Atmosphere, Not Lava Flows, for Exoplanet 55 Cancri e

Previous studies of 55 Cancri e haven’t been able to determine whether this super-Earth hosts an atmosphere. A new study settles the question.

55 Cancri e, artist's concept

New research suggests that the super-Earth exoplanet 55 Cancri e, depicted with its star in this artist's concept, likely has a thick atmosphere surrounding the whole planet.
NASA / JPL-Caltech

The exoplanet 55 Cancri e is a cipher. Astronomers have gone back and forth on its nature — waterworld, diamond world, or volcanic hellscape? Part of the riddle has been whether the planet is bare rock or has an atmosphere — previous studies have shown ambiguous results.

Now, new research from Isabel Angelo and Renyu Hu (both at JPL-Caltech) published in the December Astrophysical Journal (full text here) seems to have settled the question: 55 Cancri e probably does have an atmosphere and a substantial one at that.

A Planet of Many Faces

55 Cancri e, also named Janssen for the inventor of the optical telescope, is in a scorchingly close orbit around a star 40 light-years away. At eight times Earth’s mass and almost twice its radius, it’s classified as a hot super-Earth, bigger than Earth but smaller than Neptune.

But there’s nothing Earth-like about this planet. Its average density is so high (5.7–7.2 g/cm3, compared to 5.5 g/cm3 for Earth), that astronomers have considered a planet-wide ocean or diamond-like interior. Neither of those scenarios has panned out so far — an ocean would have to be in a super-critical state to survive the star’s intense heat and a diamond interior is unlikely due to the host star’s composition.

Equally tough to figure out is whether this planet hosts an atmosphere — observations have shown ambiguous results.

Brice-Olivier Demory (University of Bern, Switzerland) led a team in taking Spitzer Space Telescope observations of the planet as it circled its star, watching not just the planet’s transits but everything in between in order to measure the planet’s brightness over a full orbit. The infrared telescope acted like night-vision goggles, enabling astronomers to see how much heat the planet was emitting.

In Demory and colleagues’ interpretation of the data, the planet is a blazing 1400K by night, nearly five times Earth’s average surface temperature. Days are even hotter at 2700K. The planet is in a tidally locked orbit that forces the same side to point to the star at all times, but the temperature difference is so extreme, the astronomers concluded that either there’s no atmosphere at all, or there is an atmosphere but only on the dayside.

55 Cancri e, animated

This animated illustration shows lava flows on the dayside of 55 Cancri e, a scenario suggested, among other things, to explain the planet's extreme day-night differences.

Yet the Spitzer data also showed that the hottest spot on the planet is shifted at least 30 degrees from the center of the dayside hemisphere. This means something is recirculating the heat, at least on the dayside. If not an atmosphere, then what? Demory’s team suggested lava flows carry heat away from the center of the dayside hemisphere, but as Demory notes, “we always felt uncomfortable with this explanation.”

Now, theorists Angelo and Hu have taken those same Spitzer observations and compared the data to a computer model of the planet’s atmosphere. The model takes the star’s luminosity and heat into account, and it describes how well the atmosphere transports heat. The result is a different interpretation of the data. The measured temperature difference between day and night is only 900K instead of 1300K. This difference, while still far outside our earthbound experience, is compatible with an atmosphere that surrounds the planet.

In addition to eliminating the lava-flow scenario, Angelo and Hu also rule out a water- or carbon-dioxide-based atmosphere. However, the observations are consistent with an atmosphere based on nitrogen or carbon monoxide. Whatever the atmosphere is made of, it has to be quite thick to survive so close to the star.

“Angelo and Hu have taken a simple approach that I really like,” Demory says. “This broadens the view that we had two years ago.”

Now the ball lands once more in the observers’ court. While Spitzer observations have proven invaluable to understanding the planet’s heat emissions, visible-light observations will improve our understanding of the planet’s reflected light. Demory, a member of the Characterizing Exoplanets Satellite (CHEOPS) science team, is advocating hundreds of hours to monitor the strange world of 55 Cancri e once CHEOPS launches in 2019.

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