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We organise weekly seminars (on Tuesdays) focussing on exoplanet-related science. These meetings gather researchers working on an exoplanet-related field (theory and observations) from several departments at the University. Meetings usually take place in the Ryle seminar room in the Kavli Institute. They start at 13h00 and end around 14h00. As talks are usually 40-min long, there is ample time for discussion.
URL: http://talks.cam.ac.uk/show/index/47514
Updated: 1 hour 14 min ago

Tue 21 May 13:00: Exploring the atmospheric structure of water-rich sub-Neptunes

Thu, 16/05/2024 - 10:48
Exploring the atmospheric structure of water-rich sub-Neptunes

Population studies and planetary formation models predict a class of water-rich sub-Neptunes consisting of a rocky core overlain by a water-rich envelope. Characterising such planets is difficult since differing interior structures often lead to degenerate mass and radii. Recent JWST observations aim to break some of these degeneracies by retrieving atmospheric composition, however accurate atmospheric models are required to interpret data. For example, separate analyses of the JWST transmission spectrum of water world candidate TOI -270 d predicted different interior structures – a “Hycean world” scenario with a liquid water surface and a “miscible sub-Neptune” where the water in the envelope is in a supercritical state. To distinguish these scenarios, I have developed a radiative-convective model specifically designed to model water-rich sub-Neptunes. In particular, the model accounts for the inhibition of convection due to mean molecular weight gradients induced by the condensation of water vapour in a less dense background gas. I show that this can warm a liquid water surface significantly, moving the traditional habitable zone of the planet outwards and disfavouring the presence of water oceans for sub-Neptunes with Earth-like instellations. Following on from this, I will explore the possible equilibrium states of a sub-Neptune with a supercritical water envelope. Lastly, I will discuss attempts to decipher whether sub-Neptunes have surfaces using atmospheric chemistry and the implications this has on interpreting present and future observations.

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Tue 14 May 13:00: Attractor reconstruction of active stellar light curves

Fri, 10/05/2024 - 10:47
Attractor reconstruction of active stellar light curves

Stellar activity is notoriously difficult to model, being neither periodic nor purely stochastic. In light curves, the interplay between the stellar rotation period and the birth and death of spots and faculae gives rise to quasi-periodic modulation over time scales of hours to weeks. Despite the complexity of this interplay, the resulting light curves bear strong qualitative resemblance to systems known to exhibit low-dimensional dynamical chaos, such as the Rössler attractor.

In the 1980s and 1990s, a suite of techniques for nonlinear dynamical analysis, called attractor reconstruction, evolved to study exactly this type of system. Attractor reconstruction works by embedding a 1-dimensional time series, such as stellar light curve, in a higher-dimensional phase space capable of capturing its full dynamical behavior: too low a dimensionality, and the system’s trajectory will self-intersect and tangle, which we know to be physically unrealistic given the non-periodicity of the observed signal. This technique has been used successfully to model the historical sunspot record and the light curves of variable stars (both simulated and observed) and to recover important features of their underlying dynamics, including their dimensionality and the time scales over which they can be meaningfully forecast into the future. Here, I discuss the application of attractor reconstruction to the light curve of the Sun over Solar cycles 23-25, as observed by the Solar and Heliospheric Observatory.

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Tue 30 Apr 13:00: Stellar activity mitigation in radial velocity measurements

Fri, 19/04/2024 - 13:58
Stellar activity mitigation in radial velocity measurements

Over the past decades, the radial velocity (RV) community has made tremendous leaps forward in detecting and characterising ever smaller and lighter exoplanets. This trend has been interrupted in recent years, as planetary RV signals below 1 m/s are drowned out by the stars’ activity. The detection of Earth analogues producing an RV effect of about 10 cm/s is therefore currently out of reach. Several avenues are being explored to restore the trend towards the detection of increasingly less massive planets. These include improvements in (1) instruments, (2) observing strategies, (3) RV extraction techniques, (4) stellar activity monitoring, and (5) stellar activity modelling. In this talk, I will focus on points (4), and (5). I will provide an overview of stellar activity mitigation techniques and show how a proxy for stellar magnetic activity induced RV variations can be extracted from intensity spectra in the visible wavelength range, providing an independent estimate of the evolution of the magnetic field.

