Lava Flows On This Oddball World

Many planet-hunting astronomers think that super-Earths are the most common type of exoplanet in our Milky Way Galaxy. Belonging to the strange and often exotic families of parent-stars beyond our Sun, distant exoplanet “oddballs”, such as the super-Earth 55 Cancri e, hold in store a treasure trove of surprises for curious observers. 55 Cancri e circles a star about 40 light-years from Earth in the constellation Cancer (The Crab)–and like other super-Earths, it boasts a mass about eight times greater than that of our own planet, as well as a radius that is about twice that of our Earth. Extremely dark, rocky, and richly endowed with the element carbon, some astronomers speculate that about one-third of this strange planet’s mass may be composed of diamond. In March 2016, a team of astronomers proposed that this weird “oddball” world may be even more bizarre than previously thought. In fact, it may have lava flowing over much of its fiery surface, according to a new thermal map–which is the first of its kind!

55 Cancri e is famous for being known as the “diamond planet”. This is because models based on its mass strongly indicate that its interior is heavily laden with the element carbon. Studies show the presence of hydrogen and helium in this exotic world’s atmosphere–but no water vapor. This means that 55 Cancri e is very, very dry! Furthermore, this relatively large, rocky, and remote world circles dangerously close to the searing-hot fiery furnace of its parent-star, 55 Cancri, completing one full orbit in a mere 18 hours. This means that 55 Cancri e is closer to its parent-star than our Solar System’s innermost planet Mercury is to our Sun. If our planet had formed in a similar position, the soil that we now walk upon would become a roasting 3200 degrees Fahrenheit. Planetary scientists have long believed that 55 Cancri e may be an uninviting wasteland of parched rock. However, 55 Cancri e may really be even weirder than this.

In 2012, NASA’s infrared eye in the sky, the Spitzer Space Telescope (SST), obtained measurements of the extremely small quantity of light that 55 Cancri e blocks out when it floats in front of the glaring face of its parent-star. These transit events occur every 18 hours, giving astronomers repeated opportunities to collect the data that they need to estimate the density, volume, and width of this bizarre exoplanet. Only a handful of known super-Earths transit across the face of their bright parent-stars as observed from our vantage point on Earth. This makes 55 Cancri e better understood than most other super-Earths.

The astronomers who discovered 55 Cancri e back in 2004, while it was in the process of passing in front of 55 Cancri’s brilliant face, were uncertain as to whether it was a relatively small gas-giant–a diminutive version of our own Solar System’s behemoth Jupiter–or a larger version of our rocky Earth.

Carbon Planets: Diamonds In The Sky?

A carbon planet is one that contains more carbon than oxygen. Many astronomers think that these exotic, very alien, distant worlds emerge from a nourishing carbon-rich protoplanetary accretion disk. Even though it is rich in carbon, the protoplanetary accretion disk that feeds the baby star, is poor in oxygen. This means that these very alien distant worlds would not evolve in the same way as the four relatively small, rocky inner terrestrial planets of our Solar System: Mercury, Venus, Earth, and Mars. The quartet of kindred terrestrial planets, inhabiting our Sun’s family, formed out of silicon-oxygen compounds. This viewpoint has been strengthened by scientific evidence, and it has been gaining ever-increasing acceptance among planetary scientists. Different planetary systems would possess different carbon-to-oxygen ratios, with our own Solar System’s four rocky inner planets closer to being designated oxygen planets, instead of carbon planets.

It is thought that a carbon planet would probably contain a core of iron or steel–similar to the core within our Earth. Surrounding the iron or steel core of a carbon planet would be molten silicon carbide and titanium carbide. A layer of carbon in the form of graphite, could possibly sport a kilometer-thick substratum of diamond–if there is enough pressure to produce it. It has also been proposed that during fierce and fiery volcanic eruptions, diamonds from the interior of a carbon planet could be shot upward to the surface–and, as a result, form mountains made of diamonds and silicon carbide.

However, carbon planets would likely be bereft of water. Water cannot form on these very arid worlds. This is because any oxygen carried to these strange exoplanets by crashing asteroids or comets would interact with the carbon present on the exoplanet’s exotic surface. The alien surface of such a strange carbon-rich world would display liquid or frozen hydrocarbons–such as tar and methane–and carbon monoxide. In addition, there could possibly even be a weather-cycle on carbon planets sporting an atmosphere–but only if the bizarre world shows an average surface temperature that is below 77 degrees Celsius. The atmosphere belonging to a cool carbon planet would primarily be made up of carbon dioxide or carbon monoxide, which could produce a thick carbon smog.

A number of planetary scientists predict that carbon planets will be of similar diameter to silicate and water planets of approximately the same mass, and this similarity between the two types makes them difficult to distinguish from one another. Geological surface features that are similar to those on Earth may also be present on carbon planets–but they would be of different compositions. For example, the rivers flowing on these exotic, distant worlds may not be composed of flowing liquid water, but instead consist of oils. If the temperature is cool enough, gases may be able to photochemically synthesize into long-chain hydrocarbons. These hydrocarbons could then rain down on the carbon planet’s weird surface.

