It’s 2158, and you’re chugging away on your PhD in Planetary Volcanology from the University of Utopia Planitia on Mars. Graduate students still get paid a sub-living wage, so you’ve been stuck eating freeze-dried ramen for the past three years. You’ve completed studying Jupiter’s moon, Io, but now you have to leave the solar system for a good exoplanet analog. While Io’s volcanism is caused by tidal heating, you need an exoplanet whose volcanism is caused by extreme heat from its host star. You recently secured funding from the Exoplanet Research Institute for a faster-than-light (FTL) ship, but the exoplanet is required to be less than 50 light-years away.

Back in the present day, scientists have used NASA’s James Webb Space Telescope to observe 55 Cancri e (55 Cnc e), which is designated as a super Earth with a radius and mass of about 1.88 and 8 Earths, respectively. It is located approximately 41 light-years away and is tidally locked to a Sun-like star, completing one orbit in about 0.7 days. It is this extremely close orbit that researchers hypothesize cause the exoplanet’s surface to melt. For context, Mercury orbits our Sun in 88 days. Their findings were recently submitted for publication in Nature Astronomy and could help scientists better understand the formation and evolution of lava exoplanets.

Using JWST, the researchers observed five eclipses of 55 Cnc e and compared it to longstanding models of exoplanet formation and evolution. These models have inferred that lava planets have high abundances of carbon monoxide (CO) and carbon dioxide (CO2). In the end, the researchers concluded that 55 Cnc e’s atmosphere is likely comprised of large amounts of CO and small amounts of C02 while also having large amounts of hydrogen. Additionally, the researchers noted that mixed data among the five eclipses could be indicative of outgassing or clouds forming from the outgassing. They note how the clouds could temporarily cool the surface until outgassing pushes the clouds away.

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.”

For context, a planet’s redox state is the chemical balance between oxygen and hydrogen/iron within its interior. In the context of 55 Cnc e, hydrogen is heavily favored compared to oxygen, resulting in the exoplanet’s hydrogen-rich atmosphere.

Artist's illustration of lava exoplanet CoRoT-7 b. (Credit: ESO/L. Calçada)

Lava exoplanets have come into the spotlight in recent years, as several have been discovered within the last decade, with 55 Cnc e being discovered in 2004. These exoplanets include K2-141 b, L 98-59 d, TOI-561 b, HD 63433 d, and CoRoT-7 b, which have orbital periods of about 6.7 hours, 7.5 days, 10.5 hours, 4.2 days, and 20.4 hours, respectively. Like 55 Cnc e, all these exoplanets are tidally locked to their host stars and experience extreme temperatures. While 55 Cnc e has its lava on the sun-facing side, exoplanets like L 98-59 d have its entire surface covered in a magma ocean like Jupiter’s moon Io.

As noted in the fictionalized tale above, the volcanism on Jupiter’s moon Io is caused by tidal heating, which occurs from the small moon being stretched and compressed by Jupiter’s immense gravity. In contrast, all lava exoplanets listed above, including 55 Cnc e, are the result of the extreme temperatures they endure while orbiting at extremely close distances to their host stars. While Io has volcanism all over its surface, some lava exoplanets have their lava surfaces on the sun-facing side due to them being tidally locked.

What new insights into 55 Cnc e and other lava exoplanets will researchers make in the coming years and decades? Only time will tell, and this is why we science!

As always, keep doing science & keep looking up!