How Do You Find the Surface of an Exoplanet? Ask its Atmosphere!

Title: How to identify exoplanet surfaces using atmospheric trace species in hydrogen-dominated atmospheres

Authors: Xinting Yu (余馨婷), Julianne I. Moses, Jonathan J. Fortney, Xi Zhang

First Writer’s Establishment: College of California Santa Cruz

Standing: Printed in ApJ, obtainable on arXiv

Of the 4400 (and counting!) exoplanets, the inhabitants of intermediate-sized planets is likely one of the most fascinating. With sizes between Earth and Neptune not seen in our Photo voltaic System, the most commonly occurring kind of planet is usually a complicated one. A planet on this class might be a large terrestrial planet, with a stable floor and skinny ambiance (a “super-Earth”), or it might be extra like a shrunken down model of the Photo voltaic System’s ice giants (a “sub-Neptune”), with a floor situated deeper throughout the planet at high-pressure ranges, if there may be one in any respect. Despite the fact that many intermediate-sized exoplanets have been found, the interior construction of anyone planet isn’t at all times clear. Giant uncertainties within the plenty and radii of those planets, and therefore of their densities, could make understanding their precise compositions a problem, and even essentially the most delicate upcoming telescopes like JWST and ARIEL can not immediately probe surfaces, leaving many exoplanets in composition limbo. 

Nonetheless, JWST and ARIEL might be able to exactly measuring the atmospheres of such planets, so what if there was a approach to learn the way deep the floor of an exoplanet lies by finding out its ambiance? The writer’s of in the present day’s paper investigates the relation (if there may be one) between the abundances of species discovered within the exoplanet’s ambiance and the placement of an exoplanet’s floor.

Under Pressure

Because it seems, the existence of a stable floor performs a key function within the make-up of the atmospheres inside our personal Photo voltaic System. Each Jupiter and Saturn’s moon Titan include little or no ammonia (NH3) inside their higher atmospheres, because it will get destroyed by photochemical reactions which happen there, nevertheless, whereas Jupiter incorporates important quantities of NH3 deep inside its ambiance, Titan doesn’t. The distinction right here is Titan’s floor. In Jupiter, the dearth of a stable floor means the constituent elements of NH3 are transported into the recent, excessive stress decrease ambiance the place they will reform into ammonia by way of thermochemical reactions, whereas Titan’s floor prevents its ambiance from reaching excessive sufficient temperatures and pressures for the recycling reactions to happen. Titan as a substitute has a bigger abundance of nitrogen, leftover from the destroyed NH3

The authors suggest {that a} related state of affairs may happen with different species throughout the atmospheres of exoplanets. To check this principle, they modelled the impacts of getting no floor and having a floor at considered one of three completely different stress ranges on the atmospheric evolution of K2-18b, a sub-Neptune. 

Two diagrams are shown. On the left the deep/no-surface case is shown, with "UV photons" impacting at the top, followed by a series of interactions between H, N2, NH3, CO, CH4, H2O, HCN, CxHy and CO2. "Diffusion / Transport" is annotated below this, shown by vertical ~ lines. Below this, "Thermochemistry" is labelled, with interactions between CxHy, N2, NH3, CH4, H2O, HCN and CO. A flame symbol is shown below the thermochemistry, indicating that the atmosphere is hot here. The background of the plot is a dark blue at the top, and fades into orange over thermochemistry. On the right, the shallow surface is shown. The upper portion of the image is the same as before, but at the level of diffusion/transport a brown surface blocks the rest of the image. In this case, H, N2, CO and CO2 are in red, and NH3, CH4, H2O, HCN and CxHy are in blue.

Determine 1: Diagrams describing the principle chemical pathways throughout the ambiance of K2-18b for a deep or no floor (left) and a shallow floor (proper). Arrow thickness signifies the significance of every pathway, with dashed arrows being the least necessary. In each instances, UV photons impacting the higher ambiance trigger photochemical reactions which break down delicate molecules equivalent to NH3, HCN, H2O and CH4. Within the deep/no floor mannequin, thermochemistry within the deep, sizzling ambiance recreates the molecules misplaced to photochemistry. Within the shallow floor case, atmospheric temperatures are by no means sizzling sufficient for thermochemistry to be efficient, inflicting a lower in abundance of the species in blue in comparison with the no floor case, and a rise for the purple species. Determine 2 within the paper.

Very similar to throughout the Photo voltaic System, if K2-18b has a shallow floor, the ambiance isn’t sizzling sufficient for thermochemical reactions to happen, which means the abundances of photochemically fragile species equivalent to ammonia lower in comparison with when the floor is way deeper or non-existent, as proven in Determine 1. For every model of the mannequin, the adjustments within the volume mixing ratios (VMR) of key chemical species throughout the observable ambiance show the impacts of surfaces at completely different stress ranges. 

