Webb data suggest potential atmosphere around rocky exoplanet
Researchers using NASA’s James Webb Space Telescope may have detected atmospheric gases surrounding 55 Cancri e, a hot rocky exoplanet 41 light-years from Earth. This is the best evidence to date for the existence of any rocky planet atmosphere outside our solar system.
Renyu Hu from NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, is lead author on a paper published today in Nature. “Webb is pushing the frontiers of exoplanet characterization to rocky planets,” Hu said. “It is truly enabling a new type of science.”
Super-Hot Super-Earth 55 Cancri e
55 Cancri e, also known as Janssen, is one of five known planets orbiting the Sun-like star 55 Cancri, in the constellation Cancer. With a diameter nearly twice that of Earth and density slightly greater, the planet is classified as a super-Earth: larger than Earth, smaller than Neptune, and likely similar in composition to the rocky planets in our solar system.
To describe 55 Cancri e as “rocky,” however, could leave the wrong impression. The planet orbits so close to its star (about 1.4 million miles, or one-twenty-fifth the distance between Mercury and the Sun) that its surface is likely to be molten – a bubbling ocean of magma. With such a tight orbit, the planet is also likely to be tidally locked, with a dayside that faces the star at all times and a nightside in perpetual darkness.
In spite of numerous observations since it was discovered to transit in 2011, the question of whether or not 55 Cancri e has an atmosphere – or even could have one given its high temperature and the continuous onslaught of stellar radiation and wind from its star – has gone unanswered.
"Most of my research has been focused on trying to unravel the ambiguous nature of this planet”, said Mantas Zilinskas, a Postdoctoral researcher at the Netherlands Institute for Space Research (SRON) and co-author on the study. "With Webb, we can finally start putting strong constraints on super-Earths, shedding light on their atmospheres and compositions with unprecedented detail."
Unlike the atmospheres of gas giant planets, which are relatively easy to spot (the first was detected by NASA’s Hubble Space Telescope more than two decades ago), thinner and denser atmospheres surrounding rocky planets have remained elusive.
Previous studies of 55 Cancri e using data from NASA’s now-retired Spitzer Space Telescope suggested the presence of a substantial atmosphere rich in volatiles (molecules that occur in gas form on Earth) like oxygen, nitrogen, and carbon dioxide. But researchers could not rule out another possibility: that the planet is bare, save for a tenuous shroud of vaporized rock, rich in elements like silicon, iron, aluminum, and calcium. “The planet is so hot that some of the molten rock should evaporate,” explained Hu.
Measuring Subtle Variations in Infrared Colors
To distinguish between the two possibilities, the team used Webb’s NIRCam (Near-Infrared Camera) and MIRI (Mid-Infrared Instrument) to measure 4- to 12-micron infrared light coming from the planet.
Although Webb cannot capture a direct image of 55 Cancri e, it can measure subtle changes in light from the system as the planet orbits the star.
By subtracting the brightness during the secondary eclipse, when the planet is behind the star (starlight only), from the brightness when the planet is right beside the star (light from the star and planet combined), the team was able to calculate the amount of various wavelengths of infrared light coming from the dayside of the planet.
This method, known as secondary eclipse spectroscopy, is similar to that used by other research teams to search for atmospheres on other rocky exoplanets, like TRAPPIST-1 b.
About MIRI
The MIRI instrument on the James Webb Space Telescope was built in a partnership between Europe and the US. The Dutch Research School for Astronomy (NOVA) was responsible for the main optics of the MIRI spectrometer, with ASTRON and TNO as subcontractors and with contributions from SRON. The Dutch research funder NWO provided financial support.
Cooler than Expected
The first indication that 55 Cancri e could have a substantial atmosphere came from temperature measurements based on its thermal emission, or heat energy given off in the form of infrared light. If the planet is covered in dark molten rock with a thin veil of vaporized rock or no atmosphere at all, the dayside should be around 4,000 degrees Fahrenheit (~2,200 degrees Celsius).
“Instead, the MIRI data showed a relatively low temperature of about 2,800 degrees Fahrenheit [~1540 degrees Celsius],” said Hu. “This is a very strong indication that energy is being distributed from the dayside to the nightside, most likely by a volatile-rich atmosphere.” While currents of lava can carry some heat around to the nightside, they cannot move it efficiently enough to explain the cooling effect.
When the team looked at the NIRCam data, they saw patterns consistent with a volatile-rich atmosphere. “We see evidence of a dip in the spectrum between 4 and 5 microns — less of this light is reaching the telescope,” explained co-author Aaron Bello-Arufe, also from NASA JPL. “This suggests the presence of an atmosphere containing carbon monoxide or carbon dioxide, which absorb these wavelengths of light.” A planet with no atmosphere or an atmosphere consisting only of vaporized rock would not have this specific spectral feature.
“We’ve spent the last ten years modelling different scenarios, trying to imagine what this world might look like,” said associate professor and co-author Yamila Miguel from the Leiden Observatory and SRON. “Finally getting some confirmation of our work is priceless!”
Bubbling Magma Ocean
The team thinks that the gases blanketing 55 Cancri e would be bubbling out from the interior, rather than being present ever since the planet formed. "With NIRCam we see potential evidence of carbon monoxide or carbon dioxide, said PhD candidate Christiaan van Buchem, from Leiden Observatory, “lava is able to store large amounts of water and carbon dioxide. Hence, we think that the lava ocean on 55-Cnc e could serve as a reservoir, continuously supplying the atmosphere with gas.”
While 55 Cancri e is far too hot to be habitable, researchers think it could provide a unique window for studying interactions between atmospheres, surfaces, and interiors of rocky planets, and perhaps provide insights into the early conditions of Earth, Venus, and Mars, which are thought to have been covered in magma oceans far in the past. “Ultimately, we want to understand what conditions make it possible for a rocky planet to sustain a gas-rich atmosphere: a key ingredient for a habitable planet,” said Hu.
This research was conducted as part of Webb’s General Observers (GO) Program 1952.
The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and the Canadian Space Agency.
Article
A secondary atmosphere on the rocky exoplanet 55 Cancri e, Renyu Hu et al. Appears today in Nature: https://www.nature.com/articles/s41586-024-07432-x.
About NOVA
The Netherlands Research School for Astronomy (NOVA) is the partnership of the astronomical institutes of the universities of Amsterdam, Groningen, Leiden and Nijmegen. Top research school NOVA's mission is to conduct groundbreaking astronomy research, train young astronomers at the highest international level and share new discoveries with society. The NOVA laboratories specialize in building state-of-the-art optical/infrared and submillimeter instrumentation for the largest telescopes on Earth.