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Webb hints at possible atmosphere surrounding 55 Cancri e, a rocky exoplanet

By ScientifiCult 8 May 2024

Webb hints at possible atmosphere surrounding 55 Cancri e, a rocky exoplanet

Researchers using the NASA/ESA/CSA 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 a rocky planet atmosphere outside our Solar System.

Renyu Hu from NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, USA, is the lead author of a paper published today in Nature.

“Webb is pushing the frontiers of exoplanet characterisation to rocky planets,” Hu said. “It is truly enabling a new type of science.”

Super-hot super-Earth 55 Cancri e

55 Cancri e is one of five known planets orbiting a Sun-like star in the constellation Cancer. With a diameter nearly twice that of Earth and a 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 give the wrong impression. The planet orbits so close to its star (about 2.25 million kilometres, or one twenty-fifth of the distance between Mercury and the Sun) that its surface is likely to be molten – a bubbling ocean of magma. In 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.

“I’ve worked on this planet for more than a decade,” said Diana Dragomir, an exoplanet researcher at the University of New Mexico in the USA and a co-author of 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 are finally getting some answers!”

Unlike gas-giant atmospheres, which are relatively easy to spot (the first was detected by the NASA/ESA 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 vaporised rock, rich in elements like silicon, iron, aluminium, and calcium.

“The planet is so hot that some of the molten rock should evaporate,” explained Hu.

Illustration of a rocky exoplanet and its star. The star is in the background at the lower left and appears somewhat smaller in the sky than the planet. The planet has hints of a rocky, partly molten surface beneath the haze of a thin atmosphere. — This artist’s concept shows what the exoplanet 55 Cancri e could look like.
Also called Janssen, 55 Cancri e is a so-called super-Earth, a rocky planet significantly larger than Earth but smaller than Neptune, which orbits its star at a distance of only 2.25 million kilometres (0.015 astronomical units), completing one full orbit in less than 18 hours. In comparison, Mercury is 25 times farther from the Sun than 55 Cancri e is from its star. The system, which also includes four large gas-giant planets, is located about 41 light-years from Earth, in the constellation Cancer.
Observations from Webb’s NIRCam and MIRI suggest that the planet may be surrounded by an atmosphere rich in carbon dioxide (CO2) or carbon monoxide (CO). Because it is so close to its star, the planet is extremely hot and is thought to be covered in molten rock. Researchers think that the gases that make up the atmosphere could have bubbled out of the magma.
The star, 55 Cancri, is a K-type star nearly the same size and mass as the Sun, but slightly cooler and dimmer. It is just bright enough to see with the naked eye in a very dark sky. The star and planet are so close to each other that the star would appear 70 times wider in the planet’s sky than the Sun appears in our sky. In addition, because the planet is likely to be tidally locked, from any given point the star would appear fixed in the sky.
This artist’s concept is based on new data gathered by NIRCam and MIRI as well as previous observations from other ground- and space-based telescopes, including NASA’s Hubble and the now-retired Spitzer space telescopes. Webb has not captured any images of the planet.
Credit: NASA, ESA, CSA, R. Crawford (STScI)

Measuring subtle variations in infrared colours

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 the light from the whole 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.

Diagram of a secondary eclipse and a graph of change in brightness over time. Below the diagram is a graph showing the change in brightness of mid-infrared light emitted by the star-planet system over the course of about four and a half hours. The infographic shows that the brightness of the system decreases as the planet moves behind the star. — This lightcurve shows the change in brightness of the 55 Cancri system as the rocky planet 55 Cancri e, the closest of the five known planets in the system, moves behind the star. This phenomenon is known as a secondary eclipse.
When the planet is beside the star, the mid-infrared light emitted by both the star and the dayside of the planet reaches the telescope, and the system appears brighter. When the planet is behind the star, the light emitted by the planet is blocked and only the starlight reaches the telescope, causing the apparent brightness to decrease.
Astronomers can subtract the brightness of the star from the combined brightness of the star and planet to calculate how much infrared light is coming from the dayside of the planet. This is then used to calculate the dayside temperature and infer whether or not the planet has an atmosphere.
The graph shows data collected using the low-resolution spectroscopy mode on Webb’s Mid-Infrared Instrument (MIRI) in March 2023. Each of the purple data points shows the brightness of light ranging in wavelength from 7.5 to 11.8 microns, averaged over intervals of about five minutes. The grey line is the best fit, or model lightcurve that matches the data most closely. The decrease in brightness during the secondary eclipse is just 110 parts per million, or about 0.011 percent.
The temperature of the planet calculated from this observation is about 1800 kelvins (around 1500 degrees Celsius), which is significantly lower than would be expected if the planet has no atmosphere or only a thin rock-vapour atmosphere. This relatively low temperature indicates that heat is being distributed from the dayside to the nightside of the planet, possibly by a volatile-rich atmosphere.
Credit: NASA, ESA, CSA, J. Olmsted (STScI), A. Bello-Arufe (JPL)