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Tue 30 Apr 13:00: Title to be confirmed

Sun, 14/04/2024 - 12:47
Title to be confirmed

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Tue 28 May 13:00: Title to be confirmed

Sun, 14/04/2024 - 12:44
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Tue 21 May 13:00: Title to be confirmed

Sun, 14/04/2024 - 12:44
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Tue 23 Apr 13:00: The physical mechanism of the streaming instability, and whether it works in vortices

Sun, 14/04/2024 - 12:41
The physical mechanism of the streaming instability, and whether it works in vortices

A major hurdle in planet formation theory is that we do not understand how small pebbles congregate into big planetesimals. A promising way to overcome this metre-scale barrier involves a fluid dynamics phenomenon called the streaming instability (SI). It concentrates the pebbles into clumps that are dense enough to collapse gravitationally, thereby forming planetesimals.

Unfortunately, the mechanism responsible for the onset of the instability remains mysterious. This makes it hard to evaluate the robustness of the instability, or to understand how it saturates. It has recently been shown that the SI is a Resonant Drag Instability (RDI) involving inertial waves. In the first part of this talk, I build on this insight to produce a clear physical picture of how the SI develops.

Another problem is that the SI can only devellop in regions containing a high density of similar-sized pebbles. Those conditions are met in large-scale vortices, but no one knows if the SI can feed on vorticial flows. Indeed, any instability can only devellop in specific flows, and a priori the SI is tailored to Keplerian disc flows, not vortex flows. I answer this question in the second part of the talk. To do so, I develop a simple pen-and-paper model of a dust-laden vortex in a protoplanetary disc. I find that if the vortex is weak and anticyclonic, dust drifts towards its centre. I then build a vortex analog of the shearing box to analyse the local linear stability of my dusty vortex. I find that the dust’s drift powers an instability which closely resembles the SI. This result strengthens the case for vortex-induced planetesimal formation.

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Tue 23 Apr 13:00: Title to be confirmed

Sat, 30/03/2024 - 08:42
Title to be confirmed

Abstract not available

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Tue 11 Jun 13:00: Title to be confirmed

Sat, 30/03/2024 - 08:38
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Tue 04 Jun 13:00: Title to be confirmed

Sat, 30/03/2024 - 08:37
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Tue 30 Apr 13:00: Title to be confirmed

Sat, 30/03/2024 - 08:36
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Tue 14 May 13:00: Title to be confirmed

Sat, 30/03/2024 - 08:36
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Tue 12 Mar 13:00: Relative contribution from comets and carbonaceous asteroids to the Earth's volatile budget

Mon, 04/03/2024 - 14:31
Relative contribution from comets and carbonaceous asteroids to the Earth's volatile budget

Recent models of solar system formation suggest that a dynamical instability among the giant planets happened within the first 100 Myr after disk dispersal, perhaps before the Moon-forming impact. As a direct consequence, a bombardment of volatile-rich impactors may have taken place on Earth before internal and atmospheric reservoirs were decoupled. However, such a timing has been interpreted to potentially be at odds with the disparate inventories of Xe isotopes in Earth’s mantle compared to its atmosphere. In this seminar, I will talk about the dynamical effects of an Early Instability on the delivery of carbonaceous asteroids and comets to Earth, and address the implications for the Earth’s volatile budget.

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Tue 05 Mar 13:00: Reassessing the Evidence for Time Variability in the Atmosphere of the Exoplanet HAT-P-7b

Fri, 01/03/2024 - 09:20
Reassessing the Evidence for Time Variability in the Atmosphere of the Exoplanet HAT-P-7b

We reassess the claimed detection of variability in the atmosphere of the hot Jupiter HAT -P-7 b, reported by Armstrong et al. (2016). Although astronomers expect hot Jupiters to have changing atmospheres, variability is challenging to detect. We looked for time variation in the phase curves of HAT -P-7 b in Kepler data using similar methods to Armstrong et al. (2016), and identified apparently significant variations similar to what they found. Numerous tests show the variations to be mostly robust to different analysis strategies. However, when we injected unchanging phase curve signals into the light curves of other stars and searched for variability, we often saw similar levels of variations as in the HAT -P-7 light curve. Fourier analysis of the HAT -P-7 light curve revealed background red noise from stellar supergranulation on timescales similar to the planet’s orbital period. Tests of simulated light curves with the same level of noise as HAT -P-7’s supergranulation show that this effect alone can cause the amplitude and phase offset variability we detect for HAT -P-7 b. Therefore, the apparent variations in HAT -P-7 b’s atmosphere could instead be caused by non-planetary sources, most likely photometric variability due to supergranulation on the host star.

