HARPS3 and The Terra Hunting Experiment

We now know that planets are common objects orbiting stars. The results of transit (looking for the small dip in light from a star due to a planet passing in front of it) and precise radial velocity surveys (looking for tiny shifts in spectral lines due to the "wobble" of the star caused by the mass of the planet in orbit) have unravelled an amazing diversity of planets from hot Jupiter-like planets to compact multiple systems of small planets in orbits very close to their stars. In the past two decades we have acquired a wealth of knowledge about the structure and the occurrence of these planets. Despite this success, no Earth "twin" has yet been confirmed, making it difficult to set our own Solar System in context. The challenge in looking for "Earth-twins" is related to the technical difficulty needed to reach the required instrument precision and sensitivity. However, the recent exoplanet surveys have shown that the noise from the star (arising from magnetic and convective effects in the photosphere and chromosphere) is higher than expected, so signals from small planets are lost in this "star noise". Currently, progress towards detecting smaller planets on longer orbits appears to depend more on our ability to understand and remove this "star noise". In the specific case of radial velocity surveys, series of measurements that are spread thinly and unevenly over the observation period, appears to be a rather inefficient way to use telescope time to search for small mass planets.
To make significant progress towards Earth-twin detections we propose to install a fully dedicated facility equipped with a state-of-the art precision spectrograph (HARPS3) on the Isaac Newton telescope (INT) to carry out a dedicated and precise radial velocity survey. Our plan is to observe every night, for 6-8 consecutive months, over a 10-year period, a selected sample of about 40 bright old G and K stars (our Sun is a G-type star). This survey, called "The Terra Hunting Experiment" will be able to efficiently explore a range of orbital distances for Earth mass planets more alike to the rocky planets of our Solar system. Extrapolating from the statistical results of the Kepler mission we expect to find between 2 and 10 Earth-size planets in orbital ranges corresponding to the habitable zone of G and K dwarf stars. These "Goldilocks" planetary systems are likely to be the centre of attention for future programmes aiming to analyse the structure and atmosphere of planets.
To minimise risk and cost we will build a close-copy of the existing HARPS spectrograph on the 3.6m telescope in La Silla. This design has demonstrated the necessary long-term stability and achieved radial velocity performances well below the star noise. To enhance our capability to monitor and remove the star noise from our signal, a precise polarimeter will be included with the instrument. The nightly sampling rate and the extended duration of the survey requires an optimized operation strategy. The consortium plans to refurbish the INT to operate it almost completely robotically. Community observation programmes will be inserted in the nightly observing schedule.
A broad community is likely to use this facility for a wide range of observing programmes, including stellar physics, the interstellar medium and other exoplanet programmes. In the UK, the exoplanet radial velocity community is small in comparison with other areas of astrophysics but it is growing faster than any other field. Our aim is to help build a UK community by making it as straightforward as possible for new entrants to the field. Open access to a flexibly-scheduled, precise radial velocity instrument will provide a unique opportunity for UK scientists to use this technique and, for example, play a leading role in the future follow-up of TESS and PLATO discoveries.

The main scientific goal of the project is to detect Earth-mass exoplanets on longer orbits than is currently possible around nearby, bright stars, with an ultimate aim to push the detection threshold out to Earth-like planets. The scientific impact of such a discovery will be first to demonstrate that the Earth and the Solar System does not belong to a rare category of planetary systems. Second, the identification of a "Goldilocks planet" potentially able to sustain life would stimulate and encourage further studies of its nature and its atmospheric content with the prospect of identifying indirect traces of life. Such a discovery is likely to stimulate excitement and enhanced interest in science amongst the general public and young people. Our aim is to use that excitement to drive the outreach programmes at both Exeter and Cambridge, by ensuring that planet announcements are followed up by related outreach activities.

Our programme is based around the construction of a large, highly technical instrument, and so also involves transfer of technical expertise to those institutes, companies and staff involved.