NGTS Survey

NGTS is designed to reach 1mmag accuracy over a wide field, with the prime objective of detecting transiting Neptune size planets. From the ground, assuming a precision of 1 mmag, for wide-field small-size telescopes, the most optimal targets are K and early M stars. For G stars the planet size is limited by the difficulty of reaching sub-mmag photometric accuracy from the ground. While there are insufficient numbers of bright late M stars in the visible band.

Parameter space for transit detection with the yellow area indicating the prime science parameter space search of NGTS. Transit depth is indicated as a function of planet and star radius. Approximate spectral types of stars are also indicated, as well as the radii of representative Solar system planets. Known transiting systems are marked in green where they were discovered in ground-based transit surveys, blue if they were originally identified in radial velocity surveys, and red if they were discovered from space.
The optical system is designed to observe with optimal sensitivity in the 600 to 900nm range, in order to match the peak emission of the primary targets (K and early M). Large format (2k×2k) CCD cameras have been developed by our industrial partners (Andor Technology, Belfast, and e2v, Chelmsford) specifically for our requirements, and these have now been made commercially available in their product line. We are also working with Astro Systeme (Austria) to optimize one of their commercially available wide-field telescopes, specifically for our purpose.

Following the results from HARPS planet surveys on super-Earths, the work from Howard et al. 2010 as well as the early results from Kepler, one can consider a conservative planet occurrence of typically 5% for Neptune size planets with periods shorter than 10 days (corresponding to an average transit probability of about 5%). One then finds that a minimum sample of about 40,000 stars needs to be monitored to detect about 100 transiting planets of this kind. With the additional constraint to identify these planets on bright stars, to ensure an optimal capability for follow-up observations, we considered a V magnitude of 13th as the practical faint limit. For M stars a fainter V-magnitude down to 15th may be considered in view of the smaller mass of the star and the larger radial-velocity amplitude expected from the planet. An additional benefit of staying in this magnitude range is the limited crowding factor, which is less than 20% for our PSF, even for a field 30 degrees from the Galactic plane.

The most fundamental result from Kepler is the statistical information gathered on Neptune and large Earth-like planets. NGTS is designed to detect this planet population around brighter stars, allowing systematic follow-up of planetary structure and atmospheric composition. Kepler candidates are mostly too faint to be studied with ground-based instruments and are not visible from the Southern hemisphere. Interestingly the period distribution of small planets significantly rises above 4 days, motivating a facility with good detection efficiency in the 4 to 10 day period range.

The use of the V-magnitude as reference instead of the I-band, which would be more naturally suited for K and M stars, arises from the need to conduct radial velocity follow-up with precise high-resolution spectrographs like HARPS. However, the brightness of the bulk of our targets in the IR domain will provide an additional incentive to support possible future development at ESO of a near-IR stabilized high-resolution spectrograph.

Candidate location for NGTS at ESO-Paranal.
Image from Google Earth.
The 600-900nm wavelength region includes strong water bands whose rapid variations will degrade the quality of the photometry by adding short-term variable color effects. The quality of the site in this respect crucially affects the discovery potential of the survey. Another key parameter affecting the final planet catch is the number of clear nights per year. These factors have driven the selection of Paranal as the site for NGTS.