An exoplanet located about 437 light-years away with a mass comparable to that of Neptune or Uranus sheds light on how planets form around their host stars.

HAT-P-26b, also called "warm Neptune," is characterized by an atmosphere composed mostly of hydrogen and helium.


Data from the Hubble and Spitzer space telescopes helped astronomers determine the planet's metallicity, a measure of elements that are heavier than helium and hydrogen in the atmosphere that can help determine how a planet formed. If a planet has more heavier elements compared with the sun, it is described to have high metallicity.

Metallicity Of Planets In The Solar System

Jupiter has a metallicity of about two to five times that of the solar system's sun, while Saturn has a metallicity of about 10 times that of the sun. The low values suggest that these two gas giants are almost made up of hydrogen and helium.

Neptune and Uranus, the ice giants of the solar system, are smaller compared with the gas giants but have heavier elements with 100 times the metallicity of the sun.

What Metallicity Tells About The Formation And Evolution Of Planets

In the solar system, the trend is that bigger planets like Jupiter and Saturn tend to have lower metallicities.

Astronomers said that this likely happened because during the formation of the solar system, Neptune and Uranus formed in the outskirts of the disk that circled around the sun and these planets were likely pounded by icy debris carrying heavier elements.

The metallicity of Saturn and Jupiter indicates that they likely formed closer to the sun, where there was fewer icy debris present. The gas giants formed in the warmer region of the disk, which means they were less hit by those objects.

Metallicity Of Warm Neptune Hat-P-26b Has Much Closer Value With Jupiter

Researchers found that HAT-P-26b has 4.8 times the amount of heavy elements found in the sun, which means it has relatively low metallicity when compared with the standards of the solar system.

Based on the assumed metallicity of the exoplanet, researchers also argued that HAT-P-26b formed closer to its star than it is now. Despite having a mass comparable with Neptune and Uranus, its metallicity is closer in value to Jupiter's than the two icy planets.

"We were expecting [HAT-P-26b] to have a very high metallicity, but what we found is it's actually closer to Jupiter in the amount of heavy elements it has in its atmosphere," said study researcher Hannah Wakeford, of NASA's Goddard Space Flight Center in Greenbelt, Maryland.

The researchers said that their analysis shows more diversity in the atmosphere of exoplanets than expected. It also offers insights on how exoplanets can form and evolve differently from the solar system. Scientists said they often find diversity when studying extraterrestrial worlds.

"I would say that has been a theme in the studies of exoplanets: Researchers keep finding surprising diversity," said study researcher David Sing from the University of Exeter.

Results of the analysis of warm Neptune were reported in a study published in Science on May 12.

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