55 Cancri e is cooler than expected

The first indication that 55 Cancri e could have a substantial atmosphere came from temperature measurements based on its thermal emission, the heat energy given off in the form of infrared light. If the planet is covered in dark molten rock with a thin veil of vaporised rock, or has no atmosphere at all, the dayside should be around 2200 degrees Celsius.

“Instead, the MIRI data showed a relatively low temperature of about 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 JPL. “This suggests the presence of an atmosphere containing carbon monoxide or carbon dioxide, both of which absorb these wavelengths of light.”

A planet with no atmosphere or only vaporised rock in an atmosphere would not have this specific spectral feature.

“This is exciting news,” said co-author Yamila Miguel from Leiden Observatory and the Netherlands Institute for Space Research (SRON), both in the Netherlands. “We’ve spent the last ten years modelling different scenarios, trying to imagine what this world might look like. 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 since the planet’s formation.

“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 is continuously replenished by the magma ocean. Magma is not only crystals and liquid rock, there’s a lot of dissolved gas in it, too.”

In all likelihood, any atmosphere surrounding the planet would be more complex and quite variable as a result of interactions with the magma ocean. In addition to carbon monoxide or carbon dioxide, there could be gases like nitrogen, water vapour, sulphur dioxide, some vaporised rock, and even short-lived clouds made of tiny droplets of lava condensed from the air.

While 55 Cancri e is far too hot to be habitable, researchers think it could provide a unique window for studying interactions between the atmospheres, surfaces and interiors of rocky planets, and perhaps provide insights into the early Earth, Venus and Mars, which are thought to have been covered in magma oceans 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. Analysis of additional secondary eclipse observations of 55 Cancri e are currently in progress. In the future, the team hopes to capture a full phase curve with Webb in order to map temperature differences from one side of the planet to the other, to get a better sense of the planet’s weather, climate and more detailed atmospheric conditions.

Graph showing the brightness of light captured by Webb’s NIRCam and MIRI instruments plotted alongside two different model emission spectra, and an illustration of the planet and its star in the background. — A thermal emission spectrum captured by Webb’s NIRCam (Near-Infrared Camera) in November 2022, and MIRI (Mid-Infrared Instrument) in March 2023, shows the brightness (y-axis) of different wavelengths of infrared light (x-axis) emitted by the super-Earth exoplanet 55 Cancri e. The spectrum shows that the planet may be surrounded by an atmosphere rich in carbon dioxide or carbon monoxide and other volatiles, not just vaporised rock.
The graph compares data collected by NIRCam (orange dots) and MIRI (purple dots) to two different models. Model A, in red, shows what the emission spectrum of 55 Cancri e should look like if it has an atmosphere made of vaporised rock. Model B, in blue, shows what the emission spectrum should look like if the planet has a volatile-rich atmosphere outgassed from a magma ocean that has a volatile content similar to Earth’s mantle. Both MIRI and NIRCam data are consistent with the volatile-rich model.
The amount of mid-infrared light emitted by the planet (MIRI) shows that its dayside temperature is significantly lower than it would be if it did not have an atmosphere to distribute heat from the dayside to the nightside. The dip in the spectrum between 4 and 5 microns (NIRCam data) can be explained by absorption of those wavelengths by carbon monoxide or carbon dioxide molecules in the atmosphere.
The spectrum was made by measuring the brightness of 4- to 5-micron light with Webb’s NIRCam GRISM spectrometer, and 5- to 12-micron light with the MIRI low-resolution spectrometer, before, during and after the planet moved behind its star (the secondary eclipse ). The amount of each wavelength emitted by the planet (y-axis) was calculated by subtracting the brightness of the star alone (during the secondary eclipse) from the brightness of the star and planet combined (before and after the eclipse). Each observation lasted about eight hours.
Note that the NIRCam data have been shifted vertically to align with Model B. Although the differences in brightness between each wavelength in the NIRCam band were derived from the observation (the data suggest a valley between 4 and 5 microns), the absolute brightness (the vertical position of that valley) could not be measured precisely because of noise in the data.
Credit: NASA, ESA, CSA, J. Olmsted (STScI), R. Hu (JPL), A. Bello-Arufe (JPL), M. Zhang (University of Chicago), M. Zilinskas (SRON Netherlands Institute for Space Research)

Press release from ESA Webb.

