by Using the James Webb Space Telescope (JWST), astronomers have discovered that a planet of scorching hot lava, believed to be composed of diamond, grew a second atmosphere – after the star destroyed its first atmosphere .
The planet, called 55 Cancri e, is located about 41 light-years from the solar system and is almost twice the width of Earth and about nine times the mass of our planet. Among the bucket of extrasolar planets, or “exoplanets,” scientists have cataloged over the years, this world is classified as “Super-Earth.” That means it is more massive than Earth, but much lighter than planets like Neptune and Uranus. However, that’s where comparisons end with our world for 55 Cancri e.
This exoplanet is so dense that astronomers thought it was composed mostly of carbon compressed into diamond. Also, the exoplanet is only 1.4 miles (2.3 kilometers) from its sun-like star, 55 Cancri A. That’s equal to 0.01544 times the distance between Earth and the sun. This proximity means that the host star orbits 55 Cancri ea once every 17 Earth hours and has a roasting hot surface temperature of around 4,400 degrees Fahrenheit (about 2,400 degrees Celsius).
Thus the radiation from its star has stripped 55 Cancri of its original or main atmosphere, just as it appears to other rocky planets that orbit their stars so closely together. However, the new research shows that the planet is surrounded by a thick layer of gas, suggesting that it has a “growing” second atmosphere – and the scientists behind the discovery think they know how this happened.
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“We measured the thermal emissions from this rocky planet, and the measurement shows that the planet has a substantial atmosphere,” Renyu Hu, a member of the team behind this discovery and a researcher at the California Institute of Technology (Caltech), told Space.com . “This atmosphere likely supports escape from the rocky interior of 55 Cancri e, and we think this is the first measurement of a secondary atmosphere on a rocky exoplanet. It’s very exciting.”
How 55 Cancri e defied a star
55 Cancri e was discovered in 2004 by the “wobble” it caused in the motion of its host star, as seen from our perspective on Earth. This is called the radial velocity exoplanet discovery method for exoplanet discovery. Originally named Janssen, the world was the first Super-Earth to be identified orbiting a distant main-sequence star, or a star that is still converting hydrogen to helium at its core.
As the planet was further investigated, scientists also learned about its 0.7 Earth day orbit and its carbon composition. Then, in 2016, the Hubble space telescope determined the presence of hydrogen and helium in the atmosphere of 55 Cancri e during the first atmospheric investigation of an exoplanet.
There are two possible scenarios to explain the atmosphere of 55 Cancri ea.
First, Super Earth could be a lava world with a thin atmosphere of vaporized silicate. It would be made of the planet’s volatiles and chemical compounds such as carbon, nitrogen, hydrogen and sulfur that can easily be lost due to radiation from its star. Or, alternatively, the planet could be thick secondary an atmosphere created over time by volcanism.
To investigate which of these scenarios was correct, Hu and colleagues examined JWST observations of the planet as it passed behind the star 55 Cancri A, an event known as a secondary eclipse. Data from two secondary eclipses of 55 Cancri e ruled out the possibility of it being a nearly bare lava world with no substantial atmosphere.
There is no doubt that the planet is a lava-coated inferno, and the team even thinks that it is this molten nature that helped 55 Cancri ea grow a secondary atmosphere.
“55 Cancri e is so close to the host star that it receives a lot of heat in the form of radiation. That heat keeps the temperature on the planet very high,” Hu said. “At these temperatures, everything on the planet is molten. If it’s rock, it’s molten lava, which helps the outgassing process that supports a secondary atmosphere due to a molten surface.”
He explained that gas dissolves in the global 55 Cancri lava ocean, constantly “bulging out” to form the secondary atmosphere.
The researcher also said that the original atmosphere of 55 Cancri e, which it would have had since its formation around its star, would have been composed mainly of hydrogen and helium. However, the composition of the secondary atmosphere that replaced the first atmosphere is still uncertain.
“The composition of the secondary atmosphere depends on where the underlying rock is from,” Hu said. “If the rock is shrinking a lot [made of compounds that gain electrons and hydrogen], it can also create a hydrogen-helium atmosphere similar to the primary atmosphere. But if the rock is more like Earth’s mantle, the secondary atmosphere would be dominated by water, carbon monoxide and carbon dioxide.”
Hu added that while the JWST 55 Cancri e observations do not say definitively what the planet’s atmosphere is made up of, models used to explain the measurements favor a significant amount of carbon dioxide and carbon monoxide.
Can 55 Cancri e really create a secondary atmosphere?
55 Cancri e is not the only rocky planet seen orbiting so close to its host star, although Hu points out that it is one of the hottest of its kind. So does this mean that these other hotter worlds could have grown on secondary atmospheres as well? Well, the team is not sure.
That is because there is something quite unique about 55 Cancri e.
“At 1.8 times the Earth, it is a fairly large chunk of rock – and that helps keep the volatile against the radiation of a star,” explained Hu. “We expect that a very small rocky planet in a very close orbit around its star could lose its entire fluctuation budget from the entire planet, and then it will have less atmosphere.”
That means it’s not just the distance between a planet and its star that determines whether a planet will retain its atmosphere and “grow” another plant, but also the size of that world. Hu pointed out that for both reasons, 55 Cancri e appears to be “optimized” for replacing a lost atmosphere with a secondary atmosphere.
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Hu said that the analysis of 55 Cancri e in this way would have been impossible without the observing power and infrared sensitivity of the JWST, which makes it suitable for characterizing exoplanet atmospheres.
“We are definitely thinking about the next steps to study 55 Cancri e. We have some ideas for making measurements of the planet’s thermal emissions, not only during secondary eclipses, but also as the planet revolves around the star ,” Hu said. “This will tell us about the size of the atmosphere and the circulation within.”
The team’s research was published on Wednesday (May 8) in the journal Nature.
