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Remo Burn's Webpage

ABOUT ME

Enthusiastic Planet Formation Theorist

Hi, my name is Remo Burn and I'm interested in many aspects of planet formation and currently working as postdoc at the Max Planck Institute for Astronomy in Heidelberg.

Picture of me at the defense of my doctoral thesis

Welcome to my website and thank you for your interest in me and my work. I got interested in planetary sciences thanks to the presence of the reasearch group in Bern which gave me the opportunity to move from more fundamental physics to theoretical astrophysics during my PhD. This was all the way back in 2016 and I was certainly excited to learn about all the discovered exoplanets. I graduated from the University of Bern in 2020 and the excitement is still there. But I also came to the conclusion that the best model of planet formation is only as good as the input that is used. For planet formation, this is given by the protoplanetary disk that surrounds the star. Therefore, I moved to Heidelberg to start a fellowship at the Max Planck Institute for Astronomy to learn more about disks and what conditions can be found there. Recently, I explored the composition of small planets in more detail and started looking into applying modern machine learning techniques in our field.

If you like to know more about me, check out my CV (14th of March 2024)

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RINGBERG CONFERENCE

Density Matters 2024

I recently initiated and organized together with Rafa Luque a workshop on small planets. More details can be found on the conference webpage. Since the space was limited, I would like to share here six identified main messages from the participants.

  1. Sub Neptunes are the most dominant exoplanet population known to date
  2. JWST is capable of detecting the atmospheres of sub Neptunes
  3. There are multiple scenarios capable of explaining the current observations. We need a) more observations and b) testable predictions and contextual factors
  4. There is a need for models coupling the interior and atmosphere while accounting for evolution processes
  5. There is a need for laboratory work and experimental data including
    • Equations of State at high temperatures and pressures
    • Material properties
    • Line lists and continuum emission/absorption
    • Chemical reaction rates
    Theoretical sensitivity studies need to quantifiy the importance of these model ingredients.
  6. Intrinsic distributions of fundamental planetary properties would be very useful for answering important population-level questions.

What is the main direction where research is required to understand the nature of small exoplanets (Earths to sub Neptunes)?

Replies from participants to
Among the participants, we conducted a survey powered by sci-an where we asked about the future directions. The 29 free format replies were automatically categorized into pre-defined categories listed above. Individual quotes selected by me exemplify the replies. In particular, interdisciplinary research, such as the link to geology and atmospheric physics, was stressed.

Individual, anonymous quotes by participants of the Density Matters 2024 workshop.

"Better coupled atmosphere-interior models, which make testable, observable predictions about what molecules to observe in the atmosphere and how to link these to the interior state"

"How can we go from elemental budgets from formation via realistic interiors to final atmospheres?"

"More work in the lab looking at realistic compositions of sub-Neptune atmospheres and their interiors (e.g. silicates, water, and hydrogen at extremely high temperatures and pressures). Many of our models rely on extrapolations that may not be valid."

WORK

A selection of my main research topics is illustrated below. A full list of all publications can be found on ADS.


THE RADIUS VALLEY FROM A PLANET FORMATION PERSPECTIVE

For this project we used the result of planetary population synthesis models (see below) to try to understand the origins of the radius valley - an under-abundance of planets with radii of approximately twice the Earth radius separating smaller super Earths from larger sub Neptunes of a priori unknown composition. It turns out that a consistent radius valley can be produced if water-rich planets migrate towards the star and are thus identified as the larger sub-Neptunes with radii - but not masses - separated from those of smaller rocky planets.

Figure 4 from Burn et al. 2023
Schematic from Burn et al. 2024 for the four considered scenarios investigated in that work. The compositional options in agreement with observational data are either the standard rocky super Earths and sub Neptunes with pure H/He envelopes (a) or, alternatively, water-rich sub Neptunes with mainly supercritical water vapor envelopes (d). Not in agreement with observations based on the assumptions in the study are the cases of condensed water (b) or lack of photoevaporation (c). Those two options are also not realistic from fundamental physcial principles due to the high temperatures and high-energy radation of close-in planets.

