A Pathway to the Confirmation and Characterisation of Habitable Alien Worlds

Are we alone in the Universe? Since the confirmation of the first planets outside our solar system in the 1990s, we have made tremendous progress towards answering this question. Yet, the confirmation of a true Earth-analogue still evades us. On top of this, if we are truly to understand the origins of life in the cosmos, we must also create a complete picture of planetary formation, evolution, and habitability.

However, each of these aspects necessitates a detailed knowledge of solar-type stars. This is because we study exoplanets indirectly by analysing their much more luminous host stars. For example, most planet confirmation relies on the Doppler wobble of the host star, induced by the planet. Moreover, we can learn about a planet's dynamical history from mapping its projected orbit as it transits its host star. Hence, stellar surface inhomogeneities can impact planetary interpretations, and can completely swamp the signals from rocky worlds. My research aims to overcome these hurdles. For this, I study stellar surfaces from a two-pronged approach: with state-of-the-art 3D simulations and using transiting planets to empirically probe stellar surfaces.

I aim to understand and disentangle a fundamental barrier on the pathway to confirming other Earths: the stellar surface inhomogeneities from convection. Planet confirmation requires a mass measurement, which can be determined from the Doppler shift of the absorption lines in the stellar atmosphere. However, all Sun-like stars are enveloped in boiling plasma, causing hot bubbles of plasma to rise to the surface (inducing blueshifts), where they cool and fall down into the surrounding regions (inducing redshifts). The net result is spurious velocity shifts up to a m/s - completely swamping the tiny signal of an Earth- twin, which is a mere 9 cm/s. These shifts can be even larger if regions of magnetic field concentrate and inhibit the convection. Moreover, as the next generation of spectrographs (e.g. ESPRESSO) come online we are entering an era where it is technologically feasible to detect an Earth-twin signal, making this work extremely time critical.

The Sun has shown us convection does not easily average out; we must disentangle its signature to find Earth-like worlds. To do this, I use 3D magnetohydrodynamic simulations to create realistic model stars. With these, I study precisely how convection alters stellar lines, and work to optimise stellar noise reduction techniques. My present work on Solar-analogues indicates we can use the curvature of the stellar lines to remove this noise, but will this work for hotter or cooler stars? How do noise diagnostics behave if a star has a patchy distribution of magnetic field? Which lines are most sensitive to the convection and magnetic fields? These are some of the questions my research aims to answer in the next four to seven years.

Of course, these diagnostics are only as reliable as their underlying simulations. I have pioneered a new technique, using transiting planets as probes, to validate these for the first time for main-sequence stars other than the Sun. By subtracting in- from out-of-transit observations, we isolate the starlight behind the planet. With this, I can study the convection behaviour, stellar differential rotation, and determine the 3D trajectory of a planet's orbit - a key feature in understanding its formation and evolution. By applying this technique to a range of systems I will validate the simulations, quantify the impact of convection on planetary dynamic measurements, and contribute to a more global understanding of planet formation and evolution.

With this two-pronged approach, I aim to push the frontiers of astronomy towards the future confirmation and characterisation of habitable alien worlds, and help answer whether or not we are truly alone in the Universe.

The interdisciplinary proposed work will impact a diverse academic audience, and will inspire the next generation of scientists. On top of this, the space industry also generates a sizeable economic income, and my work will help ensure the next generation instrumentation comes to the UK.

My aim is to enable the future confirmation and characterisation of Earth-like alien worlds. As we have entered an era where this is technologically feasible, the fundamental limitation for discovering such rocky planets is due to noise sources originating from the host stars. Hence, solar/stellar physics is of paramount importance to this work, and the knowledge gained from this proposal will directly impact these fields, alongside the exoplanet field. An improvement in our knowledge of magnetic activity and differential stellar rotation, and stellar oscillations, has the power to revolutionise our understanding of stellar interiors and potentially impact our space weather predictions. Moreover, this will also impact a range of exoplanet sub-fields, including planet atmosphere characterisations and planet formation/evolution. This project will initiate both international and national collaborations. Knowledge gained will be disseminated through international conferences/workshops and publications in high impact journals, with open-access and cross-listing across solar, stellar and exoplanet fields. Computer code will also be made open-source through online repositories, such as GitHub.

The search for alien life excites a natural curiosity across kids and adults alike. With the help of the Physics Teaching Support staff at Warwick, dedicated Physics Impact Officer and centrally resourced Impact Team we will develop an outreach program, underpinned by my research, that will inspire the next generation of scientists (including interactive classroom demonstrations, lectures, and a travelling planetarium); emphasis will be put on ensuring we reach a diverse audience, and will aim to target typically unrepresented groups. In particular, we will target Key Stage 3 students to help increase the number of students later selecting GCSE and A-levels in STEM subjects. A further focus will be put on encouraging young women to pursue Physics, as the Royal Astronomical Society has shown that without active change it will take > 100 years to reach gender equality in astronomy. I will also disseminate research goals and outcomes through public lectures and science fairs, as well as with innovative measures such as 'Astronomy on Tap,' with the aim to reach audiences not typically interested in science, with a focus on young adults. The goal will be to instil a scientific curiosity and critical thinking skill set, aspects that will benefit society beyond science.

As this work crosses many domains and is at the forefront of the exoplanet field, it pushes and utilises the next generation of instrumentation - this will have a tangible economic impact. The science aims herein will help improve the efficiency of a number UK-invested spectrographs, such HARPS-N and the future Terra Hunting Experiment. Additionally, the ESPRESSO spectrograph, just online this year, is the first to enable the instrumental precision necessary to confirm a true Earth-twin. As such, this work is extremely timely and critical to help overcome the astrophysical barriers preventing such a discovery. Moreover, follow-up planet confirmation is an official deliverable for ESA's future space mission PLATO, and will only be possible for Earth-like planets if we can overcome the astrophysical noise (as proposed herein). In the long run, these developments will position the UK to compete for the next space mission that will be attempting to characterise the atmospheres of terrestrial alien worlds. This is likely to be a billion-pound facility - much of which will go to UK industry leading to new technological developments.