Planet Hunters NGTS: More detail on our first four Planet Candidates

You may have seen my previous post announcing that we have 4 new planet candidates discovered by Planet Hunters NGTS, if not you can find it here. We wanted to give you some more information on what we know about each candidate so far and the efforts we’ve made to gather follow-up data as we work towards ascertaining whether or not they’re real exoplanets! The first step once we decided that these candidates were worthy of further investigation was to fit the data using a more intricate model. This is typically what’s meant by phrases like “modelling estimates” or “fit results” that you may see below or in other pieces of science communication. Each of these candidates pose their own unique challenges in being able to confirm whether they’re really planets, however to already have 4 planet candidates is a promising sign for the future of the project and a real testament to the brilliant work put in by you, the Planet Hunters NGTS volunteers.

Subject 69695263: This is our most promising candidate so far as the transits look quite clear and our modelling estimates give encouraging results. We think this is a hot Jupiter orbiting a “K dwarf star,” which is a star slightly smaller and cooler than our Sun. We’ve estimated that the planet candidate has a radius equal to 1.09 times the radius of Jupiter, which we write as R_p=1.09 R_Jup. It orbits its host star every 1.74 days which means by the time you turn 18 Earth-years-old, you’d be almost 3800 years-old on this planet candidate (if it exists, and assuming you could live on it, which you can’t). We have received data from the Zorro team for this candidate (read more about the Zorro instrument and our follow-up efforts here), and it shows that the host star is isolated from other stars as far as we can tell, which means that we can be slightly more confident that this isn’t a false positive signal caused by an eclipsing binary! There’s still plenty of work to do before we can approach the possibility of confirming or validating this as a planet, but we’re cautiously optimistic about this candidate.

Subject 69593857:

This candidate has been causing something of a headache since we only managed to observe one full transit and a number of partial transits (i.e. we only see the start or end of the transit). We have estimated that the potential orbiting body has an orbital period of 3.97 days however our radius estimates range from Jupiter-sized up to a radius not much smaller than our Sun. This suggests we could have a planet candidate but it could also be what’s known as an eclipsing binary with low mass (EBLM). While these are interesting systems, they’re not the planets we’re looking for! We do have Zorro data for this candidate and we can’t see another star nearby however it’s possible that the signal could be a false positive caused by “systematics” which is a general term for any trend in the data that might not have been accounted for during processing.

Subject 69473954:

Much like the candidate above, this candidate is a troublesome one. The transits appear to be grazing (check out this post for a description of grazing transits) which makes it tricky to get a good estimate of the radius of the candidate. The radius of the planet candidate is estimated to be at least R_p=1.50 R_Jup which is already at the upper limit for what we believe can be a planet. Our upper estimate of the radius is one solar radius, meaning this could be a standard eclipsing binary. Sadly we don’t have Zorro data for this candidate to be able to check this just yet. This candidate, and the previous one, will require some more investigation to really understand what they actually are.

Subject 69654531:

Lastly, this candidate is very exciting because if our initial estimates are correct, this would be one of only a handful of discoveries to date of giant planets orbiting close to their small host star. The transit depth of ~12% is much deeper than we typically expect for exoplanet transits, however the host star has a radius equal to only 1/3rd the radius of our Sun. Therefore, we believe this could be a hot Jupiter, with a radius of R_p=1.13 R_Jup, orbiting an M dwarf star on a 2.10 day period. These types of systems are very rare and really surprising to find because our current understanding of how planets form suggests that such a large planet shouldn’t be able to form around such a small star because there simply shouldn’t be enough time for it to form or planet building material available. The Zorro data for this candidate is also encouraging as we see no companion stars. We hope to gather more data on this object in the coming months if possible, so stay tuned!

These four NEW candidates are really exciting! They weren’t spotted in the initial check of the data by the NGTS team which means that the Planet Hunters NGTS project has proven it really has the potential to be a complimentary search to help NGTS discover as many planets as possible. We couldn’t have done this without the help of everyone who’s classified subjects on Planet Hunters NGTS and we’re eager to see what else you can help us find in the future as there are still plenty of subjects to be classified! I’ll be presenting these candidates next week at the UK Exoplanet Meeting in Edinburgh before checking the latest results from the project for more possible planet candidates. We’ll also be starting to write up the first results from the project into a paper to be submitted to an academic journal!