Key Takeaways:

  • It’s a rocky planet much larger than Earth, likely tidally locked with a scorching dayside and a cooler nightside. New data suggests it may have a thin atmosphere.
  • The atmosphere might be rich in carbon dioxide or monoxide, unlike previously detected atmospheres of gas giant planets.
  • The atmosphere is likely replenished by gases continuously released from the planet’s magma ocean due to extreme heat.
  • Studying 55 Cancri e may provide insights into the conditions present on early Earth, Venus, and Mars.
  • Understanding this planet’s atmosphere helps scientists identify factors for a rocky planet to sustain a life-supporting atmosphere.

Finding the planet’s atmosphere could reveal information about the early conditions of Earth, Venus, and Mars, even though the planet is too hot to support life.

55 Cancri e is a hot, rocky exoplanet located 41 light-years from Earth. Researchers may have found atmospheric gases surrounding it using NASA’s James Webb Space Telescope. This is the strongest proof that a rocky planet atmosphere exists anywhere outside of our solar system that has been found to date.

Lead author of a paper that was published in Nature today is Renyu Hu from NASA’s Jet Propulsion Laboratory in Southern California. “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

One of the five planets known to orbit the Sun-like star 55 Cancri in the Cancer constellation is 55 Cancri e, also referred to as Janssen. The planet is categorized as a super-Earth because it is larger than Earth, smaller than Neptune, and most likely has a composition similar to the rocky planets in our solar system. Its diameter is almost twice that of Earth and its density is slightly higher.

Infographic titled “Super-Earth Exoplanet 55 Cancri e Secondary Eclipse Light Curve, MIRI Low-Resolution Spectroscopy.” At the top of the infographic is a diagram showing a planet moving behind its star (a secondary eclipse).
Data from the Mid-Infrared Instrument on NASA’s Webb telescope shows the decrease in brightness of the 55 Cancri system as the rocky planet 55 Cancri e moves behind the star, a phenomenon known as a secondary eclipse. The data indicates that the planet’s dayside temperature is about 2,800 degrees Fahrenheit. Credit: NASA, ESA, CSA, Joseph Olmsted (STScI), Aaron Bello-Arufe (JPL)

But calling 55 Cancri e “rocky” could give the wrong impression. The planet is likely covered in a bubbling ocean of magma due to its close orbit, which is around 1.4 million miles (one-twentieth the distance between Mercury and the Sun). The planet is probably tidally locked, with a dayside that faces the star constantly and a nightside that is always in the dark, due to its extremely tight orbit.

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.

“I’ve worked on this planet for more than a decade,” said Diana Dragomir, an exoplanet researcher at the University of New Mexico and co-author on the study. “It’s been really frustrating that none of the observations we’ve been getting have robustly solved these mysteries. I am thrilled that we’re finally getting some answers!

The thinner and denser atmospheres surrounding rocky planets have remained elusive, in contrast to the atmospheres of gas giant planets, which are comparatively easy to spot (the first was detected by NASA’s Hubble Space Telescope more than two decades ago).

Prior research on 55 Cancri e using data from NASA’s eliminated Spitzer Space Telescope revealed the existence of an extensive atmosphere full of volatiles (molecules that are found on Earth in gaseous form), such as carbon dioxide, nitrogen, and oxygen. However, scientists were unable to rule out the alternative theory that the planet is empty of all life except for a thin layer of vaporized rock that is rich in elements like calcium, silicon, iron, and aluminum. “The planet is so hot that some of the molten rock should evaporate,” explained Hu.

Graphic titled “Super-Earth Exoplanet 55 Cancri e Emission Spectrum, NIRCam Grism Spectroscopy (F444W), MIRI Low-Resolution Spectroscopy” shows a graph of amount of light detected from planet versus wavelength of light, with 2 model emission spectra.
A thermal emission spectrum of the exoplanet 55 Cancri e — captured by the NIRCam instrument, GRISM Spectrometer, and MIRI Low-Resolution Spectrometer on NASA’s Webb telescope — shows that the planet may be surrounded by an atmosphere rich in carbon dioxide or carbon monoxide and other volatiles. Credit: NASA, ESA, CSA, Joseph Olmsted (STScI), Renyu Hu (JPL), Aaron Bello-Arufe (JPL), Michael Zhang (University of Chicago), Mantas Zilinskas (SRON Netherlands Institute for Space Research)

Measuring Subtle Variations in Infrared Colors

The team measured 4- to 12-micron infrared light coming from the planet using Webb’s NIRCam (Near-Infrared Camera) and MIRI (Mid-Infrared Instrument) in order to differentiate between the two possibilities.

Webb is unable to take a direct picture of 55 Cancri e, but it is able to detect minute variations in the light coming from the system as the planet orbits around the star.

The team calculated the amount of different wavelengths of infrared light coming from the planet’s dayside by subtracting the brightness during the secondary eclipse, when the planet is behind the star (starlight only), from the brightness when the planet is directly beside the star (light from the star and planet combined).

Secondary eclipse spectroscopy is a technique that has been employed by other research groups to look for atmospheres on other rocky exoplanets, such as TRAPPIST-1 b.

Cooler Than Expected

Temperature readings based on 55 Cancri e’s thermal emission—heat energy released in the form of infrared light—were the first clue that the planet might have a significant atmosphere. The dayside should be approximately 4,000 degrees Fahrenheit (~2,200 degrees Celsius) if the planet is covered in dark molten rock with a thin veil of vaporized rock or no atmosphere at all.

“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.” Lava currents have the ability to transport some heat to the nightside, but not enough to account for the cooling effect.

When the team looked at the NIRCam data, they saw patterns consistent with a volatile-rich atmosphere. Co-author Aaron Bello-Arufe of NASA JPL added, “We see evidence of a dip in the spectrum between 4 and 5 microns — less of this light is reaching the telescope.” “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 10 years modeling different scenarios, trying to imagine what this world might look like,” said co-author Yamila Miguel from the Leiden Observatory and the Netherlands Institute for Space Research (SRON). “Finally getting some confirmation of our work is priceless!”

Bubbling Magma Ocean

The team believes that rather than having been there since the planet’s formation, the gases that cover 55 Cancri e would be bubbling out from the interior. “The primary atmosphere would be long gone because of the high temperature and intense radiation from the star,” said Bello-Arufe. This would be a secondary atmosphere that the magma ocean continuously restores. Magma contains a significant amount of dissolved gas in addition to crystals and liquid rock.

Although 55 Cancri e is far too hot to support life, scientists believe it may offer a unique window into the interactions between rocky planet surfaces, interiors, and atmospheres. It may also shed light on the conditions that existed in the early phases of Earth, Venus, and Mars—planets that are believed to have been submerged in magma oceans long ago.

“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 study was carried out as a part of the 1952 Webb General Observers (GO) Program. We are currently analyzing more secondary eclipse observations of 55 Cancri e.

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