critical chemical reactions and is essential to all life on Earth. “The fact that we have now detected it on a very small exoplanet in a scorching environment is a big step toward understanding the diversity of these extra-solar planets and perhaps their habitability.”
For GJ 9827d to have water, it must have formed farther away from its host star, where ice is likely present. As the planet migrated closer, its intense radiation would have boiled the ice away into water and gas. This would have left the planet with an atmosphere dominated by water vapor rather than hydrogen, as seen on the more giant Jupiter-sized planets.
Another possibility is that the planet has geyser activity similar to thatJupiter’s moon, Io, whose icy surface spews water into space at a rate hundreds or thousands of times faster than Europa’s. Observing the planet as it erupts in water vapor could reveal this by watching for specific elements and compounds to absorb certain starlight colors or by observing whether the evaporating water varies.
Having confirmed the presence of water on GJ 9827d, astronomers hope to learn more about this world by studying it with the James Webb Space Telescope, which will be launched in 2021. The $10 billion observatory will enable scientists to identify other chemicals in its atmosphere, including carbon dioxide and possibly nitrogen, which may help narrow down the possibility that it supports life.