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Astronomy

Webb Unveils Dark Side of Pre-stellar Ice Chemistry

By ScientifiCult 23 January 2023

Webb Unveils Dark Side of Pre-stellar Ice Chemistry

The discovery of diverse ices in the darkest, coldest regions of a molecular cloud measured to date has been announced by an international team of astronomers using the NASA/ESA/CSA James Webb Space Telescope. This result allows astronomers to examine the simple icy molecules that will be incorporated into future exoplanets, while opening a new window on the origin of more complex molecules that are the first step in the creation of the building blocks of life.

If you want to build a habitable planet, ices are a vital ingredient as they are the main carriers of several key light elements — namely carbon, hydrogen, oxygen, nitrogen, and sulphur (referred to collectively as CHONS). These elements are important ingredients in both planetary atmospheres and molecules like sugars, alcohols, and simple amino acids. In our Solar System, it is thought they were delivered to Earth’s surface by impacts with icy comets or asteroids. Furthermore, astronomers believe such ices were most likely already present in the dark cloud of cold dust and gas that would eventually collapse to make the Solar System. In these regions of space, icy dust grains provide a unique setting for atoms and molecules to meet, which can trigger chemical reactions that form very common substances like water. Detailed laboratory studies have further shown that some simple prebiotic molecules can form under these icy conditions.

Now an in-depth inventory of the deepest, coldest ices measured to date in a molecular cloud [1] has been announced by an international team of astronomers using the NASA/ESA/CSA James Webb Space Telescope. In addition to simple ices like water, the team was able to identify frozen forms of a wide range of molecules, from carbonyl sulfide, ammonia, and methane, to the simplest complex organic molecule, methanol (in the interstellar medium, organic molecules are considered to be complex when having six or more atoms). This is the most comprehensive census to date of the icy ingredients available to make future generations of stars and planets, before they are heated during the formation of young stars. These icy grains grow in size as they are funnelled into the protoplanetary discs of gas and dust around these young stars, essentially allowing astronomers to study all the potential icy molecules that will be incorporated into future exoplanets.

“Our results provide insights into the initial, dark chemistry stage of the formation of ice on the interstellar dust grains that will grow into the centimetre-sized pebbles from which planets form in discs,” said Melissa McClure, an astronomer at Leiden Observatory who is the principal investigator of the observing program and lead author of the paper describing this result. “These observations open a new window on the formation pathways for the simple and complex molecules that are needed to make the building blocks of life.”

In addition to the identified molecules, the team found evidence for prebiotic molecules more complex than methanol in these dense cloud ices, and, although they didn’t definitively attribute these signals to specific molecules, this proves for the first time that complex molecules form in the icy depths of molecular clouds before stars are born.

“Our identification of complex organic molecules, like methanol and potentially ethanol, also suggests that the many star and planet systems developing in this particular cloud will inherit molecules in a fairly advanced chemical state,” added Will Rocha, an astronomer at Leiden Observatory who contributed to this discovery. “This could mean that the presence of prebiotic molecules in planetary systems is a common result of star formation, rather than a unique feature of our own Solar System.”

By detecting the sulfur-bearing ice carbonyl sulfide, the researchers were able to estimate the amount of sulfur embedded in icy pre-stellar dust grains for the first time. While the amount measured is larger than previously observed, it is still less than the total amount expected to be present in this cloud, based on its density. This is true for the other CHONS elements as well. A key challenge for astronomers is understanding where these elements are hiding: in ices, soot-like materials, or rocks. The amount of CHONS in each type of material determines how much of these elements end up in exoplanet atmospheres and how much in their interiors.

“The fact that we haven’t seen all of the CHONS that we expect may indicate that they are locked up in more rocky or sooty materials that we cannot measure,” explained McClure. “This could allow a greater diversity in the bulk composition of terrestrial planets.”

