Astronomers have identified a watery atmosphere containing clouds around an exoplanet first discovered in 2010.
The planet, HAT-P-26b, orbits a moderately bright dwarf star named GSC 0320-01027, and is a similar mass to Neptune. While such exoplanets have been discovered in large numbers – astronomers have discovered several thousand exoplanets to date – knowledge about what atmospheres they may or may not have remains relatively scarce.
As a result, the discovery of an atmosphere containing water around this otherwise unremarkable exoplanet is highly significant.
Not only does it allow astronomers to infer the proportion of certain elements in the atmosphere, but it also helps to further the understanding of how atmospheres vary across exoplanets of different sizes and allow researchers to hone their models for how planets form.
The atmosphere was discovered by a team of astronomers led by Hannah R Wakeford using six separate observations. Four were made recently using the Hubble Space Telescope, while two were previously made using the Spitzer Space Telescope.
The researchers were able to use the observations not only to determine that the planet has water in its atmosphere, but how much is present. This in turn allowed them to calculate the atmosphere’s metallicity: the proportion of elements in the atmosphere that are heavier than both hydrogen and helium.
In the case of HAT-P-26b, this number was lower than had been expected, based on what is known about other similarly-sized planets such Neptune and Uranus.
It is thought that the reason for this is that this exoplanet acquired an atmosphere later than usual, when it was reaching the end stages of it formation. This is because this low metallicity suggests the planet has not experienced any major impacts from debris such asteroids since its atmosphere formed.
The discovery, which is detailed in a paper published today in the journal Science, is significant for the understanding Neptune-sized planets, both in our own solar system and beyond, which are highly abundant.
“Neptune-sized worlds are among the most common planets in our galaxy and frequently exist in orbital periods very different from that of our own Solar System ice giants,” the researchers wrote in the paper. “Atmospheric studies using transmission spectroscopy can be used to constrain their formation and evolution.”
Despite having a mass similar to Neptune, HAT-P-26b has a number of other differences to the Solar System’s eighth planet. It is, for example, almost twice the size of Neptune, despite being considered ‘Neptune-sized’ but has an orbit of just 4.23 days.