Published Planet Candidates from Planet Hunters NGTS!

Good news everyone! Our paper describing the Planet Hunters NGTS project and presenting the first 5 planet candidates discovered by the project has been published in the Astronomical Journal!

The paper goes into the nitty gritty details of how the project operates, how we combine all the responses we received for the first dataset we uploaded (over 2.6 million individual classifications!) and how the list of planet candidates are selected for further observations. The most exciting part of the paper is the section where we present the 5 new planet candidates found by you, the citizen scientists! These planet candidates were not previously identified by the NGTS team or as TESS Objects of Interest, meaning some of you were the first people to ever suspect these could be possible planets. The paper includes a link to the project results page which lists the names of all the citizen scientists who classified these candidates at any stage of the project and you can find that list here too! Although none of these candidates are “confirmed planets” yet, we’re continuing to gather and analyse more data on each of them and hope to announce the confirmation of one or more of these planet candidates in the near future. Even at this stage, these candidates are interesting discoveries, in particular the excitingly named TIC-165227846 and TIC-135251751, which I’ll give a bit more detail on below.

TIC-165227846 is an exciting system as it’s a rare find. We think this could be a giant planet (bigger than Jupiter) orbiting very close to a low-mass star known as an M-dwarf. There’s only around 10 of these systems that have been detected so far and, if confirmed, ours would be the lowest mass star to host a close-in giant planet discovered to date! These types of systems are interesting because, quite simply, we don’t think they should be able to exist. A newly formed star can be surrounded by what we call a “protoplanetary disk,” which is a load of dust and gas that provides the material necessary to form planets. We currently believe that most planets form via “core accretion” where this dust and gas sticks together and forms a planet over time. However the amount of material available in the protoplanetary disk correlates with the mass of the star, therefore a low-mass star such as this will have less material available to form such a large planet! Another barrier is the time it takes for a planet to form around a low-mass star is longer (the lower mass means that things move more slowly around it) but planet formation is on a time limit as the protoplanetary disk gets dispersed as the star evolves. By discovering interesting systems such as TIC-165227846 that challenge our current theories of planet formation, we can test and tweak our models to gain a better understanding of how planets come to exist.

TIC-135251751 is an interesting system with an even more interesting story of how we came to learn (part of) its true nature. When we first discovered this system, we thought we were seeing a giant planet orbiting close to an ageing star, known as a subgiant. Don’t be fooled by the name though as this star apparently had a radius 2.4 times that of our Sun. These kinds of systems are interesting because we don’t often expect to find planets close to stars that are evolving into the later stages of their lifecycle. This is because these stars begin to expand and can engulf any planets close to them. However, one of the key steps in validating whether or not a possible transit signal is really due to a planet is to obtain speckle imaging, which you can read more about here. In short, speckle imaging allows us to observe close to the host star to see if there are any other stars nearby that could be affecting our measurements. When we observed TIC-135251751 using the Zorro instrument at Gemini South, the wonderful Zorro team found that this star was actually two stars in a binary system! Though it doesn’t look like much, the slightly elongated shape of the red spot in the inset of the figure below is the evidence that tells us that this system is not as it initially seemed.

While initially disheartening to find that we were actually looking at two stars, meaning that further observations and analysis would be challenging, the reality is that the transiting signal we detect is possibly still due to a planet and is not a result of the 2 stars passing in front of each other (like an eclipsing binary). These stars are orbiting with a period of ~50 years while the transit signal occurs approximately every 4 days. We could be observing a planet orbiting one of the two stars in this close binary system which would be another interesting discovery for Planet Hunters NGTS as these types of systems also pose interesting questions for our theories on planet formation.

This paper and these discoveries are just the first step for Planet Hunters NGTS. We will continue to analyse these systems with the belief that we can report the confirmation of a bonafide planet soon. Meanwhile, we’ll continue to analyse the newer datasets as classifications roll in, including the latest data from NGTS which is live on the site now and hasn’t been looked at by anyone before. This means that you could be the first person to spot a new planet in the NGTS data, so get classifying and maybe one day we’ll be publishing a paper with a discovery you helped find!