Astronomers discover a rare hot Neptune planet that ‘shouldn’t exist’

Astronomers have discovered a planetary oddity: An exoplanet of an extremely rare type called a hot Neptune.

Hot Neptunes are planets around the size of Neptune which lie close to their stars. Astronomers think the reason they are so rare could be because planets of this size rapidly lose their atmospheres when they are close to their stars, and they are quickly eroded down to Earth-size. Hot Neptunes are so rare that astronomers even refer to the area close to a star as a “Neptune desert.”

But now, the team from the University of Kansas has spotted one such hot Neptune exoplanet in data from NASA’s TESS and Spitzer missions. Planet LTT 9779b was investigated using a technique called phase curve analysis, in which the infrared light being emitted by the planet is measured to see which parts of the planet are hottest.

“For the first time, we measured the light coming from this planet that shouldn’t exist,” Ian Crossfield, lead author of the paper, said in a statement. “This planet is so intensely irradiated by its star that its temperature is over 3,000 degrees Fahrenheit and its atmosphere could have evaporated entirely. Yet, our Spitzer observations show us its atmosphere via the infrared light the planet emits.”

This artist's impression shows the LTT9779 system approximately to scale, with the hot Neptune-sized planet at left and its bright, nearby star at right. The trail of material streaming off of the planet is hypothetical but likely, based on the intense irradiation of this planet.
This artist’s impression shows the LTT9779 system approximately to scale, with the hot Neptune-sized planet at left and its bright, nearby star at right. The trail of material streaming off of the planet is hypothetical but likely, based on the intense irradiation of this planet. Ethen Schmidt | University of Kansas

As well as being an unusual find, the planet’s location leads to some bizarre features as well. “This planet doesn’t have a solid surface, and it’s much hotter even than Mercury in our solar system — not only would lead melt in the atmosphere of this planet, but so would platinum, chromium, and stainless steel,” Crossfield said. “A year on this planet is less than 24 hours — that’s how quickly it’s whipping around its star. It’s a pretty extreme system.”

As an unusual object, the planet is a prime target for follow-up study using the upcoming James Webb Space Telescope. And by using new techniques to examine the planet’s atmosphere, astronomers can develop more accurate tools for searching for habitable planets in the future.

“As someone who studies these, there’s just a lot of interesting planetary science we can do in measuring the properties of these planets — just like people study the atmospheres of Jupiter, Saturn and Venus — even though we don’t think those will host life,” Crossfield said. “They’re still interesting, and we can learn about how these planets formed and the broader context of planetary systems.”

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