Water found in a Neptune-sized exoplanet’s atmosphere | Ars Technica

It’s the smallest exoplanet yet to have water vapor discovered in its atmosphere.

by Xaq Rzetelny – Sept 24 2014, 12:00pm CDT

HAT-P-11b is 4.7 times the size of Earth and has 25 Earth masses.
Harvard Center for Astrophysics

After a difficult search, scientists have found definitive traces of water on a relatively small exoplanet for the first time. The exoplanet in question, HAT-P-11b, is the size of Neptune and has copious amounts of both water vapor and hydrogen in its atmosphere.

Using the Hubble Space Telescope, the Spitzer Space Telescope, and the Kepler spacecraft, a team of scientists obtained spectrographic data as HAT-P-11b passed in front of its host star, allowing them to determine the planet’s atmospheric composition.

While other exoplanets with water have been discovered, these have mostly been gas giants larger than Jupiter. HAT-P-11b is the first significantly smaller planet with water to be discovered. The discovery paves the way for searches for water, perhaps even on smaller, more Earth-like planets.

Clouds

One of the main problems with obtaining clear evidence of a molecule in the atmospheres of exoplanets was the presence of clouds in their atmospheres. A thick cloud layer can absorb light that would otherwise pass through the planet’s atmosphere before reaching Earth. In order for scientists to determine the atmospheric composition of a planet, they need to analyze the spectra of light that has passed through the atmosphere.

Light from the star that travels past the planet will have the spectrum of the star. Any light that travels through the atmosphere of the planet will have specific parts of the spectrum absorbed by molecules in the planet’s atmosphere.
John Timmer

Assuming light can get through the clouds, water vapor (and any other molecules in the atmosphere) will absorb certain wavelengths of light but not others. This results in darkened lines on the otherwise bright spectrum of starlight that has passed through the atmosphere. By checking which lines are present, scientists can discover the presence of specific gases in the atmosphere.

HAT-P-11b is the first planet of this type with no interference from clouds, which is why the team was able to discover the water vapor.

This bodes well. After a string of exoplanets with atmospheric compositions that could not be determined, this discovery demonstrates that this approach works. That means future observations won’t be casting around blindly—we know now there’s the potential for finding something to discover and analyze.

Sunspots

Another obstacle to the observation of water in a planet’s atmosphere is the equivalent of sunspots on the host star. In certain cases, the presence of "starspot" activity could cause false signals, mimicking the presence of water in the atmosphere. For HAT-P-11, the team was able to rule out this kind of interference. Two telescopes were pointed at the planet at the same time, observing in different wavelengths. That way, they were able to isolate the starspot activity and determine that the temperatures of the spots were too hot to mimic water vapor in HAT-P-11b’s atmosphere.

With starspots ruled out as an explanation of the water vapor absorption lines, it became clear that HAT-P-11b really does have water vapor. The planet also features hydrogen and small amounts of heavier elements in its atmosphere.

The next generation

One of the key tools used in this observation was the Wide Field Camera 3 (WFC3), a special instrument attached to the Hubble Space Telescope during its final servicing in 2009.

WFC3 provided incredibly high-precision measurements, giving a glimpse of what scientists might be able to achieve with future telescopes. The James Webb telescope, in particular, is set to replace Hubble in 2018. In addition to carrying high-precision instruments like WFC3, it will have a larger mirror than Hubble’s, allowing it to detect signals even fainter than the atmosphere of HAT-P-11b. This will allow more Neptune-sized and even smaller planets to be examined (again, as long as they don’t have clouds).

The study of planetary atmospheres provides us with a window into exoplanet formation and evolution. Current models are largely derived from planetary masses and radii, but this only goes so far, since it doesn’t tell us much about the planet’s composition. By studying the atmospheres, however, scientists can gain valuable clues as to the interior structures. Already, this observation has provided critical insight into the formation of Neptune-sized planets, and future work will explore how even smaller planets formed.

Nature, 2014. DOI: doi:10.1038/nature13785 (About DOIs).

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