Understanding stellar variability with polarimetric Sun-as-a-star observations

The search for Earth-twins

The installation of the third-generation High-Accuracy Radial velocity Planet Searcher (HARPS3), on the 2.54-m Isaac Newton Telescope in La Palma (see the image below) marks the beginning of the next era of ground-based exoplanet detection using radial velocities (RVs). HARPS3 will be able to measure radial velocities with precisions of 10 cm/s. The significance of this number is that if an alien observer lightyears away from us were to view our Solar system with HARPS3, they would have the required precision to detect the Earth! HARPS3 is the new powerhouse instrument of the aptly named Terra Hunting Experiment, or THE. The THE will have half of all observing time with HARPS3 for a period of 10 years, allowing for an extended RV survey in the coming years.
The 2.5-m Isaac Newton Telescope pictured in 2023 with the now decommisioned Wide Field Camera (WFC) instrument mounted at its primary focus.

Stellar variability: The greatest hurdle

Currently, the greatest hinderance in detecting and characterising Earth-twins is stellar variability. The advent of high-resolution spectrographs such as ESPRESSO and the state-of-the-art HARPS3 brings 10 cm/s stability within reach. Signals from stars like the Sun are an order of magnitude greater than this. To mitigate these effects, the activity of Sun-like stars must be studied, and which better than our own Sun? Observing the Sun gives us high-cadence, high signal-to-noise data which is free from planetary signals. Previous iterations (HARPS, HARPS-N) have incorporated Solar observations into their programs, making use of the instruments’ extraordinary resolving power.

A visualisation of the RV scatter due to stellar activity (yellow points) compared with the signal from an Earth-like planet (pale blue line) detectable with HARPS3. Solar data from the HARPS-N data release (10.82180/dace-h4s8lp7c.)

ABORAS

Components of ABORAS during testing at the University of Birmingham.

ABORAS will make use of HARPS3 during the daylight hours, taking polarimetric data of the Sun. The Sun is free of planetary signals (excluding the potentially elusive "planet nine"), so can be used as a template for activity in stars of types F, G, and K. By measuring the line of sight magnetic field of the Sun and coupling these data with RV observations of the Sun, the physics of stellar activity can be better understood and inform mitigation techniques. Ultimately, ABORAS will contribute to our planet-hunting toolkit - bringing small, rocky around sunlike stars into view.

Light path of sunlight through the ABORAS components to the HARPS3 spectrograph.

ABORAS setup and components

The above diagram shows the light path of sunlight through the main components of ABORAS. Circularly polarised sunlight enters the ABORAS aperture and is converted into linearly polarised light by the quarter-waveplate. The quarter-waveplate is rotated through 360° to gain the observations required for full Stokes V characterisation. Four sub-exposures are taken at the angles 45°, 225°, 135°, and 315°.

The linearly polarised light is then split by the Foster prism. This consists of two quartz crystals which are joined together. The birefringent material causes the beam to split into two beams with orthogonal polarisation. These beams are then focused into separate integrating spheres.

The integrating spheres scatter the light from the image, creating a uniform Solar disc. Once the light from the disc is scattered, it is transferred to the HARPS3 spectrograph via optical fibres. In total, there are eight sub-exposures (four from each fibre) passed into HARPS3 per observation. These data are then combined to create Stokes I (intensity) and V (degree of circular polarisation) profiles.

View from the roof of the INT, where ABORAS will be mounted.