Webb Unveils Dark Side of Pre-stellar Ice Chemistry. Astronomers have taken an inventory of the most deeply embedded ices in a cold molecular cloud to date. They used light from a background star, named NIR38, to illuminate the dark cloud called Chameleon I. Ices within the cloud absorbed certain wavelengths of infrared light, leaving spectral fingerprints called absorption lines. These lines indicate which substances are present within the molecular cloud.
These graphs show spectral data from three of the James Webb Space Telescope’s instruments. In addition to simple ices like water, the science team was able to identify frozen forms of a wide range of molecules, from carbon dioxide, ammonia, and methane, to the simplest complex organic molecule, methanol.
In addition to the identified molecules, the team found evidence for prebiotic molecules more complex than methanol (indicated in the lower-right panel). Although they didn’t definitively attribute these signals to specific molecules, this proves for the first time that complex molecules form in the icy depths of molecular clouds before stars are born.
The upper panels and lower-left panel all show the background star’s brightness versus wavelength. A lower brightness indicates absorption by ices and other materials in the molecular cloud. The lower-right panel displays the optical depth, which is essentially a logarithmic measure of how much light from the background star gets absorbed by the ices in the cloud. It is used to highlight weaker spectral features of less abundant varieties of ice.
Credit:
NASA, ESA, CSA, and J. Olmsted (STScI), M. K. McClure (Leiden Observatory), K. Pontoppidan (STScI), N. Crouzet (Leiden University), and Z. Smith (Open University)

The ices were detected and measured by studying how starlight from beyond the molecular cloud was absorbed by icy molecules at specific infrared wavelengths visible to Webb. This process leaves behind chemical fingerprints known as absorption spectra which can be compared with laboratory data to identify which ices are present in the molecular cloud. In this study, the team targeted ices buried in a particularly cold, dense and difficult to investigate region of the Chameleon I molecular cloud, a region 631 light-years from Earth which is currently in the process of forming dozens of young stars.

“We simply couldn’t have observed these ices without Webb,” elaborated Klaus Pontoppidan, Webb project scientist at the Space Telescope Science Institute, who was involved in this research. “The ices show up as dips against a continuum of background starlight. In regions that are this cold and dense, much of the light from the background star is blocked and Webb’s exquisite sensitivity was necessary to detect the starlight and therefore identify the ices in the molecular cloud.”

This research forms part of the Ice Age project, one of Webb’s 13 Early Release Science programs. These observations are designed to showcase Webb’s observing capabilities and allow the astronomical community to learn how to get the best from its instruments. The Ice Age team have already planned further observations, and hope to trace out the journey of ices from their formation through to the assemblage of icy comets.

“This is just the first in a series of spectral snapshots that we will obtain to see how the ices evolve from their initial synthesis to the comet-forming regions of protoplanetary discs,” concluded McClure. “This will tell us which mixture of ices — and therefore which elements — can eventually be delivered to the surfaces of terrestrial exoplanets or incorporated into the atmospheres of giant gas or ice planets.”

Webb Ice Chemistry — Webb Unveils Dark Side of Pre-stellar Ice Chemistry. This image by the NASA/ESA/CSA James Webb Space Telescope’s Near-InfraRed Camera (NIRCam) features the central region of the Chameleon I dark molecular cloud, which resides 630 light years away. The cold, wispy cloud material (blue, centre) is illuminated in the infrared by the glow of the young, outflowing protostar Ced 110 IRS 4 (orange, upper left). The light from numerous background stars, seen as orange dots behind the cloud, can be used to detect ices in the cloud, which absorb the starlight passing through them.
An international team of astronomers has reported the discovery of diverse ices in the darkest, coldest regions of a molecular cloud measured to date by studying this region. This result allows astronomers to examine the simple icy molecules that will be incorporated into future exoplanets, while opening a new window on the origin of more complex molecules that are the first step in the creation of the building blocks of life.
The two background stars used in this study, NIR38 and J110621 are denoted on the image in white.
Credit:
NASA, ESA, CSA, and M. Zamani (ESA/Webb); Science: F. Sun (Steward Observatory), Z. Smith (Open University), and the Ice Age ERS Team

Notes

[1] A molecular cloud is a vast interstellar cloud of gas and dust in which molecules can form, such as hydrogen and carbon monoxide. Cold, dense clumps in molecular clouds with higher densities than their surroundings can be the sites of star formation if these clumps collapse to form protostars.

Press release from ESA Webb

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