Mapping a super-Earth

Exoplanetology never ceases to surprise and is constantly progressing. Recently, a team of researchers from several institutes, including the University of Liege, established a longitudinal thermal brightness map of 55 Cancri e, a “super-Earth”, which is approximately forty light-years away from Earth. The challenge was to measure the evolution of the brightness of the planet during its entire orbit around the host star. This data already exists for some gas giants, but is totally new for smaller planets. The data were obtained by photometric analysis of images taken by Spitzer, one of NASA’s space telescopes. These images made it possible to observe the hottest and coldest regions of the planet and to provide an estimate of some of its surface properties. The discovery marks the beginning of a new direction of research within the science of astrophysics: the study of the geological and atmospheric dynamics of small-sized exoplanets. 

Since it began in 1995 with the discovery of a first planet outside the solar system, exoplanetology has led to the compilation of a list of thousands of planets orbiting hundreds of stars. Today, it is estimated that there are tens of billions of them in our Galaxy alone. Some observational methods already allow us to obtain quite a quite detailed, though somewhat inert, picture of these planets. Measuring the radial velocities of their host star, for example, can help to determine their mass, and photometric observation of their transit reveals information about their size. The combination of the two methods reveals their density, thus providing a crucial clue as to their geological composition (gas, rock, metal and ice), and eventually, about the possible presence of an atmosphere. These methods of detection remain indirect. The vast majority of these exoplanets are too small and distant to be directly observable. In short, it is the behavior of their star or variations in the brightness of the entire system that reveals their presence. These constraints mean that it is not easy to get a clear picture of any activity that may exist on these planets.

These rather static portraits do not do justice to these planets. Just like the planets in our solar system, as they should be subject to significant physical and geological dynamics. The study of these planets marks a new direction in exoplanetology which has been enthusiastically initiated by Michaël Gillon, a research associate at the FNRS in the Origins in Cosmology and Astrophysics Laboratory of the University of Liege. In collaboration with the University of Cambridge and other institutes, he has just participated in a pioneering study that has been published in the journal Nature (1). The study involved the establishment of a longitudinal map of temperature differences across the entire surface of 55 Cancri e, a super-Earth which is forty light years away from Earth and which orbits its star in only eighteen hours (See: “A super-Earth is highlighted” and “55 Cancri e: enormous variations in temperature!”). 

These data were obtained by photometric analysis of several complete orbits by the planet around its star. Such ground-breaking detail concerning a small planet also enables us to learn more about the possible presence of an atmosphere. However, as is always the case with any promising new discovery, a cautious approach is advised. Michaël Gillon explains, “The thermal emission and its distribution between the two hemispheres of an exoplanet had already been measured for gas giants but these planets are essentially big balls of gas, the complete opposite of smaller planets. The latter can present a large range of possibilities in terms of their composition and surface conditions. For example, we do not know if 55 Cancri e is composed of rock or ice, if it is a truly telluric planet or the residual core of a giant planet, if it has always been close to its star or if it formed further away before eventually migrating towards its star. There are many possible scenarios, both in terms of its history and its composition”.  Of course, researchers have their own ideas on the subject. But given that this is only a first study, there is no available statistical knowledge to test their theories. Michaël Gilloncontinues, “In addition, there are no super-Earths in our solar system. It only has giant planets which are much bigger, and telluric planets which are much smaller. Unfortunately, we cannot therefore compare 55 Cancri e to a solar specimen that it is possible to study in situ”. 

A volcanic super-Earth

With a radius twice the size of that of the Earth and a mass which is eight times greater, 55 Cancri e therefore remains a small planet, a lot less easy to detect than gas giants which are sometimes even bigger than Jupiter. Nonetheless, it is an ideal candidate for observation. Its star, which is very bright and almost a neighbor, is visible to the naked eye. Its signals are very easy to detect by telescope. The short orbit of the planet means that its transit can be observed at very frequent intervals. Finally, its proximity to its star means that it is a very hot object which therefore emits sufficient amounts of light for it to be detectable. Recently, large variations in temperature were also detected on its surface, leading the researchers to an exciting hypothesis. “This system contains other planets further out”, says Michaël Gillon. “They influence the orbit of 55 Cancri e, which is not perfectly circular but slightly elliptical. “Because it is very close to its star, the elliptical nature of the orbit generates intense tidal effects which result in a constant transfer of energy to its core by internal friction. This significant “tidal heating” could be the cause of colossal volcanic activity on its surface”.  

