Imagine the year 2158. You’re pursuing a PhD in planetary volcanology at the University of Utopia Planitia on Mars, surviving on freeze dried ramen while searching for the perfect exoplanet to study. After finishing your research on Jupiter’s volcanic moon Io, you need a rocky world beyond our solar system with intense volcanism driven not by gravity, but by the searing heat of a nearby star. Better yet, it has to be within 50 light years so your faster than light (FTL) research mission stays within budget.
While that scenario is fictional, astronomers today are already studying a remarkable candidate.
Using NASA’s James Webb Space Telescope (JWST), researchers have investigated the super Earth 55 Cancri e (55 Cnc e), an extreme rocky planet located about 41 light years from Earth. Measuring roughly 1.88 Earth radii and about 8 Earth masses, the planet circles a Sun like star in only about 0.7 days. By comparison, Mercury takes 88 days to orbit our Sun.
Because 55 Cancri e orbits so close to its star, scientists believe its surface is hot enough to remain molten. Their findings, which have been submitted for publication in Nature Astronomy, could provide valuable insights into how lava exoplanets form and evolve.
James Webb Detects a Hydrogen Rich Atmosphere
The research team observed five eclipses of 55 Cancri e with JWST and compared the results with long standing models of rocky exoplanet evolution. Those models have generally predicted atmospheres rich in carbon monoxide (CO) and carbon dioxide (CO2).
Instead, the new observations point to an atmosphere containing abundant carbon monoxide, relatively small amounts of carbon dioxide (CO2), and surprisingly large amounts of hydrogen.
The researchers also found differences among the five eclipse observations. They suggest those variations could be caused by volcanic outgassing or by clouds that form from material released by the planet’s interior. According to the researchers, these clouds may temporarily cool the planet’s surface before continued outgassing disperses them.
The study notes, “Since secondary atmospheres of rocky planets are set by the composition of the interior and subsequent outgassing, the composition of their atmospheres is directly linked to their interior redox states. The preference for hydrogen-rich models, together with the steep inversions they produce, therefore suggests an interior with relatively low oxygen fugacity, consistent with outgassing from a reduced magma ocean.”
What the Planet’s Chemistry Reveals
A planet’s redox state describes the chemical balance between oxygen and hydrogen/iron within its interior. For 55 Cancri e, the results indicate that hydrogen is strongly favored over oxygen, helping explain why the planet appears to possess a hydrogen rich atmosphere.
Because an atmosphere can reflect what is happening deep inside a planet, these observations may offer a rare window into the chemistry of an alien world’s interior.
Lava Exoplanets Are Becoming Increasingly Common
Interest in lava exoplanets has grown rapidly over the past decade as more of these extreme worlds have been discovered, although 55 Cancri e itself was first identified in 2004.
Other known lava exoplanets include K2-141 b, L 98-59 d, TOI-561 b, HD 63433 d, and CoRoT-7 b. Their orbital periods are approximately 6.7 hours, 7.5 days, 10.5 hours, 4.2 days, and 20.4 hours, respectively.
Like 55 Cancri e, these planets are tidally locked to their host stars and endure extraordinary temperatures. On 55 Cancri e, molten rock is thought to be concentrated on the permanently sunlit side. Other worlds, such as L 98-59 d, may be covered by a global magma ocean resembling the volcanic landscape of Jupiter’s moon Io.
Io Versus Lava Exoplanets
Although Io and lava exoplanets can both feature extensive volcanism, the forces driving that activity are very different.
Io’s volcanoes are powered by tidal heating. Jupiter’s immense gravity continually stretches and compresses the small moon, generating enough internal heat to fuel widespread volcanic eruptions.
Lava exoplanets such as 55 Cancri e, by contrast, are heated primarily because they orbit extremely close to their host stars. Their intense stellar heating melts rock at the surface, and because many of these worlds are tidally locked, the molten regions can remain concentrated on the permanently daylit side.
As astronomers continue using powerful observatories such as JWST, 55 Cancri e and other lava worlds could reveal even more about the formation, evolution, and hidden interiors of some of the most extreme rocky planets ever discovered.