A Forge Without Iron

Imagine standing on a small rock, surrounded on all sides by a sea of boiling lava. Above you, an enormous star looms across most of the sky. Take a deep breath in: what do you smell, what’s in the air? While no humans have yet been in this situation, astronomers are making progress towards answering this question using instruments back here on Earth.

Worlds Beyond Imaginings

Ours is an era of wonder, one in which we are just beginning to discover planets beyond our own solar system and just beginning to realize how strange these worlds can be. Exoplanet astronomers have confirmed that over the years, science fiction writers largely underestimated the universe: included in the menagerie of the first discovered 5,000 planets are worlds with not one, or two, but three stars; planets that are fluffier than cotton candy; and planets where the clouds are made of rock. Each of these worlds offers the chance to contextualize our own home and to study how our galaxy creates and maintains planets.

One extreme and somewhat famous exoplanet discovered back in 2004 is named 55 Cancri e. In some ways, this world might remind us of our own. It’s a little bigger than Earth (it has a radius about twice as large), and it’s also likely rocky, with a bulk density a little higher than the terrestrial value. The comparisons to our home world abruptly end, however, when we consider where the planet is: it nearly skims the surface of its host star, whipping around it on an orbit that takes only about 17 hours. It is so close that were you to stand on the surface of the planet, the star would dominate your view of the sky. Also, you’d likely be standing in lava, since the ambient temperature is high enough to melt the upper crust.

Astronomers have been understandably fascinated with this molten world, and over the past 20 years numerous groups have labored to characterize it. Unfortunately, it is difficult to measure something as small as a planet from 41 light-years away — current estimates of the surface pressure range from a near-Earth-like 1.4 bar all the way to the pressures felt more than a mile undersea. Even so, progress has been made, and the latest step came recently from the first analysis of 55 Cancri e’s emission spectrum using a high-resolution instrument.

New Observations

A wavelength vs electron counts plot. The data appear as several dozen side-by-side Gaussian looking bumps, each of which is outlined with a black line.

One of the high-res spectra collected by MAROON-X of 55 Cancri. The black lines mark the borders between different echelle orders. [Rasmussen et al. 2023]

A team led by Kaitlin Rasmussen and Miles Currie, both University of Washington, collected observations using an exquisitely sensitive spectrograph named MAROON-X with the goal of measuring which elements were present in 55 Cancri e’s atmosphere. Before jumping straight to data analysis, however, they first determined which elements they could detect, if any were truly there. This was a valuable check: even though theories suggest that some combination of Mg, SiO, Na, K, H2O, and CO2 should be present, the researchers found that their data were insufficiently sensitive to detect these species. Happily, however, their data would be sensitive to iron: if any was present floating above the magma oceans, MAROON-X would be able to sniff it out.

A 3-panel plot of heatmaps showing radial velocity on the X axis and Kp on the Y axis, both in km/s. The bright central feature fades as you move from left to right.

The actual data (right), an injection of what an iron signature was expected to look like (middle), and an injection of an iron signal 10x stronger than expected (left). Only a very weak signal was recovered in the real data, which implies that there is very little iron present. [Rasmussen et al. 2023]

Confident they could detect iron should it be there, the team then turned to their real data and found very little sign of it. This left them confident that 55 Cancri e probably does not have a thick iron atmosphere, and gives the broader astronomy community the first constraint from high-resolution emission spectroscopy on whatever is going on in the air above this fiery world. It will likely not be the last, since 55 Cancri e is an upcoming JWST target. That brings us back to a common refrain these days in exoplanet science: to better understand this strange new world, we’ll have to wait for JWST to take a look.


“A Nondetection of Iron in the First High-resolution Emission Study of the Lava Planet 55 Cnc e,” Kaitlin C. Rasmussen et al. 2023 AJ 166 155. doi:10.3847/1538-3881/acf28e