The research was published in Nature Astronomy with an accompanying short Research Briefing.


PLANETARY POPULATION SYNTHESIS

The idea of planetary population synthesis is to model planet formation theoretically in order to statistically compare and predict what kind of planets form. This requires simulating the process over and over to enlarge the sample. In the movie below, you can see a single run of the Bern model of planet formation and evolution.



Time evolution of simulated planets in mass, semi-major axis space around a small star (10% of a Solar mass). The size of the dots and the transparent layers are proportional to the radii of the solid core and the Hydrogen/Helilum atmospheres of the planets. The fading crosses indicate the former location of bodies that collided with other planets; accordingly, the collision partner jumps in mass. Planets in Mean-Motion Resonance are connected with lines. These resonant chains can break due to interactions with other bodies. For eccentric orbits, the ap- and periastron locations are indicated and the color corresponds to the ice mass fraction of solids in the core. The circles that appear at the end are the observed Trappist-1 system planets with masses calculated by Grimm et al. 2018.

This general approach was used by me and my collaborators to study a number of effects. For example, the building-blocks of planets might dry out over time if they are heated by the radioactive decay of elements. We found that this process leads to the formation of mainly dry planets in systems with a large abundance of radioactive aluminium (Lichtenberg et al. 2019). In this particular work, we assumed that the building-blocks are large, but newer propositions of the major contribution to the planetary mass consisting of small, pebble-sized objects were recently made. We tested these two assumptions within the same framework in the work published by Brügger et al. 2020. I contributed to updating the model and we describe it in all technical details in Emsenhuber et al. (2021). Then, I applied the model to the case of the small M dwarfs where we have a lot of observational data: Burn et al. (2021).

PROTOPLANETARY DISK EVOLUTION

Recently, I improved our description of protoplanetary disks which are subject to photoevaporative winds and where some dust can be carried in these winds (Burn et al. (2022)). With these improvements, our models can also be compared to observed disks to learn about planet formation as it is happening. The developed model was used in a global comparison project of models against disk observations (Emsenhuber, Burn, et al. 2023) and constructed initial conditions for disks which under certain assumptions result in the observed distribution after several Myr of evolution. You can also watch a talk on these topics on youtube.

Figure 5 from Emsenhuber et al. 2023
Density of model solutions and observational data as a funciton of dust mass against accretion rate. The observed data is shown as points and the model output is shown as histogram. Nominal parameters as well as disks with only external photoevaporation were not able to match observations. Only after reducing the outer radius and increasing the disk mass by a factor 3 (or equivalently reducing the refernce stellar mass), the disks show sufficient accretion and millimeter emission in our modeling.

RADIAL DRIFT AND ABLATION

Drift path over iceline

Mass Fractions of different distributions of drifting bodies

Remaining mass fraction in the overall population of bodies crossing the snowline (1 kg ≤ m ≤ 109 kg) with shaded bands indicating the standard deviation due to the evolving disk. The mass shown is an integral over a distribution of masses with the indicated power-law slope and a mean over time.

Bodies with sizes from centimeters to a few hundred meters radially drift towards the central star in protoplanetary disks. In this work, we adressed the question of how much water could be transported from the region where water is frozen to the region where water is usually assumed to be present only as vapor. The figure on the left summarizes the findings: Material distributed over a size-range from centimeters to hundred meters moves a few percent closer to the star before it had time to evaporate. Therefore, a planet growing in this region would accrete water as ice, which would melt on the planet and build up an ocean, at a location where this is otherwise not expected. For Earth, it seems unlikely that this effect made a difference because it formed too far away from this region.

This work is published in Burn et al. 2019 or you can find a copy on the arXiv.

CONTACT

WHERE I WORK

Max Planck Institut für Astronomie, Office 114, Königstuhl 18, 69117 Heidelberg, Germany
Email: burn {at} mpia.de

I'm always happy to have a cup of with visitors