Welcome to my projects page.
Here you will find a brief description of the projects I have worked on and are currently published. These include my paper on Mass-metallicity trends in transiting exoplanets, my work on degeneracies in retrievals of exoplanetary transmission spectra, and some of the work on the analysis of different planetary spectra.

Mass–Metallicity Trends in Transiting Exoplanets 

You can read the full paper open access here. This research was made open access thanks to the Bill and Melinda Gates foundation.

Authors: Luis Welbanks, Nikku Madhusudhan, Nicole F. Allard, Ivan Hubeny, Fernand Spiegelman, and Thierry Leininger

One of the reasons why we care about the atmospheric composition of exoplanets is because it can help us understand how these planets were formed and where they formed. With the outstanding amount of planets found in the last 20 years, we are finding planets of all types and sizes. From giant planets the size of Jupiter but with temperatures of thousands of degrees Kelvin, to mini-Neptunes with temperate conditions in the habitable zones of their host stars! So, it is possible to ask the question, are there any trends in the composition of these planets?

To answer this question, we used the latest cross sections available. Particularly those of sodium (Na) and potassium (K), for which their cross section is broadened by interactions with atomic hydrogen. Why do we use Na and K? Two main reasons. First, besides water (H2O), Na and K are amongst the species that have been detected the most in transiting exoplanets. Second, as we found in our work on degeneracies, Na and K cross sections can affect the abundance estimates (i.e. how much ‘stuff’ there is) inferred from a spectrum; especially water abundance estimates.

Once we had the latest cross sections we set to perform the most extensive set of homogeneous Bayesian retrievals to date using optical and near-infrared observations. We used a sample of 19 planets, from cool mini-Neptunes to hot Jupiters, with temperatures between ~300 to 2700 K (i.e. from room temperature to way too hot!). We looked at the detections of Na, K and H2O, and confirmed 6 detections of Na, 6 of K, and 14 of H2O. This is exciting! There seems to be water everywhere!

Now, what about the trends we were searching for? Well, we find a mass–metallicity trend of increasing H2O abundances with decreasing mass, spanning generally substellar (less than their host star) values for gas giants and stellar/superstellar (same or more than their host star) for Neptunes and mini-Neptunes. In other words, we find that as you go from smaller to more massive planets the amount of water in their atmospheres decreases. See the blue line in the figure below.

However, the overall trend in H2O abundances, from mini-Neptunes to hot Jupiters, is significantly lower than the mass–metallicity relation for carbon in the solar system giant planets and similar predictions for exoplanets; see the coral line in the figure below. That is, we find ‘less water’ than expected based on expectations for the amount of carbon we see in the solar system. Said differently, the blue line seems to be consistently below the coral line, which is what we expect/see in the solar system.

On the other hand, the Na and K abundances for the gas giants are stellar or superstellar, consistent with each other, and generally consistent with the solar system metallicity trend. That is, Na and K fell were generally consistent with what we see in the solar system.

Mass-metallicity relation in transiting exoplanets

If you see the plot above, the idea is that the orange and yellow markers are mostly consistent with the coral line. Interesting to point out is that wherever we see all three species, Na, K, and H2O, this last one is always lower than the alkali species.

Then, this seems to indicate that the H2O abundances in hot gas giants are likely due to low oxygen abundances relative to other elements rather than low overall metallicities. This inference may provide new constraints on the formation mechanisms of these exoplanets.

A nice by-product of this study was finding that there are different trends in the abundances of the different species. This argues against the use of chemical equilibrium models with metallicity as one free parameter in atmospheric retrievals, as different elements can be differently enhanced.

On Degeneracies in Retrievals of Exoplanetary Transmission Spectra

You can read the full paper open access here. This research was made open access thanks to the Bill and Melinda Gates foundation.

Authors: Luis Welbanks and Nikku Madhusudhan

It may be to understand the formation history of exoplanets or to look for trends in the composition of exoplanets. It may be to put a new chemical detection in context or to understand other properties of the atmosphere of an exoplanet. Whatever it may be for, we perform retrievals on the transmission spectra of exoplanets and want as an output robust and tight constraints on their atmospheric composition. Therefore, it is important to understand what degeneracies affect our retrievals and how do they affect them. Do they make abundance estimates (i.e. how much ‘stuff’ there is) unreliable? Is the presence of clouds and hazes in the atmosphere of these planets preventing us from determining accurate abundance estimates? We decided to look into this.

