NGTS (Next-Generation Transit Survey)
NGTS is a ground-based project searching for transiting exoplanets orbiting bright stars. NGTS builds on the legacy of previous surveys, most notably WASP (Wide Angle Search for Planets) , and is designed to achieve higher photometric precision and hence find smaller planets than have previously been detected from the ground. It also operates in red light, maximizing sensitivity to late K and early M dwarf stars. The survey specifications call for photometric precision of 0.1 per cent in red light over an instantaneous FOV (Field of V)ew) of 100º x 100º, enabling the detection of Neptune-sized exoplanets around Sun-like stars and super-Earths around M dwarfs. The survey is carried out with a purpose-built facility at Cerro Paranal, Chile, which is the premier site of ESO (European Southern Observatory). 1)
An array of twelve 20 cm f/2.8 telescopes fitted with back-illuminated deep-depletion CCD cameras is used to survey fields intensively at intermediate Galactic latitudes. The instrument is also ideally suited to ground-based photometric follow-up of exoplanet candidates from space telescopes such as TESS (Transiting Exoplanet Survey Satellite), Gaia and PLATO (PLAnetary Transits and Oscillation of stars).
The NGTS facility is funded by by the University of Warwick, the University of Leicester, Queen's University Belfast, the University of Geneva, the DLR (German Research Center), the University of Cambridge and the UK Science and Technology Facilities Council (STFC; project reference ST/M001962/1). The facility is operated by the consortium institutes with support from STFC (also project ST/M001962/1).
ESO provided the Paranal site as well as generous in-kind support. The research leading to these results has received funding from the European Research Council (ERC) under the European Union's Seventh Framework Program (FP/2007-2013)/ERC Grant Agreement n. 320964 (WDTracer). The contributions at the University of Warwick by PJW, RGW, DLP, FF, DA, BTG and TL have been supported by STFC through consolidated grants ST/L000733/1 and ST/P000495/1. The Chilean astronomical community is currently represented by the Pontificia Universidad Católica de Chile and the Universidad de Chile.
Science with the NGTS: The primary goal of the NGTS is to search for transiting exoplanets around bright, nearby stars using a technique called transit photometry, which precisely measures the slight dimming of a star's light when a planet passes in front of it. Recent discoveries of Neptune-sized planets and super-Earths in other planetary systems have revealed that low-mass planets around solar-type stars are very common, but their composition and structure are extraordinarily diverse and remain open areas of study. The NGTS is designed to fill this observation gap between Earth-sized planets and gas giants, by searching for planets with diameters between two and eight times that of Earth. 2)
The planets discovered with the NGTS will be studied in greater detail using other larger telescopes, including ESO's VLT (Very Large Telescope). One key goal of the survey is to find small planets around stars that are bright enough to allow the planets' masses to be measured, which will provide clues about their composition. The masses of the planets will be measured with ESO's HARPS (High Accuracy Radial velocity Planet Searcher)and ESPRESSO (Echelle SPectrograph for Rocky Exoplanet- and Stable Spectroscopic Observations) instruments, using the radial velocity method.
NGTS has been operating routinely since April 2016 at the ESO Paranal Observatory, Chile. The high photometric precision – better than 1 mmag (milli Magnitude) for most stars brighter than 12th magnitude – is made possible by the quality of the Paranal site, precise autoguiding, and an instrument design that ensures stable pointing and image shape. This stability also allows the application of a centroiding method that efficiently identifies false-positive transits. Transit injections show that most signals deeper than 0.2 percent are detected at short orbital periods and most signals deeper than 0.5 percent are detected even at periods as long as 20 d (days).
The photometric precision of NGTS is comparable to that of TESS for stars fainter than the scintillation limit (I ~12.5), and flexibly scheduled NGTS photometry of single-transit candidates from TESS will extend planet discoveries to longer orbital periods than can be detected in the standard 27 d TESS dwell time. NGTS photometry will also test TESS candidates for false positives due to blended eclipsing binaries and provide precise ephemerides for atmospheric characterization with flagship facilities such as JWST (James Web Space Telescope) and E-ELT (European Extremely Large Telescope).
NGTS data are also being used for a wide range of variable star studies. When TESS data become available, a joint analysis with archival NGTS survey data will allow searches for shallow transit signals at long orbital periods that are not detectable in either instrument individually. NGTS also stands ready to support PLATO by characterizing stellar variability and activity in advance of target selection, and it will be able to search for transits of wide-separation exoplanets with edge-on orbits detected in Gaia astrometry.
