Using the James Webb Space Telescope (JWST), astronomers have discovered that a scorching hot lava planet, believed to be made of diamond, has gained a second atmosphere – and that was after its star destroyed its first atmosphere.
The planet, known as 55 Cancri e, is located about 41 light-years from the Solar System and has a width almost twice that of Earth and a mass about nine times that of our planet. Among the vast array of extrasolar planets, or “exoplanets,” that scientists have cataloged over the years, this world is classified as a “super-Earth.” That means it is more massive than Earth, but much lighter than planets like Neptune and Uranus. Yet that is where the comparisons with our world end for 55 Cancri e.
This exoplanet is so compact that astronomers have hypothesized that it consists mostly of carbon compressed into diamond. Moreover, the exoplanet is only 2.3 kilometers away from its Sun-like star, 55 Cancri A. That is equal to 0.01544 times the distance between Earth and the Sun. This proximity means that 55 Cancri e orbits its host star every about 17 Earth hours and has a scorching surface temperature of about 4,400 degrees Fahrenheit (about 2,400 degrees Celsius).
Radiation from its star has thus stripped 55 Cancri e of its original or primary atmosphere, just as is seen in other rocky planets that orbit their stars at such close distances. However, the new research suggests that a thick layer of gases surrounds the planet, implying that a second atmosphere has ‘grown’ – and the scientists behind the discovery think they know how this happened.
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“We measured the thermal emissions of this rocky planet, and the measurement indicates 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 is likely supported by degassing from the rocky interior of 55 Cancri e, and we believe this is the first measurement of a secondary atmosphere on a rocky exoplanet. It’s very exciting.’
How 55 Cancri e defied its star
55 Cancri e was discovered in 2004 because of the “wobble” it caused in the motion of its parent star, as seen from our perspective on Earth. This is called the radial velocity exoplanet discovery method for exoplanet discovery. The world, originally named Janssen, was the first identified super-Earth orbiting a distant main sequence star, or a star still converting hydrogen to helium in its core.
As the planet was further explored, scientists also learned about its 0.7 Earth-day orbit and carbon composition. Then in 2016, the Hubble Space Telescope determined that 55 Cancri e’s atmosphere contained hydrogen and helium in what was the first atmospheric survey of an exoplanet.
Two possible scenarios exist to explain the atmosphere of 55 Cancri e.
First, the super-Earth could be a lava world with a thin, evaporated silicate atmosphere. It would be made of the planet’s volatile and chemical compounds, such as carbon, nitrogen, hydrogen and sulfur, which can easily be lost by irradiation from the star. Or, alternatively, the planet could have a thick layer subordinate atmosphere created over time by volcanism.
To investigate which of these scenarios was correct, Hu and his colleagues examined JWST observations of the planet as it passed behind the star 55 Cancri A, an event called a secondary solar eclipse. Data from two secondary eclipses of 55 Cancri e ruled out the possibility of it being a nearly barren lava world without a substantial atmosphere.
There is no doubt that the planet is a lava-covered hellscape, and the team even thinks that it is this molten nature that has allowed Cancri e’s secondary atmosphere to grow.
“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 aids in the degassing process that supports a secondary atmosphere due to a molten surface.”
He explained that gas dissolves in the global lava ocean of Cancri, continuously ‘bubbling out’ to form the secondary atmosphere.
The researcher added that 55 Cancri e’s original atmosphere, which it would have had since it formed around its star, would have consisted mainly of hydrogen and helium. However, the composition of the secondary atmosphere that replaced the first remains uncertain.
“The composition of the secondary atmosphere depends on where the rock beneath it is made,” Hu said. “If the stone is very reducing [made of compounds that gain electrons and hydrogen], it can also create a hydrogen-helium atmosphere like the primary atmosphere. But if the rock is more like the Earth’s mantle, water, carbon monoxide and carbon dioxide would dominate the secondary atmosphere.”
Hu added that while the JWST observations of 55 Cancri e do not say with certainty what the planet’s atmosphere is made of, the 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 certainly not the only rocky planet orbiting so close to its host star, although Hu points out that it is among the hottest of its kind. Does this mean that these other scorching hot terrestrial worlds could also have developed secondary atmospheres? The team isn’t sure.
That’s because there’s something quite unique about 55 Cancri e.
“At 1.8 times the size of Earth, it’s a fairly large piece of rock – and that helps trap the volatiles from the irradiation of stars,” Hu explains. “We expect that a very small rocky planet in a very close orbit around its star could lose the entire volatile budget of the entire planet, and then become atmosphereless.”
That means it’s not just the distance between a planet and its star that determines whether one planet will retain its atmosphere and another will ‘grow’, but also the size of that world. Hu pointed out that 55 Cancri e appears to be “optimized” in both respects for replacing a lost primary atmosphere with a secondary atmosphere.
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Hu said analyzing 55 Cancri e in this way would have been impossible without the JWST’s observability and infrared sensitivity, making it ideal for characterizing the atmospheres of exoplanets.
“We are certainly thinking about the next steps for studying 55 Cancri e. We have some ideas about measuring the planet’s thermal emission, not only during secondary eclipses, but also as the planet orbits the star,” said Hu. “This will tell us about the size of the atmosphere and the circulation within it.”
The team’s research was published Wednesday (May 8) in the journal Nature.