This plausible theory is not without basis. “When I said that 55 Cancri e had no counterpart in our solar system, this is true with regard to our planets”, clarifies the astronomer.  On the other hand, in terms of its structure, 55 Cancri e and its star show a certain similarity to Jupiter and Io, its closest satellite. Io’s orbit, influenced by Europe and Ganymede, two moons that are further away, is also elliptical. “This situation creates volcanic activity and intense tidal effects on Io”. Michaël Gillon is delighted with this somewhat encouraging analogy between the two structures. “We could only study this kind of phenomenon in our solar system. Today, we are on the brink of extending this study to other systems!"

An opportunity to be seized

The proximity of 55 Cancri e to its star traps it into what is called a “synchronous rotation”. That is to say, its period of revolution is identical to its orbital period so that it always presents the same side to its star, just as the Moon does to the Earth. The planet therefore has a side where it is always daytime and a side where it is always night. “This synchronous rotation presents an opportunity because it makes it possible to unambiguously determine the efficiency of the thermal energy distribution between the day and night sides of the planet which depends directly on the presence of an atmosphere and the dynamic of this atmosphere. If, as is the case on Earth, the two hemispheres were successively exposed to the star, the temperature around the globe would be a lot more uniform and it would therefore be more difficult to draw any conclusions”.   

For a short fraction of time during each of its orbits, the planet passes behind the star. It is therefore totally occulted and its contribution to the flux of the system disappears. Its emission, more precisely the emission of its day side, can therefore be measured in a negative way. “The phenomenon of synchronous rotation makes it possible to observe the planet from different angles during its entire orbit. If its thermal emission varies on its surface in terms of longitude, we will therefore record a modulation, a variation that belongs to the planet itself. By combining the two measurements, occultation and the orbital modulation curve, we can draw a longitudinal map of the thermal emission on the surface of the planet: The occultation gives us an absolute value for the thermal emission of the day side, and the orbital modulation shows us how this value varies according to the longitude”. 

Eight 9-hour observations distributed over a period of one month made it possible to record these variations in emission and to verify that they were repeated in accordance with the orbital phases of the planet. The astrophysicists were therefore able to assemble the data and establish the longitudinal map of the thermal emissions of the planet. The map reveals an enormous temperature gradient between the day and night sides:  The day side has a temperature rising as high as 2700 Kelvin as opposed to 1300 Kelvin for the night side”. “Such a temperature gradient indicates an inefficient circulation of heat which corresponds to a rocky planet without an atmosphere. In fact, it is essentially the atmosphere which distributes heat on the surface of a planet by means of dynamic phenomena (winds). Without an atmosphere, the temperature gradient between the day and night sides will be very significant”. This is true, and yet…

The hotspot paradox

In theory, without an atmosphere, the hottest part of the planet should be the point nearest the star (the substellar point). Therefore, on the graph showing variations in the light of the planet (see figure above), the peak of brightness should be at the point nearest the occultation, given that these are the moments when the telescope captures the largest part of the day side. However, the brightness curve reveals a peak that has shifted, occurring at a time well before the occultation. “The hottest point on the planet is therefore not at a point exactly below the star, says Michaël Gillon, but has shifted some thirty degrees towards the East. “This observation would be easy to understand in the presence of an atmosphere, which, thanks to strong winds blowing towards the East would enable a transfer of heat to take place leading to the hot point being located at a point other than the substellar point. And yet, in the case of 55 Cancri e, this observation is difficult to reconcile with the significant thermal gradient between the two hemispheres”.  

A theory that needs to be proved seems to evade this paradox. It is to be remembered that the very close proximity of the planet to its star coupled with the influence of other planets in the system means it has an elliptical orbit. This generates considerable tidal effects. “The temperature during the day is higher than that at which rocks melt. We can imagine that these rocks form “oceans” of magma. Under the tidal effects, there could be an “oceanic” transfer, a flow of magma towards the East causing a delay between the peak in irradiation and the peak in heat”.  