In the search for planets in other stellar systems, the NGTS — Next-Generation Transit Survey — is part of Paranal's arsenal of telescopes. Built by a consortium of UK, Swiss and German universities and agencies, the NGTS is the first telescope at Paranal to not be operated directly by ESO. Instead it works in synergy with ESO's telescopes at the site, including the VLT (Very Large Telescope) and VISTA (Visible and Infrared Survey Telescope for Astronomy). For the NGTS to work effectively, it requires an atmosphere with low levels of water vapor, and excellent viewing conditions — the sort that can be found at Paranal Observatory in northern Chile. 3)
Figure 1: NGTS is located at ESO's Paranal Observatory in northern Chile. This project will search for transiting exoplanets — planets that pass in front of their parent star and hence produce a slight dimming of the star's light that can be detected by sensitive instruments. The telescopes will focus on discovering Neptune-sized and smaller planets, with diameters between two and eight times that of Earth. This engineering rendering shows the complete system in its enclosure (image credit: ESA, R. West)
This great accuracy of brightness measurement, across a wide field, is technically demanding, but all the key technologies needed for NGTS were demonstrated using a smaller prototype system, which operated on La Palma in the Canary Islands during 2009 and 2010. NGTS also builds on the success of the SuperWASP experiment, which up to now leads in the detection of large gaseous planets.
The discoveries of NGTS will be studied further using other larger telescopes, including the ESO VLT (Very Large Telescope). One goal is to find small planets that are bright enough for the planetary mass to be measured. This will allow planetary densities to be deduced, which in turn provides clues about the composition of the planets. It may also be possible to probe the atmospheres of the exoplanets whilst they are in transit. During the transit some of the star's light passes through the planet's atmosphere, if it has one, and leaves a tiny, but detectable, signature. So far only a few such very delicate observations have been made, but NGTS should provide many more potential targets.
This is the first telescope project hosted, but not operated, by ESO on Paranal. Several telescope projects operating under similar arrangements are already at work at the older La Silla Observatory. The NGTS data will flow into the ESO archive system and will be available to astronomers worldwide for decades to come.
Table 1: Overview of NGTS parameters and events
The NGTS facility was constructed during 2014 and 2015 at the ESO Paranal Observatory in northern Chile. It consists of an array of 12 independently steerable 20 cm telescopes, with a combined instantaneous field of view of 96º x 96º. The telescopes are housed in a single enclosure, with a roll-off roof, located about 900 m from ESO's VISTA telescope and at an altitude of 2440 m.
NGTS consists of a cluster of twelve identical telescope units, each unit comprising a 20 cm f/2.8 astrograph and a 2 k x 2 k deep-depleted CCD camera mounted on an independently steerable fork mount. Each unit has a field of view of approximately 3 x 3 degrees, yielding a total field of view for the whole facility of approximately 100º x 100º (roughly equivalent to that of the Kepler satellite). A photograph of the NGTS facility at Paranal is shown in Figure 4.
In general, NGTS utilizes commercial off-the-shelf products which have been modified to optimize their performance. For example, the detectors are deep-depleted e2v (CCD-42) devices packaged into a camera by Andor Technology (model iKon-L). Andor manufactured the original WASP cameras so we already had a good working relationship with this company. Optimization of these devices proved more demanding than we expected and the company expended considerable effort to produce devices suitable for the extreme accuracies we were trying to achieve. Similarly, the telescopes are H-series astrographs from ASA (Astro Systeme Austria), but fitted with a custom corrector optic designed to NGTS specifications. The NGTS enclosure was designed and fabricated by GR-PRO in the UK; it is of glass-reinforced plastic (GRP) composite construction, and has a footprint of around 10 x 15 meters.
The NGTS facility is fully robotic and operates unsupervised, following a pre-generated schedule that is prepared on a daily basis. The survey data are transferred to the NGTS Data Center at the University of Warwick, UK, to be analyzed using sophisticated automated algorithms that search for the tell-tale signatures of an exoplanet transiting its host star. Detections made by these algorithms are further vetted by automatic and manual means, and the highest quality candidates are passed for photometric and spectroscopic follow-up using larger facilities (e.g., the CORALIE spectrograph on the EULER telescope at La Silla). Processed NGTS light-curves will be made available to the community via the ESO Science Archive Facility after a proprietary period (two years for the first release, one year for subsequent releases).