A 2x2 image of plots of pressure (bar) (from 1000 at origin to 10^-8) vs mixing ratio (from 10^-9 at origin to 1) for no surface, 100 bar surface, 10 bar surface and 1 bar surface. In each plot the region of pressure between 10^-4 to 1 bar is shaded pale blue. In plots with a surface, the region below that pressure level is covered by a solid blue region. In each plot, mixing profiles for O2, C2H2, HCN, C2H6, NH3, CO2, CO, CH4, N2, H2O and H2 are plotted, each in a different colour.

Determine 2: Plots displaying how the amount mixing ratios of key chemical species change with stress by way of the ambiance of K2-18b with completely different surfaces. Greater pressures point out deeper into the ambiance. The pale blue shaded area signifies the observable a part of the ambiance. Determine 3 within the paper.

When the planet has no floor or a really deep floor, massive quantities of hydrocarbons and nitrites equivalent to hydrogen cyanide (HCN) are produced, whereas important portions of ammonia are discovered deep within the ambiance similar to within the case of Jupiter. When a floor exists at 10 bars, key nitrogen species can now not be replenished and produced as simply, resulting in lowering VMRs for HCN and NH3. For the shallowest, Earth-like surfaces, thermochemistry is prevented for almost all of species, and the ambiance is now additionally depleted in water (H2O) and methane (CH4).  As Determine 2 exhibits, altering the floor of a planet will change the abundances of an entire host of species, however are these adjustments important sufficient to differentiate between surfaces? 

A New Device For Observers?

Utilizing the discovering that a wide range of species are uniquely delicate to the presence of various surfaces, the authors are in a position to make use of the abundance ratios between a species when a floor is and isn’t current, and between completely different pairs of species to tentatively define a approach to distinguish the place a floor might be. 

The image shows a flowchart for determining the surface of a planet. The flowchart begins at the top with a blue box labelled "Atmospheric characterization". Following the flowchart, below and to the right an arrow leads to a green box labelled when "f[NH3]~1 f[HCN]~1", which then leads to a red box directly below it labelled "Deep surface (p>100 bar)". Returning to the blue box, below and to the left an arrow leads to another green box labelled when "f[NH3]<1 f[HCN]<1". Two arrows lead down and away from this box, each to orange boxes. The orange box on the left reads when "f[CH4]<1 f[C2H2]<1 f[CO/CH4]>1 f[CO/H2O]>1", and leads to a red box directly below it labelled "Shallow surface (p>10 bar). The orange box on the right reads when "f[CH4]~1 f[C2H2]~1 f[CO/CH4]<1 f[CO/H2O]<1", and leads to a red box directly below it labelled "Intermediate surface (10 bar < p <100 bar). A caption at the very top of the figure reads "When X = single species, f[X]=[X]/[X](no-surface). When X = species A/species B, f[X]=[A]/[B]"

Determine 3: Flowchart to assist within the potential willpower of the stress stage of a floor inside an exoplanet just like K2-18b utilizing the noticed abundances ([X]) ratios of various species. Tailored from Determine 9 within the paper.

With the flowchart proven in Determine 3 handy, it might be potential to find out the place surfaces lie inside exoplanets, supplied correct abundance measurements can be found. Sadly, present measurements of K2-18b’s Neptunian ambiance aren’t exact sufficient to make a prediction about any potential floor, however upcoming JWST observations of this planet may assist to make clear the thriller. 

So, does this imply the thriller of intermediate planet surfaces can lastly be resolved? Not fully. Extra modelling is required to increase the vary of planetary parameters and eventualities. Sooner or later, the flowchart might be expanded to incorporate thrilling however much less well-studied species equivalent to phosphine (PH3). The present examine additionally doesn’t contemplate the potential impacts of processes that happen on the floor, equivalent to volcanic activity and reactions with oceans or rocks, or the potential escape of gases from the highest of the ambiance – all processes which may change the noticed abundance ratios in an exoplanet. Nonetheless, in the present day’s paper outlines an thrilling new idea that extends our toolkit as we proceed to try to perceive the rising variety of unusual new worlds ready to be explored.

Astrobite edited by Alice Curtin

Featured picture credit score: Amanda Smith


About Lili Alderson

Lili Alderson is a primary yr PhD scholar on the College of Bristol finding out exoplanet atmospheres with space-based telescopes. She spent her undergrad on the College of Southampton with a yr in analysis on the Heart for Astrophysics | Harvard-Smithsonian. When not eager about exoplanets, Lili enjoys ballet, movie and baking.

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How Do You Find the Surface of an Exoplanet? Ask its Atmosphere!


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