Lava Flows On This “Oddball” World

In a paper published in the March 30, 2016 issue of Nature, a team of astronomers suggest something new. Previous studies of 55 Cancri e indicated a thick atmosphere of water vapor or carbon dioxide surrounding a rocky inner body. However, this theory was rejected when improved measurements of the bizarre exoplanet ruled out water vapor in the atmosphere.

Other planetary scientists then suggested that the planet was composed mostly of carbon–in contrast to the Earth’s oxygen-rich interior–in the form of graphite and diamond. This is the reason why 55 Cancri e came to be known as the “diamond planet”. However, even these findings have recently been questioned.

In the March 30, 2016 Nature paper, the scientists suggest an alternative interpretation of the data, proposing that this distant world’s surface could largely be covered by flowing lava.

Super-Earths are scattered abundantly throughout our Galaxy, but are absent from our Sun’s own family of planets. Therefore, exploring this solar system diversity can provide precious clues about solar system formation and planet migration, Dr. Brice-Olivier Demory explained in the March 30, 2016 issue of Eos, a publication of the American Geophysical Union (AGU). Dr. Demory is an astrophysicist at the University of Cambridge in the UK and lead author of the Nature paper.

Dr. Demory and his colleagues used the infrared camera of the Earth-orbiting SST to observe this “oddball” super-Earth. In 2013, the space telescope obtained millions of measurements in the infrared from this distant world as it orbited around its parent-star.

55 Cancri e is tidally locked to its stellar parent, just like our own Moon is tidally locked to Earth. This means that one side of 55 Cancri e always faces its star’s searing-hot and fiery face, while the other side is turned away, and only the nightside is observable as the planet transits across the glaring face of 55 Cancri. Therefore, Dr. Demory and his team could observe the dayside of 55 Cancri e immediately before and immediately after the exoplanet floated behind its stellar parent. The infrared camera aboard SST is especially sensitive to temperature variation, and this enabled the astronomers to construct the very first thermal map of a super-Earth.

The map unveiled numerous surprises about the true, and previously hidden, nature of 55 Cancri e–and these features were reported in the Nature paper of March 2016. One surprising revelation showed that the dayside temperature of 55 Cancri e is approximately 2700 kelvins–which exceeded its nightside temperature by about 1300 K! This is an unusual pattern, and it indicates that the existence of a thick atmosphere is unlikely. This is because a thick atmosphere would have circulated the heat relatively evenly around the planet, Dr. Demory explained in the March 30, 2016 issue of Eos.

The astronomers were also surprised when they discovered that the hottest spot on this bizarre world–which they had expected to be centrally situated on its dayside– was, instead, about 41 degrees of longitude to the east. This longitudinal heat map indicates that this offset from the center is likely caused by the presence of flowing lava.

“We may have a bit of (heat) circulation on the planet but not from the atmosphere, but mainly from lava,” Dr. Demory added. At the searing-hot dayside temperatures, silicate-based rocks are molten–as they are when they are buried deep below the mantle of our Earth. Therefore, the astronomers propose that on 55 Cancri e‘s dayside, the lava could be flowing almost like water. As the distant world orbits its parent-star, the lava would flow in the direction of the nightside. Here, it would cool off significantly and thicken–and possibly even solidify.

Gaseous, giant exoplanets termed hot Jupiters show similar flow dynamics of gases, Dr. Demory went on to explain, with a hot spot offset from the center.

“The suggestion of lava flow is intriguing–this is something that has never been detected before. If the response to this high temperature environment really is extrusion of molten lava, this gives us information about the internal structure of this unseen planet. The analysis is beautiful and alternative explanations have been carefully considered,” Dr. Debra Fischer noted in the March 30, 2016 Eos article, written by JoAnna Wendel. Dr. Fischer is an astronomy professor at Yale University in New Haven Connecticut, who was not involved in the study.

Dr. Johanna Teske explained in the same Eos article that “A study like this that hones in one planet may seem limited in scope, but is actually critical to our understanding of super-Earths because there are still only a handful of these planets that are characterizable in such detail.” Dr. Teske is an astronomer at the Carnegie Institution of Washington D.C., who was also not involved with the research.

Many more observations are necessary in order for astronomers to confirm that there is lava on this strange planet, as well as providing a better understanding of that planet, Dr. Demory explained. Indeed, because super-Earths are relatively common in our Milky Way Galaxy, this “makes us wonder why we don’t have any” in our own Solar System.”

Studying planets like Cancri 55 e is “paramount to better understand our own origins,” he added.

This article is dedicated to the memory of cousin Martin Braffman.

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