A second theory involves the presence of a very particular atmosphere which conceals unusual phenomena. The astrophysicist continues, “But, on this subject, we are really still in the realm of speculation. One thing is certain, a hydrogen-based atmosphere, such as those observed around gas giants is highly unlikely. This element is too light and would quickly be blown out of the gravitational field of the planet, or attracted by the star’s magnetic field. We could imagine a secondary atmosphere, composed of heavier elements resulting in a constant degassing on the surface due to the massive irradiation of the planet. It would be constantly destroyed and renewed. If 55 Cancri e is essentially composed of ice, this atmosphere should be rich in oxygen and carbon monoxide. If it is essentially rocky, which we believe today to be the case, the secondary atmosphere could be rich in silicates and metals”. 

Spitzer’s swan song

At the present time, such a study would not have been possible without the NASA space telescope Spitzer. It offers several capabilities necessary for such a precise observation. “On a planetary scale, these temperature variations are indeed very great”, concedes the researcher “And yet, they only correspond to a miniscule variation in the brightness of the system, in the order of 200 ppm, or 0.02%!” Why has Spitzer succeeded in doing what would seem to be impossible for other telescopes?” Firstly, all the instruments present on Earth are disqualified due to their unsuitability. The atmosphere limits their precision by around 0.1%. Therefore, a telescope like Spitzer that can work in infrared was required. At those wavelengthswavelengths there is a better contrast between the thermal emission of a planet and its star. As the star is much hotter, it essentially emits shorter wavelengths. Thirdly, Spitzer was able to continuously record large fractions of the planet’s orbit. No Earth-based satellite is capable of this because our planet occults their field of vision half of the time. Spitzer follows the Earth on its heliocentric orbit, though it is progressively moving further away from it.  Today, it is at a distance of one astronomical unit from our planet and is therefore very far away from the light of our parasitic light. But Spitzer is getting old. Two of its three instruments are out of use and the third is only usable to 50% of its original capacity, and its distance makes the transfer of data more and more difficult. It should however continue to be usable during the interim period leading up to the point when the JWST space telescope becomes operational. “Its mirror will be 6.5 meters in diameter as opposed to 85 centimeters for Spitzer, says a delighted Michaël Gillon. “And its instruments will record the emissions of photons at several wavelengths. Each wavelength teaches us something different about a planet, combining the different wavelengths informs us about the vertical structure of the atmosphere, the composition of the surface etc. We got an intriguing first glimpse from Spitzer but an entire series of dynamic phenomena still remains unknown”.   

Towards “habitable Earths” 

Very much in demand, the JWST telescope will be capable of pointing its mirror at some hand-picked candidates. The challenge is to detect these planets with the aid of other instruments and to discern those with the most attractive profiles. With the aptly named SPECULOOS project and with the help of TRAPPIST, Michaël Gillon and his colleagues are at the cutting edge of technology in the area of astrophysics. They are seeking to detect even smaller planets, with geological and atmospheric conditions similar to those of the Earth and which orbit their star in the habitable zone. “We want to discover Earth-like exoplanets that are suitable for detailed study by the most modern telescopes such as the JWST, particularly the search for traces of life in the composition of their atmospheres.  This will only be possible for planets which transit the smallest and coldest stars in the immediate neighborhood of the Sun. Known as “ultracool dwarfs”, these stars are very frequent in the galaxy, much more so than sun-like stars. They are similar in size to the planet Jupiter, and have a temperature which is more than two times smaller than that of the Sun. With such a small and dim host star, the signal of a planet the size of the Earth is not totally drowned out by the flux of the star. By way of comparison, the Earth, around the sun is tiny and only emits a very small amount of light. A similar object orbiting a star that is not very bright and that is around the same size as Jupiter, will provide a significant part of the overall thermal emission. It will therefore be easier to study”. 

Read : A trio of Earths 40 light years away?

(1) Brice-Olivier Demory, Michael Gillon, Julien de Wit, Nikku Madhusudhan, Emeline Bolmont, Kevin Heng, Tiffany Kataria, Nikole Lewis,Renyu Hu, Jessica Krick, Vlada Stamenković, Björn Benneke, Stephen Kane & Didier Queloz, A map of the extreme day-night temperature gradient of a super-Earth exoplanet. Nature, DOI: 10.1038/nature17169