Status and some images of NGST
• 17 April 2019: A stellar flare ten times more powerful than anything seen on our sun has burst from an ultracool star almost the same size as Jupiter. The star is the coolest and smallest to give off a rare white-light superflare, and by some definitions could be too small be considered a star. 4)
- The discovery, funded by the Science and Technology Facilities Council, is published in the Monthly Notices of the Royal Astronomical Society: Letters as the version of record today (17 April) and sheds light on the question of how small a star can be and still display flaring activity in its atmosphere. Flares are thought to be driven by a sudden release of magnetic energy generated in the star's interior. This causes charged particles to heat plasma on the stellar surface, releasing vast amounts of optical, UV and X-ray radiation. 5)
- Lead author James Jackman, a PhD student in the University of Warwick's Department of Physics, said: "The activity of low mass stars decreases as you go to lower and lower masses and we expect the chromosphere (a region of the star which support flares) to get cooler or weaker. The fact that we've observed this incredibly low mass star, where the chromosphere should be almost at its weakest, but we have a white-light flare occurring shows that strong magnetic activity can still persist down to this level.
- "It's right on the boundary between being a star and a brown dwarf, a very low mass, substellar object. Any lower in mass and it would definitely be a brown dwarf. By pushing this boundary we can see whether these type of flares are limited to stars and if so, when does this activity stop? This result takes us a long way to answering these questions."
Figure 2: A superflare on an L-dwarf (image credit: University of Warwick/Mark Garlick)
- The L dwarf star located 250 light years away, named ULAS J224940.13-011236.9, is only a tenth of the radius of our own sun, almost the same size as Jupiter in our solar system. It was too faint for most telescopes to observe until the researchers, led by the University of Warwick, spotted the massive stellar explosion in its chromosphere in an optical survey of the surrounding stars.
- Using the NGTS (Next Generation Transit Survey) facility at the European Southern Observatory's Paranal Observatory, with additional data from the Two Micron All Sky Survey (2MASS) and WISE (Wide-field Infrared Survey Explorer), they observed the brightness of the star over 146 nights.
- The flare occurred on the night of 13 August 2017 and gave off energy equivalent to 80 billion megatons of TNT, ten times as much energy as the Carrington event in 1859, the highest energy event observed on our sun. Solar flares occur on our Sun on a regular basis, but if the Sun were to superflare like this star the Earth's communications and energy systems could be at serious risk of failing.
- It is one of the largest flares ever seen on an L dwarf star, making the star appear 10,000 times brighter than normal.
- James adds: "We knew from other surveys that this kind of star was there and we knew from previous work that these kinds of stars can show incredible flares. However, the quiescent star was too faint for our telescopes to see normally — we wouldn't receive enough light for the star to appear above the background from the sky. Only when it flared did it become bright enough for us to detect it with our telescopes."
- James's PhD supervisor Professor Peter Wheatley said: "Our twelve NGTS telescopes are normally used to search for planets around bright stars, so it is exciting to find that we can also use them to find giant explosions on tiny, faint stars. It is particularly pleasing that detecting these flares may help us to understand the origin of life on planets."
- L dwarfs are among the lowest mass objects that could still be considered to be a star, lying in the transition region between stars and brown dwarfs. Brown dwarfs are not massive enough to fuse hydrogen into helium as stars do. L dwarfs are also very cool compared to the more common main sequence stars, such as red dwarfs, and emit radiation mostly in the infra-red, which may affect their ability to support the creation of life.
- James adds: "Hotter stars will emit more in the optical spectrum, especially towards the UV. Because this star is cooler, around 2000 kelvin, and most of its light is towards the infrared, when it flares you get a burst of UV radiation that you wouldn't normally see.
- "To get chemical reactions going on any orbiting planets and to form amino acids that form the basis of life, you would need a certain level of UV radiation. These stars don't normally have that because they emit mostly in the infra-red. But if they produced a large flare such as this one that might kickstart some reactions."
- Professor Wheatley adds: "It is amazing that such a puny star can produce such a powerful explosion. This discovery is going to force us to think again about how small stars can store energy in magnetic fields. We are now searching giant flares from other tiny stars and push the limits on our understanding of stellar activity."
• 3 November 2017: Exoplanets have been found in all different types and sizes, showing how much diversity there is among planets outside of our own Solar System. Now another one, a "monster exoplanet," is of interest to astronomers because according to current models of planetary formation, it shouldn't exist – but does. 6) 7)
- The giant planet, called NGTS-1b, is known as a "hot Jupiter," a gas giant similar to Jupiter but scorching hot due to orbiting close to its star. That's not so unusual, but what is odd is that the planet orbits a very small red dwarf star. Current understanding of planetary formation says that such a large planet shouldn't be able to form easily around such small stars.
- It should be noted that although this is actually the third hot Jupiter found orbiting a red dwarf star, it is also the largest planet compared to the size of its companion star ever discovered so far, making it a bit more unique.
- NGTS-1b, about 600 light years away, orbits its star in only 2.6 days and its orbit is only 3 percent of the distance between Earth and the Sun.