We performed an extensive set of retrievals on simulated data and on the real data set of the canonical hot Jupiter HD209458b. In our retrievals we tested a wide variety of assumptions. We employed models that ranged from simplistic semi-analytic prescriptions that assumed isothermal and isobaric atmospheres, all the way to fully numerical models that consider full pressure-temperature profiles, inhomogeneous cloud and haze cover, and multiple absorbers. We investigated how these models affect the retrieved H2O abundances.

Different model assumptions and their different degrees of fit to the data

The different models were applied on data sets that spanned the near-infrared only or data sets that combined spanned the optical and near-infrared. As it can be seen above, different models provide different degrees of fit to the data. How does this translate to the abundance estimates?

H2O abundance estimates from different models

As it can be seen above, different models provide better or worse constraints to the abundance estimates. In other words, the size of the error bar depends on what you assume. Then, one has to be very precise on what assumptions are made and at what cost.

Our study reveals four key insights:

  • First, we find that a combination of models with minimal assumptions and broadband transmission spectra with current facilities allows precise estimates of chemical abundances. In particular, high-precision optical and infrared spectra, along with models including variable cloud coverage and prominent opacity sources, with Na and K being important in the optical, provide joint constraints on cloud/haze properties and chemical abundances.
  • Second, we show that the degeneracy between planetary radius and its reference pressure is well characterized and has little effect on abundance estimates, contrary to previous claims using semi-analytic models.
  • Third, collision-induced absorption due to H2–H2 and H2–He interactions plays a critical role in correctly estimating atmospheric abundances.
  • Finally, our results highlight the inadequacy of simplified semi-analytic models with isobaric assumptions for reliable retrievals of transmission spectra.

When performing retrievals, it is important to use high-precision optical data and infrared spectra. It is important to model the clouds and hazes appropriately. When considering the optical data, the cross sections of Na and K play a critical role (hence why we looked carefully into this for our mass-metallicity work). If all this is done correctly, an atmospheric retrieval is capable of providing precise and robust estimates of the composition of exoplanet atmospheres.

Characterisation of exoplanets

Part of my work has been analysing the spectra of different exoplanets. This has been an exciting opportunity and has been work done in collaboration with many people. Here I highlight some of the work that has been already published.

The atmosphere and interior of K2-18b

You can read the full paper open access here. This research was made open access thanks to the Bill and Melinda Gates foundation.

Authors: Nikku Madhusudhan, Matthew Nixon, Luis Welbanks et. al

This study linked atmospheric observations with mass and radius estimates (bulk properties) to perform an end-to-end analysis of the habitable-zone exoplanet K2-18b. Here, I helped perform the retrievals on the spectra of the mini-Neptune K2-18b. We used different models ranging from clear atmospheres, to inhomogeneous clouds and hazes passing through fully cloudy atmospheres.  We constrained the atmosphere to be H2-rich with a H2O volume mixing ratio of 0.02%-14.8%, consistent with previous studies, and find a depletion of CH4 and NH3, indicating chemical disequilibrium. These results were then used to constrain the internal structure and thermodynamic conditions in the planet. This study finds that K2-18b hosts conditions at its surface that allow for liquid water. These results demonstrate that the potential for habitable conditions is not necessarily restricted to Earth-like rocky exoplanets.

Detection of haze, Na, K, and Li in the super-Neptune WASP-127b

Authors: Guo Chen, Enric Pallé, Luis Welbanks et. al
You can read the paper here.

Observations of the super-Neptune WASP-127b were taken using ground based telescopes (Gran Telescopio Canarias (GTC) and Nordic Optical Telescope (NOT)). I helped perform the analysis of the spectrum using retrievals and we found strong indications of Na, K and Li absorption. This data set was the first to exhibit Li absorption in an exoplanet.

First indication of aluminum oxide in an exoplanet WASP-33 b.

Authors: Carolina von Essen, Matthias Mallon, Luis Welbanks et. al
You can read the paper here.

Using again the ground based observatory Gran Telescopio Canarias, the ultra hot Jupiter WASP-33b was observed. I helped perform a retrieval on these spectral observations. Our analysis found that the feature at ~400 nm was better explained by the presence of AlO, aluminium oxide. This is the first indication of such species in an exoplanet. This indication is an interesting prospect in a field where we have an increasing diversity of ‘exotic’ species found in ultra hot Jupiters.

WASP 127b image credit. Artistic simulation of WASP 127b orbiting a star. Credit: Gabriel Pérez, SMM (IAC).

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