Discovery of 'Warm Neptune' Challenges Theories on Planet Births

About four hundred and thirty light-years from Earth, in a separate galaxy, lies a Neptune-sized planet that has water in its atmosphere, according to a team of global astronomers.

The study found that the atmosphere of the HAT-P-26b is relatively clear of clouds and has a strong water signature, however, the planet is not a water world.

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But though HAT-P-26b is roughly the same size as Neptune, it is closer in metallicity to Jupiter and formed closer to its host star - it's an anomaly. Scientists discovered the planet in 2010, but have just now uncovered information about its composition that could totally reshape how we think about the births and deaths of planets. The metallicity of HAT-P-26b was lower than would be expected for a planet of its size, suggesting the atmosphere is "young" and probably formed after the main body of the planet, without much subsequent contamination from space debris or impacts.

When the transit takes place, a fraction of the starlight gets filtered through the atmosphere of the planet.

It's worth noting, that collecting a transmission spectrum isn't a precise science yet, and there's still a large range of uncertainty in this measurement: the true abundance of heavy elements in the planet's atmosphere could range from 0.8 to 26 times the Sun's value.

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Like Earth in Kevin Costner's 1995 epic Waterworld, HAT-P-26b contains a surprising amount of water.

Speculated possibilities include atmospheres rich in hydrogen and helium, in carbon dioxide, or in water, depending on their formation and evolutionary history. During the transit period, some of the wavelengths of light of the host star are also absorbed by the planet's atmosphere while others are not. This pointed towards how rich the planet is in elements which are heavier than hydrogen and helium. However, the study was innovative in the sense that it applied the technique to a much smaller planet than previous efforts.

Jupiter and Saturn formed in a warmer part of the disc, meaning they weren't hit by as many of those objects. The discovery points out at a greater diversity in the atmospheres of exoplanets than was previously assumed, says Professor Sing.

"This analysis shows that there is a lot more diversity in the atmospheres of these exoplanets than we were expecting, which is providing insight into how planets can form and evolve differently than in our solar system", said David K Sing of the University of Exeter and the second author of the paper. Data are archived at the Infrared Science Archive housed at the Infrared Processing and Analysis Center at Caltech.

In recent years, telescope and telescope arrays such as NASA's Kepler have revealed several intriguing planets, greatly expanding our understanding of alien worlds and their solar systems. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington.

  • Douglas Reid