Ultra-precise readings shed new light on a 'hell planet' with an 18-hour year

Planet 55 Cnc e, also known as "Janssen", orbits so close to its sun that a year is shorter than an Earth day.
Chris Young
An artist’s impression of the planet Janssen.
An artist’s impression of the planet Janssen.

ESA / Hubble, M. Kornmesser 

Scientists shed new light on planet 55 Cnc e, known by some as the "hell planet", revealing how it became so fiery.

55 Cnc e orbits its host star so closely it performs an entire rotation around it in only 18 hours. Its surface is essentially an ocean of lava, and its interior is considered teeming with diamonds.

That wasn't always the case, though. New, exact measurements of a planet roughly 40 light-years away from Earth allowed scientists to gain further insight into the way planets can turn into fiery hellscapes over many millennia, as per a press statement.

The evolution of "Janssen"

Scientists carried out the new analysis using a new tool called EXPRES that captured incredibly precise measurements of the starlight shining from Janssen's sun, Copernicus, or 55 Cnc.

The light measurements of the planet, also known by the nickname "Janssen", shifted very slightly as it moved between the Earth and the star. The scientists analyzed these readings and discovered that Janssen orbits Copernicus along the star's equator.

This means Janssen is an outlier in that solar system, which is on incredibly different orbital paths. The scientists detailed their findings in a paper in the journal Nature Astronomy.

"We’ve learned about how this multi-planet system — one of the systems with the most planets that we've found — got into its current state," explained study lead author Lily Zhao, a research fellow at the Flatiron Institute’s Center for Computational Astrophysics (CCA) in New York City.

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Based on their findings, the scientists Janssen formed in a cooler orbit further from Copernicus and then gradually fell toward its host star. Even while it was orbiting further from the sun, the planet “was likely so hot that nothing we're aware of would be able to survive on the surface,” Zhao said.

How did the scientists measure Janssen's orbital path?

Impressively, the scientists determined Janssen's orbital path by taking advantage of the Doppler effect. When a light source is moving toward you, the wavelength of the light is shorter, making it bluer.

When it's moving away, it's longer and redder. Using EXPRES, the scientists were able to pinpoint the side of Copernicus spinning away from us (as it's redder) and the side spinning toward us (it's bluer) as it rotates.

They then measured where Janssen was when passing over each side to calculate its orbital trajectory along the star's equator.

Scientists search for similar planetary systems to our own

The new findings could help the scientific community better understand how planets form and change dramatically over time. Venus, for example, has a fiery, inhospitable surface, though it is believed to have once had an Earth-like climate.

When Janssen was first discovered, some scientists doubted its existence. The planet was the first-ever example of an ultra-shoer-period planet, with a year only 18 hours long. Its orbit has a minimum radius of roughly 2 million kilometers, which is incredibly small compared to Earth's approximately 147 million orbital radius.

Now that they've shed new light on Janssen, Zhao and her team aim to study other planetary systems. "We’re hoping to find planetary systems similar to ours,” she explained, "and to better understand the systems that we do know about."