- According to Professor Peter Wheatley from the University of Warwick, "Despite being a monster of a planet, NGTS-1b was difficult to find because its parent star is so small and faint. Small stars like this red M-dwarf are actually the most common in the Universe, so it is possible that there are many of these giant planets waiting to found."
- The planet was discovered by monitoring patches of the night sky over many months, and detecting the red light from the star with innovative red-sensitive cameras. — Hot Jupiters have been found to be quite common in our galaxy, but mostly orbiting larger stars.
- As Dr. Daniel Bayliss, lead author of the study and also from the University of Warwick, noted, "The discovery of NGTS-1b was a complete surprise to us – such massive planets were not thought to exist around such small stars – importantly, our challenge now is to find out how common these types of planets are in the Galaxy, and with the new Next-Generation Transit Survey (NGTS) facility we are well-placed to do just that."
- NGTS-1b is also the first exoplanet to be discovered by NGTS.
Figure 3: Artist's conception of the gas-giant exoplanet NGTS-1b (image credit: University of Warwick/Mark Garlick)
- "Having worked for almost a decade to develop the NGTS telescope array, it is thrilling to see it picking out new and unexpected types of planets," said Wheatley. "I'm looking forward to seeing what other kinds of exciting new planets we can turn up."
- So far, about 3,500 exoplanets have been found in our galaxy by the Kepler Space Telescope and other observatories. The total number is estimated to be in the billions for our galaxy alone, and some studies have even suggested that there are more planets than stars. Many are hot-Jupiters, but the most common appear to be smaller planets, including "super-Earths" which are rocky planets larger than Earth but smaller then Neptune. That is good news as far as the search for extraterrestrial life is concerned. In the next few years, telescopes will also be better at analyzing the atmospheres of some of these planets, looking for possible biosignature gasses such as oxygen or methane.
• September 2016: The NGTS has commenced science operations at Paranal. 8) First light at Paranal was achieved with the first NGTS telescope in January 2015 and the transit survey began with five telescopes in August 2015. The full complement of twelve telescopes became operational in February 2016 and, despite the poor weather associated with El Niño, NGTS has already acquired more than 3.5 million science images. The quality of the data produced by NGTS is exceptional, and the instrument has proven itself well capable of delivering the required sub-millimagnitude photometric precision.
Figure 4: Photograph of the NGTS facility at Paranal with the VLT in the background to the west (image credit: NGTS consortium)
- As an example of the step-change improvement that NGTS represents over previous ground-based surveys, Figure 5 compares the light-curve of a single transit of a known exoplanet (the "hot Jupiter" WASP-4b) taken with NGTS, with the best quality transit in the WASP (Wide Angle Search for Planets) survey data. The WASP data show instrumental effects that have a magnitude similar to that of the transit signal, so WASP required coverage of many transits in order to make a significant detection. NGTS on the other hand could have detected WASP-4b with a single transit.
Figure 5: Light curves of single transits of the Jupiter-sized exoplanet WASP-4b measured with NGTS (upper) and WASP (lower graph), image credit: NGTS consortium
- A preliminary analysis of the data from the first six months of the full NGTS survey has yielded several dozen candidate planets which are we currently following up. Recent simulations based on the actual performance of NGTS have shown that the full NGTS survey can be expected to detect several super-Earths (planets with a radius less than twice that of the Earth), tens of Neptune-sized planets (2–6 Earth radii), and more than 200 planets with a radius larger than Saturn's (6–22 Earth radii). NGTS is most sensitive to planets with orbits of less than 20 days.
• 14 January 2015: NGST is a wide-field observing system made up of an array of twelve telescopes, each with an aperture of 20 cm. This new facility, built by a UK, Swiss and German consortium, is located at ESO's Paranal Observatory in northern Chile and benefits from the superb observing conditions and excellent support facilities available at this site. 9)
"We needed a site where there were many clear nights and the air was clear and dry so that we could make very accurate measurements as often as possible — Paranal was the best choice by far," says Don Pollacco of the University of Warwick in the UK and one of the NGTS project leads.
NGTS is designed to operate in a robotic mode and it will continuously monitor the brightness of hundreds of thousands of comparatively bright stars in the southern skies. It is searching for transiting exoplanets and will reach a level of accuracy in measuring the brightness of stars — one part in a thousand — that has never before been attained with a ground-based wide-field survey instrument.
Figure 6: This night time long-exposure view shows the NGST telescopes during testing. The very brilliant Moon appears in the center of the picture and the VISTA (right) and VLT (left) domes can also be seen on the horizon (image credit: ESO, G. Lambert)
Figure 7: This ESOcast takes a close look at an unusual new group of small telescopes that has recently achieved first light at ESO's Paranal Observatory in northern Chile (video credit: ESO, Published on 14, 2015)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com).