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Tue 27 Feb 13:00: Protoplanetary disc: what can we learn by combining theory and observations?

Thu, 22/02/2024 - 12:49
Protoplanetary disc: what can we learn by combining theory and observations?

Protoplanetary discs serve as the cradle for planetary formation and evolution. It is then fundamental to study their evolution to gain a comprehensive understanding of exoplanetary system formation. These discs can be studied using two distinct approaches.

On one side, they can be analysed as a set of single sources, allowing for a detailed analysis of the mechanisms behind the diversity of observed morphologies using gas and dust tracers such as rings, gaps and asymmetries.

On the other side, it is crucial to study star-forming regions, understanding which physical processes are governing the global disc evolution.

In this talk, I will firstly describe results from the modelling of single sources, underlining the information we can obtain by comparing multi-wavelengths observations with results from the hydrodynamical models of specific sources (e.g., HD169142 , PDS70, GG Tau A). In particular, I will focus on how simulations can help in constraining the mass and position of the candidate proto-planets that may be responsible for the ALMA and SPHERE observational results, as well as how they can support future observational strategies.

I will then summarize some of the results obtained by testing disc evolution models by comparing them with the Lupus star forming region. In these works, we tested the secular evolution of the observed dust and gas radius of disc populations and their ratio, to test the efficiency of radial drift and the viscous evolution theory.

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Tue 20 Feb 13:00: Uncovering Long-Period Transiting Exoplanets with TESS and CHEOPS

Mon, 12/02/2024 - 19:06
Uncovering Long-Period Transiting Exoplanets with TESS and CHEOPS

Long-period transiting exoplanets are incredibly important, allowing us to study planets with temperatures similar to those in our own solar system. However, due to its observing strategy, the Transiting Exoplanet Survey Satellite (TESS) is heavily biased towards the discovery of short-period planets. To increase the yield of long-period planets, I am using TESS “duotransits” – planet candidates with two observed transits separated by a large gap, typically two years. From the two non-consecutive transits, the period of the planet is unknown, but there exists a discrete set of period aliases. As a member of the CHaracterising ExOPlanet Satellite (CHEOPS) Duotransit Program, I perform targeted follow-up of TESS duotransits to recover their true periods. To identify the best targets for CHEOPS follow-up, I developed a specialised pipeline to discover TESS duotransits. In this seminar, I will present my pipeline, its five discoveries and the sample of small, long-period planets being uncovered by TESS and CHEOPS , including the Neptune-mass planet TOI -5678 b and the bright multi-planet system HD 15906 .

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Tue 13 Feb 13:00: Disc evolution in young intermediate-mass stars

Mon, 05/02/2024 - 09:05
Disc evolution in young intermediate-mass stars

Planets are born from circumstellar discs and the evolution of these discs determines the final architecture of planetary systems. The stellar mass range between 1.5 and 3.5 solar masses presents a particularly interesting circumstellar disc evolution; most notably, it is dominated by the EUV /FUV photoevaporation regime on the pre-main sequence, it contains the majority of gaseous debris discs, and it also shows the highest giant planet frequency. In our recent spectroscopic VLT /X-Shooter survey (UV to nIR), combined with WISE data (nIR to mIR), we identified 135 pre-main sequence (PMS) intermediate mass stars (IMSs) in the Southern sky. This is the first unbiased sample of IMSs in the PMS , allowing a study of disc evolution. Our sample, encompassing protoplanetary and debris discs, also revealed a significant number of discs between these two evolutionary stages. We find that the IR excess evolution of IMSs differs from that seen for low-mass stars (LMSs), exemplified by samples drawn from nearby star forming regions. We observe that, in IMSs, the inner disc regions are vacated in their entirety, in contrast to the LMSs where we note a more gradual inside-out dissipation. We also investigated the presence of gas absorption features in our sample via optical high-resolution spectroscopy to identify gas-bearing debris discs. This requires detailed comparisons to spectra of nearby stars to eliminate objects with foreground cloud absorption as cause of the absorption features. In particular, we apply this effective method to one such disc, eta Tel, discarding the earlier claim of disc wind as the origin for the absorption features. Finally, we discuss our several ongoing and future surveys investigating the nature of circumstellar discs around IMSs.

