Understanding Stellar Variability and Improving Exoplanet Characterisation

Almost everything we know about exoplanets, those planets outside of our solar system, is derived from the stars they orbit. This is because, like our Sun relative to the Earth, the host stars vastly outshine any companion planets. As a result, inhomogeneities and variability on the surface of the host stars can significantly impair the ability to detect and characterise exoplanets; this is especially true for temperate, rocky planets as their signals are inherently small.

The first exoplanet around a Sun-like star was discovered through the radial velocity technique (Mayor & Queloz 1995), i.e. measuring the Doppler reflex motion induced in the host star by the planet as they orbit about their common centre of mass. Since this discovery we have come a long ways, detecting thousands of exoplanets and significantly improving the precision of our instruments. However, we are yet to find a true analogue to our Earth, the only known planet to support life. Thanks to instruments like the ESPRESSO spectrograph on the VLT, we now are within the technological capabilities to detect rocky, temperate planets around Sun-like stars, yet this remains an enormous challenge. Perhaps the biggest stumbling block on the pathway to habitable alien worlds originates from the host stars themselves.

Inhomogeneities on the surfaces of the exoplanet host stars can produce spurious radial velocity signals that may mask or even mimic the Doppler wobble of an exoplanet. This happens both because the plasma on the stellar surface is moving, and thus emitting its own Doppler shifts, but also because magnetic field concentrations can suppress these motions and alter brightness of the surface in these regions. Moreover, the variability originating from the surface of the Sun (and Sun-like stars) is orders of magnitude larger than the ~9 cm/s Doppler wobble induced by the Earth.

This project focuses on improving our knowledge of stellar physics so that we may disentangle stellar and planetary signals to enable the future confirmation and characterisation of rocky, temperate worlds around Sun-like stars. Almost all stellar variability is driven by the interplay between convection and magnetic fields; hence, understanding this interplay is key to unveiling low-mass, long-period planets. On the surface of our Sun, hot bubble of gas (known as granules) rise to the surface, cool, and fall back down into the star (within intergranular lanes); the uprising motions give rise to Doppler shifts towards the bluer end of the spectrum, while the falling motions shift the light towards the redder end of the spectrum. This introduces asymmetries into the stellar absorptions lines used to measure Doppler shifts, as well as net shifts of several 10s of cm/s to a several m/s, depending the interplay with the magnetic field.

This project will use state-of-the-art 3D magnetohydrodynamical (MHD) simulations to create the most realistic stellar simulations to date. Creating a suite of stellar models will allow us to study how the convection is altered at various magnetic field strengths and how this imprints itself in a number of different (typical) stellar absorption lines. The goal is to develop a toolkit to identify and mitigate stellar variability in exoplanet data, thereby pushing our detections levels even further and helping to open a pathway to the confirmation and characterisation of Earth-like alien worlds.