Very pleasing news yesterday. After many years hard work, our project to find extra-solar planets transiting their host stars, called the Next Generation Transit Survey, has announced its first discovery, a planet called NGTS-1b.
It’s a very unusual planet too. NGTS-1b the most massive planet ever found transiting a type of star called a red dwarf (M dwarf in astronomy classification). These are some of the most common stars in the galaxy, but they are much fainter and cooler than our own Sun. The planet has a mass about 80% that of Jupiter, and a radius somewhere between 1 and 2 times that of Jupiter. There’s a bit of uncertainty because the planet is so large compared to its host star. In fact, this planet is the largest ever found in comparison to the size of its host star. Rather than completely transit the star, its orbital path grazes along the side.
Artist’s impression of NGTS-1b and its red dwarf star (credit Mark Garlick & Uni Warwick)
Like Jupiter, the planet is a gas giant. Out of about 3500 confirmed exoplanets, it’s only the third known gas giant that transits a red dwarf. This discovery confirms not only that large planets can form around red dwarfs, but that they must migrate inwards through the red dwarf’s proto-planetary disk to end up in such tiny orbits. NGTS-1b orbits its host star every 2.6 days, which is too close for the dust and ice grains which originally coalesced together to build the planet to have existed. It’s just too hot there. So the planet must have formed much further away from the star, and then migrated inwards. This process must have occurred for the hundreds of gas giants previously discovered near Sun-like stars, but hadn’t been well established for planets around red dwarfs.
When a planet passes in front of its parent star, as seen along our line of sight, the star’s brightness drops by a small fraction for a few hours.
The dip that gives away NGTS-1b: note that it is V-shaped rather than U-shaped. That’s because the planet is so large compared to the host star, and grazes in front of the star’s edge.
NGTS works by searching for the tiny dips in the brightness of a star when an orbiting planet passes in front of it, as seen by us. This is called a transit. NGTS uses a suite of twelve small, robotically controlled telescopes housed together at the Paranal observatory in the Atacama desert in Chile. Paranal is the home of the European Southern Observatory’s four giant 8m telescopes called the Very Large Telescope, and we are privileged to enjoy what is basically the best site in the world to study the night sky.
The NGTS telescopes monitor millions of stars for months at a time, taking exposures every ten seconds at a precision of 0.1%. This makes it the world’s most sensitive ground-based survey for transiting planets. Full operations began in early 2016 after a lengthy prototyping and commissioning phase (I blogged about the foundation of the project and running the prototype back in 2014). The data are returned to Warwick University where they are processed. Automatic algorithms search for the signatures of transiting planets, but they find far more “candidates” than real planets.
In the end, human eyes have to scan many thousands of candidates to pick out the most likely planets. The best are then sent to some of the world’s largest professional telescopes for follow-up measurements to confirm whether the transits are real, and to measure the masses of the candidates. Many turn out to be binary stars, but in this case we have a genuine planet. NGTS-1b was spotted by us over a year ago, among some of the first data returned from Chile. But it took time to gather all these follow-up data, confirm it was a planet, and write the paper.
This summer and autumn, together with lots of colleagues across our partner institutions (Warwick, Leicester, Belfast, Cambridge, Geneva, DLR Berlin, and the University de Chile), I’ve spent many hours scanning thousands of NGTS light curves looking for the signs of transiting planets, and other interesting stars. It’s been a lot of fun, and I’ve learnt a lot along the way.
All this has been made possible by years of hard work by the team members, in building the telescopes, writing software, and calibrating the detectors (a lot of this latter work was done by my Leicester colleague Mike Goad with PhD students Andy Grange, Alex Chaushev, and Liam Raynard). I’m very grateful to and proud of everyone in the NGTS consortium for their hard work, dedication and enthusiasm. There will be plenty more exciting discoveries to tell you about in the near future…..
The paper, “NGTS-1b: a hot Jupiter transiting an M dwarf” is available from the arXiv
University of Leicester press release
Matt Burleigh's Blog 
Leave a Reply