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Tue 06 Feb 13:00: Photoevaporation from Exoplanet Atmospheres: Understanding the Role of Stellar Winds and Considering Water-rich Atmospheres Room changed

Tue, 30/01/2024 - 10:54
Photoevaporation from Exoplanet Atmospheres: Understanding the Role of Stellar Winds and Considering Water-rich Atmospheres

The atmospheres of close-in exoplanets are extremely vulnerable to the effects of stellar UV to X-ray radiation. Photoevaporation can significantly alter planetary atmospheres or even strip them entirely, potentially rendering a planet uninhabitable. Understanding how these atmospheres evolve, persist, or fade away remains a fundamental challenge. In this talk, I will discuss two distinct but interconnected areas of photoevaporative research.

Firstly, I will discuss the interaction between the stellar wind and photoevaporating atmospheres. I will present 3D magnetohydrodynamic simulations of the interaction between the stellar wind and the photoevaporating outflow of a planet orbiting an M dwarf. This analysis reveals a diverse range of magnetosphere morphologies and plasma distributions due to the wind-outflow interaction. I consider how these changing morphologies might impact observable hydrogen Lyman-alpha signatures during planetary transits.

In the second part, I will delve into our current understanding of photoevaporation from water-rich atmospheres. Conventional analytic approaches often oversimplify the process, assuming two scenarios: the escape of only lighter hydrogen, or the dragging of oxygen along with escaping hydrogen. These two scenarios lead to two end cases: a planet that has retained its water-rich atmosphere or a planet which has lost its atmosphere, becoming dry and desiccated. I will challenge these oversimplifications by presenting results from a novel 1D multifluid hydrodynamic model of photoevaporation from a water-rich atmosphere, which shows oxygen escape should no longer be described by a simple on/off switch but instead requires careful modelling.

Room changed

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Tue 06 Feb 13:00: Photoevaporation from Exoplanet Atmospheres: Understanding the Role of Stellar Winds and Considering Water-rich Atmospheres

Mon, 29/01/2024 - 11:37
Photoevaporation from Exoplanet Atmospheres: Understanding the Role of Stellar Winds and Considering Water-rich Atmospheres

The atmospheres of close-in exoplanets are extremely vulnerable to the effects of stellar UV to X-ray radiation. Photoevaporation can significantly alter planetary atmospheres or even strip them entirely, potentially rendering a planet uninhabitable. Understanding how these atmospheres evolve, persist, or fade away remains a fundamental challenge. In this talk, I will discuss two distinct but interconnected areas of photoevaporative research.

Firstly, I will discuss the interaction between the stellar wind and photoevaporating atmospheres. I will present 3D magnetohydrodynamic simulations of the interaction between the stellar wind and the photoevaporating outflow of a planet orbiting an M dwarf. This analysis reveals a diverse range of magnetosphere morphologies and plasma distributions due to the wind-outflow interaction. I consider how these changing morphologies might impact observable hydrogen Lyman-alpha signatures during planetary transits.

In the second part, I will delve into our current understanding of photoevaporation from water-rich atmospheres. Conventional analytic approaches often oversimplify the process, assuming two scenarios: the escape of only lighter hydrogen, or the dragging of oxygen along with escaping hydrogen. These two scenarios lead to two end cases: a planet that has retained its water-rich atmosphere or a planet which has lost its atmosphere, becoming dry and desiccated. I will challenge these oversimplifications by presenting results from a novel 1D multifluid hydrodynamic model of photoevaporation from a water-rich atmosphere, which shows oxygen escape should no longer be described by a simple on/off switch but instead requires careful modelling.

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